economical structural steelwork student edition - 2004
AUSTRALIAN STEEL INSTITUTE
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(ABN)/ACN (94) 000 973 839
economical structural steelwork student edition - 2004
AUSTRALIAN STEEL INSTITUTE (ABN)/ACN (94) 000 973 839
2
ECONOMICAL STRUCTURAL STEELWORK
Student Edition 2004 Limit States Edition to AS 4100 -- 1990 AUSTRALIAN STEEL INSTITUTE --1979, 1984, 1991, 1996, 2004 NATIONAL LIBRARY OF AUSTRALIA CARD NUMBER AND ISBN 0 909945 77 2 FIRST EDITION 1979 SECOND EDITION 1984 THIRD EDITION 1991 REPRINTED 1992, 1995 FOURTH EDITION 1996
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E Australian Institute of Steel Construction 1996 While every effort has been made and all reasonable care taken to ensure the accuracy of the material contained herein the Authors, Editors and Publishers of this Publication shall not be held to be liable or responsible in any way whatsoever for any loss or damage costs or expenses howsoever incurred by any person whether the purchaser of this work or otherwise including but without in any way limiting any loss or damage costs or expenses incurred as a result of or in connection with the reliance whether whole or partial by any person as aforesaid upon any part of the contents of this publication. Should expert assistance be required, the services of a competent professional person should be sought.
FOREWORD
’Economical Structural Steelwork’ was first published in 1979 and quickly became the Institute’s most popular publication among practising designers and students. This fourth edition has been up--dated in its references to Australian Standards, practices and cost concepts, and has other amendments. It continues to provide the useful practical advice given by its previous editions towards the achievement of the optimum result in structural steelwork.
While every effort has been made and all reasonable care taken to ensure the accuracy of the material contained herein the Authors, Editors and Publishers of this Publication shall not be held to be liable or responsible in any way whatsoever for any loss or damage costs or expenses howsoever incurred by any person whether the purchaser of this work or otherwise including but without in any way limiting any loss or damage costs or expenses incurred as a result of or in connection with the reliance whether whole or partial by any person as aforesaid upon any part of the contents of this publication.
E Australian Institute of Steel Construction 1996
Should expert assistance be required, the services of a competent professional person should be sought.
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Preface
When considering steel structures it is not difficult to obtain information on engineering and technological aspects, but very little is available on how to choose steelwork economically. Yet more and more the viability of a building project depends upon critical financial considerations. Thus it is important for designers to have a good general appreciation of the components that make up the cost of fabricated steel, and of how decisions made at the design stage can influence these costs.
metre for sections and cost per square metre for plates, depending on the size, in lieu of cost per tonne. The reasoning behind this is presented in a paper entitled: ”A Rational Approach to Costing Steelwork” by T Main, K B Watson and S Dallas. This paper was presented at the International Cost Engineering Council/The Australian Institute of Quantity Surveyors International Symposium, Construction Economics -- The Essential Management Tool, Australia, May 1995.
This publication aims to supply some of this information. It is not a design manual, but rather a publication that discusses from a cost point of view all of the matters that a structural steel designer should consider. It takes into account current fabrication practices and material/labour relationships, both of which have changed markedly over the last few years.
We wish to thank all those who have contributed to this publication and special acknowledgment goes to all AISC Staff who submitted comments on the technical and editorial content of this publication.
Adherence to the principles outlined in this publication will do much to assist designers in reaching decisions that will lead to effective and economic structures. It should be noted that this edition has substantially adopted the rationalised approach to the costing of fabricated steel by using a cost per
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edited by: Jose’ R Zaragoza
BScCE, MScCE, CP Eng. AISC State Manager--N SW
ECONOMICAL STRUCTURAL STEELWORK Contents
FOREWORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
1.
1
Preliminary Considerations . . . . . . . . . . . . . . . . . 1.1. 1.2. 1.3.
2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Factors influencing Framing Cost . . . . . . . . . . . . Integrated Design . . . . . . . . . . . . . . . . . . . . . . . . . .
General Factors Affecting Economy . . . . . . . . . 2.1.
Steel Grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1. 2.1.2. 2.1.3. 2.1.4. 2.1.5.
2.2.
Economy in use of Material . . . . . . . . . . . . . . . . .
2.2.1. 2.2.2. 2.2.3. 2.2.4.
2.3.
6
STEEL PRICING . . . . . . . . . . . . . . . . . . . . . . . . . . . . PLATES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SCRAP AND WASTE . . . . . . . . . . . . . . . . . . . . . . . .
Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1. 2.3.2. 2.3.3. 2.3.4.
2.4.
STRUCTURAL STEEL . . . . . . . . . . . . . . . . . . . . . . . WEATHERING STEEL . . . . . . . . . . . . . . . . . . . . . . . HOLLOW SECTIONS . . . . . . . . . . . . . . . . . . . . . . . . QUENCHED AND TEMPERED STEEL . . . . . . . . . CHOICE OF STEEL GRADE . . . . . . . . . . . . . . . . . .
GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . BEAM AND COLUMN FABRICATION . . . . . . . . . . GIRDER AND TRUSS FABRICATION . . . . . . . . . . SUMMARY FOR ECONOMIC FABRICATION . . .
Erection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1 1 2 3 3 3 3 3 4 4
6 6 6 7 7
7 7 8 8 9
9
2.4.1. 2.4.2. 2.4.3. 2.4.4. 2.4.5. 2.4.6.
2.5.
Surface Treatment . . . . . . . . . . . . . . . . . . . . . . . . .
2.5.1. 2.5.2. 2.5.3. 2.5.4. 2.5.5. 2.5.6. 14 2.5.7. 14
2.6.
12
SUMMARY CHECKLIST FOR SURFACE TREATMENT . . . .
GENERAL CONSIDERATIONS . . . . . . . . . . . . . . . REGULATORY REQUIREMENTS . . . . . . . . . . . . . MATERIALS FOR FIRE PROTECTION . . . . . . . .
Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7.1. 2.7.2. 2.7.3. 2.7.4. 2.7.5. 2.7.6. 2.7.7.
9 10 10 11 12 12
GENERAL CONSIDERATIONS . . . . . . . . . . . . . . . 12 STEEL PERFORMANCE . . . . . . . . . . . . . . . . . . . . . 13 SURFACE PREPARATION . . . . . . . . . . . . . . . . . . . 13 PAINT SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 HOT--DIP GALVANIZING . . . . . . . . . . . . . . . . . . . . . 14 DESIGN AND DETAILS FOR CORROSION RESISTANCE .
Fire Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6.1. 2.6.2. 2.6.3.
2.7.
GENERAL CONSIDERATIONS . . . . . . . . . . . . . . . HANDLING AND TRANSPORT . . . . . . . . . . . . . . . CONNECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . FIELD BOLTING . . . . . . . . . . . . . . . . . . . . . . . . . . . . FIELD WELDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . BRACING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
GENERAL CONSIDERATIONS . . . . . . . . . . . . . . . WORKMANSHIP STANDARDS . . . . . . . . . . . . . . . TOLERANCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CAMBERING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TEMPORARY BRACING . . . . . . . . . . . . . . . . . . . . . INSPECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SUMMARY FOR SPECIFICATION WRITERS . . .
15 15 15 16
16 16 17 17 18 19 19 20
3.
Framing Concepts and Connection Types . . . . 3.1. 3.2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Connection Types . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1. 3.2.2. 3.2.3.
3.3.
Basic Framing Systems . . . . . . . . . . . . . . . . . . . .
3.3.1. 3.3.2. 3.3.3. 3.3.4. 3.3.5.
3.4.
4.
MULTI--STOREY BUILDING . . . . . . . . . . . . . . . . . . SINGLE--STOREY INDUSTRIAL BUILDING . . . .
Framing Details . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.5.1. 3.5.2. 3.5.3. 3.5.4.
3.6.
TWO--WAY RIGID FRAMEWORK (Fig 3.3) . . . . . ONE--WAY RIGID FRAMEWORK (Fig 3.4) . . . . . . TWO--WAY BRACED FRAMEWORK (Fig 3.5) . . SUMMARY OF FRAMING SYSTEMS . . . . . . . . . . STABILISING ELEMENTS . . . . . . . . . . . . . . . . . . . .
Cost and Framing System . . . . . . . . . . . . . . . . . .
3.4.1. 3.4.2.
3.5.
DESIGN METHODS IN AS 4100 . . . . . . . . . . . . . . FLEXIBLE CONNECTIONS . . . . . . . . . . . . . . . . . . . RIGID CONNECTIONS . . . . . . . . . . . . . . . . . . . . . .
SYMMETRY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RATIONALISATION OF MEMBERS . . . . . . . . . . . STANDARDIZATION . . . . . . . . . . . . . . . . . . . . . . . . . SIMPLICITY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Industrial Buildings . . . . . . . . . . . . . . . . . . . . . . . . 4.1. 4.2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Warehouse and Factory Buildings . . . . . . . . . . .
4.2.1. 4.2.2. 4.2.3. 4.2.4. 4.2.5. 4.2.6. 4.2.7. 4.2.8.
4.3.
Large Span Storage Buildings . . . . . . . . . . . . . . .
4.3.1.
7
GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . STANDARDIZED PORTAL FRAMES . . . . . . . . . . . CUSTOM DESIGNED PORTAL FRAMES . . . . . . BRACING OF PORTAL FRAMES . . . . . . . . . . . . . CRANES IN PORTAL FRAME BUILDINGS . . . . . PURLINS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FLY BRACING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SHEETING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SPANS OF 45--70 METRES . . . . . . . . . . . . . . . . . .
21
4.3.2.
4.4.
21 21 21 22 23
24
4.4.1. 4.4.2. 4.4.3. 4.4.4.
5.
24 24 25 26 27
5.3.
31 32 32 32
5.4. 5.5.
33
5.6.
34 34 34 35 36 37 40 41 42 43
43 43
GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FULLY RIGID FRAME . . . . . . . . . . . . . . . . . . . . . . . FULLY BRACED FRAMES . . . . . . . . . . . . . . . . . . . STABILITY BY MEANS OF SERVICE CORES . .
Floor Support Systems . . . . . . . . . . . . . . . . . . . . . Composite Construction . . . . . . . . . . . . . . . . . . . .
5.5.1. 5.5.2.
6.
FULLY STEEL--FRAMED . . . . . . . . . . . . . . . . . . . . . COMPOSITE FRAMES . . . . . . . . . . . . . . . . . . . . . .
High--Rise Commercial Buildings . . . . . . . . . . . .
5.3.1. 5.3.2. 5.3.3. 5.3.4.
31
34
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Low--Rise Commercial Buildings . . . . . . . . . . . . .
5.2.1. 5.2.2.
30 31
ERECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SITE WELDING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . BOLTED CONNECTIONS . . . . . . . . . . . . . . . . . . . . FUNCTIONAL CONSTRAINTS . . . . . . . . . . . . . . . .
Commercial Buildings . . . . . . . . . . . . . . . . . . . . . . 5.1. 5.2.
29
SPANS IN EXCESS OF 70 METRES . . . . . . . . . .
Heavy Industrial Structures . . . . . . . . . . . . . . . . .
FLOOR SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . COLUMNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bolting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1. 6.2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bolt Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2.1. 6.2.2.
6.3. 6.4.
COMMERCIAL BOLTS . . . . . . . . . . . . . . . . . . . . . . . HIGH--STRENGTH STRUCTURAL BOLTS . . . . .
Bolting Categories . . . . . . . . . . . . . . . . . . . . . . . . . Factors Affecting Bolting Economy . . . . . . . . . . .
6.4.1. 6.4.2. 6.4.3. 6.4.4. 6.4.5. 6.4.6.
BOLT GRADE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . BOLT DIAMETER . . . . . . . . . . . . . . . . . . . . . . . . . . . BOLTING CATEGORY . . . . . . . . . . . . . . . . . . . . . . . THREADS IN OR OUT OF SHEAR PLANE . . . . . BOLT FINISH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . INSPECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
44
44 45 45 45 45
46 46 46 46 47
47 47 50 50 51
52 54 54 55
56 57 57 57 57 57
58 59 59 59 59 60 60 60
6.5.
Summary for Economic Bolting . . . . . . . . . . . . . .
6.5.1. 6.5.2. 6.5.3.
7.
Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1.1. 7.1.2.
7.2.
7.3. 7.4.
FILLET WELDS (see Fig 7.1) . . . . . . . . . . . . . . . . . BUTT WELDS (see Fig 7.2) . . . . . . . . . . . . . . . . . . . BUTT WELDS vs. FILLET WELDS . . . . . . . . . . . .
Welding Processes . . . . . . . . . . . . . . . . . . . . . . . . Other Cost Factors . . . . . . . . . . . . . . . . . . . . . . . .
7.4.1. 7.4.2. 7.4.3.
7.5.
PRINCIPLES FOR ECONOMY . . . . . . . . . . . . . . . . COST COMPONENTS . . . . . . . . . . . . . . . . . . . . . . .
Types of Welds . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2.1. 7.2.2. 7.2.3.
8.
CHECKLIST . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . BOLT USAGE -- FLEXIBLE JOINTS . . . . . . . . . . . BOLT USAGE--RIGID JOINTS . . . . . . . . . . . . . . . .
WELD CATEGORIES . . . . . . . . . . . . . . . . . . . . . . . . WELDING SPECIFICATIONS . . . . . . . . . . . . . . . . . WELDING INSPECTION . . . . . . . . . . . . . . . . . . . . .
Economical Design and Detailing . . . . . . . . . . . .
Detailing for Economy . . . . . . . . . . . . . . . . . . . . . . 8.1. 8.2.
8
Detailing on Design Engineer’s Drawings . . . . . Beams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
61
8.2.1. 8.2.2. 8.2.3. 8.2.4. 8.2.5.
61 62 63
64
8.3.
64
Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.3.1. 8.3.2. 8.3.3. 8.3.4. 8.3.5. 8.3.6.
64 64
65 65 66 66
8.4. 8.5.
67 68
8.6.
CONNECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . PORTAL FRAME PRE--SET . . . . . . . . . . . . . . . . . .
Connection Detailing . . . . . . . . . . . . . . . . . . . . . . .
8.6.1. 8.6.2.
70
GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . COLUMN BASE PLATES . . . . . . . . . . . . . . . . . . . . . HOLDING--DOWN BOLTS . . . . . . . . . . . . . . . . . . . . COLUMN SPLICES . . . . . . . . . . . . . . . . . . . . . . . . . . COLUMN STIFFENERS . . . . . . . . . . . . . . . . . . . . . . BUILT--UP COLUMNS . . . . . . . . . . . . . . . . . . . . . . .
Trusses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Portal Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8.5.1. 8.5.2.
68 69 69
GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PLATED SECTIONS . . . . . . . . . . . . . . . . . . . . . . . . . WEB PENETRATIONS IN BEAMS . . . . . . . . . . . . . CASTELLATED BEAMS . . . . . . . . . . . . . . . . . . . . . . THREE--PLATE GIRDERS . . . . . . . . . . . . . . . . . . . .
GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . SPECIFIC CONNECTIONS . . . . . . . . . . . . . . . . . . .
76 76 76 77 78
80 80 80 82 82 84 85
87 90 90 91
92 92 93
75
9.
References & Further Reading . . . . . . . . . . . . . .
103
75 76
10. Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
105
List of Tables
TABLE 2.1 -- AVAILABILITY OF PRODUCTS BY GRADE (check currency of information with steel suppliers) . . . . TABLE 2.2 INDICATIVE COST RATIOS FOR DIFFERENT GRADES OF STRUCTURAL STEEL (per tonne, supply only) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TABLE 2.3 PREFERRED STEEL PLATE THICKNESSES (in mm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TABLE 2.4 TRANSPORTATION COSTS . . . . . . . . . . . . . . . . . TABLE 2.5 SURFACE TREATMENT COSTS . . . . . . . . . . . . .
9
4 5 7 10 13
TABLE 2.6 PASSIVE FIRE PROTECTION COSTS . . . . . . . TABLE 5.1 SHEAR WALL vs LATTICE BRACING . . . . . . . . . TABLE 6.1 BOLT TYPES AND BOLTING CATEGORIES . . . TABLE 6.2 INDICATIVE COST RATIOS OF DIFFERENT BOLT DIAMETERS . . . . . . . . . . . . . . . . . . . . Table 6.3 INDICATIVE COST RATIOS OF DIFFERENT BOLTING CATEGORIES . . . . . . . . . . . . . . . . TABLE 7.1 FILLET WELD COMPARISON . . . . . . . . . . . . . . . . TABLE 8.1 WRENCH CLEARANCES . . . . . . . . . . . . . . . . . . .
16 52 58 59 60 65 89
List of Figures
Fig. 2.1 Deep end plates can cause jamming . . . . . . . . . . . . . Fig. 2.2 Two ways of avoiding the problem of access to column web connections . . . . . . . . . . . . . . . . Fig 2.3 Typical connections where allowance for mill tolerance is needed . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 3.1 Flexible connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 3.2 Rigid connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 3.3 Two--way rigid framework . . . . . . . . . . . . . . . . . . . . . . . . Fig 3.4 One--way braced, one--way rigid framework . . . . . . . . Fig 3.5 Two--way braced framework . . . . . . . . . . . . . . . . . . . . . Fig 3.6 Stabilising elements built in steel . . . . . . . . . . . . . . . . . Fig 3.7 Stabilising elements built in reinforced concrete or masonry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig. 3.8 Floor deck bracing systems . . . . . . . . . . . . . . . . . . . . . Fig 3.9 Action of lateral force resisting systems (from Ref 5.2) Fig. 3.10 Frame example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 3.11 Relationship between mass/unit area and span . . . Fig 3.12 Relationship between cost/unit area and span . . . . Fig. 3.13 Beams for economic fabrication . . . . . . . . . . . . . . . . Fig 4.1 Steelwork Cost Components for Warehouses . . . . . Fig 4.2 Configuration of framing systems for a factory building . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 4.3 Details of bolted portal frame . . . . . . . . . . . . . . . . . . . . Fig 4.4 Details for welded portal frame (with bolted rafter splice for field erection) . . . . . . . . . . . . . Fig 4.5 Transportation limitations for portal frames . . . . . . . . . Fig 4.6 Bracing panels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 4.7 Bracing for long buildings . . . . . . . . . . . . . . . . . . . . . . . . Fig 4.8 Details for rod bracing . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 4.9 Details for angle bracing . . . . . . . . . . . . . . . . . . . . . . . . .
10
11 11 18 22 24 24 25 26 27 28 28 29 30 31 31 32 35 35 36 37 37 38 38 39 39
Fig 4.10 Details for tubular bracing . . . . . . . . . . . . . . . . . . . . . . Fig 4.11 Types of supporting columns . . . . . . . . . . . . . . . . . . . . Fig 4.12 Crane runway brackets . . . . . . . . . . . . . . . . . . . . . . . . Fig 4.13 Commonly used sections for crane runway girders and their relative fabrication cost . . . . . . . . . . . . . . . . . . . . . Fig 4.14 Standard purlin cleats . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 4.15 Zed section purlins with lap . . . . . . . . . . . . . . . . . . . . . Fig 4.16 C section purlins with butt joint . . . . . . . . . . . . . . . . . . Fig 4.17 Method of fixing fly bracing to standard punching . . Fig 4.18 Three--pinned portal truss . . . . . . . . . . . . . . . . . . . . . . Fig 4.19 The basic square grid double layered space frame Fig 5.1 Framing system for low--rise commercial building . . . Fig 5.2 Stability by masonry . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 5.3 Stability by concrete panels . . . . . . . . . . . . . . . . . . . . . . Fig 5.4 -- Optimum steel framing systems for buildings of various heights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 5.5. Field welded connection details . . . . . . . . . . . . . . . . . . Fig 5.6 Shop welded connection details . . . . . . . . . . . . . . . . . . Fig 5.7 Forms of bracing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 5.8 Bracing should connect to column . . . . . . . . . . . . . . . . Fig 5.9 Service core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 5.10 Service core at end of building . . . . . . . . . . . . . . . . . . Fig 5.11 Floor support members . . . . . . . . . . . . . . . . . . . . . . . . Fig 5.12 Composite floor beam system . . . . . . . . . . . . . . . . . . . Fig 5.13 Welded stud shear connector . . . . . . . . . . . . . . . . . . . Fig. 5.14 Profiles of composite galvanized steel decking . . . Fig 5.15 Composite columns incorporating a steel erection column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 5.16 Composite column comprising a concrete--filled tubular section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 5.17 Cost Components for a Multi--Storey Building . . . . .
40 40 40 41 42 42 42 43 43 44 46 47 47 49 50 50 51 51 52 52 53 54 54 54 55 56 56
Fig 7.1 Types of fillet welds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 7.2 Types of butt welds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 7.3 Weld cost graph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 7.4 Welded beam--to--column moment connection . . . . . Fig 7.5 Stiffened web plate girder with web penetration . . . . Fig 7.6 Some common detailing faults resulting in poor accessibility for welding . . . . . . . . . . . . . . . . . . . . . . . . Fig 7.7 Use of bending to reduce welding and give clean corners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 7.8 Beam flange with many different plate thicknesses -- avoid when steel mass saved is less than 100 times mass of weld metal required . . . . . . . . . . . . . . . Fig 7.9 Exterior column/spandrel sub--assemblies for Sears Tower, Chicago . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 7.10 Angle seat detail -- (a) preferable to (b) . . . . . . . . . . Fig 7.11 These joints are difficult to weld and the welds may be of questionable quality . . . . . . . . . . . . . . . . . . . . . . Fig 8.1 Chain of communication . . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.2 Plated sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.3 Web penetrations in beams (in descending order of cost, (d) being least costly) . . . . Fig 8.4 Typical castellated beam geometry . . . . . . . . . . . . . . . Fig 8.5 Evaluation of economics of castellated beam . . . . . . Fig 8.6 One--sided intermediate web stiffener . . . . . . . . . . . . . Fig 8.7 Stiffened and unstiffened webs in three plate girders Fig 8.8 Column base plate details (moment resisting or fixed) Fig 8.9 Typical pinned base plates (full dimensional details can be found in Ref l) . . . . . . . . . Fig 8.10 Holding--down bolt details . . . . . . . . . . . . . . . . . . . . . . Fig 8.11 Typical holding--down bolt cage . . . . . . . . . . . . . . . . . Fig 8.12 Minimise number of column splices -- 1 is preferable to 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.13 Preferred column splice locations . . . . . . . . . . . . . . .
11
66 66 67 68 68 72 72 73 73 74 74 75 76 77 77 78 79 80 81 81 82 83 83 83
Fig 8.14 Economic details for built--up columns in ascending order of fabrication cost . . . . . . . . . . . . . . . . . Fig 8.15 Welded corner details for box columns . . . . . . . . . . . Fig 8.16 Connections to box columns . . . . . . . . . . . . . . . . . . . . Fig 8.17 Spreading of columns to allow for weld shrinkage . Fig 8.18 Equivalent truss detailing . . . . . . . . . . . . . . . . . . . . . . . Fig 8.19 Single angle welded truss . . . . . . . . . . . . . . . . . . . . . . Fig 8.20 Split tee welded truss . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.21 Use of universal sections in welded trusses . . . . . . . Fig 8.22 Use of rectangular hollow sections in welded trusses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.23 Types of open web joist . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.24 End plate details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.25 Clearance at apex joint . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.26 Termination of haunch . . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.27 Attachment of purlins and girts . . . . . . . . . . . . . . . . . . Fig 8.28 Precambering details of a rigid frame . . . . . . . . . . . . Fig 8.29 Typical beam details for fabrication economy . . . . . Fig 8.30 Typical floor beam layout . . . . . . . . . . . . . . . . . . . . . . . Fig 8.31 Angle seat connection . . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.32 Flexible end plate connection . . . . . . . . . . . . . . . . . . . Fig 8.33 Angle cleat connection . . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.34 Web side plate connection . . . . . . . . . . . . . . . . . . . . . Fig 8.35 Bearing pad connection . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.36 Welded moment connection . . . . . . . . . . . . . . . . . . . . Fig 8.37 Moment end plate connection . . . . . . . . . . . . . . . . . . . Fig 8.38 Welded splice connection . . . . . . . . . . . . . . . . . . . . . . Fig 8.39 Bolted splice connection . . . . . . . . . . . . . . . . . . . . . . . Fig 8.40 Stiffener connections . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.41 Bracing connections . . . . . . . . . . . . . . . . . . . . . . . . . . . Fig 8.42 Connections to concrete cores . . . . . . . . . . . . . . . . . .
85 85 86 86 87 87 87 88 89 89 89 90 91 91 91 92 92 94 94 95 96 96 97 98 99 100 101 102 102
1.
Preliminary Considerations
1.1.
Introduction
It is generally accepted that the objective of engineering design is the achievement of an acceptable probability that the structure being designed will retain its fitness for purpose during its planned lifetime. It is also of utmost importance that the initial costs plus the maintenance costs of the completed structure be within the limits provided by the Client. For the design to be successful in the sense just outlined, the designer should search for design alternatives which consider strength and serviceability on the one hand, and economic feasibility on the other. In other words, out of a number of alternative structural solutions which comply with accepted design criteria for strength and serviceability, the designer should select the alternative likely to be the lowest overall cost. To do this successfully, the designer should develop an appreciation of the basic sources of expenditure in building construction and their effect on the overall cost of construction. In practice, the design problem is an optimisation problem. The solution to any optimisation problem involves having some means of judging the overall merit of alternatives. With regard to a building, the measure of overall merit, usually provided by the Client, will involve one or more of the following criteria: (a) Functional requirements (b) Strength and serviceability (c) Aesthetic satisfaction (d) Economy in relation to capital and maintenance costs. This publication deals almost entirely with item (d) above. In the preliminary and final design, the designer often deals primarily with member design and consequently tends to consider the minimisation of the mass of the structure as a guiding criterion towards achieving minimum
1
cost. That is, the designer substitutes the more straightforward criterion of mass minimisation for the more involved criterion of minimum cost. In regard to steel structures, a minimum mass solution does not necessarily result in a minimum cost solution. Connection detailing and the resulting cost of fabrication and erection are more often the major influences affecting overall cost. Undue preoccupation with the minimisation of the mass of a steel structure can lead to serious errors of judgement. This publication is intended to highlight the manner in which a number of factors affect the cost of fabrication and erection. It will also highlight the influence these costs have on the total final cost of a steel structure.
1.2.
Factors influencing Framing Cost
Fabricated steel has been traditionally costed on a per tonne basis. Consequently in discussing the cost of fabricated steel the question often raised relates to how much is the cost per tonne of fabricated steel. Such a question usually ignores the fact that a large number of factors have a significant influence on the final cost of fabricated steel. A more rationalised approach to the costing of fabricated steel is based on a cost per metre for sections and cost per square metre for plates depending on the size of the member. Fabrication costs for connections and erection costs, etc can then be added on a component by component basis (Ref 3.). In the design, detailing, fabrication and erection of a steel structure, the following factors influence the cost of the framing: (a) Selection of the framing system (b) Design of the individual members (c) Design and detailing of the connections (d) Fabrication processes used
(e) Erection techniques used (f)
Specification for fabrication and erection
(g) Other items such as corrosion protection, fire protection, etc. The selection of the most efficient framing system is fundamental to achieving an economical framing solution and aspects relating to this item are discussed in Sections 3. 4. and 5. Efficient member design remains an important cost factor tempered by the comments made in Clause 1.1. Detailed consideration of this item does not fall within the scope of this publication. One point that does deserve mention however is the avoidance of the individual design of every beam and column in an attempt to achieve least mass. The aim should be to group similar members (e.g. similar main beams in a floor grid) and adopt the one size for all members of the group. An experienced designer will optimise the design by being aware that if too much grouping is done, there will be material wastage. However, if little grouping is done, then there is a great waste of time on the part of the draftsperson and the erector. Economic fabrication and erection are significantly affected by economical connection details. This publication is very concerned with economic detailing of steelwork and the manner in which detailing influences the cost of fabrication and erection. Sections 6. 7. and 8. deal with a variety of points which need consideration. The specification (item (f) above) is a major influence on the cost of both the fabrication and erection since it specifies the quality of materials and workmanship required. Similarly, the costs of both corrosion protection and fire protection (item (g) above) are important influences on the final cost. All these items are discussed in greater detail in Section 2.
1.3.
Integrated Design
One of the obstacles to achieving maximum economy is that three of the most important activities in steel frame construction, namely structural design, detailing and fabrication, are usually done in isolation from one another. This is partly due to specialisation in each of the disciplines and
2
partly because of a lack of an effective dialogue among the people involved. As a result of this, there often occurs a total preoccupation with the analytical phase of the design, and a complete absence of rational thinking about the detailing phase. Consequently, the problems that arise during the detailing phase are solved by complicating the detail rather than by modifying the design concept. When the job reaches the fabrication shop, there is little alternative but to carry out whatever happens to be shown on the drawings. A much improved situation results when the design effort is integrated so that the framework, its members and its connections are considered as a whole. In this way, it becomes possible to modify the structural framing concept to allow the use of simpler and less costly connections in the interest of overall economy. The cost factors listed in Clause 1.2 should be considered in an integrated manner so that interactions between the framework, its members and its connections are considered during the design process. In this way, one aspect can be altered to enable another to be improved. This enhances the overall cost efficiency of the final structure. Obviously, such an approach ideally requires an extensive and up--to--date knowledge of the steel fabrication and erection industries. Since such knowledge is not always easily achieved, communication with fabricators is a useful method of establishing the optimum practical solution. An interchange of ideas among fabricators, erectors and designers is an ideal situation for achieving optimisation. It should be appreciated that what constitutes ”design” and ”good (i.e. economical) design” will vary depending on whose viewpoint is being considered. To the designer, an economical design is usually the lightest member to carry the load. To the fabricator, a ”good design” means high tonnage output with minimum amount of labour. To the erector a ’good’ design is one where most members are the same size and can be interchanged without any problems. Clearly such different viewpoints are best resolved by an integrated and interactive approach on the part of the steelwork designer.
2.
General Factors Affecting Economy
2.1.
Steel Grades
2.1.1.
STRUCTURAL STEEL
Throughout the world the least costly and most commonly used grades of steel for structural purposes are those generally referred to as normal strength structural steel. In Australia such steel is covered by AS 3678 or AS 3679 (Parts 1 & 2). It has a typical design yield strength of 250/300 MPa (varying above and below this figure depending on thickness), a tensile strength of at least 410/430 MPa, a minimum elongation of 22% and a carbon equivalent of 0.43/0.44 so as to assure good weldability. AS 3678 and AS 3679 (Parts 1 & 2) are omnibus standards covering a family of structural steel grades including variants of the main grades having superior low temperature toughness.
3
Plates, rolled sections, welded sections and bars are all produced to these standards, although not every product is available in every grade. This is explained more fully in Table 2.1.
2.1.2.
WEATHERING STEEL
AS 3678 and AS 3679 (Parts 1 & 2) also deal with so--called ’weathering steel’. Weathering Steel contains alloying elements which cause it to weather to a uniform patina after which no further corrosion takes place. By nature of the chemical composition the steel is high strength (Grade 350) steel. However in Australia it is available in only a limited number of products----see Table 2.1.
2.1.3.
HOLLOW SECTIONS
In Australia structural hollow sections are produced to the product standard AS 1163. This standard covers a number of cold--formed (C) grades. Rectangular hollow sections are available in Grade C350 and Grade C450. Circular hollow sections (CHS) are available in Grade C250 and Grade C350.
TABLE 2.1 -- AVAILABILITY OF PRODUCTS BY GRADE (check currency of information with steel suppliers)
--
= not manufactured
Notes:
1. Applies to TFB, TFC and small angles. 2. Sections not considered in Note 1 above.
2.1.4.
QUENCHED AND TEMPERED STEEL
Steel Grade
Plates (or Floor plates)
Rolled Sections
Welded Sections
Structural Hollow Sections
Grade
AS 3678
AS 3679.1
AS 3679.2
AS 1163
200
Y
--
--
250
Y
Y (1)
--
250L0
--
X
--
250L15
+
--
--
300
+
Y (2)
Y
300L15
+
--
+
2.1.5.
350
Y
+
--
350L0
--
X
--
350L15
+
--
--
400
+
--
Y
Table 2.1 lists the availability of various products by steel grade. The indicative relative cost of grades is shown in Table 2.2. For most structures the greatest economy will be achieved by the selection of the least costly and most readily available steel, i.e. Grade 300.
400L15
X
--
+
WR350/1
+
--
--
WR350/1 L0
+
--
--
Australian Standard AS 3597 covers these steel plates for structural steel applications and for use in pressure vessels.
Heavy plate members such as bridge girders are one instance where higher grades may prove economical. Other applications include: Y
C350
Y
C450
Y AS 3597
60
Y
70
Y
80
Y
Y
= regular grade commonly produced, readily available from stockists
+
= regular grade not commonly produced, availability subject to time limitations and order size
X
= non--regular grade, availability subject to time limitations and order size
4
CHOICE OF STEEL GRADE
In large structures with longer lead times the use of higher grades will often be worth considering at least for parts of the frame.
Grade C250
Quenched & Tempered Structural Steel
Steel plates are produced in Australia in very high strength heat--treated grades known as ’quenched and tempered steel’. These steel plates are useful in special applications where mass reduction is important (eg. crane booms) or where their high wear resistance is needed (e.g. dump truck bodies).
(a) Multi--storey structures, particularly with composite steel beams; also in maintaining the same column size down a building by varying steel grades; (b) Trusses and lattice girders. Grade 350 steel costs around 10% more than Grade 300, and generally about 5% more to fabricate. To offset these cost extras, it provides greater yield strength but no increase in stiffness. In some frames, significant reduction in steel mass may overcome the increase in material cost and fabrication cost by the use of higher grades. Each individual frame must be assessed on its merits, but there are undoubtedly applications where the use of higher grades is economical. While the information presented in Table 2.1 is indicative of the general situation, it must be remembered that the steel suppliers are always willing
to discuss special cases where, for example, the economics of a high strength steel has been considered by the designer and the sections required are not normally manufactured in that grade. For a project requiring large tonnage of specific sections, it may be possible to negotiate a special order with the supplier, provided that an arrangement has been agreed at an early enough phase in the design. Conversely, on average projects the designer should always be careful to keep within the range of readily available products so as to ensure that no problems of steel procurement occur at the fabrication stage.
TABLE 2.2 INDICATIVE COST RATIOS FOR DIFFERENT GRADES OF STRUCTURAL STEEL (per tonne, supply only) Grade
Plates
Rolled Sections
Welded Sections
Grade 250
100
100
--
250L0
--
105
--
250L15
105
105
--
300
100
110
130
300L15
105
--
140
350
110
120
--
350L0
--
130
--
350L15
120
--
--
400
115
--
150
400L15
120
--
155
WR350/1
125
--
--
WR350/1 L0
135
--
--
AS 3678, AS 3679.1 & AS 3679.2
AS 1163 Grade C250
130
C350
130
C450
130
AS 3597 Quenched & Tempered Steel
5
Grade 60
150
70
160
80
160
2.2.
Economy in use of Material
As well as having a knowledge of the factors affecting the choice of steel grade, the designer should also be aware of how design decisions can avoid unnecessary material cost or wastage. This will involve a study of the factors discussed below.
2.2.1.
STEEL PRICING
Mill prices are expressed in terms of a base price and various extras. The base price relates to the type of mill product such as plate or sections, while extras relate to specifics of the particular product or section. The most common extras for structural quality steel include the size or designation, standard or non--standard lengths, quantity extras or discounts related to the total mass of individual order items, and the grade extras which apply to the quality specification for the material chosen.
able to offer prompter delivery than would be available through the normal steelmaker’s rolling programs. On the other hand, the larger fabricators are able to meet the mill requirements for direct purchase and prefer to procure their material this way. In recent years there has been a trend among some larger fabricators to limit their mill purchases to main material, and to obtain low--volume ancillary material from steel merchants as and when required. In Australia there is not much difference in the cost of steel procured in one way or the other if all expenses are truly accounted. The argument about using, wherever possible, the preferred grades and sizes applies equally to steel obtained from a steel merchant, because these firms naturally tend to concentrate their stockholding on popular, fast--moving, items.
2.2.2.
PLATES
Quality extras for structural steel relate to the material specifications and reflect the costs of alloying elements, of tighter controls on such elements as carbon, manganese, phosphorus and silicon, and of tighter controls on manufacturing techniques to meet the specified chemical and mechanical properties. The cost of additional tests and greater frequency of testing, necessary for increased stringency of yield strength and notch ductility, are also reflected in increased quality and testing extras.
In Australia there is a rationalised series of ’preferred’ plate thicknesses as listed in Table 2.3.
Designers should recognise that the more exotic the requirements of the steel specification, the greater is the probability that other costs associated with its use, ranging from procurement through all stages of fabrication, will also be increased. Unnecessary demands by specifiers for mill heat certificates for standard sections to be used on routine projects is another example of unnecessary costs added onto projects.
Similarly there are preferred lengths and widths of plates which should be borne in mind. Major plate elements should be dimensioned as far as possible so that they can be cut from standard plates with a minimum of scrap. Smaller plate details such as brackets and gussets should be considered in the same way, especially when there is a large number of them. The most common sizes for plates up to 25 mm thick are 1.8m x 6m, 2.4m x 6m, 2.4m x 9m, and 3m x 9m.
The foregoing relates to purchases made direct from the steel mill, but in Australia most fabricators obtain their steel through steel merchants. These steel merchants aim to carry comprehensive stocks and are thus
6
For practically all structures the designer should operate within this standard range. Non--preferred thicknesses incur cost premiums and extended delivery times, and should only be considered on major projects where the overall saving in using a special thickness is greater than the direct and indirect cost penalties.
(Note: Small plate components may be substituted by flat bars which are considered as Sections.)
TABLE 2.3 PREFERRED STEEL PLATE THICKNESSES (in mm) 3
25
70
4
28
80
5
32
90
6
36
100
8
40
110
10
45
120
12
50
140
16
55
160
20
60
180
2.2.3.
SECTIONS
Australia produces a range of welded products, universal sections, channels, angles, and hollow sections which provides the designer with a reasonable choice without the proliferation which can lead to problems of availability. The lowest weight in each nominal size of universal section is the most structurally efficient and they account for over two--thirds of all UB sales. The designer should therefore make every endeavour to keep to the lowest weights, although this will not always be possible. Very long lengths of sections become difficult to keep straight and to handle, and the mills impose a price extra for them. It should be especially noted that although universal sections are listed as being available up to 18m long (and up to 27m by inquiry), the usual maximum length found in stock is around 15m. The available lengths of structural hollow sections are usually restricted to 6.5m (circulars) or 12m (rectangulars and squares).
7
2.2.4.
SCRAP AND WASTE
The real cost of material is affected by the quantity of scrap and waste, and designers should be receptive to suggestions for minimising and controlling the generation of waste. This may include greater standardisation of structural sizes, or of plate widths and thicknesses, in order to take advantage of size and quantity discounts. It might also include a more liberal approach to the splicing of beams or other structural sections using standard lengths. Random splicing, which involves welded splices anywhere within the length of a rolled structural member, can be particularly effective when material is sawn to length and fabricated on a conveyorised production line. When carefully controlled, it can dramatically reduce the accumulation of shorts and thus reduce the total cost. The only real restriction to random splicing applies to its use for beams subject to severe dynamic loads. Of course the savings in scrap have to be balanced against the welding costs, and the designer should be receptive to this technique where it is appropriate.
2.3. 2.3.1.
Fabrication GENERAL
Fabrication costs are a function of complexity and are influenced by: (a) Size of the component (b) Size and type of sections involved (c) Amount of stiffening and reinforcing required (d) Amount of repetition (e) Shop and field details (f)
Space requirements in the shop, and
(g) Facilities available for handling, lifting and moving the structural components. Fabrication costs are sensitive to simplicity or complexity of detail, and the degree to which production line techniques can be applied. They are controlled by the quality of the shop detail drawings, which must reflect the
designer’s concept for the structure, but must also permit the optimum utilisation of the fabricator’s facilities and equipment. Shop drawing preparation should be guided by the basic principle that they must provide for economy of fabrication and for economy of erection. Shop operations basically involve cutting material to size, hole--making for mechanical fasteners, and assembling and joining. Other operations include handling, cleaning and corrosion protection. All shop operations require facilities for lifting and for moving or conveying the structural steel. Cutting operations include shearing, sawing and flame cutting; hole--making operations include punching and drilling; assembly operations include welding and bolting. Increased use of computer numerically controlled (CNC) fabrication processes is changing the economics of steel fabrication. Cutting, drilling and welding operations can now be undertaken by the CNC fabrication process. Information from computer drafted shop drawings can be fed directly into CNC fabrication equipment to further improve operational efficiency. Some fabricators are now bar coding steelwork to facilitate control and monitoring of projects. Generally welding is the preferred method for shop assembly, with bolting for field assembly. There are, however, some fabricators with sophisticated hole--making equipment, who prefer shop bolting to shop welding for standard connections. Some steel merchants also provide basic cutting and drilling services to the steel fabricators.
2.3.2.
BEAM AND COLUMN FABRICATION
A large part of structural steel fabrication consists of beam and column work. It embraces framing members consisting of standard rolled shapes connected by shear or moment connections, and also includes highly irregular framing members with custom designed built--up sections and complex connections designed for combinations of shear, moment and direct tension. Simple beam and column fabrication lends itself to production line methods, in which the members are transported on a series of conveyors to saws which cut the material to length, and to hole--making equipment which provide holes in either the web or flange or both.
8
Any additional requirements, such as the attachment of cleats or brackets, are off--line operations. It is important therefore that connections and other details be selected so as to provide the maximum number of members with only cutting and holing. Otherwise the economy of using CNC equipment and the conveyorised beam--line system will be less apparent (see Figs 3.13 and 8.29).
2.3.3.
GIRDER AND TRUSS FABRICATION
Fabrication of plate girders and trusses differs from beam and column work in that it involves assembly in the shop, and calls for adequate space and handling facilities. Both girders and trusses require special fit--up jigs for assembly and welding, and the availability of heavy lifting equipment. Just as with beam and column work, however, the key to productivity and economical fabrication is the use of simple standard details for stiffeners, splices, gussets, etc. For plate girders all details should be designed for automatic welding, allowing adequate clearances for the welding machines to pass and for termination of welds at the ends of web stiffeners. Maintaining constant width flanges within a shop fabricated length of girder permits splicing of multiple width plate and subsequent stripping to finished width. This will reduce weld set--up time, eliminate weld starts and stops, and require only one set of run--on and run--off tabs. Reductions of flange widths, web depths and plate thicknesses purely to reduce mass should be considered very carefully as they can significantly increase fabrication costs. Control of distortion in plate girder fabrication is a major problem, which can be helped by design which minimises the amount of welding and avoids the use of significantly non--symmetrical sections. It is false economy to design for minimum web thickness only to require web stiffeners, thereby increasing the amount of welding and distortion; or to use very light top flanges in composite girders only to compound the problem of camber control. See also Clause 8.2.5. Trusses can be designed in a large variety of configurations which depend on the truss span, depth and loads to be carried. Therefore, it is impossible to make general statements regarding the most economical design for fabrication, other than to stress again the importance of simplicity of detail. Designers should avoid situations that can cause weld restraint and problems resulting from weld induced distortion. As far as possible trusses
in the one project should have the same configuration so that they can all be fabricated from the one jig.
determine the best solution to optimise the use of material and labour in the fabrication shop. This industry communication can often be facilitated through the services of AISC.
In truss work, the correct selection of chord members can often remove the need to turn the truss over during the fabrication (see Clause 8.4). This will enable the fabricator to complete the entire welding on the truss component without further handling.
2.4.
2.3.4.
The rate of erection of steel in a structure is controlled by five main factors:
SUMMARY FOR ECONOMIC FABRICATION
The key to economic fabrication is the use of standards at all stages. This includes standard procedures, standard schedules, standard drawings, and above all standard connections and details. Non--standard details are usually handled as ’special job standards’; however, the net effect of any specials is to slow production with some loss of fabrication economy. In the selection of connections the designer should observe the following principles: (a) Select members and connections to provide a maximum of repetition throughout a structure. This provides the fabricator with the opportunity to make up jigs and fixtures to speed up the fabrication process. (b) As far as possible, select connections so that the assembly of fitments on a member can be carried out in one position. This will reduce the number of handling or rotating operations during fabrication. (c) Keep the number of components in a connection to a minimum. (d) Select connections so that assembly of components occurs on the least number of members. (e) As far as possible use connections that are standard in the industry (see Standardized Structural Connections, Ref 1). (f)
Ensure a minimum standard of documentation in line with AISC’s publication: ” A Guide to the Requirements for Engineering Drawings of Structural Steelwork” (Ref 2.12).
(g) Most importantly, keep an open mind on the selection of members and connections. Before finally committing a design to the detail design phase, communicate with the industry and try to
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2.4.1.
Erection GENERAL CONSIDERATIONS
(a) Connection simplicity (b) Number of members (c) Number of bolts and/or amount of field welding (d) Size and efficiency of erection crew, and the equipment at their disposal (e) Timely supply of steel. It is interesting to note that of these factors, the first three are under the control of the designer. Connections should be simple, and of such a type that the allowable tolerances (in member size and shape, detailing and fabrication) can be accommodated during the placing of the members. The number of members should be kept to a practical minimum, and so should the number of bolts or amount of field welding. There should be sufficient access for welding or for tightening bolts using power wrenches. Bolted connections should be used wherever possible, and field welding kept to a minimum. Connection plates should be shop welded to one member, rather than field bolted to both, unless other considerations govern. Every endeavour should be made to standardise as far as possible (member sizes, bolt sizes, type of connection, gauge lines, member spacing, etc.), and careful consideration should be given to how a member is to be installed with minimum interference by other members, gusset plates, etc. (see Ref 1). With an increasing awareness of the importance of employee safety in the work place, erection methods are changing. Designers and erectors have a duty of care and should consider safe erection methods. The use of
equipment such as cherry pickers is becoming more common during erection. Designers need to include anchorage points for safety lines and harnesses for riggers. These issues are resulting in steelwork being erected on the ground and then craned up to final position in many projects to reduce the amount of work done at great heights. This may require alternative design and detail methods, and utilisation of additional short term cranage but provides a safer work site. A safer work site will lead to faster and more economical erection.
2.4.2.
HANDLING AND TRANSPORT
As a general rule it is more economical to erect fewer large pieces than many small pieces, due to the number of lifts involved and the number of joints to make. Generally this means fabricating larger pieces in the shop to reduce the number of pieces and field connections. On the other hand, transportation constraints may limit the size of a piece for delivery to the site and require additional field splices. For example, with long flexible trusses, the transportation length may have to be curtailed to avoid damage during transfer to site or to avoid obstructions along the way. Large sub--assemblies may require to be transported using special vehicles attended by police escort, and this may add greatly to the final price of the structure. However, projects outside capital cities could use this approach as it minimises the size of the site crew required to be mobilised on a remote or semi--remote site. With greater availability of larger mobile cranes and trucks, the balance between transport costs and site costs is changing. Where projects require large site crews, minimising time spent on site is essential to economical erection. The erection or trial erection of large components in a fabricator’s yard before delivery to site is good practice and a cost savings exercise. Trial erection guards against fabrication errors being discovered on site which may prove expensive to rectify. Generally, transport costs are high, and therefore considerable economy can be achieved if vehicles travel fully laden. The dimensions of a typical load of structural steelwork which requires no special escort are in the order of 15m long x 3m wide x 2m high. It is important that like pieces are loaded together to optimise truck capacity, but also that the components be delivered to site in the order required by the erection sequence (i.e. columns followed by beams from ground upwards). This will save double
10
handling on site and also reduce the cost of site storage and possible damage. The virtue of designing for repetitive components has already been stressed. The gains can be partly lost on site if interchangeable parts are given individual mark numbers. This will require the erector to search for a particular number mark on a member when any one of a considerable number of members would fit. After completing a design it is worth looking at marking plans with this idea in mind. Indicative transportation costs are given in Table 2.4. Costs include the loading of steelwork onto and off the truck. TABLE 2.4 TRANSPORTATION COSTS Transport
Fabrication Shop to Site (see Note)
Section Mass (kg/m)
$/member
60.5 and less
15
60.6 to 160
56
160.1 to 455 225 Note: Allow for twice the cost of transportation if the surface treatment is applied at premises other than the fabrication shop.
2.4.3.
CONNECTIONS
It is in the final fixing of members that the greatest scope for erection economy lies. Connections selected to permit flexibility in fit up should be of prime concern to designers. The use of one type of bolt and one bolting procedure throughout a structure will allow the use of a minimum variety of tools on site and provide for speedy erection sequence (see Section 6). Similarly where site welded connections are required, cleats should be incorporated to allow mating members to be held together in place for actual welding. Angle seat, angle cleat and web side plate connections (see Clause 8.6.2) provide considerable flexibility in fit--up, and are preferred in braced frames from a purely erection viewpoint. The flexible end plate connection is not quite so easy to erect, although its selection may be decided by other considerations.
In rigid frames, the following should be taken into consideration for the design of bolted connections:
connections even if it involves increasing column weight, and/or fabrication costs (see Fig. 2.2).
(a) The end plate depth should be kept to a minimum to reduce the tendency to jam during installation (Fig. 2.1). (b) The tolerance between the face of the end plate and the face of the column should either be tightly controlled so that the building plumbs itself automatically, or allowance should be made for shimming in order to plumb the building. Shimming, however, can be expensive. (c) In end plate connections for portal frames careful consideration should be given to access for installing and tensioning bolts, (see Table 8.1). If welded connections are preferred, the following should be taken into consideration: (a) Welded connections are normally erected using a bolted erection connection. The same criteria should apply to the design of these connections as described above.
Fig. 2.1 Deep end plates can cause jamming
(b) Substantial erection clearance between the end of the girder and column face should be provided where permitted by the design of the connection. (c) Field welding should be kept to a minimum and overhead welding should be avoided. (d) Attention should be paid to access for welding and welding inspection. (e) Consideration should be given to plumbing the building. The most significant time delays in the erection of a girder can be expected to occur when it is installed with the end connection against a column web. The girder can normally only be manoeuvred in a vertical plane and frequently jams. Gusset plates, stiffeners, and other members tend to interfere with its installation. Access for bolting is usually difficult and sometimes impossible. Every effort should be made to get the connection outside the flanges of the column, or at least as far out from the web as possible. This is especially important when the column section is compact. Consideration should always be given to excluding direct girder/web
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Fig. 2.2 Two ways of avoiding the problem of access to column web connections
2.4.4.
FIELD BOLTING
In projects with a predominance of large connections, threads may be excluded from the shear plane for bearing type connections as this will help to reduce the number of bolts. However with Australia’s ISO metric long--thread bolts, care should be taken that the long ’stick--through’ that occurs does not cause fouling or access problems. In projects with small connections the saving in number of bolts is not so evident, and it is more
economic to design for threads included in the shear plane. This then means that bolt lengths can be selected so as to avoid excessive stick--through. However the two systems (threads--in, threads--out) should not be mixed on the one job (see Ref 6.1). Generally, the smaller the bolt the easier it is to install. Bolt diameters should therefore be kept small if this can be done without compromising the objective of keeping the number of bolts to a minimum. M12 bolts are normally adequate for stairs and girts, while M20 bolts are the maximum size which should be considered if access for tensioning is poor; otherwise M24 bolts are acceptable. Bolts should be specified as ’snug--tight’ unless there are compelling reasons why fully tensioned bolts are necessary. The cost of full tensioning, including associated inspection, is very high and can double the cost of each installed bolt. Access for wrenches is also less critical where only snug tightening is to be carried out. Care should be exercised, however, where a project is designed to overseas codes because some of these require high strength structural bolts to be always fully tensioned. It is preferable that only one bolting category (see Section 6) be used on any one structure. When a departure from the general category (e.g. to fully tensioned bolts, to threads excluded from shear plane, etc.) is unavoidable, this should be highlighted on erection and detail drawings to reduce the possibility of the requirement being overlooked by erection crews. More information on structural bolting is given in Section 6 and Ref 6.1.
2.4.5.
FIELD WELDING
Where site welding is used for connections the total amount of welding on the job should be sufficient to justify the cost of bringing and setting up welding equipment on the site.
most structural work difficult out--of--position welds such as overhead are very slow and costly. See also Section 7.
2.4.6.
BRACING
Bracing is usually difficult and time consuming to install. To reduce erection time the number of braced bays should be kept to a minimum, i.e. fewer braced bays with heavier bracing is preferred. Wherever possible, wall bracing should be connected to columns rather than beams. This allows bracing to be installed before the beam above is in position, hence reducing any interference this beam may cause during erection. Connecting the brace to the column at its lower end eliminates interference to the floor system resulting from a gusset plate on the top flange of a beam. Connecting wall bracing to the column also usually results in lower fabrication costs.
2.5. 2.5.1.
Surface Treatment GENERAL CONSIDERATIONS
With the development in recent years of a large variety of surface treatment methods, the designer may experience considerable difficulty in selecting the optimum system for a particular application. Furthermore, it is often not fully realised that the cost of a sophisticated multi--coat treatment system can easily be more than the cost of the raw steel itself. Thus care is needed to avoid unnecessary, and sometimes unexpected, surface treatment costs. These costs are a function of surface area which can vary with both: the type of section used and the class of construction.
Access for welding is also important, and it should be remembered that a welder generally requires a substantial and carefully placed working platform.
For example, a structural hollow section has typically only one--half to two--thirds of the surface area of an ”open” structural section (UB, UC...) of equivalent capacity. For this reason, hollow sections are well worth bearing in mind for applications requiring any significant amount of surface treatment.
Otherwise the normal rules for economic welding apply. Fillet welds are preferred to butt welds, and down--hand welding to any other position. In
Heavy steel construction such as for power stations usually averages out with comparatively less surface area (despite the higher tonnage) than a
12
typical factory or warehouse where light trusswork may have a much greater surface area (despite the lower tonnage). Obviously treatment costs on a per square metre basis will vary widely depending on the actual surface area to be treated.
2.5.2.
STEEL PERFORMANCE
Bare steel will corrode only in the presence of both oxygen and moisture. Corrosion will be accelerated if traces of pollutants such as sulphur dioxide or chlorides are present -- the so--called ’aggressive environments’. Steel inside a building is rarely a corrosion risk except in the occasional case where the building houses an aggressive atmosphere as a result of its purpose, e.g. a fertiliser factory. It follows therefore that steel needs no corrosion protection whatsoever in most interior applications such as multi--storey buildings where the steel framing is eventually concealed. Where the steelwork remains exposed to view as in a factory or warehouse the same negligible risk applies but in these instances the owner may require a surface finish for a more attractive appearance. The designer should distinguish between treatment specified to achieve protection from corrosion and that specified merely to provide decoration. In practice, of course, any surface finish will attempt to do both. Detailed advice on the classification of environments and the selection of appropriate surface treatment systems is contained in AS 2312 ’Guide to the protection of iron and steel against exterior atmospheric corrosion’ (see Section 10).
2.5.3.
Various methods of surface preparation are covered by AS 1627 ’Metal finishing -- preparation and pretreatment of surfaces’ (see Section 10), and advice on their selection is contained in AS 2312 (see Section 10). The most commonly used methods in Australia are wire brushing (suitable for low cost paints) and abrasive blasting to Class 2--1/2 of AS 1627 Part 4 (needed for high performance paint systems). Acid descaling (’pickling’) is encountered mainly as part of the hot--dip galvanizing process (see Clause 2.5.5). An idea of the costs of various methods of surface preparation is given in Table 2.5. TABLE 2.5 SURFACE TREATMENT COSTS
Galvanize ROZP
ALKYD GLOSS
ZnSi
MIO
$/sq m
$/sq m
$/sq m
$/sq m
$/sq m
60.5 and less 6
6
18
9
17
60.6 to 160
5
5
16
8
23
160.1 to 455
4
4
14
7
33
(kg/m)
Notes:
1. Red Oxide Zinc Phosphate (ROZP) 50 µm paint thickness including wire brush. 2. Inorganic Zinc Silicate (ZnSi) 75 µm paint thickness including Class 2--1/2 blast cleaning. 3. Alkyd Gloss 40 µm. 4. MIO 100 µm. 5. For double--dip galvanizing, add 30 % to the above rates. 6. For lengths greater than 12 m, check with local galvanizers. 7. Rates include cost of finish painting.
2.5.4.
PAINT SYSTEMS
An important part of any steel treatment system is the preliminary surface preparation. This can range from simple degreasing and brushing, to costly chemical or mechanical descaling.
13
Hot-- dip
Mass
SURFACE PREPARATION
The surface preparation should be matched to the applied finish. Expensive paint systems will not last if applied to only partially prepared (e.g. wire--brushed) surfaces. Conversely it is a waste of money applying a low--cost porous alkyd primer to a descaled ’white metal’ surface.
Paint Types
Section
There is a very large selection of paint systems available for structural steel -- too many to be discussed within the scope of this publication. However,
excellent guidance on the performance and capabilities of various paint formulations is given in AS 2312.
2.5.6.
Probably the most commonly used paint is ’red oxide zinc phosphate primer’, often referred to as ROZP, which is applied over a wire brushed preparation. Paints of this type provide an economic base for possible further decorative coats of conventional oil paint. However being permeable, ROZP cannot be expected to last if left in the open for more than normal construction periods.
In a severe environment where steelwork is exposed to aggressive conditions the designer can vastly enhance the corrosion resistance of the structure by careful attention to a few simple principles. Conversely a structure with bad details will not perform satisfactorily no matter how much has been spent on elaborate multi--coat protective systems. Fortunately, the principles of good corrosion detailing are generally much the same as those for economic fabrication. Connections and other details should be kept as simple as possible with the minimum number of components. Depressions, pockets, ledges, narrow crevices and anywhere where water and foreign matter may lodge permanently should be avoided whenever possible. In really severe situations the use of box sections, CHS or RHS might be considered. Several examples of good and bad practice are given in AS 2312.
Another regularly used paint is ’inorganic zinc silicate primer’ which is applied over a Class 2--1/2 abrasive blast preparation. It forms an excellent base for most high performance paint formulations, or gives good results as a single coat protection for steel in all but the most aggressive environments. Paint is normally applied to steel by spraying. It is sometimes suggested that better coating is achieved by brush application, but there is little evidence to support this claim. Brush application costs two to three times as much as spraying, and cannot be used at all for some modern paints; inorganic zinc silicate is an example. Table 2.5 includes the cost of the finish painting in the surface treatment costs. It should be noted that transportation cost should also be considered if the treatment is done at premises other than the fabrication shop. Table 2.4 gives an indication of transportation costs.
2.5.5.
HOT--DIP GALVANIZING
Galvanizing is carried out by specialist firms and the process requires precleaning and surface preparation, usually by pickling. The cost of galvanizing includes these preparatory processes. Advice on the performance of hot--dip galvanizing, either as a single coat protection or as a base for paint systems, is contained in AS 2312. When considering galvanizing the designer should ascertain the scope of local facilities, and in particular the size of the available galvanizing baths. The galvanizing bath determines how big an individual component can be dipped. (Items larger than the bath can sometimes be galvanized by ’double dipping’ but at extra handling cost). Information on bath sizes in Australia is given in ’Hot--dip Galvanizing’ (Ref 2.4).
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2.5.7.
DESIGN AND DETAILS FOR CORROSION RESISTANCE
SUMMARY CHECKLIST FOR SURFACE TREATMENT
1. The required level of surface treatment and/or corrosion protection should be decided at the very earliest stage of the design, so that all design decisions can be made with this in mind. 2. In benign atmospheres such as the interiors of most buildings, or exposed steelwork in non--polluted non--marine environments, corrosion rates are generally so low as to not require corrosion protection. Any painting carried out would therefore be only for aesthetics. 3. Where corrosion protection is required, the extent needs to be carefully evaluated to ensure that it is appropriate to the circumstances. Too much protection is a waste of money, as also is too little. Obviously professional judgement is needed. 4. The degree of surface preparation should match the surface treatment system to be applied (see Clause 2.5.3). 5. As painting is substantially a labour intensive process, the current trend is to replace multi--coat (3 or 4 coat) systems with one or two coat systems. Zinc--rich paint systems are consequently increasingly used, particularly on blast cleaned surfaces. In these systems, however, film thickness build is vital to a satisfactory performance.
6. Good design practice is essential -- e.g. avoid pockets where water and debris can lodge and accelerate coating failure, (see Clause 2.5.6). 7. Allowance should be made for easy future repainting. 8. Shop painting is always cheaper and more effective than site painting, but no steel can be handled, transported and erected without damage to the coating from crane slings, etc. Touching up of the base coats and the final top coat must therefore be done on site. 9. Hot--dip galvanizing is a high performance protective system which is not prone to damage during transport and handling. In some circumstances it may cost the same as an alternative paint system (see Table 2.5). 10. Recent developments in the field of corrosion protection have evolved protective systems greatly superior to those available some years ago. These systems are expensive but are invaluable when appropriate, as in exposed structures in severe industrial or marine environments. However, this has led to waste of money by the specification of such sophisticated treatments in circumstances where they are not necessary. 11. Some paint systems require special application techniques, controlled temperature and humidity when being applied, long drying times or may have a tightly constrained time interval between successive coats. Designers should be careful of such sensitive systems, as experience has shown that they are almost impossible to apply correctly in normal construction industry conditions.
2.6.
Fire Resistance
2.6.1.
GENERAL CONSIDERATIONS
All structural material can be damaged in severe fire conditions and steel, although non--combustible and making no contribution to a fire, can have its function impaired. For this reason, building regulations require it to be protected, usually by a non--combustible insulation, when used for certain elements of construction in some types of building. Building regulations
15
prescribe statutory levels of fire resistance for structural steel members in many types of applications. The fire resistance level of a building element or structure is determined by constructing a truly representative prototype of that element or structure incorporating fire protection materials, systems or coatings where necessary and submitting that prototype element or structure to the Standard Fire Test. The Australian Standard Fire Test is given in AS 1530 Part 4 which enables a fire tested element or structure to be assigned a fire resistance level in accordance with the criteria laid down in the fire test standard. Fire resistance ratings are expressed in minutes such as 30 min, 60 min, 90 min, 120 min, 180 min or 240 min. Traditionally, building regulations have been based on the trial--and--error concept of the practical fire test. This is administratively convenient, but has two main disadvantages. Firstly, until recently it has been difficult to predict from a particular test the fire performance of a similar but slightly different configuration -- calling perhaps for further expensive tests. Secondly, it has been shown that the conditions of the standard fire test do not replicate the observed behaviour of actual building fires. The present day trend is toward the development of fire engineering design rules whereby the engineer can design for fire performance in the same way as he or she does for structural performance. The Australian design code AS 4100 contains a comprehensive section on design for fire, and it seems likely this approach will soon become standard procedure.
2.6.2.
REGULATORY REQUIREMENTS
Australian Building Regulations require that elements of a structure achieve specified fire resistance levels (FRL). The level of fire resistance required for a particular application is related to the expected fire load within the building (which is in turn related to type of occupancy), to the building height and area, and to the fire zoning of the building locality and the on--site positioning. It is not within the scope of this publication to repeat the requirements of the various Building Regulations. The fire ratings of common building elements have become well established by virtue of accumulated testing and accepted values are specified in the various Codes and Regulations. Unprotected steelwork does not normally attract any FRL, except where specialised approaches are adopted. One example is in open car parks where full scale tests have
demonstrated that bare steel will not reach a critical temperature should a car catch fire (Ref 2.5). Another example is composite steel deck floor systems utilising fire emergency reinforcement (Refs 2.6, 5.4, 5.5).
2.6.3.
MATERIALS FOR FIRE PROTECTION
Where steel has to be protected, the most practicable way is to cover it or encase it in a protective material. Such material should be: (a) Non--combustible (b) Unable to produce smoke or toxic gases at elevated temperature (c) Able to be efficiently and uniformly applied (d) Durable to prevent dislodgment, and
(e) Thermally protective. Table 2.6 compares passive fire protection costs and gives an approximate indication of their costs. These costs may not tell the whole story where a protected member is exposed to view and will be given a decorative finish -some systems are less costly than others to decorate. Another important factor to be borne in mind is that dry systems cause less disruption to other trades and the building schedule, and therefore can bring significant indirect cost savings in terms of shorter overall construction time. Commercially available materials must be able to demonstrate their capability of achieving a fire resistance level as part of building systems. The various manufacturers can supply the necessary accreditation and technical data by reference to tests conducted at recognised fire testing stations. See also Ref 2.11.
TABLE 2.6 PASSIVE FIRE PROTECTION COSTS Intumescent Paint
Vermiculite Spray
Vermiculite Spray
Fire Rating Level
Fire Rating Level
Fire Rating Level
60 min
120 min
180 min
(kg/m)
$/sq m
$/sq m
$/sq m
60.5 and less
108
27
37
60.6 to 160
106
23
35
160.1 to 455
104
17
33
Section Mass
Notes:
16
1. Rates include supply and paint/spray. 2. Intumescent paint cost includes ZnSi and Class 2--1/2 blast cleaning costs.
2.7. 2.7.1.
Specifications GENERAL CONSIDERATIONS
The specification is important because it forms part of the tender documents and ultimately becomes part of the contract documents. Its purpose is to cover aspects of the work that fall between the legal contract clauses and the technical data shown on drawings. Such aspects may include: (a) Workmanship standards (b) Tolerances (c) Inspection levels etc. In past years the specification was essential for the designer to convey to the contractor exactly what was wanted. Nowadays so many of these matters have been codified that a detailed specification has become less necessary.
The specification should not repeat material that is already in the relevant codes or standards. Nor should it become a repository for information which should more properly be shown on the drawings -- nowadays most design offices use standard notes on their drawings in order to handle this aspect more efficiently. A set of guideline notes are provided in AISC’s Steel Construction Journal, Volume 29, Number 3, September 1995 (Ref 2.1). However, such standard notes should always be checked as each drawing is prepared to ensure that they are relevant. A specification should be precise so that both parties to a contract know what is required, and should clearly state what the contractor is required to do and what he/she is to refrain from doing. Great care must be taken in the wording, with definitive requirements being stated and all allowable alternatives clearly specified. Vague general statements which could mean different things to different people should be avoided. The requirements specified should be designed only to produce work of appropriate quality to the building requirements, while avoiding unnecessarily tight requirements which only add to the cost. Experience has shown that short and precise specifications help considerably in the smooth flow of the work, and thus have a beneficial influence on costs. Conversely, long and repetitious documents can easily lead to misunderstanding, contractual arguments and expensive delays.
2.7.2.
(b) Select inspection procedures and frequencies appropriate to the class of work, using Code guidance (e.g. AS 1554) where available. (c) Select the fabrication and/or erection contractors on the basis of proven capability, using their previous work as the most reliable indicator of their quality. Check that they have quality assurance programs.
2.7.3.
TOLERANCES
Tolerances on the ex--mill dimensions of steel sections and plates are listed in AS 3678 and AS 3679 (Parts 1 & 2). The necessity for these tolerances arises because of factors in the steel--rolling process, including rolling speed, roll wear, roll adjustment and differential cooling. A study of the Standards shows that these dimensional tolerances can be significant enough to warrant consideration in detailing and fabrication; Fig 2.3 gives some examples. (a) allow for variation in beam depth in flange splice and for off-centre of webs in web splice.
WORKMANSHIP STANDARDS
Standards of workmanship and quality are extremely difficult to define in words. In the past many specifications attempted to do so by incorporating such phrases as ’workmanship shall be of first class quality’ or ’members shall be true to line and neatly finished’. However, when tested such clauses are meaningless and fortunately are becoming rare in modern specifications. In practice the owner’s and designer’s interests are best protected by observing these three principles:
17
(a) Use the tolerance and workmanship standards specified in the appropriate Code, e.g. AS 4100.
(b) any connection to column web or column flaange must make allowance for out of square, especially end plate connections -- allow for shimming where necessary (may involve tapered shims).
(c) web side plate connection -- allow for out of square of column flange and off centre of beam web.
2.7.4.
CAMBERING
The practice of cambering beams is intended to provide an upward ’set’ that will counteract the downward deflection due to normal working loads. Several obvious problems present themselves with this procedure: (a) It is difficult to calculate accurately the true deflection of a member under working loads;
Fig 2.3 Typical connections where allowance for mill tolerance is needed Experienced fabricators are aware of the possibility of dimensional variations, and it is normal practice to match members at splices in such a way as to minimise the effect of these variations. Tolerances on the dimensions of fabricated members and erected frames are given in AS 4100. The tolerances specified can be considered as related to the design provisions of the Code. Thus for structures designed in accordance with AS 4100, there is no case for specifying tighter tolerances since the tighter tolerances are not then consistent with the design assumptions, nor with the manufacturing tolerances of the raw steel. These fabrication and erection tolerances can be realistically and economically achieved and are consistent with world wide practice. They should not be varied without compelling reason. It must be particularly noted that the specifying of tighter tolerances can be a costly decision which, in most applications, will serve no purpose and destroy consistency. It is also recommended that tolerances be specified by simple reference to the provisions of AS 4100. Where dimensional tolerances are not defined, there is plenty of room for argument and contractual dispute, as most experienced designers and fabricators know. Conversely, where allowable tolerances are clearly stated, it is a simple matter to decide whether a component or structure complies or not.
18
(b) It is difficult to control accurately the degree of camber induced in a member; and (c) Cambering requires the fabricator to perform a difficult, and hence expensive, fabrication operation. There are two main methods by which rolled sections are cambered. The first involves the use of some form of heavy press, such as a hydraulic side--press. These machines are massive and costly, and are found in the shops of only the largest companies. Most fabricators employ the alternative method of controlled heating and shrinking using a standard flame--cutting torch. Both of these methods involve a degree of trial--and--error in the setting of the member so that cambering is a slow, labour--intensive and therefore rather costly procedure in the fabrication process. On simple, well--detailed beams it can more than double the actual fabrication cost. It is therefore an operation to be called for only when absolutely necessary. Generally, where members are ultimately concealed from view, or if exposed are unlikely to cause visual offence, cambering is pointless. An exception is sometimes found in steel beam/metal deck composite floor systems where it is desirable to camber against the deflection due to the wet concrete because of the ’springiness’ of the whole system during pouring. If the requirement to camber is based on a need to offset increased deflections in light members, consideration should be given to using a stiffer member without camber. There is certainly scope to do this, as the saving on cambering costs would, to a large extent, offset the increase in the cost of the heavier member. Camber is measured with the member flat on the floor with the web horizontal. Where a member is specified to be cambered, it is reasonable
to accept a tolerance on the specified camber similar to the out--of--straightness tolerance of AS 4100. To maintain tolerances closer than this can be very costly indeed (Ref 2.10).
2.7.5.
TEMPORARY BRACING
Problems often arise when the specification requires the erector to supply temporary bracing for a structure. Sometimes the erector is required to design this bracing and be responsible for its performance. In line with new occupational health and safety regulations, erectors should develop erection plans including temporary bracing requirements with the principal contractor. These plans may need to be checked by the design engineer.
In most contracts most of the inspection is directed at high--strength bolting, welding and surface treatment. Guidance on inspection levels and methods is given in the relevant codes and standards: AS 1554
Structural Steel Welding
AS 2312
Guide to the Protection of Iron and Steel against Exterior Atmospheric Corrosion
AS 4100
Steel Structures
(a) Erection Bracing -- the bracing or guys required to support individual members during their erection.
The specification should define the nature of inspection to be carried out and the methods to be used. This latter is especially important in the case of non--destructive weld testing where there is a range of methods available with widely varying costs. Specifications requiring 100% x--ray testing on all butt welds in standard industrial buildings impose significant and wasteful costs on projects. The welding test requirements for the oil and gas industry should not be applied on everyday industrial or commercial structures. Appropriate testing levels are essential for economical structures.
(b) Temporary Bracing -- required in order that the steel skeleton remains plumb and in a safe condition after erection is completed, until permanent bracing elements such as shear walls are built.
Where an independent inspection authority is to be engaged it should be made clear in the tender documents whether or not the fabricator is to cover the cost in his price quotation.
Erection bracing is the principal contractor’s and erector’s responsibility in relation to the supply and its removal on completion.
The following guidelines will assist in setting up effective and economic inspection procedures:
However, temporary bracing which is to be left in place until other stabilising elements are built is a different matter. Its design requires knowledge of the building sequence and of other factors. Normal prudence would suggest that it must be designed by the Engineer. Any special or unusual features of the structural design that may limit or affect stability during erection should be emphasised on the construction drawings.
(a) Inspection methods and levels should be compatible with the quality and tolerance requirements of the codes applying to the particular class of work. Inspectors should not seek to impose higher standards.
So--called ’temporary bracing’ actually falls into two categories:
2.7.6.
INSPECTION
Whilst some level of routine inspection is obviously necessary in the owner’s interest, it should always be remembered that inspection in itself is a non--productive expense. It should therefore be specified with discretion.
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(b) Early inspection efforts should be directed towards checking that the fabricator’s procedures will produce the required results. Thus inspection will be more intensive at the start of the job, and can be relaxed to a nominal level when production methods are proven. (c) The inspectors themselves should not only be experienced in their particular fields but should also have a steel fabrication background. This allows the inspector and fabricator to come to agreement quickly on many day--to--day matters on the basis of
common experience, rather than hold up the work unnecessarily on minor details.
2.7.7.
SUMMARY FOR SPECIFICATION WRITERS
1. Specifications are not as important as in previous years because so much has now been codified.
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2. Omit meaningless clauses, no matter how well--sounding. They can achieve nothing but may exacerbate disputes. 3. Do not include information in specifications that should be more properly shown on drawings. 4. Call up AS 4100 and associated documents. 5. Keep it brief.
3.
Framing Concepts and Connection Types
3.1.
Introduction
The framing system and framing layout chosen for a particular application will be influenced by: (a) Nature and level of the loads to be resisted (b) Requirements and restrictions on useable space within the framework, and (c) Constraints imposed by architectural requirements. One advantage of steel framing is the diversity of solutions that are possible for any given application. There are available to the designer two basic connection types, namely: •
Rigid connections, and
•
Flexible connections.
The above connections may be used in the three basic framing systems available: 1. Two--way rigid frameworks 2. One--way rigid/one--way braced frameworks, or 3. Two--way braced frameworks. Judicious selection of the appropriate framing system and connection types is a prerequisite to an economical structural design. Once a framing system is selected, the connection types to be used follow directly, thus setting bounds to the final cost of the structure. Economy in detailing, fabrication and erection can only serve to move the final design towards the lower bound of cost established by the framing system.
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In the discussions of connection types and framing systems which follow, no distinction will be made between single or multi--storey buildings since the basic principles apply to most buildings.
3.2. 3.2.1.
Connection Types DESIGN METHODS IN AS 4100
AS 4100 allows the use of three different design methods, wherein the behaviour of the connections is fundamental to the design method. These methods are: (a) Rigid Construction, in which it is assumed that the connections have sufficient rigidity to hold the original angles between the members unchanged. (b) Semi--Rigid Construction, in which the connections may not have sufficient rigidity to hold the original angles between the members unchanged, but are assumed to have a capacity to furnish a dependable and known degree of flexural restraint. (c) Simple Construction, in which the connections are assumed not to develop bending moments. The stability of the structure is therefore provided by triangulation (i.e. bracing) or by separate shear walls -- see Section 3.3 et seq. Clearly from these brief descriptions it is seen that connection behaviour has a significant influence on design. Allied to design methods (a) and (c) above are the basic connection types noted in Clause 3.1, namely: •
Rigid connections, and
•
Flexible connections.
Design method (b), Semi--Rigid Construction, will not be considered further in this publication.
3.2.2.
FLEXIBLE CONNECTIONS
Flexible connections are used in steel structures designed using the simple design method of AS 4100. These connections offer low restraint to beam rotation, being close in behaviour to that of an ideal pin. Typical flexible connections are shown in Fig 3.1. The most common flexible connections in use in Australia are the flexible end plate (Fig 3.1 (c)), the angle cleat (Fig 3.1 (d)), and the web side plate (Fig 3.1 (e)). Such connections are:
(a) Angle Seat
(b) Bearing Pad
(a) Assumed to behave as a simple support (b) Simple to fabricate (c) Simple to erect, and (d) Less costly of the two connection types. Flexible connections shown in Fig 3.1 are totally standardised in the AISC Standardized Structural Connections publication (Ref 1). (c) Flexible End Plate
(d) Angle Cleat (single or double)
(e) Web Side Plate Fig 3.1 Flexible connections
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3.2.3.
RIGID CONNECTIONS
Rigid connections are used in steel structures designed using the rigid design method of AS 4100. These connections offer very high restraint to beam rotation, being close in behaviour to fully fixed (or encastre) connections. Typical rigid connections are shown in Fig 3.2. The most common rigid connections in use in Australia are the stub girder connection (Fig 3.2 (b)) and the bolted moment end plate connection (Fig 3.2 (c)). These are also covered in the AISC Standardized Structural Connections publication (Ref 1). Rigid connections are: (a) More complex in fabrication (b) More difficult to erect where tight tolerances are involved, and
(a) Field welded moment connection -- with erection cleat (also use fillet welded web cleats in lieu of beam web welds).
(c) More costly of the two connection types.
(b) Stub girder connection -- fully shop welded beam stub, pliced on site.
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(c) Bolted Moment End Plate Connection Fig 3.2 Rigid connections
3.3.
Basic Framing Systems
3.3.1.
TWO--WAY RIGID FRAMEWORK (Fig 3.3)
Two--way rigid frameworks comprise two planes of rigid frames intersecting at right angles using common columns at their intersection. Such frameworks resist lateral forces in both planes by frame action without the need for any separate stabilising elements. All the beam--to--column connections must of necessity be of the rigid type, and the columns may need to have approximately equal stiffness in both directions, so that boxed or tubular columns may be employed due to their high stiffness about both principal axes. Under the action of lateral forces, there is always some sway as a result of the elastic deformation of the framework, but there is no problem in designing the structure in such a way that this sway is kept within an acceptable limit. The main advantage of the two--way rigid framing system is in the complete freedom in planning it offers. On the minus side is the necessity for the more costly rigid connections and columns. Since the rigid design method of AS 4100 is used for this framework, the analysis can be either by the elastic or the plastic method, the latter being more mass economical due to a better utilisation of material. It does, however, require slightly more costly connections. The main design advantage of a rigid beam--to--column connection lies in the reduction in the sizes of the floor beams due to the end fixity. Increased column section mass may, however, counterbalance this saving since larger bending moments need to be considered in the columns. The resulting increase in material cost should not exceed the extra cost involved in the rigid connections for the resulting framework to be an economical selection.
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Fig 3.3 Two--way rigid framework Typical applications that may use this type of framing include: (a) Multi--storey frames (b) Iow--rise rectangular frames (especially where architectural requirements restrict the use of bracing elements) (c) Heavy industrial structures (especially where planning needs restrict the use of bracing elements), and (d) Architectural structures that can be modelled as two--way rigid frames.
3.3.2.
ONE--WAY RIGID FRAMEWORK (Fig 3.4)
One--way rigid framework has been used quite extensively for the simple reason that the most commonly employed structural sections, (namely, universal sections) exhibit high bending resistance about the x--axis and inferior bending resistance about the y--axis. The relatively more expensive rigid beam--to--column connection is required in the unbraced plane, while simple connections of the flexible type can be utilised in the braced plane. In comparison with the two--way rigid framing system, there is slightly more restriction in planning the floor layout since space must be reserved for the stabilising elements. However, this is seldom a problem since the bracing can be arranged within the thickness of the perimeter walls or alternatively be tied back to a bracing element.
As a general rule, it is necessary with this arrangement to construct a rigid system consisting of either wind girders or a diaphragm having great rigidity in its own plane and being properly connected to the framing system. With such a system, it becomes possible to distribute the lateral forces to the individual stabilising elements. A reinforced concrete floor slab resting on steel beams is one example of a reliable diaphragm action. In the unbraced plane, the frame can be analysed as a rigid frame using the methods outlined in Clause 3.3.1. In the braced plane, ’pinned’ connecting beams are usually assumed, although rigid connections may be employed in order to provide beam continuity and/or reduce the lateral deflection of the frame in this direction. Such a procedure, however, may not be an economical overall solution. Typical applications that may use this type of framing include: (a) Low--rise industrial frames (portal frames) (b) Rectangular frames (especially where bracing can be accommodated within the perimeter) (c) Industrial structures, and (d) Architectural structures (bracing elements are often used as part of the architectural feature).
Fig 3.4 One--way braced, one--way rigid framework
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3.3.3.
TWO--WAY BRACED FRAMEWORK (Fig 3.5)
Two--way braced frameworks depend on stabilising elements arranged so that lateral forces from all directions can be effectively resisted. The framework itself can be constructed in the form of beams ’pin’ connected to the columns, in which case the beams are designed as simply supported, and the columns as essentially axially loaded members, with beam reactions acting at small eccentricities off the column face. It is most important with this system to have a relatively rigid floor system capable of preventing distortion of the framework in plan. From the design engineer’s point of view this is the easiest framing system to analyse since there is very little interaction between the framing members. Not surprisingly the two--way braced system is also very attractive from the cost point of view, since the simplicity of the member connections can offset the cost of the somewhat heavier floor beams required with this system. The stabilising elements can be orthogonally arranged shear walls, braced panels or cores (Clause 3.3.5). These stabilising elements have to be located to give a well balanced system and the floor plan must accommodate this. In most cases it is possible to utilise the walls around service blocks or external walls (Clause 3.3.5). External bracing, forming part of the architectural feature, can also be utilised. In this type of design, all beams are assumed to be pinned at their connections to the columns. In fact the connections are not pins but a flexible type so that free end--rotation can be assumed. The design of the beams can be carried out without reference to the framing as a whole. However since the beams, designed as pin--ended, tend to be larger in size than if fixed connections are used, it is imperative to design them to be as efficient as possible. One of the ways of securing economy is by making use of any concrete floor slab present to achieve composite action. The main advantage of composite action is that it augments the beam with a ’concrete flange’ and also increases its depth. Ref 5.3 contains a full discussion of composite steel beam design. The columns carry only the gravity loads. Some bending is present due to the eccentric application of the beam reactions, but the effect of this bending is usually small. The bracing system is usually assumed to take most of the lateral forces.
Typical applications that may use this type of framing are Iow to medium--rise rectangular frames (up to 50--storeys -- especially using cores, either steel framed or slip--formed concrete).
Fig 3.5 Two--way braced framework
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3.3.4.
SUMMARY OF FRAMING SYSTEMS
Framing System Two--way rigid
Advantages
Disadvantages
No stabilizing elements required for lateral forces in any plane. Freedom of layout planning. Plastic design methods can be used if desired -economical in material. Continuous beam design leads to reduced beam size.
Requires the use of rigid connections, which are more costly than simple connections. Columns ideally should have near equal stiffness in both directions -hence fabricated box columns may be needed. Large column movements. Rigid connections used in unbraced plane. Some restriction on planning layout; stabilizing elements required in one plane.
One--way rigid / Simple connections One--way (least costly type) used braced in the braced plane. Can use I columns -usually rolled sections. Can use plastic design methods and continuous beam design in plane of rigid connections -- saving in material. Two--way Simple connections braced possible -- least costly type. Usually use I columns. Beams assumed simply supported for design; columns designed for axial load only at small eccentricity.
Restriction on planning layout because of requirement for provision of stabilizing elements. Little interaction between elements. Heavier beam sizes.
3.3.5.
STABILISING ELEMENTS
Construction elements whose function is to provide a means of stabilising the framework in either one or two planes may be divided into the following categories: (a) Triangulated steel bracing panels using the X, K, or diamond pattern of diagonal members -- Fig 3.6(a); (b) Vertical Vierendeel cantilevers in steel -- Fig 3.6(b);
(c) Triangulated core
(c) Triangulated steel core -- Fig 3.6(c);
Fig 3.6 Stabilising elements built in steel
(d) Reinforced concrete or masonry shear walls -- Fig 3.7(a); (e) Reinforced concrete or masonry cores or shear tubes -- Figs 3.7(c) and (d); (f)
Brick in--fill panels and walls -- Fig 3.7(e);
(g) Light metal cladding used on the stressed skin principle.
(a) Triangulated bracing systems
(b) Vertical Vierendeel cantilever
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When stabilising elements are constructed of concrete or masonry, it is well to remember that some means of temporary bracing may be required during the early construction phase, since the steelwork may not have sufficient in--built resistance to withstand lateral forces prior to construction of the stabilising elements. Rigid systems of wind girders or diaphragms (Fig 3.8) may also be required to distribute lateral forces to the stabilising elements. Openings can readily be incorporated in all types of stabilising elements, although there is some restriction on the maximum size of openings. It is important, however, to distinguish between the low--rise building which does not require large stabilising elements, and tall building where the stabilising elements are required to carry very large forces and have a relatively high stiffness.
(a) Shear Wall
(b) Opening may be accommodated in shear wall
(c) Shear tube
(d) Corner walls
(b) Concrete floor slab as diaphragm Fig. 3.8 Floor deck bracing systems
(e) Brick in--fill wall Fig 3.7 Stabilising elements built in reinforced concrete or masonry
(i) Rigid frame action
(ii) Steel lattice bracing
(a) Wind girders as sole means of transfer of wind forces
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(iii) In--fill wall panel
(iv) Transverse wall (ii) Horizontal wind girder
(v) Stairwell walls (a) Vertical systems
(iii) Use of floor as diaphragm (b) Horizontal bracing systems Fig 3.9 Action of lateral force resisting systems (from Ref 5.2)
3.4.
Cost and Framing System
The type of framing system selected to satisfy all the design constraints will have a profound effect on the structural cost. The labour cost in the fabrication of a fully braced system employing simple flexible connections is much less than the labour cost in fabricating a fully rigid system using more complex moment connections. On average the rigid framework requires about 2.5 times the labour cost input in the fabrication process. (i) Lateral force transmitted to foundation at every column -- no horizontal bracing
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To achieve the most economical final structure the designer has to find a solution which, within the various constraints, will provide for maximum cost effect in both material and fabrication labour input.
3.4.1.
MULTI--STOREY BUILDING
The following example illustrates the way in which cost effective solutions can be achieved and the importance of selecting a framing system of least cost to serve function. A minimum mass solution may not always produce the best cost effect-- in this case the minimum mass fully rigid frame requires substantial additional labour input for connections in comparison with the simpler flexible connections used in the braced system. Thus the apparent savings in material cost are less than the increase in labour costs. The adoption of a fully rigid frame, although of significantly lower mass of material, will not produce the best economical solution unless such a system is demanded by constraints such as freedom of layout or architectural bias against cross bracing. In structures such as city buildings even greater benefit in cost is achieved by using the service core as a stabilising element in lieu of cross bracing.
Unraced Frame Plastic Design 38.2 tonnes of beams and columns at cost ratio 2.0 = 76.4 38.2 tonnes, Total Cost Fig. 3.10 Frame example
Braced Frame Elastic Design 48.1 tonnes of beams and columns at cost ratio 1.0 = 48.1 5.5 tonnes of bracing at cost ratio 0.8 = 4.4 53.6 tonnes, Total Cost
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= 52.5
= 76.4
3.4.2.
SINGLE--STOREY INDUSTRIAL BUILDING
Similarly in other types of structure the framing system will influence final cost. In typical factory buildings, for instance, which were once framed by column--and--truss systems, it is quite clear that the rigid portal frame is the most economical system. Fig 3.11 shows that truss systems are obviously more efficient on a mass/ unit area basis. However, on a cost basis, the inherent simplicity of the portal frame renders it less costly to fabricate and shows up as the economical solution within the range shown (see Fig 3.12).
Fig 3.12 Relationship between cost/unit area and span These examples are intended to illustrate the importance of carrying out an examination of framing system costs at the earliest design concept stage. The best end result will be obtained by selecting the framing system which will satisfy function and economy.
3.5. Fig 3.11 Relationship between mass/unit area and span
Framing Details
Having thus selected the framing system as previously discussed, it is important to consider framing details for that particular system so that the best cost effect will be achieved. In general the following points must be considered.
3.5.1.
SYMMETRY
In many cases symmetry is available in framing systems simply as a result of functional requirement e.g. city building frames. However in other types of structure it is often possible to arrange symmetrical layout without prejudice to function. Symmetry will invariably lead to the possibility of repetition and this will provide for the most economical fabrication and erection.
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3.5.2.
RATIONALISATION OF MEMBERS
The grouping of members in a framework with respect to type and size will also have advantages in fabrication and erection economy. Series of members of the same size and length will be processed more efficiently in the shop. At the erection stage the greater number of identical items will provide for speedy erection. Obviously in grouping of members considerable skill is required of the designer. Too much grouping of member size can be wasteful of material and too little will add to detailing, fabrication and erection costs. In general it is advisable to minimise the number of highly individualised members and thus provide for maximum repetition and interchangeability.
3.5.3.
STANDARDIZATION
Connections The AISC publication Standardized Structural Connections (Ref 1) contains highly standardized data for both simple flexible connections and rigid connections. The use of such a system, with constant dimensional criteria, allows for efficient fabrication by optimising the use of modern automated equipment in the fabrication shop. It is also recommended that the designer consider the various suitable alternatives within a particular connection group (i.e. either flexible or rigid). This will allow the fabricator to select from the Standardized Structural Connections publication the connection type which can most economically be fabricated with the equipment available and which will satisfy the designer’s requirements. The important thing to remember is that the greater part of the fabrication process is involved in preparing members to be connected to one another and the more standardization, especially with respect to connection geometry, which can be incorporated in a design, the better will be the final economy. Finally, in selecting connection types, try to consider groups of members requiring only one operation in the shop. This can be accomplished by arranging for a series of members (e.g. primary floor beams) to require only
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cutting to length and holing, while another series (e.g. beams connecting to primary beams) to require only cutting and welded fitments (see Fig 3.13).
Group 1 Cutting and holing only
Group 2 Cutting and welding only Fig. 3.13 Beams for economic fabrication Bolts and Welds (Fasteners) It is advisable to consider the standardization of fasteners within a given structure. Where possible, adopt the use of one bolt size, grade and procedure within the structure. See Section 6. Similarly, use one electrode strength grade, one weld category and if possible one weld size (in the case of fillet welds). See Section 7.
3.5.4.
SIMPLICITY
Simple detailing for such things as stiffeners, bracing gussets, attachment cleats and base plates, will produce the greatest economy in fabricated work. The number of man--hours spent can increase dramatically if such details become complex. See Section 8. The following general examples show how cost extras can be incurred: Structure A Commercial Building A relatively simple beam and column framework with repetition of bay size and minimum bracing components; standard connections (two types) used throughout with snug--tightened bolts. Structure B Similar Building This example is considerably more complex having varying bay sizes, spandrel periphery trusses and extensive bracing in the wall planes;
connections are of several types and custom designed, some using fully--tensioned bolts. Cost Index Material Shop Labour Painting Shop Detailing Erection Notes:
Structure A 1.00 1.00 1.00 1.00 1.00
Structure B 1.00 2.08 1.22 1.67 1.25
1. Cost indices are presented for the purpose of comparison only. 2. Some common items such as administrative overheads, profit and builder’s mark--up have been excluded from this comparison.
It can be seen therefore that for two structures performing similar function the final cost of structural steel is sensitive to the complexity of work required. For example, the introduction of truss work into the framing system together with more complex connections has more than doubled the shop labour component for Structure B. Also costs are higher for shop
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detailing (increased complexity required additional time), painting (increased surface area for truss work) and erection (complex connections and fully--tensioned bolts add to cost).
3.6.
Conclusion
The selection of the system for a steel framework is the most fundamental determinant of the final cost of the erected structure. Once the basic framing system is selected, the connection types which may be used are chosen. Thus, the basic cost of the erected framework is predetermined, recognising that this cost may vary within a certain range. Economic detailing, fabrication and erection can only move the final cost towards the minimum possible within this range. It is essential that at the preliminary design stage the full range of alternative framing systems are evaluated and compared before making the final selection. This comparison of alternatives must be done on the basis of erected cost -- not on the basis of mass. Good design i.e. economical design, should take into account all the influences which have an effect on the form and cost of the final structure. The economics of design must be considered in this context since the client is mainly concerned with what he/she pays for-- a complete building which meets his/her needs at least cost.
4.
4.1.
Industrial Buildings
Introduction
Steel--framed buildings in common use for industrial purposes can be classified into three broad categories: (a) Warehouse and factory buildings (b) Large span storage buildings, and (c) Heavy industrial process plant structures. In the design of industrial buildings, function more than any other factor will dictate the degree of complication and hence the economy possible. Towards this end, the designer should obtain as much knowledge as possible of the industrial process or purpose for which the building is intended, and of the limitations this might force on the structure. In this way, an optimum balance between function and economy can be achieved. The main dimensions of an industrial building are usually determined from a combination of functional and design considerations. Its width is derived first from an owner’s study of the space required to carry out the processing or storage operations. The designer then needs to consider whether this width can be provided economically by a single clear span, or whether multi--bay spans are feasible. Likewise the overall length is usually readily determined by the owner, but the designer should give thought to the optimum bay length. Some of the factors affecting the choice are: 1. Foundation conditions, and their ability to accept the column loads 2. Crane runway girder considerations (see Clause 4.2.5) 3. Purlin and girt capacities (see Clause 4.2.6)
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4. Masonry bond dimensions, and 5. Tilt--up concrete panel size and available cranage. The building height is again usually a functional consideration; for buildings with overhead travelling cranes the critical dimension is the clearance required under the hook. In most areas of Australia there is no snow, and therefore fairly low roof pitches are practicable. The steeper the slope the better the structural action, but this benefit is usually outweighed by additional sheeting costs. In practice, roof pitches between 5_ and 10_ are preferred. These pitches are suitable for any of the continuous length steel sheet roofing profiles, some of which are adequate for pitches down to 1_.
4.2. 4.2.1.
Warehouse and Factory Buildings GENERAL
In the early days of steel--framed industrial buildings the economic solution was a column--and--truss configuration (Fig 4.2 (a)). However, since truss fabrication is inherently labour intensive, rising labour costs have excluded these truss systems from normal factory or warehouse applications. Presently, rigid ’portal frames’ fabricated from universal beams offer the most economic structural solution in the usual span range of 15 to 45 metres. For very large spans, portal trusses (see Fig 4.18) are often used in lieu of the portal frame. Although the portal frame may require a greater mass of steel than the equivalent column--and--truss arrangement, the savings in the cost of fabrication and erection due to the relative simplicity of the work almost always make it the optimum system in the span range given above. To minimise the overall cost of warehouse and factory buildings, designers should be aware of the major steelwork cost components. Effort can then
be focused on cost components that can reduce the overall cost. Figure 4.1 shows the various cost components in relation to a warehouse.
4.2.2.
STANDARDIZED PORTAL FRAMES
Overseas, particularly in North America, the portal frame structure has been developed to the stage where many companies offer a standard range of buildings in spans up to as much as 50m. Economies of scale and production line manufacture have made these ’catalogue’ buildings a cost--effective choice for many industrial as well as commercial applications. The same manufacturing and marketing techniques have been attempted in Australia, but with limited success, probably due to our much smaller and more widespread demand. As a consequence, practically all larger portal frame structures built in Australia today are custom designed and manufactured. This is not as inefficient as it may sound, because there are many standardized routines in both the design office and the fabrication shop. On the other hand, smaller buildings (sheds, garages, etc) are widely available in Australia as standard catalogue items. Nowadays these are often manufactured entirely from cold--formed sections rather than from traditional hot--rolled sections. Roof & Wall Sheeting supply & fix = 37%
Steel Supply = 20%
Fabrication = 15% Purlins & Girts Supply & Fix = 24%
Surface treatment 2% Steel erection 2%
Fig 4.1 Steelwork Cost Components for Warehouses
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(a) Column and Truss
(b) Portal Frame Fig 4.2 Configuration of framing systems for a factory building
4.2.3.
CUSTOM DESIGNED PORTAL FRAMES
In this case, a client engages an Architect and Consulting Engineer who prepare design drawings and submit the project to tender. The contract is usually awarded to a builder who then sub--contracts the structural steelwork to a steel fabricator on the basis of the Consulting Engineer’s drawings. The portal frames will usually consist of universal sections in order to be economic in fabrication -- see Fig 4.3. A variety of connection details are encountered, but only a limited number are truly economic for such frames. Fig 4.4 shows examples of economic details using bolted knee and apex joints, while Fig 4.5 shows examples of economic details for frames using shop welded knee and apex joints and bolted rafter splices. For spans up to 20m a uniform column and rafter section is the most economic but for greater spans haunching of the rafter may provide a more economical system. Haunching is most economically achieved by using a cut universal beam section in the manner shown in Fig 4.3, with the depth of the section at the haunch about twice the rafter depth. The haunch length is usually of the order of 10%--15% of the span of the rafter.
Fig 4.3 Details of bolted portal frame
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Fig 4.5 Transportation limitations for portal frames Fig 4.4 Details for welded portal frame (with bolted rafter splice for field erection)
4.2.4.
BRACING OF PORTAL FRAMES
The selection of either bolted or shop--welded knee and apex joints will be governed by the span of the frame and the transport and erection facilities available for a particular job.
Bracing Disposition
In general the dimensions given in Fig 4.5 are a guide to limitations on maximum size imposed by transportation considerations.
For shorter buildings (up to 60--80m), a single end braced bay is all that is necessary to stabilise the building structure. However, this arrangement requires wind forces on the opposite end to the braced bay to be transferred along the building length by way of longitudinal eave and ridge struts. This may require heavy struts, and it is often more economic to provide braced panels in each end bay and remove the necessity to provide these substantial struts.
For frames of larger dimensions than those indicated in Fig 4.5, consideration would have to be given either to special transport facilities or additional field splices. A further discussion on portal frame details can be found in Clause 8.5.
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The typical disposition of bracing panels for portal frames buildings is shown in Fig 4.6.
Temperature expansion of buildings 60--80m long is most commonly and economically catered for either by allowing the expansion force to act on the end bay bracing or by the use of slotted holes (or oversize holes) in the connections of the longitudinal struts to the columns. In longer buildings (over 60--80m), corner bracing can be a disadvantage since the expansion involved is too much to be accommodated by the above methods. In such cases, a central expansion joint can be provided (thus effectively making two buildings -- Fig 4.7(a)) or alternatively, the bracing can be provided near the central interior bays -- Fig 4.7(b). For the latter alternative, substantial longitudinal struts may be required to transmit wind forces from the end walls through to the braced bays. Whether this solution is economic depends on the increase in size of the longitudinal struts required for the latter solution compared to the additional cost of the extra column in the expansion joint solution.
(a) Use of central expansion joint for buildings over 60--80m long
(b) Alternative bracing system for buildings over 60--80m long Fig 4.7 Bracing for long buildings Bracing Details For sheds and small buildings rod bracing, tensioned by turnbuckle or by deliberately ’detailing short’, is the most economic solution, although there is an alternative school of thought which uses angle bracing. With rod bracing, the ability to plumb frames and square the buildings by using the turnbuckle adjustment makes for easier erection.
Elevation
Plan Fig 4.6 Bracing panels
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For wide frame spacing, rod bracing will tend to sag over the longer span involved and may present some problems in effectively bracing the roof. As well, rod bracing in the walls may become subject to physical damage during occupancy. Angle bracing can overcome these difficulties. Tubular sections are efficient members for bracing in larger structures. Their inherent properties provide high load carrying capacities for low mass of material and make circular and rectangular hollow sections (CHS and RHS) very attractive from a design point of view. However, for these advantages to be reflected in the overall economy of the fabricated structure attention should be paid to the end connections since their preparation involves the largest part of the fabrication cost, (see Ref 4.7). Economic connection details for bracing members are shown in Figs 4.8, 4.9 and 4.10.
(a) End connection (b) Simple crossover intersection (a) End connection
(b) Typical intersection
Fig 4.9 Details for angle bracing
(c) Intersection using a pipe piece (no turnbuckles needed) Fig 4.8 Details for rod bracing
(a) Flattened end (CHS only)
(b) Welded tee end
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heavier structure providing lower maintenance cost in the future operation of the crane.
(c) Slotted end plate
Fig 4.11 Types of supporting columns (d) Typical intersection Fig 4.10 Details for tubular bracing
4.2.5.
CRANES IN PORTAL FRAME BUILDINGS
The most common crane type used in portal frame industrial buildings is the electric overhead travelling crane. The crane bridge travels on two longitudinal girders which are supported at each portal frame of the building structure. The design of a crane runway girder must be considered as an integral part of the whole building. At the same time, it must be recognised that because of the dynamic forces imposed on the runway girder, extreme economy in member and connection design is not recommended and is considered unwise. The best solution may be a
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Fig 4.12 Crane runway brackets
The cost of continuous girders is usually higher than for simply supported girders since the efficiency of the member is offset by higher erection costs. However, the most economical compromise is often to design and detail the girder as continuous over two frame spans. This allows the fabrication of either rolled members or plate girders from stock material and therefore minimises fabrication costs while still reducing the total number of girders to be erected.
4.2.6.
Fig 4.13 Commonly used sections for crane runway girders and their relative fabrication cost The method of supporting the crane runway girder depends on the magnitude of the crane wheel reactions (i.e. on the crane capacity and the crane classification) and upon the structural characteristics of the portal frame column. Fig 4.11 shows some typical arrangements as follows: (a) Separate crane column, acting with the frame column (b) Combined frame and crane column (c) Separate crane column, acting separately from the frame column (d) Light frame column bracket, with the frame column acting as both frame and crane column. Generally types (a), (b) and (c) in Fig 4.11 will be chosen for heavier capacity cranes as classified in AS 1418. In most factory type buildings, cranes will be of low to medium capacity (up to 5 tonnes) in which case the crane runway girders could be supported on a column bracket (type (d)). This bracket should be proportioned to minimise stiffening of the frame column (see Fig 4.12). Ref 4.4 is a publication on the design of crane runway girders and outlines the factors which affect the overall economy of both the crane girder and the enclosing structure. Fig 4.13 shows the most commonly used crane girder sections in portal frame industrial buildings, and gives an indication of their relative fabrication cost. Ref 4.4 gives more detail and discusses other types of runway girders.
41
PURLINS
The sheet cladding of industrial buildings is attached to a framework of secondary members which is itself connected to the main frame. These secondary members are known as purlins (for roof sheeting) or girts (for wall sheeting); the term purlin is used when referring generally to both types. In Australia industrial purlins consist almost exclusively of cold--formed members -- usually Zed or C sections, often formed from hot--dip galvanized strip. These members are available from several manufacturers and in a variety of depths ranging from 100mm up to 350mm in 50mm increments. The availability of section depths varies in each State. Availability of the larger sections should be confirmed with suppliers before being specified to avoid unnecessary delays and cost to the project. For average industrial buildings a purlin 200 mm deep appears to represent an economic optimum, and it is the capacity of this size that often fixes the frame spacing typically 6 to 8m. The supply and fixing of purlins and girts represent about 24% of the total steelwork cost for a warehouse. Judicious selection of purlins, and attention to design loads and details can contribute to a significant reduction in overall project cost (see Fig 4.1). Purlins are bolted to the rafters by means of a simple welded cleat (Fig 4.14). Most manufacturers specify M12 bolts and some provide special purlin bolts having an M12 thread and an M16 shank. Purlin and cleat bolt--hole geometry has been standardized by the Australian Institute of Steel Construction in the ’Standardized Structural Connections’ and most manufacturers conform to these standards (Ref 1). Zed section purlins are shaped so that they can be lapped, and this feature allows the designer to take advantage of partial or complete continuity at the splices (Fig 4.15). However in some cases the structural advantages of
continuity may be off--set by extra cost and complication in the purlins themselves. C section purlins are normally used simply supported at the ends (Fig 4.16) or continuous over two spans. For shorter bay lengths purlins can be obtained long enough to be used continuously over two spans. This reduces deflection compared with simple spans but does not give the same structural performance as a fully lapped system. The performance of purlin systems requires in most cases the provision of adequate lateral stability by means of ties or bridging. Purlin manufacturers supply such systems, and some also offer accessory items such as raking girts, fascias, etc.
Fig 4.15 Zed section purlins with lap
Details of proprietary purlin systems, design information and load tables can be obtained from manufacturers’ literature.
Fig 4.16 C section purlins with butt joint
4.2.7.
Fig 4.14 Standard purlin cleats
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FLY BRACING
In a portal frame building either flange of both the rafters and the columns can be a compression flange depending upon the assumed magnitude and direction of wind loading. The exterior flanges are normally adequately laterally braced by the purlins and girts, but sometimes the design may require the provision of bracing to the otherwise unrestrained interior flanges.
This is most conveniently accomplished by the inclusion of so--called ’fly bracing’ at purlin intersections (see Fig 4.17). This can easily become a very costly detail, and unnecessary expense can be avoided by the use of the simple flat bar arrangement as shown.
It can be seen that the choice of cladding determines the purlin spacing which in turn can influence some of the basic design parameters such as purlin size and bay length. Steel sheeting is readily fixed to cold--formed purlins by means of self--tapping screws. Special heavy duty self--drilling self--tapping screws with in--built neoprene seals are normally used. Concealed--fix profiles are secured by separate clips or straps which are normally attached to the purlins. On the finished job these straps are hidden and there is no piercing of the cladding surface. Where sheeting is to be painted for decorative purposes or to provide added protection, considerable economy can be gained by the use of pre--painted cladding. The factory--applied finish avoids costly site painting and provides far superior paint adhesion and quality. Full details of steel sheet cladding profiles, accessories, design and fixing data etc., are obtained from manufacturers’ literature.
4.3. Fig 4.17 Method of fixing fly bracing to standard punching
4.2.8.
SHEETING
Coated steel sheeting are the most popular and economic cladding material for both the roof and walls of industrial buildings. (There may in some circumstances be regulatory constraints on its use in walling).
4.3.1.
Large Span Storage Buildings SPANS OF 45--70 METRES
When buildings of over 45m clear span are required for such purposes as container storage, etc., consideration should be given to the use of portal--truss systems for economy. Spans of 45 to 70m are economically satisfied with such systems (Fig 4.18).
A variety of profiles is available, ranging from traditional corrugated sheeting to sophisticated ’concealed fix’ products. All of these sheets are manufactured from continuous strip and therefore can be supplied in most cases so as to eliminate end laps. It is usual practice for sheeting to be ’custom cut’ by the manufacturer in the precise quantities and lengths needed for each particular project. Except in cyclonic areas, steel roofing is capable of spanning about 1200 mm in the case of corrugated sheeting up to as much as 2700 mm for stronger and deeper profiles. These figures relate to interior spans. End spans for screw--fixed products should normally be limited to about three--quarters of these figures. For walling, spans can be 25% to 50% greater.
43
Fig 4.18 Three--pinned portal truss The factors affecting the economy of the fabricated structure in such truss systems are those common to truss--work in general and these are
discussed in Clause 8.4. Other considerations such as bracing, sheeting, etc., are as discussed in Clause 4.2.
4.3.2.
SPANS IN EXCESS OF 70 METRES
Spans greater than 70m are required for structures such as aircraft hangars, large stadia or storage buildings. Several buildings have been built in recent years using a space frame system of the ’flat double layer’ type (Fig 4.19), although other types are also available. The success of space structures, as in all structures, greatly depends on the use of an efficient jointing method (or connection). In Australia there are several proprietary joints readily available and a full discussion of space frame systems may be found in Refs 4.5 and 4.6. The inherent economy of space structures lies in the fact that the frame is made up of a large number of similar elements which can be fabricated in a mass production operation. The erection of the frame can be often accomplished by assembling the frame on site at ground level and jacking it into position on the column supports. From an overall economy point of view, however, space frames should be considered only for applications where extremely large clear spans are required to satisfy building function. They may be selected for other applications purely for architectural reasons. Fig 4.19 The basic square grid double layered space frame
4.4.
Heavy Industrial Structures
These structures can be considered as almost entirely custom designed to fulfil the function demanded of the engineering or manufacturing process involved. It is therefore most important that the designer adopt a rationalised approach to member selection and standardized connection details in order to achieve the most economic frame within the functional constraints. In structures such as steel--mill buildings or power stations, the members are often massive in comparison with normal building structures and certain considerations assume greater importance.
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4.4.1.
ERECTION
The proposed method and sequence of erection should be considered at the preliminary design stage. The columns in such structures are often of very stiff box--section with fixed bases and it is obviously not possible to ’spring’ such a column during the erection of a girder. The girder--to--column connection must be selected to permit easy placing of the girder between columns and ready access to complete the connection fastening. End plate connections are usually not preferred in cases such as these, since the need to fabricate girders short and subsequently shim on site adds greatly to the final cost of the erected structural work. Web side plate or angle cleat connections, on the other hand, provide flexibility in fabrication and erection tolerances, and generally will be more economic for simple flexible connections in industrial structures. Connections are discussed in more detail in Clause 8.6.
4.4.2.
SITE WELDING
In heavy industrial structures there is usually a great number of large connections each involving a considerable amount of site work. In these circumstances it may be worthwhile considering field welded connections. This is because the cost of establishing welding equipment on the job, and of moving it around, can readily be spread over the total amount of work to give an economic result (see Section 7).
4.4.3.
BOLTED CONNECTIONS
Although the general rule for economy is to design bolted connections with threads included in the shear plane, this may not apply in projects with a
45
predominance of large connections -- for example 50 or more bolts per connection. For these connections significant savings in the number of bolts (and therefore in the physical size of the details, the number of holes to be drilled and the time needed for erection) can often be made by designing for ’threads excluded’ (see Clause 6.4.4).
4.4.4.
FUNCTIONAL CONSTRAINTS
In large process plants and similar structures it is sometimes impractical to adhere to all the guidelines for economy in fabricated steelwork. For example the need to accommodate a variety of machinery, equipment and services can make it difficult to maintain uniform column spacings or to rationalise on a single floor beam size. Likewise bracing can often present a problem, and may have to be fitted in by the designer. While these departures from optimum practice may be unavoidable, the designer should nevertheless maintain an overall philosophy of: •
Simplicity -- keep the number of members down to a minimum to satisfy the structural and functional requirements.
•
Standardization -- use as many beams and columns of the same size and mass as possible; standardize the connections used.
•
Symmetry -- although in these custom designed structures it is often difficult, it should be remembered that connection selection and bracing disposition can lead to symmetry in members and layout. Obvious economy will be gained by providing for repetition in the fabrication shop.
5.
5.1.
Commercial Buildings
Introduction
In contrast to the industrial structures discussed in Section 4, where the criterion controlling the framing arrangement was often building function, the commercial or office type building is usually of a more regular layout. It is this characteristic which allows the greatest economy to be obtained through standardization and repetition of structural elements and connections. This category of steel building comprises a grid of steel beams connected to steel columns (or composite columns or concrete shear walls) using either simple or rigid connections. Resistance to lateral loads may be provided by using some form of bracing with steel elements or other types such as in--fill walls or shear walls, or by frame action using rigid connections.
5.2.1.
FULLY STEEL--FRAMED
Low--rise buildings fully framed in steel offer advantages in building speed and therefore in the overall economy of the final building. Because low--rise buildings do not require large stabilising elements, a steel frame using only simple connections can be used, offering economy in both fabrication and erection. The stabilising element is usually provided in the form of a steel cross--bracing system in one or two directions which can be incorporated in a facade treatment so as not to intrude into window openings. Another framing system which has been used successfully for low--rise buildings is the one--way--rigid, one--way--braced system (see Fig 5.1).
This type of building can be divided into two categories: (a) Low--Rise Commercial -- e.g. suburban office blocks of up to 4 storeys, schools, shopping centres, etc. (b) High--Rise Commercial -- e.g. city office buildings, hospitals.
5.2.
Low--Rise Commercial Buildings
This category can be further sub--divided into: 1. Fully steel--framed structures. 2. Composite frames (steel frames connected to concrete cores or utilising masonry in--fill panels).
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Fig 5.1 Framing system for low--rise commercial building This is essentially an extension of the industrial portal frame structure and results in an economic solution for small commercial buildings where freedom of layout and planning can be provided across the building width since no internal columns or bracing elements are necessary. In the design of such a building, it should be recognised that bays of equal size will assist in gaining maximum economy by allowing the repetitive use of similar sized beam and column sections. The economic detailing of
beams and columns is most important in achieving overall economy and aspects of this are contained in Section 8. Undoubtedly the greatest advantage of a fully steel framed structure lies in the ability to erect the entire structural framework on prepared footings, as a self sustaining system before any other building trades are required on site. With proper planning, this feature can lead to faster building speed and the elimination of many of the problems associated with diverse trades on site simultaneously.
5.2.2.
For the case shown in Fig 5.2 it should be remembered that the steel frame must be effectively temporarily braced during erection and properly plumbed before the brickwork or blockwork can be laid. If the temporary bracing has to be removed after stability is provided by the infill panels it could be placed on the inner flange of the columns in order to facilitate later removal and in order not to interfere unduly with the masonry work.
COMPOSITE FRAMES
Currently a favoured type of construction for steel low--rise commercial buildings is the provision of a stabilising element comprising a masonry or reinforced concrete core, with the steel floor beams connected with simple connections between periphery steel columns and the concrete core. For the low--rise commercial building, it is also common to use in--fill masonry panels to provide lateral stability. Examples of these systems are shown in Fig 3.7. Typical details of such a framing arrangement are shown in Fig 5.2 for the case where masonry panels are used to provide the stabilising element in a building frame.
Fig 5.3 Stability by concrete panels Fig 5.3 shows an alternative method of providing a stabilising element in the form of a concrete panel cast between two adjacent steel columns and tied into each. In this case, the wall thus produced would normally be considered as load--bearing and would support stair--landings etc., throughout the height of the building. In addition to the concept of composite frames, the use of composite beam--slab systems will provide best economy in these buildings. This is discussed in Clause 5.5.
5.3. 5.3.1.
High--Rise Commercial Buildings GENERAL
In Australia at present a high--rise commercial building will usually be a city office block of up to 50 floors. In these buildings, a regular column grid can be established resulting in repetitive bays in one or both directions. As previously mentioned, regularity of bays is important since it leads to maximum economy due to repetition.
Fig 5.2 Stability by masonry
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The architectural and aesthetic requirements usually control the exterior column spacing and therefore the bay sizes. A panel wall design with columns contained within the wall thickness allows maximum freedom in bay size selection, whereas when columns are exposed externally as an
architectural feature this results in the least flexibility in bay size selection. Bay sizes should be selected to produce minimum storey height. It is noteworthy that a saving of 75 mm per floor in a 20 storey building will save 1500 mm of exterior and interior wall, partitioning, columns, lifts, etc. On the other hand, columns cannot be spaced so closely as to detract from the usefulness of the space through which they pass. Selection of bay sizes is always a compromise between these two considerations. In a way similar to low--rise commercial buildings, high--rise commercial buildings can be sub--divided into: (a) Fully steel--framed structures, and
48
(b) Steel frames connected to reinforced concrete cores. In the selection of the best framing system the most important consideration is to find a structural form which is highly efficient under lateral loadings and which does not require an unreasonable premium in frame cost to resist those forces. A vast number of alternative steel framing systems have been successfully used in the past, but not all of these are economic under today’s conditions. Fig 5.4 shows some of the frame types suitable for buildings of various heights.
Fig 5.4 -- Optimum steel framing systems for buildings of various heights
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5.3.2.
FULLY RIGID FRAME
From a planning and layout point of view this system obviously creates maximum freedom since no stabilising elements are required in the vertical planes of the building framework. The system is suitable for buildings up to 30 storeys in height but should be considered only when constraints of planning and layout are unavoidable. It has the advantage of allowing efficient use of material because of the considerable interaction between beams and columns due to the use of rigid connections with resultant continuity in beams. However, in today’s situation, rigid connections are more costly to fabricate and this will often offset any savings in material. In addition columns will generally be more expensive because equal stiffness about both axes is required.
In view of the relative costs of shop and field welding, the stub girder shop welded to the column will generally prove a more economic solution for rigid framework.
5.3.3.
FULLY BRACED FRAMES
Fully braced frames of the type mentioned below are ’braced tubes’ where stability against lateral forces is provided by the braced action of the external building wall framing.
In the USA where frames of this type have been in use for many years, the basic method was to erect columns and field--weld beams at floor levels -see Fig 5.5.
Fig 5.6 Shop welded connection details Bracing across full building width Fig 5.5. Field welded connection details However, since this method required the field welding of the most critical joints in the structure, where both high quality welds and high construction speed was required (both being subject to weather and operator skill), this method has been refined by transferring the welding operation from the field back into the shop. This is accomplished by using the ’Christmas Tree’ concept as shown in Figs 5.6 and 7.9.
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If the total facade width of the building can be considered as a vertical truss, the resulting frame offers maximum stability against lateral forces and this system can be used for almost unlimited storey height. The advantages of braced frames lie in the use of simple flexible connections throughout and these are the most economical to fabricate. In addition, smaller columns can be used, often merely rolled sections. The floor beams on the other hand will tend to be heavier because no beam continuity is available but this mass addition will almost always be more than compensated by the less costly fabrication required.
Bracing by shear truss in external walls For buildings up to 50 storeys a shear truss in the plane of the external walls provides good stability characteristics and has the advantage of not intruding into facade treatment as much as the full width bracing mentioned. Architecturally, cross bracing has never been readily accepted. Some exceptions to this do exist overseas and in Australia, but in general engineers are expected by their architect to conceal bracing in building facades, and this can often be done by accepting a compromise between the space possible in a bay opening and the bending induced in floor beams-- see Fig 5.7.
5.3.4.
STABILITY BY MEANS OF SERVICE CORES
Since building structures of the type under discussion invariably require a ’Core’ in which are contained lifts, stairs, service ducts etc., it is convenient to consider the core as a major stabilising element to resist lateral forces. The floor beams are ’simply’ connected between steel periphery columns and the core structure, with resultant economies in fabrication and erection. Steel framed service core A fully braced core structure using steel elements can be erected very quickly as a free standing structure and provides convenient access to all levels of the building throughout the construction phase. Bracing can normally be placed to accommodate the necessary openings and provide adequate stabilising function for buildings up to 50 storeys. Slip--formed concrete core
Fig 5.7 Forms of bracing
Development of efficient slip--forming techniques has resulted in the construction of concrete cores becoming a fast, economic building process. Because such a central core is essential to house building services such as lifts, stairs, ducting, etc., it is logical to consider using the strong core as the major stabilising element for a multi--storey building (see Fig 5.9). This system has been successfully used in many recent buildings constructed in Australia and overseas. Using this method of stabilising the frame, the lateral forces on the external walls of the building are transmitted to the core through the floors. The floor, which usually consists of a concrete slab acting compositely with its steel supporting beams (see Clause 5.4), is considered as a deep horizontal diaphragm, and is extremely effective in transmitting lateral forces to the central core.
Fig 5.8 Bracing should connect to column
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The position of the concrete core within the building has a significant effect on its structural behaviour under lateral loads. If the core is asymmetrical, rotation in addition to translation will be generated under lateral loads. This is an important consideration when the core is situated at the extreme end of a rectangular shaped building -- see Fig 5.10.
connection of the floor beam to the concrete core must also be executed economically and methods of making such connections are discussed in Section 8. Table 5.1 summarises situations where the use of shear walls or cores are advantageous, and also lists situations where steel lattice bracing may be more appropriate. TABLE 5.1 SHEAR WALL vs LATTICE BRACING
5.4.
Floor Support Systems
Supporting members, suitable for use in floor systems for steel--framed commercial buildings include the following (see Fig 5.11):
Fig 5.9 Service core
Fig 5.10 Service core at end of building In such a case, it is often necessary to employ the use of an auxiliary steel bracing system in the end wall remote from the core. Thus the stability of the building in the direction shown is shared by the core and the bracing system. In general, when building structures using concrete cores as stabilising elements, connections of steel beams to periphery columns and connections of floor beams to floor beams can be of the flexible type. The
52
•
Universal sections (UB)
•
Welded Beams (WB) or plate girders
•
Hybrid girders
•
Castellated girders.
Universal sections are in general use in steel framed construction, except where long spans and/or heavy loads necessitate the use of larger members. The universal beam sections cover a reasonable range of spans and loading conditions, and are best suited for use as main or secondary beams. Cover plates can be welded to the flanges to increase capacity but it is usually more economic to use a standard welded I--Section. Electrical services and airconditioning ducts can penetrate through the web to avoid adding to overall floor depth. Simple and economic detailing of such openings is essential (Section 8 contains suggested details). Universal and standard welded I--Sections require little fabrication except at the beam--to--column or beam--to--beam connections. Non--standard welded Beams or plate girders cater for larger spans and heavier loads than universal sections. The flange plates are normally fillet welded to a single web plate. However, box girders can be fabricated using two web plates where very heavy loads are involved. Like universal sections, plate girders can have web holes to enable the electrical services and airconditioning to pass through. Economic fabrication of these members is possible using automatic submerged arc welding (see Section 7).
Hybrid girders are plate girders using a stronger grade of steel on the tension flange of the beam and possibly part of the web. One economical way of fabrication is to cut two universal sections of different grades symmetrically and reweld them with a central web butt weld. The beams may be made castellated or can have a solid web. These girders are particularly suited where the beam is to be made composite with a concrete floor slab, but have been rarely used in Australia. The profiled cutting and rewelding of a universal section to form a castellated girder containing web openings results in a girder which is deeper, stronger and stiffer than the original section. The web openings can be used for ducts and piping. Consequently, castellated girders can permit a reduction in the overall mass of the floor system, leading to savings in total building cost. The savings in material must, however, be considered against the increased cost of fabrication with this type of girder. Computer numerically controlled (CNC) cutting and welding equipment has improved the economic viability of castellated beams. Further discussion of these beam types is contained in Section 8. One other type of floor support system deserves some mention -- the stub girder system. This is a novel system reported from its use in the United States to increase the economic span limit for steel beams while, at the same time, providing space for mechanical ducts without any increase in floor height. The system consists of short lengths (stubs) of rolled section welded at intervals to the top flanges of the girder, the stub beams being spaced at intervals which allow secondary floor beams and ducts to pass between the floor slab and the girder. Fig 5.11 Floor support members
53
5.5.
Composite Construction
The current trend to steel framing for commercial buildings has been due to a large extent to the development of composite construction techniques. This concept is based on designing a structure to rely on some degree of interaction between elements of different materials. The economical use of materials should be the keynote in all modern building design. Composite steel--concrete construction in slabs, beams and columns, using both steel and concrete to maximum advantage, is one of the most effective means of achieving this objective.
5.5.1.
connection between the beam and slab at the interface. These elements are known as shear connectors, of which the most economic type is the welded stud (see Fig 5.13).
FLOOR SYSTEMS
In composite structural framing the term composite steel beam refers to a floor system comprising a steel beam acting with a concrete slab component on its top flange, interconnected to the slab such that both form an integral unit. The principal advantage of this lies in the fact that the concrete slab not only spans between and distributes the loads to the main beams but also forms part of the beams themselves (Fig 5.12).
Fig 5.13 Welded stud shear connector A conventionally formed slab system could be used as an alternative, but rising costs of the removable formwork material and the associated labour are making steel decking systems more attractive. In addition, the provision of extensive propping to the underside of formwork and the time delay in its removal mean that following trades are hindered in proceeding, thus negating the advantage of steel’s fast construction. If steel decking is to be used it is probably better to use a type which will also act compositely with the slab by becoming the positive reinforcement. Several forms of composite steel decking are currently available in Australia and are made from high--strength zinc--coated steel. Typical profiles are shown in Fig 5.14.
Fig 5.12 Composite floor beam system In the types of composite beam--slab systems in this discussion, the concrete slab can be constructed in several ways. One of the best nowadays is to cast it on profiled steel sheeting -- the sheeting serving as permanent formwork when the slab is poured. The method of achieving composite beam action involves the provision of some form of mechanical
54
Fig. 5.14 Profiles of composite galvanized steel decking
The use of composite steel decking provides for double economy. Firstly, it provides a low cost and efficient floor slab by eliminating the need for all or most of the lower reinforcement. Secondly, it has the benefits of permanent formwork such as speedy installation, a weather and safety cover, and an immediate working platform for other trades. Steel decking is used to its optimum advantage in steel framed buildings because full advantage can be taken of sheet continuity to increase slab load capacity and because the resultant slab can also be made composite with the steel beams. This means that composite action is achieved in two ways: (a) Within the slab, and
Fig 5.15 Composite columns incorporating a steel erection column
(b) Between beam and slab.
By proceeding in this way the erection of the structural frame is not controlled by the time taken for the forming, pouring and curing the final shape of a wholly concrete column. The steel column can be designed to support say 6 to 10 floors of structure, and the building program is planned so that the encasement of the lower columns becomes a relatively non--critical item in the construction sequence.
Design methods for composite floors are readily available (see Refs 5.3, 5.4, and 5.5).
5.5.2.
COLUMNS
The concrete encased steel column is a further example of composite action. Encasing of columns is often required to satisfy the architectural features of building facades and to provide fire protection to the steel column. The opportunity exists to consider a relatively small steel column section, designed to carry construction loadings, which can be subsequently encased and, as a composite section, designed to carry total vertical loading. The steel column can be used as reinforcement in the final composite column, or, where a larger final section is required additional reinforcement can be introduced (see Fig 5.15).
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The converse of a concrete encased steel column is a steel tubular column filled with concrete, which also provides composite action. Small or medium sized columns might be RHS or CHS; larger columns are box or tubular sections fabricated from steel plates -- see Fig 5.16. These tubular composite columns make for quick and easy erection and of course they eliminate the need for concrete formwork. In the larger sizes their overall economy depends upon the ability of the fabricator to manufacture the tubular sections efficiently.
remember is that such buildings, in order to be viable business ventures, require to be constructed with maximum economy of time, materials and labour. Many city buildings in Australia in recent years have been constructed using the steel frame to concrete core method and it is apparent that this system is proving economic in the current situation. High on--site labour costs are causing a return to the principle of prefabricating building elements off--site and then simply assembling them to form a building structure. As tall buildings, by virtue of their large number of identical floors, require a vast number of repetitive structural members, it is in these structures that economy can be achieved by the adoption of rationalised member design and standardization of connections. Steel beams which connect periphery columns to a central core and carry the slab on steel sheet decking (composite with the steel beams) will usually prove a most economic solution in commercial buildings. When assessing different structural systems, designers should be cognizant of the relative cost components (see Fig 5.17) to enable a more rational approach to the framing system. Steel Supply = 31%
Fig 5.16 Composite column comprising a concrete--filled tubular section
5.6.
Summary
From a technological point of view, the design of commercial buildings is relatively well understood. However, in today’s scene the important point to
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Fabrication = 8% Surface Treatment = 13%
Slab = 23%
Steel Deck Supply & Fix = 21% Steel Erection = 4 %
Fig 5.17 Cost Components for a Multi--Storey Building
6.
6.1.
Bolting
Introduction
The selection of a bolt for use in a structural steelwork connection will need to have regard to a variety of factors including: (a) Load capacity of available bolt types (b) Cost of the installed fastener (c) Amount of joint slippage (d) Nature of the forces to be resisted (e) Degree of flexibility/rigidity desired in the joint;
6.2.
Bolt Types
The two basic metric bolt types in use in structural engineering in Australia are: (a) the commercial (Strength Grade 4.6) bolt; (b) the high--strength structural (Strength Grade 8.8) bolt. The identification of high--strength structural bolt and nut assemblies can be readily made from the bolt head and nut markings (see Ref 6.1). In addition, a distinguishing feature is the larger bolt head and nut of the high--strength structural bolt compared to the commercial bolt.
in order to obtain, at least cost, a safe bolted connection.
Only a limited range of sizes of these bolts is of interest to structural engineers.
’Bolting of Steel Structures’ (Ref 6.1) contains a detailed discussion of all of the above factors and provides a state--of--the--art summary of matters related to the use of bolts in steel structures.
6.2.1.
This section concentrates on aspects which affect the economic use of bolts. Ref 6.1 should be consulted for more details of all aspects of the use of bolts in steel structures. The cost of a bolted connection includes: 1. Cost of obtaining, cutting and holing components 2. Cost of the bolts
COMMERCIAL BOLTS
The commercial bolt is commonly used in the following diameters (the prefix M is used to designate ISO metric bolts): M12
-- purlin and girt applications
M16
-- cleats, brackets (relatively lightly loaded)
M20, M24 -- general structural connections, holding down bolts M30, M36 -- holding down bolts.
3. Cost of installing the bolts
6.2.2.
4. Cost of inspection.
The high--strength structural bolt is most commonly used in diameters:
Every bolt specified should be a bolt that is needed -- bolt numbers should be kept to the minimum needed from strength considerations. The cost of installing bolts can vary considerably, depending on the bolting category.
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M16
HIGH--STRENGTH STRUCTURAL BOLTS -- designed connections in small members:
M20, M24, M30, M36 -- flexible connections, rigid connections. Larger sizes (M30, M36) ofthe high--strength structural bolt should be avoided when full tensioning is required, since
on--site tensioning can be difficult and requires special equipment to achieve the minimum bolt tensions.
6.3.
Bolting Categories
In Australia a standard bolting category system has been adopted for use by designers and detailers. This system is summarised in Table 6.1. Category 4.6/S refers to commercial bolts of Strength Grade 4.6 conforming to AS 1111 tightened using a standard wrench to a ’snug--tight’ condition. Category 8.8/S refers to any bolt of Strength Grade 8.8, tightened using a standard wrench to a ’snug--tight’ condition in the same way as for category
4.6/S. Essentially, these bolts are used as higher grade commercial bolts in order to increase the capacity of certain connection types. In practice they will normally be high--strength structural bolts of Grade 8.8 to AS 1252, but any other bolt of Grade 8.8 would be satisfactory. Category 8.8/TF and 8.8/TB (or 8.8/T when referring generally to both types) refer specifically to high--strength structural bolts of Strength Grade 8.8 conforming to AS 1252, fully tensioned in a controlled manner to the requirements of AS 4100. The system of category designation identifies the bolt being used by using its strength grade designation (4.6 or 8.8) and identifies the installation procedure by a supplementary letter (S -- snug; T -- full tensioning).
TABLE 6.1 BOLT TYPES AND BOLTING CATEGORIES Bolting Category 4.6/S 8.8/S
Method of Tightening Snug Snug
Nominal Bolt Tensile Strength (MPa) 400 830
Nominal Bolt Yield Strength (MPa) 240 660
8.8/TF (Friction type joint) 8.8/TB (Bearing type joint)
Full tensioning to AS4100
830
660
Full tensioning to AS4100
830
660
Bolt Name Commercial High Strength Structural High Strength Structural High Strength Structural
Standard Specification AS1111 AS1252 AS1252 AS1252
For 8.8/T categories, the type of joint is identified by an additional letter (F -friction--type joint; B -- bearing--type joint).
the level of tensioning being, of course, the same for both 8.8/TF and 8.8/TB categories.
As a consequence the high--strength structural bolt may be specified in three ways:
Two symbols have been added to the bolting category designations 4.6/S, 8.8/S, 8.8/TB.
(a) Snug--tightened-- category 8.8/S (b) Fully tensioned, friction--type -- category 8.8/TF (c) Fully tensioned, bearing--type -- category 8.8/TB;
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N:
bolt in shear with threads iNcluded in the shear plane (e.g. 8.8 N/S).
X:
bolt in shear with threads eXcluded from the shear plane (e.g. 8.8 X/S).
In practice 8.8/S category would mainly be used in flexible joints where the extra capacity of the stronger bolt (compared to 4.6/S category) makes it economical. It is recommended that 8.8/TF category be used only in rigid joints where a no--slip joint is essential. Note also that 8.8/TF is the only category requiring attention to the contact surfaces. A summary of the usage of Grade 4.6 and 8.8 bolts is contained in Clauses 6.5.2 and 6.5.3.
6.4.
Factors Affecting Bolting Economy
6.4.1.
BOLT GRADE
For a given diameter, and assuming snug--tight category, Grade 8.8 bolts offer far better structural economy than Grade 4.6. This is because a Grade 8.8 bolt costs only around 30% more than Grade 4.6, but has over twice the shear capacity; moreover the installation labour cost is the same for both. TABLE 6.2 INDICATIVE COST RATIOS OF DIFFERENT BOLT DIAMETERS Bolt Diameter
Of course where fully tensioned categories are used; Grade 8.8 bolts to AS 1252 are mandatory -- see Clause 6.4.3. One application for Grade 4.6 is in foundation bolts, especially where welded cages are used.
6.4.2.
BOLT DIAMETER
Bolts of M20 and M24 diameter represent an optimum in many respects such as: purchase price (see Table 6.2), hole drilling, and site installation. They should be preferred in all applications wherever possible. Where special circumstances demand the choice of larger diameters (M30 or M36) they should be specified with the knowledge that a cost premium will be involved. M30 and M36 bolts are not recommended for applications requiring full tensioning (8.8/TF or 8.8/TB) because it is difficult to obtain suitable portable equipment capable of inducing the high shank tensions required by AS 4100.
6.4.3.
Cost Index (supply Cost Index per kN of only) shear capacity
Table 6.3 shows that snug--tightened bolts of Grade 8.8 (i.e. 8.8/S category) offer the best value in terms of cost per kN of shear capacity. This is therefore the preferred bolting method.
90
1.4
M20
100
1.0
M24
170
1.2
M30
360
1.6
M36
670
2.1
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For this reason Grade 8.8 bolts are rapidly taking over as the standard grade for structural engineering.
High--strength structural bolt (Grade 8.8) x 100 mm long, with nut & hardened washer
M16
Notes:
2. Shear capacity calculations are based on strength limit state design.
1. The indicative cost ratios quoted are valid only within this table
BOLTING CATEGORY
Category 8.8/TB provides no greater structural capacity and would therefore be used only where some other consideration warrants it. An instance is where connection behaviour depends on the rigidity afforded by tensioned bolts as in rigid portal frame construction. 8.8/TB category has also been used on bolted bridges where the tensioning is merely a safeguard against nuts working loose in service. Category 8.8/TF (friction--type joint) offers the poorest economy of all the options on a cost per kilonewton basis (see Table 6.3). It should be used only in applications where joint slippage cannot be tolerated. An example is a structure supporting vibrating machinery such as a coal washery.
6.4.4.
THREADS IN OR OUT OF SHEAR PLANE
The plain shank area of a bolt is approximately 30% greater than the core area at the threads. Thus an apparent gain of 30% in shear capacity is available if the threaded part of the bolt can be kept out of the joint shear plane. However, this benefit can often be illusory, especially on average connections with up to only 10 or so bolts. Any savings in bolts must be measured against the cost of longer bolts required, possible installation problems and the higher cost of supervision needed to ensure ’threads out’. On the other hand on major structures with joints of around 50 bolts or more, a good case can be made for basing the design on threads excluded. Savings accrue from fewer bolts, fewer holes, smaller gusset plates and reduced installation time, while there is usually already a high level of supervision on these large projects to ensure correct installation.
Table 6.3 INDICATIVE COST RATIOS OF DIFFERENT BOLTING CATEGORIES (One M20 galvanized bolt installed in a group, ”threads included”) Bolting Category
Shear Capacity (kN)
Cost Index Cost Index per (installed) kN of Shear Capacity
4.6/S
44.6
80
1.66
8.8/S
92.6
100
1.00
8.8/TB
92.6
200
2.00
Notes:
2. The above comparison is based on strength limit state. Since serviceability generally governs for 8.8/TF bolts, they have been excluded from this table.
One final point to be borne in mind is that there is never a case for considering 4.6/S category with threads excluded, i.e. Category 4.6X/S. It will always be more economic to use Category 8.8N/S. The topic of threads in vs. threads out is discussed in more detail in Ref 6.1.
6.4.5.
BOLT FINISH
The available choices are ’plain uncoated’, ’zinc coated’ or ’hot--dip galvanized’. Galvanized bolts do not cost very much more than plain bolts and are now supplied as standard finish for Grade 8.8 bolts. In general the bolt finish should be matched to that of the structure itself. Uncoated bolts are satisfactory in low corrosion environments; galvanized bolts are needed where corrosion may be a consideration. They perform better and are much less costly than site--painted bolts. Care is needed when galvanized bolts are to be fully tensioned, although proper procedures and good housekeeping on site will obviate problems -see Ref 6.1.
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1. The indicative cost ratios quoted are valid only within this table.
6.4.6.
INSPECTION
Part of the cost of bolt installation is the necessary inspection. With 4.6/S and 8.8/S categories such inspection is minimal, and requires only a visual check that the correct type and number of bolts have been installed. Since the level of tightening is only ’snug’, and this is achieved in the normal course of erection, no further checking is required. In contrast fully tensioned bolts (8.8/TF and 8.8/TB categories) require detailed inspection in accordance with AS 4100 to confirm that the tensioning procedure has been carried out. The inspection cost is a big component of the total in--place cost of a bolt. Inspection procedures are outlined in AS 4100 and are discussed in Ref 6.1.
6.5.
Summary for Economic Bolting
6.5.1.
CHECKLIST
(e) Arrange for a minimum number of field connections by making large sub--assemblies in the shop. (f)
The essential points to be considered in the economical design of bolted connections are: (a) Standardize as much as possible for a project. (b) Adopt simple detailing. (c) Only one bolt diameter and one bolting category should be used in smaller structures; more variety may be justified on a larger structure, but different diameters or categories should be used in accordance with a predetermined philosophy. (d) Only one nominal size of bolt should be used in any single connection to facilitate the operation of punching or drilling holes, regardless of the size of the structure.
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Bolts in double shear are markedly more efficient and thought should always be given to arranging the connection details accordingly if practicable. In some instances (e.g. flange splices) such an arrangement can be negated by increased erection difficulty.
(g) If possible, avoid bolted connections with more than 5 bolts in line parallel to the force, otherwise reduction in bolt efficiency will result (see Ref 6.1). (h) Try not to mix 8.8/S and 8.8/T bolting categories on the one job. (i)
For economy, it may appear desirable to exclude threads from the shear plane. However, practical reasons dictate that usually threads are considered included in the shear plane, unless detailing of the bolts indicates exclusion is possible (see Ref 6.1).
(j)
Corrosion protection of the bolts should be matched to the end use of the structure.
The following flow chart is for bolt usage with flexible joints:
6.5.2.
BOLT USAGE -- FLEXIBLE JOINTS SIMPLE (FLEXIBLE) JOINTS
Structural Joints
Not calculated or very low stress levels
Commercial Bolts Property Class 4.6 to AS 1111 -- Snug tightened
Commercial Bolts Property Class 4.6 to AS 1111 -- Snug tightened
Category 4.6/S Category 4.6/S
Threads in shear or bearing plane is most common situation
Low capacity
Threads included in shear plane No “stick through” problem Most realistic from erection viewpoint
GENERALLY PREFERRED (See Clauses 6.4.4)
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Lower capacity (30% less) than threads excluded
High Strength Structural Bolts Property Class 8.8 to AS 1252 -- Snug Tightened.
Category 8.8/S
Twice capacity of 4.6/S
Threads excluded from shear plane Possible “stick--through” problem Difficult to inspect Greater capacity than threads included
The following flow chart is for bolt usage with rigid joints:
6.5.3.
BOLT USAGE--RIGID JOINTS RIGID JOINTS High Strength Structural Bolts Property Class 8.8 to AS 1252 Fully tensioned to AS 4100 (Category 8.8/T)
Friction Type
Bearing Type
Category 8.8 / TF
Category 8.8 / TB
No slip
Slip occurs
Lower Capacity than 8.8 / TB
Threads permitted in shear plane
Design for no slip in the serviceability limit state but also check for strength limit state.
Higher Capacity than 8.8 / TF
Threads included in shear plane
Threads excluded from shear plane
No “stick--through” problem
Possible “stick--through” problem
Most realistic from erection viewpoint Difficult to inspect GENERALLY PREFERRED (See Clauses 6.4.4)
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Lower capacity (30% less) than threads excluded
Maximum Capacity
7.
Welding
7.1.
Introduction
6. Whether to use shop or field welds 7. Accessibility
7.1.1.
PRINCIPLES FOR ECONOMY
The aim of weld design should be to provide the necessary structural performance throughout the lifetime of the structure for the lowest completed cost. To achieve this attention must be given to: (a) Eonomical design and detailing
8. Responsible specification 9. Inspection.
7.1.2.
COST COMPONENTS
The cost of welding can be considered as follows:
(b) Good welding procedure and correct process selection, and (c) Responsible inspection. The design and detailing will greatly dictate whether or not an economical welded connection can be produced and consequently is one area where great attention should be paid. Whereas the selection of the welding procedure and process to be used is the province of the fabricator, the detailing of the welded connection can often influence or limit the range of options available. Consequently, the design and detailing of the welded connection must have some regard to the processes and procedures available if an economical welded connection is to result. Responsible inspection is also a vital item in keeping the final cost to a minimum. The design engineer can best approach the objective of obtaining, at least cost, a safe welded steel structure or connection by considering the following influences during the design:
Cost of actual welding =
Time to weld Length of Cost per per unit length × weld × hour OperatingFactor
where: Time to weld Weld Volume per unit length = Deposition Rate Operating Factor =
Actual Arc Time Total Time
Cost per Hour = labour rate plus oncost These relationships indicate that a designer or detailer can minimise the cost of welding by attention to the following items:
1. Available welding processes that might be used
(a) Minimising weld volume
2. Welding consumable selection
(b) Allowing for the use of high deposition rate processes; in some connections, the detailing can restrict the use of a particular process thus forcing the fabricator to use a less efficient process
3. Code requirements (AS 4100, AS 1554) 4. Joint details and type of weld 5. Size of weld
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(c) Considering other factors which influence the deposition rate. For example, downhand welding is far more productive than over-
head or vertical welding, so that details should be oriented for downhand welding wherever practicable
TABLE 7.1 FILLET WELD COMPARISON Fillet size (mm)
Weld strength relative to 4mm size
Weld area relative to 4mm size
Increase in weld area for next size (%)
(e) Aiming to permit as much welding in the shop as possible, because the cost per hour and the operating factor are both more favourable in the shop than in the field
Increase in weld strength for next size (%)
4
1.00
1.00
25
56
5
1.25
1.56
20
44
(f)
6
1.50
2.25
33
78
8
2.00
4.00
25
56
10
2.50
6.25
20
44
12
3.00
9.00
33
78
16
4.00
16.00
(d) Using clean and simple detailing to assist in maintaining as high an operating factor as possible
Selecting the material grade to assist in eliminating or minimising the costs of preheating or post weld treatment.
7.2.
Types of Welds
7.2.1.
FILLET WELDS (see Fig 7.1)
The features of fillet welds are: (a) Economically attractive up to 12--16 mm leg size (b) Minimum edge preparation (c) Easy fit--up without tight tolerances (d) Poorer load carrying capacity than equivalent complete penetration butt weld and poorer fatigue characteristics (e) Intermittent fillet welds are permitted but these are usually only economical for limited applications involving the use of manual or semi--automatic processes; in many applications, a full length fillet weld of one size may be placed more economically using a fully or semi--automatic process (f)
In the horizontal--vee (HV) fillet position, up to 8 mm fillet sizes may be placed in a single pass using manual metal arc processes; with other processes (semi--automatic or automatic) a larger single pass fillet weld is possible. Such processes are now commonly used.
(g) If more than a single pass fillet weld is used, the cost of the weld can increase significantly.
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The cross--sectional area of a fillet weld varies as the square of the leg size while the strength of a fillet weld (which is based on the effective throat) varies only linearly with the leg size. As indicated in Table 7.1, there is a heavy cost penalty in over--welding. Automatic processes can reduce the cost of a fillet weld since, in addition to improving productivity, the increased penetration allows a reduced leg size for the same throat thickness.
Cruciform Fig 7.1 Types of fillet welds
(a) Complete penetration butt welds
(b) Partial penetration butt welds Fig 7.2 Types of butt welds
7.2.2.
BUTT WELDS (see Fig 7.2)
Two forms of butt weld are permitted in AS 1554 and AS 4100: (a) Complete penetration -- used where the full strength of the connected parts is required. Such a joint is given the full strength of the joined components. (b) Partial penetration -- used where less than full strength is acceptable, such as in low stress areas. These welds are less costly
66
than complete penetration, although attention is needed to ensure that the specified depth of penetration is achieved in practice. These welds are permitted to carry only shear and compression loads and have low ratings for fatigue conditions. Typical details of both types are shown in Fig 7.2. Butt welds usually require special edge preparation which (depending on the preparation type and the cutting practice) can add to the cost. Types of edge preparation normally in use are: 1. Square (no special preparation) 2. Single or double bevel 3. Single or double V 4. Single or double J 5. Single or double U. When selecting joint preparations for butt welds, prequalified preparations should be used wherever possible to obviate the need for qualification testing of the weld geometry. In selecting the included angle in a butt weld preparation, it has been demonstrated that, in general terms, the smaller the included angle in the preparation the less is the weld volume (Ref 7.2). There is a need to temper this provision with a consideration for leaving sufficient angle for electrode access -- the requirements will vary between processes. It is therefore probably better for the design engineer to specify the requirements (e.g. ’complete penetration butt weld’ or ’partial penetration butt weld, depth of penetration 12mm’) and allow the fabricator to select the best weld geometry/welding process combination to achieve the desired result. All such proposals can be submitted to the designer for approval if necessary.
7.2.3.
BUTT WELDS vs. FILLET WELDS
It is important to note that the volume of weld metal in a butt weld (partial penetration or complete penetration) depends on the type of preparation used as well as the depth of penetration. In contrast, the fillet weld increases in weld volume as the square of the leg size. In comparing the relative costs of butt welds and fillet welds, these differing relationships should be borne in mind, in addition to the fact that the butt weld usually requires edge preparation while the fillet weld does not.
The relative economics of the two will depend on the application and on the fabricator’s equipment and methods, and it is quite feasible for individual fabricators to cost various sizes of both types and plot a graph which will look something like Fig 7.3. The crossover point of weld size below which a fillet weld is the cheaper solution lies generally in the range 12--16 mm for many applications.
since it is necessary that a design is realistic in terms of both weld cost and weld quality. Manual metal arc welding (’stick electrode’ welding) is the simplest and most flexible of all the processes and is suitable for welding in all positions both in the shop and in the field. However, it is capable of only low deposition rates and has an intrinsically poor productivity because of the stop--start nature of the process. It is gradually being superseded by more efficient and economic continuous wire processes. Flux cored arc welding employs a continuous hollow electrode which contains the flux. It is capable of relatively high deposition rates, is suitable for all positions and in its gasless form is ideal for field welding. Gas metal arc welding uses a continuous solid wire electrode shielded by inert gas. It too is a high productivity flexible process, and is replacing manual metal arc welding in many fabrication shops.
Fig 7.3 Weld cost graph
7.3.
Welding Processes
The welding processes of interest in the welding of structural steel are: (a) Manual metal arc (MMAW) (b) Flux cored arc (FCAW) (c) Gas shielded metal arc (GMAW) (d) Submerged arc (SAW) (e) Electroslag (ESW) (f)
Stud welding.
For efficient design, it is necessary to understand the basic features of each welding process, to know its advantages and disadvantages and to understand the implication that the design can have on process selection,
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Submerged arc welding is another continuous wire process, where the arc is submerged under a layer of flux. It is essentially a very high deposition method intended for automatic or semi--automatic set--ups in the shop; automatic machines for welding plate girders use this process. Some specialised field applications have also been developed. Electroslag welding is a special automatic process normally used by the larger fabricators to butt weld plates. It is a single pass vertical process and is economic for plates 25 mm thick and above. Stud welding uses special equipment for the attachment of shear studs to steel members in composite construction. It is a portable process suitable for field use, but can be readily adapted to an automatic or semi--automatic set--up in the shop. These welding processes are described in greater detail in Ref 7.1. There can be startling savings in the cost of welds produced by the more modern processes. For example, considering a 6 mm downhand fillet weld made by manual welding using traditional rutile electrodes, the cost can be halved if iron powder electrodes are employed. This cost in turn can be halved again by adopting a suitable continuous wire process. Thus the designer should take great care to avoid introducing unnecessary costs in a job by restricting, through the details or the specification, the use of the optimum welding process.
7.4.
Other Cost Factors
7.4.1.
WELD CATEGORIES
The Structural Steel Welding Code, AS 1554 specifies two categories of weld, these being: GP -- General Purpose
category those welds which could otherwise, according to the guidelines given in the Standard, be categorised as GP. Mixing Weld Categories Weld categories can be mixed on a project but should not be mixed along a weld. In Fig 7.4, for example, it would be quite in order in a welded beam--to--column moment connection to have SP weld category for the flange butt welds but either SP or GP for the fillet welds along the web or for the fillet welds along the column stiffeners.
SP -- Structural Purpose The difference between the two arises from the more stringent quality and inspection requirements of the SP category over the GP category. The Steel Structures Design Code AS 4100 has been used as the reference standard from which the permissible levels of imperfections for GP and SP welds have been set. In other words, AS 1554 and AS 4100 are compatible. Category GP
Fig 7.4 Welded beam--to--column moment connection
The GP weld is the less stringent of the categories. It is intended for use in joints which are statically loaded, and where the design load on the weld is significantly below its full design capacity. It should be noted that for GP Category, the capacity factor is 0.6 as compared to a range of 0.70 to 0.90 for the SP Category (see Table 3.4 of AS 4100 -- 1990).
The web--flange fillet welds in a three--plate girder (Fig 7.5) may have stress levels which vary along the beam such that an SP category weld may be required at the ends of the beam, while GP category welds are sufficient elsewhere. Obviously, in this case an SP category weld should be specified for the full length, but weld inspection should be concentrated at the ends of the beam. If a length of weld which does not comply with the SP category was found in the central portion, it could still be accepted if it complied with GP category.
Category SP The SP category is the full--strength structural weld for use in static applications where the higher range of capacity factors is used. SP category is also mandatory for dynamic (fatigue) applications -- see AS 4100 and AS 1554.
It would, however, be quite in order to specify GP category welds for intermediate web stiffeners or stiffening around a web penetration.
Choice of Weld Category GP category welds will occur quite frequently in certain types of application. The designer should always endeavour to specify GP weld category where appropriate in order that advantage may be taken of the lower production costs associated with it. Only under circumstances where weld failure could cause a complete collapse of the structure or lead to severe risk or loss of life, should a designer contemplate specifying as SP
68
Fig 7.5 Stiffened web plate girder with web penetration
7.4.2.
WELDING SPECIFICATIONS
It is essential that the drawings and specifications detail the functional requirements of the design clearly and concisely but avoid needless over detailing or over specification of items which are better left to the fabricator or erector. It is advisable to avoid generalising with such items as ’no under--cut permitted’ or ’all welds to be smooth and free from defects’ or ’weld all round’ as these too often lead to confusion and extra cost. Flexibility in the approach to design is important particularly in considering proposals for alternative welding details or procedures. The fabricator or erector may have alternative methods to improve productivity and reduce costs and these should not necessarily be excluded by a rigid specification. If tendering is involved, prices for the tender specification and for viable alternatives could be useful. It is generally quite sufficient to nominate only the functional requirements plus compliance with an appropriate welding code, such as AS 1554, for satisfactory results. Standards are prepared for use as reference documents and it is not usually necessary to depart from them unless very good reasons exist. Where welding is specified in accordance with an Australian Standard, it should be the one relevant to the service conditions, e.g. specifying pressure vessel standards for a multi--storey office building is poor design. Fitness for service should be the sole criterion for the quality level specified and for the specification of the appropriate levels of inspection. Any departure from normal levels is likely to increase costs and should be called for only when really required.
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7.4.3.
WELDING INSPECTION
Fabrication costs are very sensitive to the required weld quality and the type and standard of inspection. Modern equipment and techniques for welding and testing of welds make it possible to provide near perfect weldments if so required. However, this also adds considerably to the cost. If such standards are not necessary, the benefits previously gained by careful economic design are frequently negated. It rests with the design engineer to determine the critical areas of a structure requiring close inspection and then to set a realistic standard for the inspector to follow. In setting guidelines for the inspector, the best results are achieved by nominating the use of the Structural Steel Welding Code, AS 1554. This Standard is well understood by both fabricators and inspection organisations and usually results in a good job being achieved at a reasonable cost. A confusing and often expensive practice sometimes adopted is to rewrite some existing Standard clauses into the specification in an attempt to achieve a higher standard than that provided by the Standard. This should be avoided because it usually leads to anomalies and contractual problems. Fitness for purpose should be the rule in setting inspection standards and AS 1554 provides realistic levels of both workmanship and inspection suited specifically for various weld quality levels required in structural fabrication .
7.5.
Economical Design and Detailing
The essential requirement of weld design is that adequate structural performance be provided. Usually a variety of alternative methods of achieving this aim are available and the cost aspects of the alternatives need to be looked at. The principal considerations in economical detailing of weldments are:
Specifying oversize welds can be harmful in two ways. Firstly, the cost is unnecessarily increased and secondly, oversize welds cause increased shrinkage forces which may lead to distortion. As an example, an 8 mm fillet is only 33% stronger than a 6 mm fillet, yet the volume of weld metal is 78% higher (Table 7.1). Thus, the cost of production of a joint can be significantly increased, not only due to the increased volume of weld metal required but more importantly due to the increased time in welding the joint.
(a) Simplicity -- details of welded attachments and details of end connections should be simple and consist of the fewest possible number of component parts.
The only qualifying point that should be raised is that the minimum weld sizes required by AS 1554 have to be observed and hence some oversize welds may be unavoidable.
(b) Weld volume -- only the minimum required weld volume, as determined by structural calculations, should be specified.
The ’weld all round’ philosophy should be avoided as it can lead to unnecessary additional cost.
(c) Accessibility -- welding electrodes must be able to be positioned in such a way that good quality welding can be achieved without difficulty and without undue strain on the operator. (d) Erection -- proper detailing should allow for reasonable fit--up tolerances and weld preparations. (e) Inspection -- all welds should be located in positions so that visual examination and/or nondestructive testing can be carried out easily.
(3) Use welding judiciously when using it to reduce material mass. If welding is used to reduce the amount of material (e.g. by splicing to change flange plate thicknesses or to provide stiffeners to a thin web in a three--plate girder), then be sure the cost of the welding is less than the cost saving in material cost. Weld metal costs many times more than parent material (somewhere from 50--100 times), and it is often cheaper to increase component mass so as to reduce weld metal volume. (4) Keep the number of pieces to be welded to the minimum practicable.
The following rules are suggested as basic to economical weld design and detailing (see also Refs 7.2 and 7.3):
A simple design with the fewest number of pieces is the most economic and often results in a better product.
(1) Design with welding in mind.
(5) Remember the special effects of welding such as distortion (Ref 7.2).
This requires an appreciation of the cost components in welding, the types of weld available, the types of processes and procedures available and their limitations. (2) Do not specify oversize welds. The most cost effective weld is the smallest weld that provides the required strength. It is good weld design practice to provide only that amount of welding which ensures that the welded fabrication can perform its intended function.
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(6) Allow welding to be used to maximum advantage. This particularly applies to allowing the fabricator to take advantage of high production processes, and in many cases may be best achieved by consultation with the fabricator. The detailing of a weldment can often restrict the fabricator to only the one process, and this may not always be the most suitable. (7) Aim for as much shop fabrication as possible. (8) Keep in mind the economics of fillet welding (Clause 7.2.1).
Fillet welds are usually limited to 6 mm leg size for most processes (notably manual metal arc), although with other processes, under certain conditions, a 10 mm or larger single pass fillet weld is possible; (for example a 20 mm single pass fillet weld is possible using tandem submerged arc welding but such processes are not commonly used when welding short runs on most simple connections). Before specifying large fillet welds, the situation should be checked with the fabricator. Larger single pass fillet welds can be placed in the flat natural vee position. If more than a single pass is required, the cost of the weld increases significantly. Single run continuous fillet welds are usually more economic than intermittent fillet welds of a larger size.
Examples of bad accessibility -- together with suggested improvements are shown in Fig 7.6.
(a) Gussets too close to flanges
(9) Keep in mind the economics of butt welding (Clause 7.2.2). Complete penetration welds need only be specified when they are really required, and the use of partial penetration welds can reduce weld metal and give other gains which add up to an improvement in productivity. If complete penetration welds are demanded, the use of backing bars with welds from one side which do not need back gouging or turning of the work piece, may lead to improvement. If selecting joint preparations, use prequalified preparations (AS 1554) to avoid qualification testing.
(b) Angle seats too tight against flanges
Select the smallest included angle consistent with achieving the desired penetration. Better still, specify only, say, ’complete penetration butt weld’ (or specify acceptable alternative details) on the drawing and allow the fabricator to select the method he can do best and most economically. (10) Use fillets in preference to butt welds wherever possible.
(c) Correction: Use butt weld in lieu of fillet
Butt welds usually involve edge preparation, which adds to costs, and as a result fillet welds are cheaper than butt welds up to about 16 mm thickness of connected plates. (Other considerations, such as joints which may be subjected to fatigue, may dictate the use of a butt weld in preference to a less costly fillet weld.) (11) Provide adequate access. Another way the designer can significantly help productivity is to ensure adequate access for welding. This is vital as it is essential to ensure always that the appropriate quality of weld can be made.
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(d) Correction: Use larger channel
(e)
(f) Column stiffener details Fig 7.6 Some common detailing faults resulting in poor accessibility for welding (12) Consider the method of fabrication. Allow welds to be made in the downhand position wherever practicable. This can often be achieved by the fabricator using special jigs and positioners.
Fig 7.7 Use of bending to reduce welding and give clean corners
Always try to aid fabrication by designing to allow the maximum use of jigs and positioners -- certainly try to make designs so that their use is not hampered.
Be prepared to accept alternative welded joints/details proposed by the fabricator which have clear advantages.
(13) Avoid dictating the manner of making a welded joint. The fabricator knows the best joint preparation and welding procedure for ease, economy and quality of joint using the facilities available. The designer who details the fabrication method must accept responsibility for any fabrication problems and extra cost. Ensuring the method of fabrication is acceptable can be achieved by calling for compliance with a recognised Code or Standard (AS 1554) and requiring the proposed fabrication and welding procedure to be submitted for concurrence on important jobs.
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(14) Be receptive to alternative proposals.
(15) Recognise the value of consultation with the fabricator. (16) Use minimum number of joints by: (a) Using largest size of plate/section available consistent with the functional requirements (b) Bending or forming in place of welding (Fig 7.7) (c) Considering the use of castings, forgings in lieu of complex welded joints (d) Avoiding excessive detail to reduce parent metal mass -- see item (3) and Fig 7.8.
Fig 7.8 Beam flange with many different plate thicknesses -- avoid when steel mass saved is less than 100 times mass of weld metal required (17) Standardise joint details as much as practicable to reduce variety. Different sized welds at a joint will require changes in current and electrode size by the operator. This causes lost time and a drop in the operating factor. Aim to have the minimum variety of weld sizes and types on a member or at a joint. (18) Use sub--assemblies to give: (a) Easier handling and positioning for downhand welding (b) Better access for welding (c) Less site welding and more shop welding (see Fig 7.9).
Fig 7.9 Exterior column/spandrel sub--assemblies for Sears Tower, Chicago (19) Use non--destructive testing judiciously. The use of non--destructive testing of welds is very disruptive to the flow of work and adds considerably to the cost of a structure. Much of this cost will be avoided if non--destructive testing is restricted to critical joints and carried out on a random basis only after careful development of weld procedures. Modern welding Codes encourage this approach. (20) Test only where required. Testing of welders and weld procedures for each job is expensive. Where practicable, consideration should be given to accepting welders and procedures approved by recognised authorities for other similar work. (21) Specify weld quality consistent with service requirements.
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Fitness for purpose should be the guiding rule in specifying weld quality. Higher quality specified unnecessarily or for its own sake is wasteful and costly (see Clause 7.4.2). Specify tolerances to limits consistent with the purpose of the weld. Adequate tolerances are necessary in order to allow for ease of fit--up. (22) Avoid, as far as practicable, requiring turning of members to weld on other side.
(23) Avoid joints which create difficult welding procedures. Joints which create difficult welding procedures, such as two round bars side by side, acute angle intersections, etc., should be avoided. Such welds prove time-- consuming and are of questionable quality (see Fig 7.11). Such joints also cause difficulties with any post--weld treatments, (deslagging, brushing, grinding and corrosion protection).
Examples are: (a) Avoid putting stiffeners on both sides of a plate girder web (b) Truss detailing which requires one side welding only (see Clause 8.4) (c) Angle seat to column flange connections -- a narrow seat in lieu of wide seat avoids turning the member (see Fig 7.10).
(a)
(b)
Fig 7.10 Angle seat detail -- (a) preferable to (b)
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Fig 7.11 These joints are difficult to weld and the welds may be of questionable quality (24) Consult ’Economic Design of Weldments’ (Ref 7.3) for further advice on ways to use welding effectively and economically.
8.
Detailing for Economy
8.1.
Detailing on Design Engineer’s Drawings
It is in the design office that the potential economy of any steel structure is effectively determined. Judicious decisions on details at this stage can provide for simple, economic methods to be used at the fabrication stage. The designer is faced with the problem that a different fabrication and erection technique could be favoured by each individual fabricator likely to tender for the project. It is a good idea at the outset for the designer to have some preliminary discussions with likely fabricators to check on latest techniques prevailing in the industry. From these discussions the design and detailing approach for the structure can be carried out with factors influencing economics firmly in mind. In the normal course of events a steel structure passes through several separate stages involving design, detailing, fabrication and erection. With this in mind, it is important for designers to remember that a minimum of design detailing by them will assist towards economy, since the shop detailer is then left free to make the most efficient use of the particular fabricator’s capabilities (Ref 2.12). The need for this flexibility is often overlooked by designers in their anxiety to specify their requirements. Such things as a fabricator’s ability to fabricate large sub--assemblies in the shop and subsequently transport to site and erect them will obviously have a bearing on the design of connection types and therefore on the economy of the overall project. In this regard it must be stressed that a maximum of work done in the shop will almost always produce better quality and more economical structures. In the presentation of working drawings therefore, the basic key is ’communications’ which normally take place through a chain as illustrated in Fig 8.1.
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Fig 8.1 Chain of communication The processes involved in the design can be summarised in the following sequence: (a) Initial communication (b) Structural concept including consideration of connection types (c) Integrated design (d) Connection detailing (e) Framing plans. The Engineer’s structural framing plans must contain all the necessary information to enable the fabricator to have shop drawings prepared for the individual members, as well as the marking plans to identify each member for the erection phase. The following discussion is intended to highlight aspects of the detailing of both members and connections to achieve economy in the overall fabrication and erection of structural elements.
As an additional consideration the use of AISC Standardized Structural Connections (Ref 1) will enable designers to specify standardized connections directly from the publication without detailing, and if necessary permit alternatives to be offered by the fabricator with the confidence of assured design capacity and behaviour.
8.2.
will allow the welding of both plates to the beam to be done in the downhand position without the need to turn the member during fabrication.
Beams
8.2.1.
GENERAL
The simplest and therefore the most economic beams in structures will be of rolled universal sections. Wherever possible, it will almost always prove more economic in one--off types of steel structures to use a universal section or welded beam section as a beam, even if a heavier solution results. The alternative fabrication of a three--plate girder introduces plate preparation, assembly and welding, the costs of which will generally exceed the cost of additional material in the rolled universal section or standard WB section, unless a vast amount of repetition is required.
8.2.2.
PLATED SECTIONS
Where headroom limitations apply (distance from ceiling soffit to floor level), it may be necessary to consider plating a universal section of a limited depth instead of choosing a deeper beam. Here, the extra cost of supplying plates, assembling and welding causes the cost of the member to rise, and a plated solution should only be used when a net saving in cost results compared to other feasible alternatives. Attention to the detailing of the member will assist in keeping fabrication costs down. For example, selecting cover plate widths as shown in Fig 8.2
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Fig 8.2 Plated sections
8.2.3.
WEB PENETRATIONS IN BEAMS
Holes cut in the webs of beams to provide access for service ducts have proved to be very costly in the past due to uneconomic detailing. This is due to the fact that, traditionally, these openings have been compensated for by the provision of extensive stiffening systems around the openings (see Fig 8.3(a)). The position of such openings in the beam length obviously has a major effect on the degree of stiffening required -- openings near the centre of uniformly loaded beams will require little or no stiffening, while openings placed near the supports may require stiffening. An early dialogue between the structural engineer and the building services designer can lead to ducting being located in a favourable position structurally without detriment to service requirements. Plain circular openings as shown in Fig 8.3(d) obviously represent the most economic solution. These can be cut by automatic means and result in minimum additional fabrication costs. If additional stiffening is required for round holes, it is most economic to use a pipe piece, fillet welded to the beam web (see Fig 8.3(c)).
8.2.4.
CASTELLATED BEAMS
Castellated beams are fabricated by cutting a profiled line in the web of a universal beam -- Fig 8.4. Circular profiles in lieu of the hexagonal profiles are also available from fabricators using computer controlled fabrication equipment. The beam halves are then offset longitudinally and the part webs welded on member centreline.
Fig 8.3 Web penetrations in beams (in descending order of cost, (d) being least costly) Where rectangular holes cannot be avoided and stiffening is necessary, this can be economically accomplished by a web hole with half--pipe cuttings and make--up plates or, alternatively, simply reinforcing the beam web using square edge flat bars fillet welded to one side of the beam web as shown in Fig 8.3(b). By judicious planning, the duct penetrations required in beams should be selected in position, size and shape to gain maximum economy in the fabrication of such beams.
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D c = overall depth of castellated section D n = nominal depth of original section D = actual depth of original section D Dc = D + n 2 Example: Original section = 530UB82 D = 528 D n = 530 D c = 528 + 265 = 793 Fig 8.4 Typical castellated beam geometry
Design paramters Span 7m full restraint, Grade 300 steel W* = 900 kN Rolled Section Solution 610UB113, Mass = 113 kg/m Castellated Beam Solution 800CUB82 cut from 530UB82 Mass = 82 kg/m
must again be remembered that a fabrication cost has been introduced which could be larger than the saving made in material cost -- depending upon the quantities required and the methods used. The cost involved for this additional fabrication varies depending on the equipment available within individual fabrication shops. In some cases, problems can be encountered with distortion of the beam during cutting thus requiring subsequent straightening of the members and adding further to the cost. In general, most fabricating shops are now well--equipped to undertake the fabrication of castellated beams, but designers should carefully investigate the relative cost differences with the industry before specifying this type of section. In the example shown in Fig 8.5 the heavier 610UB113 would be more economic than the castellated 530UB82. This example highlights the need to consider each case on its merits by applying up--to--date cost data to the examination of the alternative solutions.
8.2.5.
Comparison of cost indices: Rolled section (610UB113) Castellated Beam using CNC equipment Castellated beam w/o CNC equipment
1.00 1.02 1.39
Conclusion Rolled section is more economic in this solution in this instance, although using CNC (Computer Numeric Control) Equipment could be just as economical. Each individual situation should be readily assessed based on using updated cost information. Fig 8.5 Evaluation of economics of castellated beam The use of castellated beams in steel structures is often seen as a method of increasing beam strength while using the same mass of material. While many instances have been reported where savings have been effected, it
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THREE--PLATE GIRDERS
Where beams are required of greater depth than the largest universal beam, consideration should be given to three--plate girders or the standardized range of welded sections. These will most often offer more economic solutions than trusses for such applications as floor supporting beams. Three--plate girders are fabricated in modern automatic assembly and welding machines using the submerged arc welding process. In designing and detailing three--plate girders the following considerations are important in achieving economy: (a) Use flat bar or preferred plate widths and thicknesses for the flange and web plates (b) Use edge trimmed plate of preferred width wherever possible for the web plate to avoid additional cutting in the fabrication shop. This type of prepared plate can be fillet welded to the flange plate without further preparation of the edge. (c) When considering changing the flange width or thickness in order to reduce mass, take account of the lengths of plate available and whether continuation of an ’oversize’ plate is a more economical solution than introducing butt welded splices in the
flange plate. As a rule of thumb, it is probably economic to change the flange thickness when: Steel mass saved in flange
> 100 x mass of weld metal required
UNSTIFFENED WEB Load bearing stiffeners in both cases 8mm & 12mm webs
Where lengths of girders are such that butt welded splices are necessary, locate the changes of flange plate size to suit the available lengths of plate.
Load bearing stiffeners in both cases 8mm & 12 mm webs
(d) The cost increase for three plate girders with stiffened webs against unstiffened webs is about 10--25%, depending on the detailing adopted. Consequently, when evaluating whether to use a stiffened rather than an unstiffened web, the cost saving due to the reduced mass of the web plate with a stiffened web must exceed this cost differential, for the stiffened web solution to be economic. (e) If using a vertically stiffened web, use one sided stiffeners to avoid having to turn the girder during fabrication (see Fig 8.6). Terminate intermediate stiffeners by the allowable ’4t’ from the flange (see AS 4100) -- this avoids cutting stiffeners accurately to length (see Fig 8.6). (f)
Avoid the use of horizontal web stiffeners if at all possible.
The example shown in Fig 8.7 illustrates an evaluation of the relative economics of stiffened vs. unstiffened webs in a typical three--plate girder application. Total Mass Cost Ratio Mass x Cost Ratio
= 5.5 tonnes = 1.0 = 5.5
[ Cheaper Solution in this case ] INTERMEDIATE STIFFENED WEB
Fig 8.6 One--sided intermediate web stiffener
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Either: Stiffeners: 90x6 square edge flat bars, both sides, at 1500 mm centres (18 off) Or: Stiffeners: 90x6 square edge flat bars, one side, at 1500 mm centres (9 off)
8.3. 8.3.1.
Total Mass Cost Ratio
= 5.0 tonnes = 1.25 for two sided (average) = 1.15 for one sided (average) Mass x Cost Ratio = 6.3 two sided; 5.7 one sided The unstiffened web solution is most often the most economic solution but it is not intended to suggest that this is always so. Each individual situation can be readily assessed by the above process using updated values of the cost ratio for the stiffened web solution.
Columns GENERAL
The most economical columns in most building frames will usually be universal beam or column sections. These sections are available in a range of sizes which suit most applications. For applications where good appearance is important, square hollow sections could be considered. In high--rise buildings it is often economical to consider composite columns, where a relatively small universal column is sufficient to carry dead and construction loads and which, when encased in concrete, becomes a composite column able to carry additional live loads (see Clause 5.5.2).
8.3.2.
COLUMN BASE PLATES
In the design of column base plates, it is advisable once again to question the wisdom of minimising the mass of material and so introduce extensive fabrication, compared to a heavier base plate simply welded to the column shaft. Fig 8.8 shows three alternative details for moment resisting base plates.
Fig 8.7 Stiffened and unstiffened webs in three plate girders
(a) Slab base plate
(b) Extended flange slab base
(c) Gusseted base plate -- avoid, too expensive
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(d) A pipe sleeve allows easy entry of anchor bolts in a double baseplate Fig 8.8 Column base plate details (moment resisting or fixed) Slab base plate (a) is used widely. It calls for a thicker base plate than the gusseted base plate (c) but requires far less labour for fabrication and therefore it is more economical. Column flanges can be extended as shown in (b) to present a larger bearing surface. Fillet welds should always be preferred for welding the column shaft to the base plate. Only in very rare instances will complete penetration butt welds be required -- these should be avoided if possible for maximum economy. Typical details for pinned base plate connections are shown in Fig 8.9. For the nominally pinned base, there is no need to provide true pin or rocker connections as these are unnecessarily expensive to fabricate. Standardised dimensions for ’pinned base’ plates are available in AISC Standardized Structural Connections (Ref 1).
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Notes: Weld: 6E41 continuous; Bolts: 4.6/S; Column shafts with cold sawn ends provide full bearing contact; All dimensions in millimetres Fig 8.9 Typical pinned base plates (full dimensional details can be found in Ref l)
8.3.3.
HOLDING--DOWN BOLTS
One of the greatest problems facing the fabricator/erector of structural steelwork is inaccuracies in the placing of holding--down bolts. This operation is beyond the fabricator’s control and if corrective measures are required on site they usually lead to cost extras and subsequent contractual difficulties. Several methods have been adopted to overcome this problem and it is essential that the designer presents to the builder very explicit instructions on the method to be used in fixing the bolts. Fig 8.10 shows two typical holding--down bolt details. More detailed information may be found in Ref 1. In addition to providing flexibility in individual bolt location to ensure matching with base plate drilling, it is good practice to cage bolt groups as shown in Fig 8.11.
Fig 8.10 Holding--down bolt details
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8.3.4.
COLUMN SPLICES
In high--rise buildings economies can be achieved by running column shafts through three or four floors rather than providing splices at say every second floor (Fig 8.12). Since lengths up to 18m (but see Clause 2.2.3) are now available in most column sections, the greatest economy will be gained in maintaining the same section mass for 3 or 4 floors thus reducing the number of splices required. Column splices can be welded or bolted. The relative economics of field welding should be checked with the fabricator before deciding on adopting this method. Bolted splices will almost always be an economical detail. Fig 8.38 shows typical economic welded splices in columns; Fig 8.39 shows typical economic bolted splices. Further information on column splices can be obtained in Ref 1. It is essential to locate column splices at a convenient level above the floor beams in order to provide comfortable access for the erection personnel to field weld or install the bolts (Fig 8.13).
Fig 8.13 Preferred column splice locations Fig 8.11 Typical holding--down bolt cage
Fig 8.12 Minimise number of column splices -- 1 is preferable to 3
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8.3.5.
COLUMN STIFFENERS
In rigid framed structures, the connections between the beams and columns very often require special stiffening of the column section in order to provide for the satisfactory transfer of forces. These stiffeners add considerably to the fabricated cost of the columns and consideration should be given at the design stage to investigating the alternative use of a heavier column section which requires no stiffening. The example shows how such an evaluation can be carried out. For the case investigated, it is seen that to increase the size of the column section from a 250UC89 to a 310UC137 is a more economical solution than using the smaller UC with stiffening.
SOLUTION 2: Increase Column Size to Avoid Stiffening
DESIGN PROBLEM:
Requires 310UC137 to avoid any column stiffening at all. Note: 250UC89 = $101 /m 310UC137 = $160 /m Cost difference = $59 /m
SOLUTION 1: Stiffen 250UC89
Consider 3m column lift: Solution 1: Requires 4 stiffeners at $61 = $244
EXAMPLE TAKEN FROM AISC ”DESIGN OF STRUCTURAL CONNECTIONS” (Ref 2)
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COMPARISON OF SOLUTIONS
Solution 2: Requires 3m x $59 /m = $177 Solution 2 is the more economic The use of a heavier column with a thicker web and flange may prove more economic in situations such as that illustrated, especially for short column lifts. Each individual situation can be readily assessed by the above process using updated cost information. Evaluation of economics of the use of column stiffeners at rigid beam--to--column connection
8.3.6.
BUILT--UP COLUMNS
Where universal column sections have insufficient capacity for a particular application, the use of built--up columns has to be considered. Such columns can be fabricated in a variety of shapes. Fig 8.14 shows economic details for built--up columns in ascending order of fabrication cost.
Fig 8.14 Economic details for built--up columns in ascending order of fabrication cost
In box columns the detail at the corner can heavily influence fabrication costs. Where possible the use of fillet welds will afford the best economy -Fig 8.15 (a) and (b). Where fillet weld sizes required are greater than 12--16mm, partial penetration welds should be considered (Fig 8.15(c)) as a more economic solution. Complete penetration butt welds at corner joints will be rarely required and should only be considered in the vicinity of very heavily loaded rigid beam--to--column connections. Fig 8.15 Welded corner details for box columns Splices in box columns can be either welded or bolted, but more often than not the welded alternative is selected because a bolted splice is only practicable in large box columns where access can be provided to the inside of the box. A partial penetration welded box column splice can be carried out using the detail shown in Fig 8.16 (a). Fig 8.16 (b) shows a girder connection to box column -- site welded. This connection requires accurate fabrication in the overall length of the girder and may present problems if a considerable run of beams in a line are delivered to site with tolerances in length cumulative. In addition, allowance must be made in
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column erection for weld shrinkage, since the relatively large weld volume required in heavy girder flanges will cause significant shrinkage in length. Columns must be spread by the shrinkage dimension, as shown in Fig 8.17 and for heavy box columns this can lead to erection difficulty. Fig 8.16(c) shows a girder--to--column connection which avoids the problems encountered with the direct welded connection shown in Fig 8.16(b). In the case of a girder stub welded to column in the shop, the control of welding procedures and fabrication tolerances generally will lead to a more economic weld and better quality assurance. The subsequent site splicing of the girder to the stub can be either welded or bolted, but the bolted alternative will normally be less costly. In the case of heavy industrial structures using grid flooring however, the bolted flange splice will interfere with this type of flooring, and consideration should be given to welding the splice for such applications. Fig 8.16(d) shows a bolted girder--to--box column connection. Where flexible connections are used, the angle cleat connection provides good site fit--up. The web cleats are usually loosely shop--bolted to the girder and allow movement for any out--of--tolerance during erection. For box columns, provision must be made in this connection for access to the inside of the column for bolt installation.
Fig 8.16 Connections to box columns
Alternatively, where flexible girder--to--box column connections are employed, the web side plate connection will provide about equal economy. The web side plate can be welded to the column face, thus avoiding the problem of internal access.
Fig 8.17 Spreading of columns to allow for weld shrinkage
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8.4.
Trusses
Welded trusses have in the past provided very efficient building elements because of the favourable mass/span ratio possible. Although for many industrial building applications, such systems as saw--tooth trusses have been superseded by the portal frame system, there are still many long span applications where truss portals provide an economic solution (see Clause 4.3). In general, trusses fabricated by welding should preferably use specially developed details suitable for economical welded truss fabrication rather than details borrowed from the days of riveted construction. For too long the old riveted details have been used on welded trusses, on the basis of simply replacing rivets by equivalent welding (see Fig 8.18). This leads to uneconomic fabrication, since it introduces an unnecessary amount of welding and, most importantly, since it requires the truss to be turned during fabrication to weld the angles to the gussets on each side. Several alternative details offer far more economic welded truss fabrication. Fig 8.19 shows a detail where single angles have been used as both the truss chords and the web members. This provides for the most economic truss fabrication since all welding can be done from one side, thus avoiding turning of the truss during fabrication. Additionally, the gussets have been eliminated by using a long leg angle as a chord member. Obviously this detail requires the designer to consider the eccentricities involved in the design, but it appears in most cases that the use of slightly heavier angles will cater for these eccentricities.
Alternatively a T--section can be used for truss chord members with single angle web members welded to the vertical leg of the tee (see Fig 8.20). The T--sections would usually be split universal beam or column sections -- an operation that can be economically carried out by most fabricators.
Fig 8.19 Single angle welded truss
Fig 8.20 Split tee welded truss
Fig 8.18 Equivalent truss detailing
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cases the detail at the intersection of members can lead to very costly fabrication and it is suggested that the spreading of intersection points can provide a better detail where members can be plain mitre cut to length rather than having double mitre end preparations. The resulting eccentricity can usually be accommodated by the relatively massive chord members in such trusses. Fig 8.21 illustrates the use of universal sections in a welded truss while Fig 8.22 illustrates the use of rectangular hollow sections. In both cases, detail (b) is preferable to detail (a). Although trusses are usually considered as roof framing members there are other areas where they offer economical light framing members. (a) Coincident intersection points. Double mitred member ends
(b) Preferred. Spread intersection points. Single mitred member ends Fig 8.21 Use of universal sections in welded trusses In large heavy trusses, (i.e. those fabricated from universal beam or column sections), care must be taken with detailing to ensure optimum economy. In these
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Such a case is in multi--storey construction where secondary floor members at relatively close centres are required. Economy can be achieved by the fact that a large number of these members will be required and the use of mass--produced truss members can be considered. In other parts of the world the open web joist lends itself to this application and many notable buildings have incorporated such joists as floor members. Fig 8.23 shows the traditional open web joists (a), as well as a proprietary light weight truss (b). These light weight joists are no longer made as a standard item and are usually uneconomic for structural applications unless large quantities are required.
(a) Coincident intersection points. Double mitred member ends
(a) Non preferred
(b) Preferred
Fig 8.24 End plate details
(b) Preferred. Spread intersection points. Single mitred member ends Fig 8.22 Use of rectangular hollow sections in welded trusses
TABLE 8.1 WRENCH CLEARANCES θ degrees
0 5 7.5 10 *
Fig 8.23 Types of open web joist
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Recommended Minimum Dimensions (mm) b1 X (M20 and M24 bolts only) for Air for Hand Wrench* Wrench 60 60 60 60 100 60 60 100 60 60 100 60
The use of a universal joint does offer some possibility of reducing this dimension, and while this may be seen as an advantage from a design
point of view, it should be noted that an impact wrench with a universal joint and socket is generally difficult to handle for an operator some height from ground level and sitting on only the width of the beam flange. In addition, the use of a universal joint reduces the efficiency of the impact wrench and this can be a problem in tensioning M24 bolts or larger, especially if located some distance from the source of the compressed air supply.
8.5.
Portal Frames
8.5.1.
CONNECTIONS
A discussion of various aspects of the economics of portal frame steel buildings is contained in Clause 4.2. A number of other items of concern to the economic detailing of these frames is contained in this Section. In portal frames using bolted end plate connections for the knee and apex joints (see Fig 4.2), close attention must be paid to the detailing of these connections, especially where tensioned bolts (8.8/TB category) are employed --the most common practice. Any cost savings obtained by simplifying connection details to make fabrication simpler can be lost during site erection if clearance problems are encountered during site assembly. Recommended dimensions for such connections, extracted from Ref 1, are given in Table 8.1. These dimensions are sufficient to ensure that the bolts can be installed and tensioned, since sufficient clearance is provided to accommodate either hand or air wrenches.
excessively stiffened end--plates is that insufficient clearance may then exist to allow the bolts to be installed. Design guidance on the design of end plates without stiffening may be found in Ref 2. At a bolted apex joint, care must also be taken to allow sufficient clearance between the adjacent purlin cleat and the end plate to enable the end plate bolts to be installed and tensioned. The dimension ’Z’ (see Fig 8.25) must be larger than the bolt length to be installed plus a clearance dimension, and also be large enough to permit the wrench socket to be placed on the nut. Where split universal sections are used to haunch a portal frame rafter (see Fig 4.2), stopping short the fillet weld joining the split haunch to the flange of the rafter is suggested as an economical and structurally sound device. Any fillet weld placed in the tight confines of the junction is likely to be of doubtful quality due to the difficult access involved -- see Fig 8.26. The recommended method of attaching purlins and girts in portal frame buildings is illustrated in Fig 8.27.
In the design of the end plates, designers can approach the proportioning of the end plate to resist the bending moment developed due to the behaviour of the plate under loading in two ways; viz: (a) Use a thick unstiffened end plate (b) Use a thin stiffened end plate. Fig 8.24(a) shows an excessively stiffened thin end plate which would be an extremely expensive detail compared to the thicker end plate detail of Fig 8.24 (b). For this reason, (b) is much preferred. Another problem with
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Fig 8.25 Clearance at apex joint
8.5.2.
PORTAL FRAME PRE--SET
In order to ensure that the columns of a portal frame will be within the basic erection tolerances in the final erected position, it is necessary to provide a ’pre--set’ of the frame during fabrication. This is done by determining the deflection at the frame ridge under dead loads and calculating the resultant horizontal deflection at the knee joints. This latter dimension is then used in the set--out for fabrication to pre--set the geometry of the frame -- see Fig 8.28. Fig 8.26 Termination of haunch
NOTES: 1. Place girts and purlins to most effectively shed water and debris with due consideration to ease of erection. 2. Ensure adequate clearance to avoid interference with cleat welding. 3. Design cleats to accommodate standard punching -- refer to manufacturers’ brochures. 4. Ensure adequate capacity in top girt to carry load from sag rods. Fig 8.27 Attachment of purlins and girts
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Fig 8.28 Precambering details of a rigid frame
8.6.
Connection Detailing
8.6.1.
GENERAL
In general, the greatest economy in detailing of beam--to--column and beam--to--beam connections is achieved by selecting combinations of connections to require only one type of operation to be executed on each member in the fabrication shop. Preferred ways in which this can be achieved are suggested in Fig 8.29. Such a method of selecting connections enables the fabricator to reduce the handling operations required to fabricate the member and lends itself readily to a ”flow--through” system in the shop.
On both grounds the logical choice is Fig 8.34, web side plate (WP), for every connection to the box columns. By the rule of symmetry (Clause 4.4.4.) use the WP connection at the other end of the beams in question, B1, B4, B8 and B9. By the rule of standardization use the WP connection on both ends of the other longitudinal beams B7 and B10, checking that there will be adequate clearance at those ends of B7, B8, B9 and B10 which frame into the webs of the l--section columns. Standardise further by using the WP connection also at both ends of B3 and at the column end of B6 (see Summary below). For the connections selected so far, the beams require only to be cut to length and drilled. Therefore the connections for the transverse members framing into them should be chosen so that the beams require only further drilling (as in Fig 8.29 upper). Choosing Fig 8.33, angle cleat (AC) will achieve this aim. Another option is Fig 8.32, flexible end plate.
Preferred -- Holed only
Preferred -- Welded Fitments Only Fig 8.29 Typical beam details for fabrication economy The designer and detailer should look at rationalising the selection of details and connections in this way. Naturally, holing operations on any group of similar members would use the same set--out parameters (gauge lines, pitch, hole diameter, etc). An example of this type of selection process can be illustrated using the beam marking plan shown in Fig 8.30. In this instance, the frame is braced in both planes and flexible connections only are to be used. In this frame the critical connections are those to the two box columns. If these columns are small they cannot accept connections requiring bolting through their walls. If they are large, bolting through may be possible (with some difficulty and expense) but the connections must be of a type where the beams can be entered without the need to ’spring’ the very rigid columns.
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Fig 8.30 Typical floor beam layout Summary: We now have a frame requiring only two different connection types, selected in such a way as to minimise fabrication and erection costs.
The columns themselves require welded fitments only. Beams B1, B3, B4, B7, B8, B9 and B10 require only cutting to length and drilling. Beams B2, B5, and B6 again require only cutting to length and drilling (assuming the AC connection). All beams have the same type of connection at each end except B6 where it is necessary to make a minor compromise of WP at one end and AC at the other.
8.6.2.
SPECIFIC CONNECTIONS
This Clause presents notes on the efficient and economic detailing of a variety of individual connection types, as follows: Fig 8.31 Angle seat connection 8.32 Flexible end plate connection
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8.33 8.34 8.35 8.36 8.37 8.38 8.39 8.40 8.41 8.42
Angle cleat connection Web side plate connection Bearing pad connection Welded moment connection Moment end plate connection Welded splice connection Bolted splice connection Stiffener connections Bracing connections Connections to concrete cores
The recommendations made in this Clause follow the AISC Standardized Structural Connections (Ref 1).
• • •
•
Use bolted restraint cleats for maximum economy and to allow margin for rolling tolerances on rolled section beams. For welded seats, it may be necessary to taper the vertical leg of the seat in cases where the seat is welded to an H--section column web between flanges to allow access for welding see Fig 7.6 (b). Check length of seat to ensure satisfactory fit onto column. Where the seat is wider than the column flange, welded angle seats require welding from behind the column flange. This involves turning the column and may prove costly -- see Fig 7.10. Observe recommendations on economical aspects of the use of bolting (Section 6) and welding (Section 7).
Fig 8.31 Angle seat connection
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•
Select gauge ’g’ to ensure bolt clearance, (usually 90 mm).
•
Fabrication of this type of connection requires close control in cutting the beam to length. Adequate consideration must be given to squaring the beam ends such that both end plates are parallel and the effect of any beam camber does not result in out--of--square end plates which makes erection and field fit--up difficult. Shims may be required on runs of beams to compensate for mill and shop tolerances.
•
The use of this connection for two sided beam--to--beam connections should be considered carefully. Installation of bolts in the end plates can cause difficulties in this case. When unequal sized beams are used, special coping of the bottom flange of the smaller beam may be required to prevent it fouling the bolts.
•
Since the end plate is intended to behave flexibly, damage of the end plate during transport is not normally of concern and may be rectified on site.
•
Observe recommendations on economical aspects of the use of bolting (Section 6) and welding (Section 7).
Fig 8.32 Flexible end plate connection
Cleat holes must allow for variations in beam depth due to standard rolling tolerances and also provide for erection tolerances. Standard holes (2 mm larger than nominal bolt diameter) are usually sufficient. • Check that cleat components will fit between column flanges for connections to column webs. • The use of this connection for two sided beam--to--beam connections should be considered carefully. Installation of bolts in the outstanding legs of the angle cleats can cause difficulties in this case. When unequal sized beams are used, special coping of the bottom flange of the smaller beam may be required to prevent fouling the bolts. • For double angle cleats, the nominal gauge required in the supporting member is (2 g3 + t). Standard gauges can hence accommodate only certain web thicknesses (t) of the supporting member when using normal holes (2 mm clearance). Drifting widens the range of web thicknesses that can be accommodated, but may result in some distortion of the cleat. Alternatively, a special gauge may be used in the supporting member. • In order to obviate both drifting or the use of a special gauge, custom detailed horizontal slotted holes may be used in the outstanding leg of the angle cleat component. Alternatively, oversize (4 mm larger than nominal bolt diameter) holes could be used, but this may complicate levelling the supported member during erection. • Observe recommendations on economical aspects of the use of bolting (Section 6). Fig 8.33 Angle cleat connection •
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• •
•
•
•
Bolt holes must allow for variations in beam depth due to standard rolling tolerances and also provide for erection tolerances. Standard holes (2 mm larger than nominal bolt dia.) are usually sufficient. In connections to column webs, a check must be made on the length of bolt to ensure sufficient clearance is available between the side plate and the inside of the column flange to permit the bolt to be installed. Erection clearances must be especially considered for this detail because of the necessity to angle beams into place during erection. This consideration is most important for the case of a series of beams in the one row, all connected between the same main supporting members. Observe recommendations on economical aspects of the use of bolting (Section 6) and welding (Section 7).
Fig 8.34 Web side plate connection
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•
•
•
The connection may need to be shimmed to suit during erection. The connection detail consequently includes provision for shims of 0--5mm nominal thickness. Shims will need to be holed to the same gauge as the end plate. Sawn or machine flame cut edges are recommended at the bearing interface in order to avoid edges with slopes, such as
Check width of components when welding to H--section column web to allow access for welding -- see Fig 7.6 (b). Where the bearing pad is wider than a column flange, welding is required from behind the column. This involves turning the column and may prove costly. Observe recommendations on economical aspects of welding (Section 7).
Fig 8.35 Bearing pad connection
(a) Stub Girder Connection, Fully shop welded beam stub, spliced on site. (c) Field Welded Moment Connection -- using fillet welded web cleat(s)
(b) Field Welded Moment Connection -- including erection cleat. • • • • •
The economics of field welding should be checked with the fabricator before it is specified. Flange weld preparation assumes the use of a backing strip -- which requires coping of the beam web. Details (b) and (c) are not considered as economical in Australia. Observe recommendations on economical aspects of welding (Section 7). Site welding should be kept to a minimum, and should be used in an integrated manner.
Fig 8.36 Welded moment connection
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• • • • • • • •
Holes are normally 2 mm larger than the nominal bolt diameter, although oversize or slotted holes may be used. Fillet welds or butt welds may be used as the beam flange to end plate weld. A discussion of the use of fillet welds larger than 8 mm as related to available welding processes is contained in Section 7. Fillet welds only are recommended for the beam web to end plate weld. Fabrication of this type of connection requires close control in cutting the beam to length and adequate consideration must be given to squaring the beam ends such that end plates at each end are parallel and the effect of any beam camber does not result in out--of--square end plates which makes erection and field fit--up difficult. Shims may be required to compensate for mill and shop tolerances. Select a gauge for the end plate bolts which allows sufficient clearance to install the bolts. Bolts adjacent to the tension flange should be as close as possible to the flange. Dimensions must be sufficient to ensure that bolts can be installed and tensioned -- sufficient clearance must be provided, see Table 8.1. Stiffeners on the end plate should be avoided -- a thicker end plate is recommended instead. Observe the recommendations on economical aspects of the use of bolting (Section 6).
Fig 8.37 Moment end plate connection
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• • • • • •
The economics of field welding should be checked with the fabricator before it is specified. Flange weld preparation assumes the use of a backing strip -- which requires coping of beam web. The backing strip should be required to be removed only in special instances. Details avoid accurate fitting up of member sections. A shop splice with complete penetration welding without web plate is a detail used at the discretion of a fabricator and is not a detail in use as a site connection. Edges required to be prepared for bearing can be obtained satisfactorily and economically by cold sawing. Column splices should be located in positions where access can be easily obtained for site welding -- as in Fig 8.13.
Fig 8.38 Welded splice connection
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•
• •
Where flange splice plates are used, assemble joints with nuts to outside of splice plate as in (a). This arrangement is recommended for ease of tensioning, since in universal sections sufficient clearance is not always available between flanges for a standard air wrench. Members can be prepared for bearing satisfactorily and economically by cold sawing. The cap plate detail of (d) is usually reserved for column splices between members with significant differences in member depth.
Fig 8.39 Bolted splice connection
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•
In order to accommodate out--of--alignment of member webs at a splice, the use of shims may be necessary. To mitigate the effects of any out--of--alignment, holes in member flanges should be located using the centre--line of the member web as a reference point.
•
In order to accommodate out--of--square of member flanges at a splice, the use of tapered shims may be necessary.
•
Column splices should be located in positions where access can be easily obtained for the installation of the bolts -- as in Fig 8.13.
NOTES: 1. The use of column stiffeners should be kept to a minimum for maximum economy, commensurate with design requirements.
6. Compression stiffeners should be fillet welded to the column web. When diagonal shear stiffeners are used, it is recommended that compression stiffeners be fillet welded to the column flange adjacent to the shear stiffener. 7. Tension and compression stiffeners need to be cropped 30 mm to clear column section radiused fillets.
2. All welding of stiffeners should be shop welding. 3. Only tension stiffeners need be welded to the inside face of the column flange(s). Compression stiffeners may be fitted against the inside face of the column flange. 4. Fillet weld sizes on stiffeners should be 6 or 8 mm, to ensure single pass welds. Welds to column web may be one--sided.
8. Shear (diagonal) stiffeners are fillet welded at their ends in the manner shown below. Fillet welding along the stiffener length may be introduced either to increase the capacity and/or to reduce the l/r of the stiffeners.
5. Where tension stiffeners extend across the full column depth (A2), the tension stiffeners should be (fillet) welded to the column flange and only fillet welded to the column web where flange fillet welds have insufficient capacity to transmit the design force in the stiffener. Where tension stiffeners extend only part way across the column depth (A1), welding to the column web is required.
Fig 8.40 Stiffener connections
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(a) A steel plate of fairly generous proportions is presented flush with the exterior wall of the core to which is welded a web side plate at the time of erection. Such a connection does not impose strict tolerances on (i) beam overall length (by using slotted holes in the web side plate) or (ii) beam level and lateral location (catered for in the site positioning of the web side plate provided the embedded plate is reasonably oversize). If anchor lugs are tack-welded into the general reinforcement cage, little drift of the embedded plate will occur during slip forming.
(a) Bracing gussets should be detailed as rectangular shapes to reduce marking--off and cutting time. (b) In braced frames it will generally prove more economic to weld bracing gussets to columns rather than to beams. The eccentricity caused by spreading intersection points can usually be easily accommodated by the column section. (c) For roof bracing, the most economic solution will be to weld gussets to the rafter top flange. Where this cannot be done, the gusset can be welded to the rafter web but sufficient clearance must be provided for welding electrode access. Fig 8.41 Bracing connections
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(b) The older method employed for this connection is that of leaving a cored hole in the wall of the slip--formed core. Originally it was thought necessary to embed a steel seating in this opening in which to bolt the bottom flange of the beam. This is not now recommended since the accurate positioning of this cored hole, including an embedded seating, is almost impossible to achieve on site. It is now considered better to leave a simple cored opening in the wall, pack the beam to level alignment during the erection phase, and fully grout up the remaining opening. From an economy viewpoint the alternative (b) should normally be better. However, in the overall building design it is suggested that designers consult with the slip--core contractor to check the more economical method. It is possible that in some cases a large number of cored openings, with resultant complication of reinforcement pattern, would be more expensive than the embedded plate shown in alternative (a). Fig 8.42 Connections to concrete cores
9.
References & Further Reading
COMPLEMENTARY REFERENCES:
8.
Potter, P. D. ”Fast Steel Erection”, Steel Fabrication Journal No. 46, Australian Institute of Steel Construction, February 1983.
1.
”Standardized Structural Connections’: Australian Institute of Steel Construction.
9.
2.
Hogan, T. J. and Thomas, 1. R., ”Design of Structural Connections”, Australian Institute of Steel Construction, 4th ed., 1994.
Oakes, D. L. T. ”Philosophy for Economical Design, Fabrication and Erection”, Steel Construction Vol. 17 No. 4, Australian Institute of Steel Construction, 1983.
10.
”Hot--Dip Galvanizing’ Galvanizers Association of Australia, 13th ed., 1993.
11.
Macpherson, 1. J. ”Unprotected Steel Framed Open Deck Car Parking Structures -- A Case Study”, Metal Structures Conference Adelaide 1976, Institution of Engineers Australia.
12.
Main T., Watson, K.B., and Dallas S., ”A Rational Approach to Costing Steelwork”, International Cost Engineering Council/The Australian Institute of Quantity Surveyors International Symposium, Construction Economics -- The Essential Management Tool, Australia, May 1995.
Resevsky, C. G. ”Economical Fire--Rated Composite Steel Floor now established in Australia”, Steel Construction Vol. 7 No. 3, Australian Institute of Steel Construction 1973.
13.
Steel Structures Manual, Part 8 -- Fabrication, SAA MA1.8, Standards Association of Australia, 1982.
14.
4.
Standards Association of Australia/Australian Institute of Steel Construction, ”Steel Structures, Part 1 -- Planning”, SAA MA1.1 --1973.
Steel Structures Manual, Part 9-- Erection, SAA MA1.9, Standards Association of Australia, 1975.
15.
5.
Standards Association of Australia/Australian Institute of Steel Construction, ”Steel Structures, Part 7 -- Design”, SAA MA1.7 --1977.
Hogan, T. J. and Firkins, A, ”Welding in a Limit State Steel Structures Code”, Tables 1, 2 and 3, Proceedings of 31 st Annual Conference, Australian Welding Institute, October 1983.
16.
6.
Firkins, A., ”Design for Economy” Third Conference on Steel Developments, Australian Institute of Steel Construction, 1985.
Quinn, N. ”Specifications: the Fabricator”, Steel Fabrication Journal No. 40, Australian Institute of Steel Construction, August 1981.
17.
”Handbook of Fire Protection Materials for Structural Steel’ Australian Institute of Steel Construction 1990.
18.
”A Guide to the Requirements for Engineering Drawings of Structural Steelwork”, Steel Construction Journal, Vol. 29, No. 3, September, 1995.
SPECIFIC REFERENCES BY SECTIONS: Note: References not mentioned specifically in the text are listed for the purpose of further reading or as additional references. Section 1. PRELIMINARY CONSIDERATIONS 3.
Section 2. GENERAL FACTORS AFFECTING ECONOMY 7.
Day, G. A. ”Fabrication and its Future”, Steel Fabrication Journal No. 42, Australian Institute of Steel Construction, February 1982.
103
Section 3. FRAMING CONCEPTS AND CONNECTION TYPES 19.
Steel Structures Manual, Part 3 -- Forms of Construction, SAA MA1.3, Standards Association of Australia, 1971.
Section 4. INDUSTRIAL BUILDINGS 20.
Gaylord, E. H. and Gaylord, C. N., ”Structural Engineering Handbook”, Section 19.2, McGraw Hill Book Co., 2nd ed., 1979.
21.
Macdonald, A. J., ”Wind Loading on Buildings”, Applied Science Publishers Ltd., 1975.
22.
Gorenc, B. E., Tinyou, R., and Syam, A. ”Steel Designers Handbook”, University of New South Wales Press, 6th Edition, 1996.
32.
Johnson, R. P. and Smith, D. G. E. ”A Simple Design Method for Composite Columns”, Steel Construction Vol. 16 No. 4, Australian Institute of Steel Construction 1982.
33.
Firkins, A., ”City Buildings”, Steel Construction Vol. 17 No. 1, Australian Institute of Steel Construction 1983.
34.
Firkins, A., ”City Buildings -- The Steel Solution’ Structural Steel Conference, Singapore Structural Steel Society, 1984.
35.
Hogan, T. J. and Firkins, A., ”Economic Design and Construction of Medium Rise Commercial Buildings using Structural Steel’ Pacific Structural Steel Conference, NZ Heavy Engineering Research Association 1986.
Section 6. BOLTING 36.
Firkins, A. and Hogan, T. J., ”Bolting of Steel Structures”, Australian Institute of Steel Construction .
37.
Fisher, J. W., Kulak, G. and Struik, J. H. A., ”Guide to Design Criteria for Bolted and Riveted Joints”, John Wiley, 1987.
23.
Gorenc, B. E. ”Crane Runway Girders”, Australian Institute of Steel Construction, 1983.
24.
”Wide Span Structures”, Steel Construction Vol. 16 No. 2, Australian Institute of Steel Construction, 1982.
25.
”Australian Conference on Space Structures”, Australian Institute of Steel Construction, Papers, Melbourne 4/5 May, 1982.
38.
The Lincoln Electric Company, ”The Procedure Handbook of Arc Welding”, 1 2th Edition, 1973.
26.
Firkins, A., ”Connections for Tubular Bracing Members”, Steel Fabrication Journal No. 46, February 1983.
39.
Blodgett, O. W., ”Twelve Commandments to Design Engineers”, reprinted in Steel Fabrication Journal, No’s. 9, 10 and 11, Australian Institute of Steel Construction, November 1973/May 1974.
Section 7. WELDING
Section 5. COMMERCIAL BUILDINGS 27.
Schueller, W., ”High--Rise Building Structures”, John Wiley, 1977.
40.
28.
Hart, F., Henn, W. and Sontag, H., ”Multi--Storey Buildings in Steel”, Crosby Lockwood Staples, English Edition edited by G. B. Godfrey, 2nd ed., 1985.
Australian Welding Research Association, ”Economic Design of Weldments”, AWRA Technical Note 8, March 1979.
41.
29.
Patrick, M. and Poon, S.L., ”Composite Beam Design and Safe Load Tables’ Australian Institute of Steel Construction, 1989.
Magnusson, D. J., ”Using the Structural Welding Code’ Steel Fabrication Journal NQ 48, Australian Institute of Steel Construction, August 1983.
42.
30.
CONDECK Technical Design Manual, Stramit Metal Industries.
Firkins, A., ”Design for Welding’ Australian Welding Institute Conference, 1988.
31.
BONDEK II Composite Slabs, BD II -- 2A Composite Design Manual, BHP Building Products.
43.
Firkins, A., and McGeachie, 1., ”Fillet Welds -- What Size is Normal?’ Asian Pacific Regional Welding Conference, International Institute of Welding, 1988.
104
10. Standards
This list does not purport to be exhaustive, but covers most of the standards currently in print that are likely to concern the structural steel fabrication industry. MATERIALS Steel
AS 1275
Metric Screw Threads for Fasteners.
AS 1559
Fasteners -- Bolts, Nuts and Washers for Tower Construction.
Electrodes
AS 1085.1 Railway Permanent Way Material, Part 1: Steel Rails.
AS 1167.2 Welding and Brazing -- Filler Metals, Part 2: Filler Metal for Welding.
AS 1163
Structural Steel Hollow Sections.
AS 1553
Covered Electrodes for Welding (Parts 1 to 3).
AS 1450
Steel Tubes for Mechanical Purposes.
AS 1858
AS 1594
Hot--Rolled Steel Flat Products.
Electrodes and Fluxes for Submerged--Arc Welding (Parts 1 and 2).
AS 3597
Structural and Pressure Vessel Steel -- Quenched and Tempered Plate.
AS 3678
Structural Steel -- Hot--Rolled Plates, Floorplates and Slabs.
AS 2203.1 Cored Electrodes for Arc--welding, Part 1: Ferritic Steel Electrodes. WORKMANSHIP, DESIGN
AS 3679.1 Structural Steel, Part 1: Hot--Rolled Bars and Sections.
AS 1418.1 Cranes (including hoists and winches), Part 1: General Requirements.
AS 3679.2 Structural Steel, Part 2: Welded Sections.
AS 1538
Cold--formed Steel Structures Code.
Bolts
AS 1554
Structural Steel Welding (Parts 1 to 6).
AS 1110
ISO Metric Hexagon Precision Bolts and Screws.
AS 1111
ISO Metric Hexagon Commercial Bolts and Screws.
AS 1112
ISO Metric Hexagon Nuts including Thin Nuts, Slotted Nuts and Castle Nuts.
AS 1214
Hot--dip Galvanized Coatings on Threaded Fasteners.
AS 1237
Flat Metal Washers for General Engineering Purposes.
AS 1252
High--Strength Steel Bolts with Associated Nuts and Washers for Structural Engineering.
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AS 1562.1 Design and Installation of Sheet Roof and Wall Cladding, Part 1: Metal. AS 1657
Fixed Platforms, Walkways, Stairways and Ladders -Design, Construction and Installation.
AS 1796
Certification of Welders and Welding Supervisors.
AS 2214
Certification of Welding Supervisors -- Structural Steel Welding.
AS 2327.1 Composite Construction in Structural Steel and Concrete, Part 1: Simply Supported Beams.
AS 4100
Steel Structures.
AS 1710
Non--Destructive Testing of Carbon and Low Alloy Steel Plate -- Test Methods and Quality Classification.
AS 1929
Non--destructive Testing -- Glossary of Terms.
AS 2177
Non--destructive Testing -- Radiography of Welded Butt Joints in Metal (Parts 1 and 2).
AS 2205
Methods of Destructive Testing of Welds in Metal (Set of Parts).
AS 2207
Non--destructive Testing -- Ultrasonic Testing of Fusion Welded Joints in Carbon and Low Alloy Steel.
SURFACE TREATMENT AS 1627
Metal Finishing -- Preparation and Pretreatment of Surfaces (Parts 0 to 7, 9, and 10).
AS 1650
Hot--dipped Galvanized Coatings on Ferrous Articles.
AS 2311
The Painting of Buildings.
AS 2312
Guide to the Protection of Iron and Steel against Exterior Atmospheric Corrosion.
TESTING AND INSPECTION AS 1391
Methods for Tensile Testing of Metals.
AS 1530.4 Methods for Fire Tests on Building Materials, Components and Structures, Part 4: Fire Resistance Test of Elements of Building Construction. AS 1544.2 Methods for Impact Tests on Metals, Part 2: Charpy V--Notch.
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WELDING TERMS AND SYMBOLS AS 1101.3 Graphic Symbols for General Engineering, Part 3: Welding and Non--Destructive Examination. AS 2812
Welding, Brazing and Cutting of Metals -- Glossary of Terms.
