Final Contractor’s Report
October 2005
Tower crane stability
Hilary Skinner Tim Watson Bob Dunkley Paul Blackmore
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CONTENTS Acknowledgements 1 Foreword 1.1 Scope 2 Introduction 2.1 Causes of failure 2.1.1 Hazards Erection, climbing or dismantling Overload High wind loads Structural failure Load failures or impact Proximity hazards Overhead power lines 2.2 Regulations and standards 2.3 Health and Safety Regulations 2.4 Tower crane types and features 2.4.1 Common types of crane (issues for safety and selection) 2.5 Selection 3 Design 3.1 Types of loading Structural loads Wind loading In service loading Out of service loading Other loading Loads applied to the foundations 3.1.1 Loading information supplied by manufacturers 3.2 Factors of safety 3.2.1 Principles of factors recommended by the report Foundations Structural members 3.3 Influence of siting on design 3.4 Foundations 3.4.1 Foundation selection 3.4.2 Site investigation 3.5 Tying 3.5.1 Types of Tie 3.5.2 Internally Climbing Tower Cranes 3.6 Erection/ Climbing/ Dismantling 4 On site 4.1 Management 4.1.1 Planning 4.1.2 Design 4.1.3 Procurement 4.1.4 Erection 4.1.5 Operation 4.1.6 Maintenance 4.1.7 Re-configuration 4.1.8 Dismantling 4.2 Communication 4.3 Operational issues 4.3.1 Monitoring wind speed Wind speed indicators Operational strategies 4.3.2 Verticality 4.3.3 Foundation checks
v 1 1 2 2 2 2 3 3 3 3 4 4 5 5 6 7 9 12 12 12 12 14 14 14 14 16 17 18 18 18 18 19 22 23 23 23 24 25 27 27 27 28 29 29 30 31 31 31 32 35 35 35 36 36 37
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Pre-erection Inspection Regular Monitoring Frequency Records Certification Deformation of Foundations 4.3.4 Maintenance and Thorough Examination 4.3.5 Daily Checks 4.3.6 Weekly Inspections 4.3.7 Maintenance 4.3.8 Thorough Examinations 4.3.9 Overload Testing APPENDIX 1 Loading and structural elements A1.1 Information supplied A1.2 Wind A1.3 DESIGN FOR Fatigue A1.4 Temporary structural design (ties and anchors) Design and Manufacture of Bespoke Tie Components Tie Installation A1.5 Permanent works design APPENDIX 2 Foundations A2.1 Site investigation and determination of ground parameters A2.2 Design A2.2.1 Principles A2.2.2 Factors of safety Stability (equilibrium) Geotechnical capacity Structural capacity A2.2.3 Foundation construction issues A2.3 TRADITIONAL Design of typical bases A2.4 BASE DESIGN TO Eurocode for geotechnical design, BSEN 1997-1 and 2 A2.4.1 Principles of EN 1997 A2.4.2 How to ensure designs comply with EN 1997 APPENDIX 3 Standards and timetables for change Glossary Glossary References Bibliography
38 39 39 39 39 40 40 40 40 40 40 41 43 43 43 44 45 47 48 48 50 50 50 50 51 51 52 53 53 54 78 78 78 89 91 91 93 95
List of figures Figure 1 Tower crane erection Figure 2 Proximity hazards (Photo courtesy HTC Plant Ltd) Figure 3 Parts of a tower crane Figure 4 'A' frame tower crane Figure 5 Flat top crane Figure 6 Luffing jib cranes Figure 7 Self erector Figure 8 Tower cranes on site. Figure 9 Tower crane erection requires careful planning to ensure that access is available Figure 10 Nearby buildings can potentially alter wind speeds. Figure 11 Loading applied to tower crane foundation: concrete gravity base Figure 12 Loading applied to tower crane foundations: cruciform on pads Figure 13 Some design issues relating to crane siting. Figure 14 Example of a foundation completion certificate Figure 15 Typical Tie Figure 16 Proprietary Tie System Figure 17 Typical Internal Climbing Sequence (Courtesy HTC Plant Ltd)
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Figure 18 Clear communication between operator and the ground is required (Courtesy HTC Plant Ltd) Figure 19 Wind speed assessment and its location (Courtesy HTC Plant Ltd).
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Acknowledgements This publication was produced as a result of CIRIA Research Project 707, “Tower Crane Stability – Best Practice Guidance”, by Hilary Skinner, Paul Blackmore, Tim Watson and Bob Dunkley Hilary Skinner has been at BRE or 10 years and is now Head of the geotechnics group. She has published widely, including guidance documents for industry relating to BSEN 1997, temporary platforms, brownfield geotechnics and ground treatment as well as on a variety of research topics in numerous journals and conferences. Paul Blackmore is Head of the wind engineering group at BRE. As a specialist in fluid mechanics: wind engineering and wind tunnel testing he has 20 years of expertise in wind tunnel testing, wind loading and weathertightness and the analysis of wind damage and development of wind damage assessment methods. He has acted as an expert witness in cases of wind effects and damage including tower cranes. He is a member of a number of BSI and CEN committees related to wind loading and temporary structures. Tim Watson worked for the Laing Group for 33 years and when he left in 2001 was Engineering Director of Laing Plant. He then spent a year with Hewden Tower Cranes before becoming an independent engineering and safety consultant, specialising in cranes, hoists and access. Whilst with Laing he was heavily involved in the first CIRIA crane stability project and has been involved in the drafting of a number of crane related industry standards and publications. Bob Dunkley spent 31 years with John Laing in their temporary works department, and developed a specialism in tower crane bases. He was responsible for the design of many foundations, of all types, for tower cranes and wrote the firm’s in-house design manual for tower crane bases. He has previously been involved in the preparation of the CIRIA Site Guide “Crane Stability on Site”. The project was initially managed by Ms Natalya Brodie-Hubbart, project manager, with Dr Andrew Pitchford, project director, and subsequently led by Dr Das Mootanah, project manager. Following CIRIA’s tradition of collaboration, the study was guided by a steering group of experts involved, or with an interest in the planning and management of tower crane use, and related health and safety issues and mitigation. CIRIA would like to express its thanks and appreciation to all members of the project steering group for their helpful and valued comments and advice throughout the project. The steering group comprised: Mr Paul Phillips (Chairman) Mr Mark Blundy Mr David J Butterworth Mr Robert Cameron Mr David Carter Mr Charles S Dye
HTC Plant Ltd RB Wilbrey (Consultants) Limited Health & Safety Executive Sir Robert McAlpine Ltd George Wimpey UK Ltd Lewis & Hickey Safety Management Ltd
Mr Paul Forrester
Royal & Sun Alliance Engineering Business (Representing Safety Assessment Federation Limited HTC Plant Ltd Bovis Lend Lease Gary Global International Plant Sales Gleeson-homes Interserve Project Services Limited National Construction College, CITB-Construction Skills Peter Oram Consulting Alan Baxter & Associates (Representing the Department of Trade and Industry) Health & Safety Executive
Mr Bernard Holman Mr John Huddart Mr Ken Kirby Mr Peter Leveridge Mr Alan J Miles Mr Andy Newell Mr Peter Oram Mr David Rathbone Mr Ian Simpson
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Mr Haydn Steele Mr Rob Sweeney Mr Andrew Thomsett Mr Kanji Vekaria Mr Chris Watson
Construction Plant-hire Association CITB-Construction Skills Taylor Woodrow Developments Limited Bovis Lend Lease Carillion Engineering Services
CIRIA’s project manager was Dr Das Mootanah. Funders The project was funded by Department of Trade and Industry, through its Partners In Innovation (PII) programme; Health and Safety Executive; HTC Plant and CIRIA’s Core Members.
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1 Foreword 1.1
Scope
The document is intended to promote the safe design of foundations for, and use of, tower cranes through an improved understanding of temporary works design and health and safety issues. The initial sections of the report are aimed at those groups who need to understand the issues related to the safe use of tower cranes – planners, architects, permanent works engineers and site supervisors. The report highlights some key situations in which a specialist should be consulted. It covers specific guidance for designers of temporary works involving tower cranes. Issues include the understanding of wind effects, other loading and support considerations, and particularly factors of safety and design of foundations. Readers of this document may wish to refer to BS 7121 Part 5 Tower cranes and C703 Crane safety on site for detailed guidance and this is highlighted in the text. Tower cranes are a vital element in the construction process. There are around 1500 cranes in the UK and at any time around 1000 are in use. Tower cranes are often in use on construction sites in urban areas and, although rare in the UK, any collapse of the crane is likely to result in injury to members of the public outside the boundaries of the site as well as personnel working inside the site. Collapse of tower cranes also presents a risk to adjacent railways and roads. The guidance aims to bring together important practical and design issues that impact on health and safety and to present a current understanding of best practice based on the experience of a wide cross-section of the industry.
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2 Introduction This best practice guide is intended to: • •
provide basic information to all those involved in the planning and management of tower crane use enable temporary works design for tower cranes to be standardised and experience shared. The report recognises that the standards relating to the design of cranes and the design of temporary works are changing. It is anticipated that within the next 5 years information routinely available from crane owners for the purposes of structural and geotechnical design will be more detailed and will align with the design philosophy of the Eurocodes for structural design that are due within the next 10 years. Foundation design examples are given that relate to the current approach and to the Eurocode design.
Whilst it articulates best practice based on the experience of a wide cross-section of the industry, it does not deal with day to day lifting operations, rather with temporary works design, planning, communication and management of tower cranes. Throughout the report, the “owner” is generally defined as “the Company, firm or person owning and/or letting the plant on hire”. The “user” is generally defined as “the Company, firm, person, Corporation or public authority taking the owner’s plant on hire.”
2.1
Causes of failure
Whilst the collapse of tower cranes is rare, accidents and near misses do occur. These generally result from events, either singly or in combination, that have not been anticipated; events or actions that cause unexpected loads or from errors during erection, use or dismantling. Failures of any part of the crane or load carrying systems are likely to cause serious accidents - which generally involve both the crane operators and other site personnel or the general public.
2.1.1 Hazards Operations that involve erection, reconfiguring and dismantling cranes are particularly hazardous. When the crane is in use, poor operation, or failure of warning devices or structural members are most likely to result in a serious incident. All personnel involved in specifying, procuring, planning, erecting and operating the tower crane, as well as those on site around it, should understand the major hazards to the safe use and stability of tower cranes. It is vital that the personnel carrying out the erection, reconfiguring, use and dismantling of tower cranes are trained and competent. Some safety critical devices may require the tower crane to have an uninterrupted power source and this must be taken into account early in the planning. The following list of hazards is not exhaustive, further hazards may be identified by a site specific risk assessment. Erection, climbing or dismantling Failure of cranes during these critical operations is the most common cause of fatal accidents involving tower cranes in the UK. The weather conditions under which these operations can be carried out, in particular relating to maximum wind speed, must be adhered to (and for this the wind speed should be assessed at a suitable location). During these operations, the crane cannot be used for lifting!
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Crane components may be lifted by a second crane (Figure 1) and may be in an inherently unstable condition until properly bolted together or when unbolted and disassembled. The correct sequence of component assembly or dismantling is vital to ensure that the part-completed crane remains stable.
Figure 1 Tower crane erection Climbing a tower (increasing its height) by means of an external frame involves particular hazards relating to the carrying of an unbalanced load during the operation. The HSE discussion paper published in 2003, on the Safe use of external climbing frames discusses the particular hazards that arise from the disconnection of the slewing section and jib assembly from the rest of the crane, its support and jacking upwards via a climbing frame before the insertion of additional mast sections. Overload The use of rated capacity indicators or limiters has reduced cases of overload, but the structure of the crane and its stability can be impaired when loads are lifted that are in excess of the rated capacity at the given radius, are inappropriate in the wind conditions or for the crane configuration. It is therefore vital that all lifts are covered by a lifting plan and that such activities are controlled by an appointed person. High wind loads The crane structure and any load are both subject to wind forces. Loads in excess of design can impair the stability of the crane. Advertising banners etc fixed to cranes can add significantly to the wind loading. Structural failure Tower crane collapse could be caused by the failure of elements of the structure and its foundations that have either not been correctly assembled, or fail due to overload or fatigue. Load failures or impact Although not related to tower crane stability, a significant number of injuries have been caused by loads either slipping, or lifting accessories failing, resulting in the load falling to the ground or into the structures under construction. Other hazards are related to poor control of the load during lifting and
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moving such that it may hit workers or structures. The sudden release of load, or impact of falling loads can also damage the crane and impair stability. Proximity hazards Tower crane collapses or failures have been caused by crane impacts with other cranes, plant or buildings. It is critical that the crane(s) operational procedures take into account the proximity hazards and that a safe choice of crane and system of work has been developed. It is important not to rely on anti-collision systems alone to warn of proximity to other structures. Zoning systems are a useful aid in preventing the load and/or parts of a tower crane from entering a prohibited space (Figure 2). Overhead power lines Whilst not a stability issue, collision with power lines is reported, in a recent worldwide survey (OSHA Census), to be the cause of the greatest number of electrocution fatalities outside the UK and merits a mention. Whilst this is not a common cause of accidents involving tower cranes in the UK, it is critically important to site the crane such that it and its loads avoid power lines, or to ensure that, where possible, lines are powered down when lifting. There should be protection against electrocution for the crane operator, slinger signaller and other site personnel. Devices are available that are designed to be fitted on cranes to determine the presence and give warning of overhead electric lines and cables. Such devices are not recommended and should not be considered as a substitute for a safe system of work and safe clearances as set out in HSE Guidance Note GS 6.
Figure 2 Proximity hazards (Photo courtesy HTC Plant Ltd)
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2.2
Regulations and standards
Tower cranes are designed to current standards and must be supplied and used according to key Health and Safety and other relevant legislation. The standards related to tower crane design, published by FEM, DIN, CEN and BSI are in the process of re-drafting. The current standards and anticipated timetable for change are summarised in the table in Appendix 3. The designer of foundations or other temporary works will not generally have to calculate loading applied by tower cranes, but it is useful to understand which standards have been applied in the calculation of loads that are supplied by the manufacturer. This is particularly pertinent when assessing whether the standards applied in the calculation of wind loads are appropriate to the particular site. If in doubt the temporary works designer should consult the crane owner or hirer. Standards relevant to the design of temporary or permanent works and the use of tower cranes are shown in Table 2.1. 1
Table 2.1 Standards relating to tower crane temporary works design and operation Standard Comment BS 7121 Code of practice for the safe use of cranes – Part 1 general Under revision. Replacing 1997 edition. BS 7121 Part 5 Tower cranes Significant overhaul of all clauses relating to operation. (only applies to Part 5) BS 7121 Part 2 Inspection, testing and examination Part 2 revised 2003 BS 8110-1:1997 Structural use of concrete (in 4 parts) BS 5950 Code of practice for the use of structural steelwork in building (in 6 parts) BS8004 Code of practice for Foundations BS5930 Code of practice for site investigation BS1377 (9 parts) Method of tests for soils for engineering purposes BSEN 1990 The basis of design BSEN 1991 Actions BSEN 1992 Concrete BSEN 1993 Steel BSEN 1997 Geotechnical Design
2.3
Currently cover site investigation and foundation design in the UK. Codes of practice giving guidance rather than standards, little coverage of temporary works design. Expected to be in period of 'co-existence' with equivalent BS until at least 2008. Will govern structural and foundation design. BSEN 1997 is supported by a suite of testing and execution standards covering site practice that become requirements when they are published.
Health and Safety Regulations
The hiring, erection, use and dismantling of tower cranes must be carried out with due regard to the requirements of UK Health and Safety legislation. The responsibilities outlined in the Health and Safety at Work Act and the Construction (Design and Management) Regulations provide the framework under which cranes on construction sites must be operated. There are particular issues in crane use relating to the use of machinery and the regulations governing lifting. Detailed guidance on tower crane use is given in BS 7121-5.
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ISO standards have not been included in this document
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Table 2.2 Health and safety legislation and guidance The Health and Safety at Work etc. Act 1974 Management of Health and Safety at Work Regulations. 1999 The Construction (Design and Management) Regulations 1994 (As amended) The Work at Height Regulations 2005 The Lifting Operations and Lifting Equipment Regulations 1998 (LOLER) The Provision and Use of Work Equipment Regulations 1998 (PUWER) The Supply of Machinery (Safety) Regulations 1992 (As amended) The Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 1995 (RIDDOR) The Construction (Head Protection) Regulations, 1989. HSE Guidance Note HS (G) 141, Electrical Safety on Construction Sites. HSE Books. CIC CDM Guidance Note I002 Provisions for the safe use of cranes on construction sites (2004) Other standards, such as those applied by the rail network and operating companies, airports, ports, underground or tram owners and operators must also be adhered to where relevant.
2.4
Tower crane types and features
Tower cranes have a number of different features, which may vary in their construction. The tower or mast provides the height required for the crane operation. The tower supports the operators cab, the main or load-carrying jib and the counter jib on which the crane motors, winches and counterweights are located.
Jib
Tower
Non-slewing tower slewing ring at top of tower Slewing tower slewing ring at base of tower Base
Figure 3 Parts of a tower crane2
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Note slewing towers cannot be tied to adjacent structures and therefore rated capacity is only in the configurations supplied
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2.4.1 Common types of crane (issues for safety and selection) (a) Horizontal trolley or saddle jib (A frame type, Figure 4).
Figure 4 'A' frame tower crane • • • • •
The jib is held in a horizontal or slightly raised position by tie bars or ropes connected to an “A” frame on the top of the crane tower. Fixed length horizontal jib - the hook is suspended from a trolley which moves along the jib to alter the hook radius. High capacity available. Can be erected with the tower inside building. But, needs access for erection and dismantling; semi-permanent structure; jib radius unalterable for out of service condition. Consequently fitting in multiple cranes on a congested site may be a problem.
(b) Horizontal trolley or saddle jib (Flat top type, Figure 5).
Figure 5 Flat top crane
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• • • • • •
The horizontal cantilever jib is connected directly to the tower top and does not require tie bars or ropes connected to an “A” frame. Reduced overall height of the crane which can be important on congested sites, near airports and where adjacent cranes oversail. The hook is suspended from a trolley which moves along the jib to alter the hook radius. Higher weight for a given capacity because of jib being a pure cantilever Can be erected with the tower inside building But, needs access for erection and dismantling, semi-permanent structure, jib radius unalterable for out of service condition. Consequently fitting in multiple cranes on a congested site may be a problem.
(c) Luffing jib (Figure 6) • • • • • • •
The jib angle can be changed to reposition the load at various radii. The jib may be single- or multi-component, and if multi-component, may be articulated (goose necked). Lower capacity than trolley jib cranes for a given tower size but may be lower tower height Radius can be altered to avoid obstructions which can be useful on congested sites (higher crane density likely to be possible) or to avoid oversailing other sites/ buildings Care is needed with out-of service position and high wind speeds as the jib can be blown over backwards if the radius is too small and there may be insufficient wind area to allow the crane to weathervane – check manufacturers instructions Can be erected within building Slow relative to trolley jib cranes Generally require a bigger power supply than trolley jib cranes of equivalent capacity as additional motor power is needed to alter the angle of the jib.
Figure 6 Luffing jib cranes (d) Self-erector (Figure 7) •
Alternative to smaller top slew tower cranes or mobile cranes or tele-handlers (forklifts with telescopic booms)
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• • • • •
Space for access, erection and dismantling still required Lower capacity Normally mounted on wheeled chassis to facilitate transport to and movement around site (although not whilst carrying loads!) Can also be mounted on rail-going or crawler chassis to enable a greater area to be covered on site. Generally outside the scope of this report.
Figure 7 Self erector
2.5
Selection
It can be seen that tower cranes are available in a number of forms and the characteristics of the various cranes should be considered in relation to the job requirements. Having decided upon the type(s) of crane and knowing the overall requirements involved, cranes that will safely meet all the requirements of the planned lifts should be selected. The following section should be read in conjunction with BS 7121-5, which gives further guidance on crane selection. Raising and lowering people by equipment that is not specifically designed for this purpose should only be carried out in exceptional circumstances, when it is not practicable to do so by other less hazardous means. Details of specific requirements are given in BS 7121-5. Careful planning should be carried out prior to each raising and lowering operation. Points to be considered in making the selection include the following: a) weights, dimensions and characteristics of loads The dimensions and characteristics of the load, as well as any potential for abnormal loading (e.g. additional wind loads), will determine the requirements for rated capacity of the crane. Whilst cranes have rated capacity indicator/limiters preventing the lifting of items heavier than the crane and equipment can carry, these must not be used to routinely limit the loads. Loads close to the capacity of the crane at the required radius should be identified before lifting is attempted. Some loads may cause non-vertical loading on the crane if lifted incorrectly. b) operational speeds, radii, heights of lifts and areas of movement The operational rate and area covered by the crane needs to be checked against the construction schedule and plans. Check for proximity hazards, numbers of cranes needed and oversailing. The freestanding height of the crane may be important.
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c) number, frequency and types of lifting operations The number and frequency of lifts needs to be evaluated to ensure that the requirements of the construction programme can be met. Will the loads need to be held for a period of time whilst being fixed as with steel work which restricts availability of the crane? Or will the crane be required to operate with high load cycles as with skipping concrete? The rate of usage should be discussed with the crane supplier. d) length of time for which the crane will be required or anticipated life expectancy for a permanently installed crane Short durations may be best served by a tower crane that can be moved across site, such as rail mounted crane or a self-erector. e) site, ground and environmental conditions, or restrictions arising from the use of existing buildings The ground conditions may determine the type of crane base that can be constructed and may restrict the crane capacity. The loads that must be carried by the ground, whether during erection, use or dismantling, need to be considered. The designer of the tower crane support should take into account any additional loading applied to the ground and whether this may affect the foundations of existing buildings or other construction. The ground conditions should also be considered if a mobile crane is needed for erection or dismantling - these cranes need support beneath their wheels and outriggers (refer to Ciria C703). Figure 8 shows cranes on site - requirements for access, ground support and other structures should all be considered in crane selection
Figure 8 Tower cranes on site. Where existing buildings are to be used to support the crane tower using ties, the load capacity of the existing structure must be assessed. The location of existing structures, the location and any requirements for continued operation of overhead power cables will also need to be established.
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f) space available for crane access, erection, travelling, operation and dismantling Tower crane components may be carried to site on articulated trucks, or may be towed as a complete unit in the case of a self-erector. In either case, suitable access to the crane location is needed. In some cases space will be needed for parts of the crane prior to erection. Space around the crane location is required for erection, raising and dismantling operations. These may need a mobile crane and the working area for this should be identified. Self-erecting cranes will also need space for the erection process. Where a crane is required to travel with a load, on a rail base for example, the space on site needed for the rails may be an issue within the construction schedule.
Figure 9 Tower crane erection requires careful planning to ensure that access is available g) any special operational requirements or limitations imposed The use of a tower crane near airports, railways, highways or over other buildings not part of the construction site requires particular care. The airports code of practice (AOA, 2003) can be followed to ensure that an appropriate height of crane is selected. Particular guidelines also apply to the use of cranes near railways and the rail operator's safety requirements must be taken into account. Agreement with the local council and other property or land owners is required to oversail either a public highway or other buildings. h) prevailing wind-speeds, which can restrict the use of tower cranes in certain locations The location of the site, if positioned on an escarpment or other area of high wind speeds, may reduce the physical dimensions of loads that can be carried. The likelihood of high in-service wind speeds may reduce the availability of the crane. High out-of-service wind speeds may require the crane to be designed against additional wind loading.
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3 Design This section is intended to give an overview of design issues relevant to tower crane operation – including the information that should be collected and understood by both temporary works and permanent works designers and what information needs to be passed on. In addition, issues such as the impact of crane siting and design requirements during erection and dismantling are covered. Foundation design examples cover both traditional calculations to British codes and those based on the forthcoming European structural codes (Eurocodes).
3.1
Types of loading
Loads applied to the tower crane structure consist of those that can be applied when the crane is working - when lifting and moving loads and those that will be imposed even when the crane is not working. The tower crane structure, components and the foundations are all required to safely carry these loads. The crane structure will have been designed to carry these loads, which will include dynamic effects of the load, movement of the crane and wind, within the scope of the prevailing standards for crane design. Structural loads The structural loading arises from the weights of the tower crane, its components and loads carried and their position in relation to the tower. Weights carried at the tip of the jib generate much larger overturning moments than those carried close to the axis of the tower. Any non-verticality of the tower may also add to the moment. Wind loading Wind is comprised of a random fluctuating component, called turbulence, superimposed on a steady mean wind speed. These are summed to give the "gust wind speed". Wind speed increases with height above ground. Therefore, what feels like a gentle breeze at ground level will be stronger in the cab of a tower crane. For example, in city centre locations the gust wind speed at a height of 100m will be approximately twice as strong as the gust wind speed at pedestrian level (excluding effects from nearby buildings). The occurrence of a particular gust wind speed at a particular location and time is not something that can be predicted. Because of the random nature of the wind, all modern wind loading Codes and Standards use a combination of statistical and empirical methods to predict wind speeds for a given "probability of exceedance" usually presented in terms of a return period. The return period should not be confused with the lifetime of the structure. The return period is just a statement of annual risk, which for a 2 year return period is 0.5 in any one year. The dynamic wind pressure, used to calculate wind loads, is proportional to the square of the wind speed. Any structure that impedes the flow of the wind will experience loading. The loading depends on the nature of the disruption to the flow, which will vary with the shape of the structural element. The wind force on the structure or element is obtained by multiplying the dynamic wind pressure by pressure (or force) coefficients and the area over which the wind load acts. Determining wind loads on cranes The determination of wind loads is conceptually simple although quite complex in practice. The calculation is carried out by the crane manufacturer according to the prevailing standards. The same sequence is used, although different codes may specify different wind speeds and force coefficients. For example, when carried out using the FEM rules (1.001, 1998), wind loading is one of many load cases applied to the structure during the crane design. The FEM calculation of wind speed assumes a horizontal wind from any direction, and considers the static reaction of the crane structure. The inservice and out-of-service wind speeds are determined from:
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• •
The maximum wind speed that the crane is designed to operate in (varying between 14 and 28 m/s) The maximum (storm) wind speed at which the crane is designed to remain stable in the outof-service position (varying between 36 and 46 m/s depending upon height above ground level).
The total wind load on the crane structure is calculated from the sum of the loading on each component. Each component load is calculated from the dynamic wind pressure and a shape coefficient depending on the configuration. The magnitude of each component load and its centre of pressure are then used to calculate the total wind load and the point through which it acts. Advertising banners and the like attached to the crane structure form additional impedances to the wind and therefore generate very large additional wind loads that will not generally have been taken into account in the crane or foundation design. Reference should be made to the manufacturer if any sign other than that included in the tower crane manual is to be mounted on a tower crane.
Differences between wind speeds calculated using various codes There are a number of Codes, Standards and Design guides which can be used to determine wind loads on cranes, and the differences between them may have an impact on the crane selection, foundation and tie design. This is further discussed in Appendix 1. Effects of nearby buildings All buildings obstruct the free flow of the wind, causing it to be deflected and accelerated, resulting in very complex flow patterns. Wind flow around isolated buildings is relatively well understood, although the interaction between groups of buildings that dominate the local wind environment is less well understood (Figure 10.). Nearby buildings can have a very significant influence on wind forces. If they are the same height as the crane then they will mostly provide shelter, although local wind loads can be increased in some situations. Where surrounding buildings are significantly taller they will often generate increased wind loading on nearby lower cranes.
Figure 10 Nearby buildings can potentially alter wind speeds.
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In any cases where the crane user is unsure of the suitability of the crane for the location of the site, the supplier should be consulted. In service loading • dead loads - weights of the tower crane components • imposed loads - weight of the load being lifted • live loads - wind loading Dynamic effects may result in additional loads caused by movements such as : • hoisting • slewing • trolleying • luffing • travelling Note: These affect both dead and live loads, and are included in the crane manufacturer’s calculated foundation loads that are supplied with the crane. When the crane is in service, there will be limitations on the wind speed that can be safely tolerated these will be specified by the crane manufacturer or supplier. Out of service loading • dead loads - weights of the tower crane components • live loads - wind loading The out-of-service loadings will only be valid for the crane left in the manufacturer’s specified out-ofservice condition. This is normally with the crane in the free-slew condition so that the jib will “weathervane” and always present the minimum wind area, thereby minimising the overturning moment due to wind pressure. On luffing jib cranes it is also important to set the jib radius to the specified value to ensure that the crane can “weathervane”. Travelling cranes should be clamped to the rails. In some cases a crane jib may need to be locked - for example to prevent weather-vaning over a railway; this special condition must be discussed with the crane supplier as it will cause higher loads than anticipated to the tower crane and its ties and foundations. Other loading The particular forces applied during erection, reconfiguration or dismantling operations should also be considered when designing the foundation or ties to other structures. These forces may in some circumstances be the highest loads transmitted by the tower crane to its support structures. There will be limits on the weather conditions in which these activities can take place. All tower cranes assembled on site are required to be tested with an overload after erection. The loads applied during this operation, when following the manufacturer’s instructions, will have been accounted for in the crane design and loading information supplied for temporary works design. In some locations, for example in high-earthquake hazard areas or near to plant that generates significant ground-borne vibration, further loads may be applied that the tower crane and its foundation must resist. Loads applied to the foundations Tower crane foundations need to be designed taking into account both in-service and out-of-service loads. These result in forces and moments at the base of the crane tower which must be resisted by the foundation if the crane is to remain upright (Figure 11 and Figure 12).
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Wind
Weight of structure and ballast
Weight of load
Combined to form
Weight of structure and load under hook. Moment - due to hook load, structure and wind Slewing moment Horizontal load
Figure 11 Loading applied to tower crane foundation on cast-in anchorages
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Weight of structure
Weight of load
Wind
Weight of ballast and cruciform
Combined to form
Vertical loads
Figure 12 Loading applied to tower crane foundations: cruciform on rails (or similar)
3.1.1 Loading information supplied by manufacturers Information on the design loads that are required for structural and foundation design for the temporary works is supplied by crane manufacturers, and varies with the crane type and configuration. The temporary works designer must be able to interpret the information supplied - this will be particularly important in terms of: (a) the standards by which relevant loads have been derived Whilst wind loading is generally calculated according to the FEM 1.001 Booklet 5, manufacturers may be able to supply cranes designed and loads calculated specifically for higher wind speeds if they are relevant to the site in question. Wind speeds calculated in accordance with the procedures in EN19911-4 could be used. Local meteorological data could also be used but this will need to be analysed statistically to give wind speeds for the required return period.
any load factoring that has been taken into account Whilst safety factors are taken into account in designing the crane, these should not be judged to give an additional safety factor to account for higher wind speeds. Loads that are supplied relating to ties or foundation loads have not in general been factored and appropriate partial or safety factors should be applied.
(b)
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The technical documentation supplied with each crane should detail the required out-of-service procedures for the crane to ensure that the loads supplied are correct. Crane manufacturers normally quote these loads as a vertical force, a horizontal force and an overturning moment. In addition they should provide a value for the torsional moment due to slewing. Some manufacturers may provide three sets of loads covering the crane in-service, out of service and during erection. Others may only provide the worst case, without specifying in which situation it occurs. In most cases the loading supplied by the manufacturer will include some allowance for erection tolerance. Crane manufacturers and suppliers in general issue loading information in line with the prevailing standards. In due course this is anticipated (for new cranes) to result in an increase in the detail of loads supplied. It is anticipated, therefore, that in the future manufacturers will be able to supply a split of loads according to dead weight, imposed and live loading that will enable the Eurocode system of partial factors to be fully utilised for the structural and geotechnical design of the temporary works (see section A.2.2.2). Table 3.1 Checklist for crane loading information supplied Information Detail Crane type Crane design Crane dimensions Crane weight Design wind speed In-service
In service loads
Design wind speed out-of-service
Out-of-service loads
Out-of-service requirements
3.2
Crane type and model Standard no.s and references Factors applied to supplied load values Height, radius, mast and anchorage dimensions in m Verticality tolerance Kg m/s to be measured where? (ground / cab) return period, duration of gust or average? Standard used Vertical load (TC+ max hook load) Horizontal wind load Moment (from wind, TC and load -it would be desirable to split up these components) Slewing moment (torque) OR wheel, bogie or pad loads and ballast requirements for ballasted foundation design m/s to be measured where? (ground / cab) return period, duration of gust or average? Standard used Vertical load TC Horizontal wind load Moment from wind and TC (desirable to spilt up) OR wheel, bogie or pad loads and ballast requirements for ballasted foundation design Position(s) specified to obtain loads
Essential (E)/ desirable (D) E D D E E E E D D D E E E E E D D D D E E E E E
Factors of safety
Tower cranes and associated foundations are part of the temporary works essential for some forms of construction. The average time that a tower crane is installed and working on a site is likely to be
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around 9 months. Over this period, the tower crane may undergo many cycles of loading both in working and out-of-service conditions. The consequences of failure are likely to be serious, which will influence the selection of factors of safety. A number of factors of safety are recommended by this report in Appendix 2. These minimum values have been discussed and agreed by a wide cross section of industry and the HSE. However, there will be some circumstances where different factors may be more appropriate. These would need to be justified on a case by case basis.
3.2.1 Principles of factors recommended by the report The Eurocodes adopt, for all civil engineering materials and structures, a common design philosophy based on the use of limit states and partial factors. This is a substantial departure in particular from much traditional British geotechnical design practice as embodied in BS Codes that uses an allowable stress approach (see Appendix 2). The factors recommended in this report are in general based on those contained within the Eurocode system. The Eurocode system provides minimum recommended factors which may be altered by each country in order to determine local standards for safety. Foundations The factors recommended by the geotechnical Eurocode, BSEN 1997-1, have been used as the basis for the derivation of minimum load and resistance factors for foundation design. The rationale for this is that, although tower crane foundations are temporary works, over the period that they are in service both short term and long term effects in relation to soil strength and deformation (undrained and drained consolidation) should be considered. For simplicity, where possible in the calculation, partial factors have been aggregated into single lumped factors. Tower crane foundation design has traditionally been carried out by checks against three scenarios • loss of stability (equilibrium), • failure in the soil • failure in the structural foundation using a mixture of factors of safety and allowable stresses. Where a traditional design is carried out using a permissible (allowable) stress approach, a factor of safety has already been applied in the calculation of allowable stresses. BSEN 1997-1 explicitly considers the same scenarios of failure as 'limit states'. Each limit state is checked to ensure a safe design, with factors appropriate to each case. In Appendix 2 the design in relation to each of these limit states is covered. Structural members The tower crane structure will have been designed to either the DIN 15018 or FEM 1.001 standards or the new EN14439. Where proprietary ties are specified the loads to be carried and tie positions should be given by the manufacturer. The elements that may be required in addition are any non-proprietary ties or other fixings required between the crane and the permanent works or the foundations. The structural connections are subjected to fluctuating loads and fatigue may be an issue. The partial factors for these design cases can be derived from BSEN 1993.
3.3
Influence of siting on design
Siting issues that affect the foundation requirements are the location of the base in relation to other construction activities - excavations or basements, and services (whether existing or required during the construction process, Figure 13). If the crane needs to be tied to the structure, the implications for the siting of the crane, its foundations and the tie forces into the permanent works need to be considered.
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The requirements for mobile cranes needed for erection, climbing and dismantling (access, space and foundations) should also be taken into account. The person responsible for siting the crane, the designer of the crane foundations and the permanent works designer must communicate the requirements for each element of the temporary and completed structure.
Excavations near TC base may cause failure TC may apply additional loads to a retaining wall that should be considered in design
Figure 13 Some design issues relating to crane siting.
3.4
Foundations
The tower crane foundation is required to carry complex and varying loads during the period the crane is installed - both in-service and out-of-service. The values of the applied loads are generally supplied by the crane manufacturer or owner and will consist of loading originating from the masses of the crane, ballast, hook load and the effect of wind. These are often aggregated into three components - a vertical load, a horizontal load and a moment that must be resisted by the foundation. In future, when design of tower cranes is carried out to EN13001 and prEN14439, it is anticipated that the components of these three loads will also be given for the design of cast-in bases. The tower crane foundation is the responsibility of the user, who will often have to certify that it has been designed and installed satisfactorily (Figure 14).
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TOWER CRANE CANNOT BE ERECTED UNTIL THIS FORM HAS BEEN COMPLETED AND RETURNED TO OUR OPERATIONS DEPARTMENT TOWER CRANE FOUNDATION / GRILLAGE APPROVAL CERTIFICATE The foundation / grillage for a tower crane, reference …………………………….. at………………………………………………………………………………...(site name) Details of crane:
Make and Model:…………………………………… Height under Hook:………………………………… Jib length:…………………………………………... Type of base:………………………………………..
Foundation / Grillage Designed by: ………………………………………..….....(Name) ……………………………………….…(Company) Foundation / Grillage Design approved by:
………………………………………..…(Signature) ...…………………………………….……..(Name) ……………………………………….…(Company)
NOTE: A SEPARATE APPROVAL CERTIFICATE IS REQUIRED FOR EVERY TOWER CRANE
PERMIT TO ERECT I confirm that the tower crane foundation / grillage has been constructed in accordance with the Specification listed above and all foundation / grillage loads are in accordance with the Tower Crane Manufacturer’s recommendations. We confirm the foundation anchors / base pads are level and plumb as required. The Tower Crane can be erected
……………………………………………….…..(Signature) ………………………………………………..……..(Name) …………………………………………………….(Position) ……………………………………………………….…(Company)
…………………………………………………..…….(Date) Figure 14 Example of a foundation completion certificate
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Due consideration should be given to manufacturers' requirements for certain foundations, reinforcement and verticality. The appropriate codes and standards should be followed. The connections between the crane and the foundation elements must be capable of: • Transmitting the loading to the foundation safely for the duration of service • Distributing the loading between foundation elements adequately e.g. pile caps must be designed to transmit the vertical, moment and lateral loading to the piles • Carrying the required loads at the appropriate interval after construction e.g. if the crane is installed 7 days after concrete pouring the 7 day strength must be adequate Tower crane foundations may be of several types, but five generic forms are covered by this guide: • Gravity base - stability is provided by the size and weight of the base; the soil provides bearing capacity at shallow depth. • Piled base - stability is provided by a pile group. The piles carry compressive loads and usually so long as the soil is suitable, tensile loads. (If the piles are not suitable to carry tension the pile cap must be sufficiently large to eliminate tensile loads). The piles may also be subjected to bending moment, if the soil immediately below the pile cap cannot resist horizontal forces. The pile cap must be designed to transmit the loads and moments imposed by the crane. • Rail mounted - stability is provided by ballast carried on the crane chassis, the foundations to the rails carry vertical and horizontal loads and may be mounted on sleepers, pads or a concrete slab or beams. In each case the underlying granular platform (if required) and soil provide bearing capacity at shallow depth. • Cruciform mounted cranes may be mounted on individual pads or piles and may be ballasted. • Grillage - a steel grillage forms the foundation base, sometimes ballasted Table 3.2 provides more detail on each generic type of foundation and the nature of fixings. Table 3.2 Types of foundations and fixings Type ANCHORED Tower1 anchored directly to base
Cast-in, using special anchor sections or holding-down bolts (a) Pad base (b) Piled base (c) Cast-in to permanent works foundations (d) RC grillage Bolted to a steel grillage
Comments Stability is provided by the base, which is therefore subjected to an overturning moment as well as vertical and horizontal forces (and slewing torque) 2 Anchors or holding-down bolts designed by crane manufacturer
Or “gravity base” see Example 1 See Example 2 Liaise with permanent works designer Unusual; but may be suitable for for an awkward layout Substantial stiffening of steel members probably required. Connection subjected to cyclic loading - bolts must be designed to appropriate rules to avoid fatigue failure.
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BALLASTED
Rail mounted
Static
1
Tower on cruciform frame (designed and supplied by crane manufacturer) Rail-mounted (i.e. cruciform on bogies) (a) Rail on sleepers (on ballast) (b) Rail on slab (c) Rail on twin RC beams (ground-bearing) (d) Rail on twin RC beams (piled)
Stability is provided by ballast on the cruciform (or occasionally by tying down the corners or a combination of the two). Only vertical and horizontal forces are applied to the foundation
Cruciform on anchor shoes
If there is any uplift force at the corners, the connections must be capable of transmitting it.
(a) Cruciform on slab (b) Cruciform on 2 pads (c) Cruciform on piles (d) Cruciform on piers (down to competent stratum) (e) Cruciform on grillage supported by the permanent structure
See Example 3
Use of the standard cruciform may reduce the weight of the grillage
Cranes mounted on the structure itself (generally with an intermediate grillage) will require no separate foundations but the permanent (and temporary) structural elements must be designed to carry the additional loads, as must the permanent works foundations. The crane foundation can in some cases be incorporated as part of the permanent works. The complex loading history must be taken into account when determining the long term adequacy of the foundation system.
3.4.1 Foundation selection A number of practical and geotechnical issues will determine the foundation selection on any given site. Any requirement for the tower crane to travel will determine the necessity for rails. The choice between the other foundation support systems will be governed by safety, economics and the prevailing ground conditions. In general, where the required allowable soil bearing capacity is high, a deep foundation rather than a shallow base will be required. As a rule of thumb, if the required allowable bearing pressure (or
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ultimate bearing capacity divided by 3) is in excess of 200kPa a shallow foundation may not be practical. Clearly the final base design should be confirmed by the appropriate calculations. Where the permanent works foundation is being carried on piles, often a piled tower crane base will be selected.
3.4.2 Site investigation Appropriate and sufficient ground investigation is vital to ensure the provision of adequate foundations. An understanding of the soil types, strength and variability of the volume of ground that will be required to support the foundations must be obtained. Any major changes in site level or excavations near the proposed site of the crane should be identified. Investigations should also cover other relevant issues such as the location of services. Geotechnical information for the crane foundation will often be obtained from the site investigation for the main construction. This should have included a desk study to identify past uses of the site and information on the soils and their parameters. Because the foundation levels should be similar for the crane and the main structure, the information should be transferable. However, it will be important to assess how relevant the information from the SI is if it has been obtained some distance from the tower crane location. In the absence of suitable information from the site investigation, specific investigation for the tower crane foundation may be necessary.
3.5
Tying
Tower cranes are generally self supporting, with the tower attached to a foundation or base that is capable of absorbing all the forces imposed by the crane, both in-service and out-of-service. In this case the crane is described as “freestanding”. Occasionally it is necessary to provide external support for the tower from an adjacent structure and the crane is “tied” to the structure using specially designed struts or “legs”. The need for a crane to be tied may arise from a number of circumstances, such as:• Insufficient tower strength On a tall crane the tower sections may have insufficient section modulus to resist the moments imposed by the crane and it may be impractical or uneconomical to use a wider tower section. A larger tower section may not be available or space constraints may not permit the use of a larger section. • Insufficient base capacity On a tall crane it may not be possible, due to constraints on site, to install a sufficiently large foundation or base to safely absorb the forces from the crane. In this case some proportion of the horizontal forces and overturning moments can be taken by ties to an adjacent structure.
3.5.1 Types of Tie Ties between the crane and adjoining structure generally consist of three pin-jointed struts configured as shown in Figure 15, which provides a rigid, statically determinate and economical arrangement. Figure 15 Typical Tie
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The struts or legs connect onto a collar surrounding the tower forming a stiff “picture frame” which distributes the tie forces concentrically to the tower legs and inhibits racking distortion of the tower. Internal cross bracing may also be added inside the tower at the level of the collar to resist crushing forces. The other ends of the struts connect to suitable points on the supporting structure. When planning the vertical position of ties on the tower it is important to remember that most manufacturers will only allow tie collars to be positioned at certain points on each tower section. Tie legs normally consist of tubular members with clevises for pin connections at either end. Most crane manufactures supply modular systems which can be used to build up struts of varying length, with some means of fine adjustment for final assembly of the tie (Figure 16). In practice most crane owners arrange to have job-specific legs fabricated as this may prove less expensive. It is however important that these fabricated legs also incorporate some means of making fine adjustments to the tie length as this considerably eases the task of fitting the ties and adjusting the plumb of the tower (see Section 4.3.2).
Figure 16 Proprietary Tie System Formatted: Bullets and Numbering
3.5.2 Internally Climbing Tower Cranes Whilst not strictly part of tying, internally climbing cranes also rely on a building or other structure for their support and the interface between the crane and supporting structure requires careful consideration. With an internally climbed crane, the crane and its tower are located inside the building and climbed up inside the structure as construction progresses, using the completed part of the structure to take all the forces generated by the crane. The crane is supported in collars which surround the tower at two different floor levels, generally about 12m apart. The lower collar, at the bottom of the tower, takes vertical forces and part of the overturning moment, whilst the top collar takes the horizontal forces of the remainder of the moment. Whilst the crane is working the tower is clamped to both collars allowing
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the forces generated by the crane to be transferred to the collars and into the building structure, usually via a steel grillage. To climb the crane up to the next level an additional collar is assembled around the tower at the prescribed distance above the top collar. The climbing supports, up which the crane climbs, are hung from what has now become the middle collar (Figure 17). The devices clamping the tower to the collars are released and the crane is climbed to the next level using a hydraulic climbing section at the bottom of the tower, which reacts on the climbing supports. Once the bottom of the tower has reached the middle collar, the tower is clamped to the middle and top collars, leaving the bottom collar to be removed and available for the next climb (Figure 17).
Figure 17 Typical Internal Climbing Sequence (Courtesy HTC Plant Ltd) As with most external ties, the tower crane manufacturer will normally supply the reaction forces at the climbing collars. These are then used by the temporary works designer to design a means (usually a steel grillage) of transferring these forces into the permanent works of the supporting structure. Design and manufacture of the grillage should be carried out to a suitable standard (see A1.4). On high rise buildings where internal climbing is carried out, the climbing collars and their supporting grillages are often “leapfrogged” up the building once they have been left behind during climbing. Re-use in this way can save fabrication costs but the need for ease of dismantling, transfer to the new level and reassembly should be considered at the design stage. Early and effective consultation between the temporary and permanent works designers is essential to ensure that the installation and climbing process can be carried out effectively.
3.6
Erection/ Climbing/ Dismantling
The loading on the ground and access requirements for mobile cranes and transport during crane erection, climbing and dismantling should be considered early in the planning process (see 4.1.8 for example).
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Loading imposed by mobile crane outriggers and tracks can be calculated from the weights and lift radii of the tower crane elements. Suitable foundations must be provided or the ground confirmed sufficient to carry the loading. Design methods for load spreaders can be found in CIRIA C703 or the design of ground supported working platforms from CIRIA SP123 or BR470.
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4 On site It is a basic requirement of Health and Safety legislation that, in any work situation, there must be a safe system of work. Most accidents happen because of the absence of such a system or because it has been ignored. Competent personnel should be used at all times. The development of the safe system of work for lifting includes (but is not limited to) the following main steps: • Defining the lifting requirements; • Gathering information; • Establishing the organisation; • Identifying hazards and assessing risk; • Planning the operation; • Communicating the system; • Implementing the plan; • Monitoring to ensure that what should happen does happen. Day to day operational requirements are laid out in BS7121 Parts 1, 2 and 5.
4.1
Management
For a tower crane installation on site to be safe, effective and economical it must be effectively managed at all stages of its procurement and use by the contract, from initial planning to removal from site. The stages involved are generally:• Planning; • Design; • Procurement; • Foundation construction and checking (including requirements for mobiles needed for erection etc) • Erection (including thorough examination and testing); • Operation; • Maintenance; • Reconfiguration; • Dismantling. For the purposes of this document the stages will be referred to as the “tower crane project”. On a construction project all these stages are covered by the Construction (Design and Management) (CDM) Regulations. These require all those involved to consider health and safety matters throughout all stages of the project from conception, design and planning through to carrying out the work, including maintenance, repair and/or demolition.
All lifting operations, including those involving tower cranes, must be included in the Health and Safety Plan required by the CDM Regulations so that factors influencing crane safety can be assessed at a sufficiently early stage.
4.1.1 Planning Planning includes:•
Identifying the need for lifting from the construction plan;
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• • • • • • • •
• • • •
Determining the weight, dimensions and characteristics of the load and specifying the lifting accessories; Determining the number, frequency and types of lifting operations; Estimating operational speeds, radius, height of lift and areas of movement; Estimating task durations. Identifying site, ground and environmental conditions and restrictions arising from other buildings etc that may affect installation and lifting operations; Identifying restrictions imposed by others (railways, airport, neighbours etc.); Identifying the space available for crane component access, mobile crane access, erection tasks, travelling, operation and dismantling; Selecting the type and number of tower crane(s) ; Identification of tower crane location(s); Deciding on the type(s) of base to be used; Establishing if the crane(s) will need to be tied to the structure under construction, or to another structure; Investigating the availability of a suitable power supply on site; Ensuring that safe erection, alteration and dismantling of the tower crane(s) can be carried out; Determining the control of the lifting operations, including the relevant personnel Establishing maintenance arrangements.
4.1.2 Design The design of the permanent works for the project can have a significant effect on methods of construction and the type and size of lifting operations required. Designers should look critically at their designs and modify them as necessary taking into account the information they have available to create safe conditions for lifting operations. The design requirements for the installation of a tower crane are generally confined to the design of: • The tower crane base (see section 3.4); • Ties connecting the crane to a supporting structure (see section 3.5); • Foundations for mobile cranes involved in the erection, alteration and dismantling of the tower crane (see CIRIA C703 - Crane Stability on Site). The design process and the installation of these elements must be managed effectively by the Principal Contractor to ensure that constraints are identified at an early stage. This means early involvement is needed to avoid delays and expensive modifications to bases, ties and foundations. Case study - need for early communication Four tower cranes were to be erected alongside the four jump-formed pylons of a cablestayed bridge. As the structure rose in height each crane was to be climbed and tied to the completed part of the pylon, enabling it to reach a final height of 150m. As construction was starting, a chance conversation between the tower crane supplier and the project engineers revealed that the incomplete pylon structures were predicted to vibrate and deflect significantly under wind loading. This raised the concern that if the resonant frequencies of the tower crane and pylon coincided, large deflections and forces would occur which could well overload either or both structures, with catastrophic results. Immediately work was put in hand with the project’s vibration consultants to carry out computer and wind tunnel modelling of the combined structure. Fortunately it was found that the resonant frequencies of the crane and pylon would not cause a problem and the erection of the cranes was able to go ahead, However the additional dynamic analysis had taken six weeks, delaying construction and costing a significant amount of money, all of which could have been saved by better communication
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and understanding at the initial planning stage. This highlights the importance of early communication between the various parties to the construction project.
4.1.3 Procurement Tower cranes are generally hired from either an external supplier or an in-house plant company. In both cases it is important to get potential suppliers involved at an early stage so that the most effective crane solution can be found. An important part of the procurement process is the effective assessment of potential suppliers to ensure that they have adequate knowledge and resources to carry out the supply, erection, maintenance and dismantling of the cranes safely and efficiently. One person, with appropriate knowledge and experience, should be responsible for the procurement process to ensure that the requirements of all parties involved are taken into account. This may well involve tower crane suppliers, planners, permanent works designers, structural engineers, geotechnical engineers, temporary works designers, construction managers and safety advisors. It is important to resolve the often conflicting requirements of all parties early to avoid potential delays and cost overruns.
4.1.4 Erection The erection of tower cranes is almost always carried out by the tower crane supplier, who should be experienced in this type of work. The overall responsibility for the management of the process will however remain with the Principal Contractor who should appoint a suitably knowledgeable and experienced person to take responsibility for ensuring that the operation is carried out safely and efficiently. Responsibility for the direct planning and execution of the work rests with the tower crane supplier who should submit the results of the planning to the Principal Contractor’s appointed person before the erection starts. The supplier should carry out a site specific risk assessment and having evaluated the outcome, produce a method statement fully describing the erection process and highlighting specific risks and measures to be taken to control those risks. Further advice on the content of method statements for tower crane erection is given in BS 7121 Code of Practice for the safe use of cranes – Part 5: Tower cranes. The method statement should be reviewed by the Principal Contractor’s appointed person to ensure full awareness of the process and timescale. The method statement should be integrated into the overall Health and Safety Plan together with items that are the direct responsibility of the Principal Contractor. The items may well include: • • • • • • • • • • • • •
Liaison with trade and sub contractors on site to ensure clear access and possible suspension of other activities whilst the erection is being carried out; Establishment and enforcement of exclusion zones; Provision of mobile crane foundations; Provision of suitable power supply; Negotiation with neighbours (oversailing rights); Negotiations with Local Authorities for road closures and lorry parking arrangements; Negotiations with rail and airport authorities; Design and construction of tower crane bases; Design and construction of tie attachment points on adjacent structures; Arrangements for representation on site during the erection process; Emergency and contingency arrangements. Traffic management Environmental restrictions (e.g. hours of work, noise etc)
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4.1.5 Operation The management of the use of the tower crane, once it has been erected, thoroughly examined (including testing, see BS 7121-2), commissioned and handed over by the supplier, should be considered at the planning stage. A lifting plan should be drawn up, once the items to be lifted at each stage of construction have been established, together with their mass, shape, size, storage position and final location. This, together with the construction programme, will enable the size, capacity and number of tower cranes to be determined (See 4.1.1). To effectively implement the safe system of work for lifting operations, before the crane(s) is taken into use, the Principal Contractor should appoint one person to be responsible for all lifting operations. The appointed person should establish and lead a “Crane Team”. This should consist of: • • • • • •
The appointed person; Crane coordinators; Crane supervisors; Crane operators; Slinger/signallers; Other personnel as deemed necessary by the appointed person.
The composition of this team will change as contractors, cranes and equipment arrive on, and leave, site and will include all trade sub contractor’s crane team personnel as deemed appropriate by the Principal Contractor’s appointed person. It is essential that all members of the crane team be given adequate time and resources to enable them to discharge their duties effectively. Regular meetings are essential to ensure that the team communicate effectively, and that advice and comments from all members of the team are heard. Lifts should always be carried out in accordance with method statements prepared from the outcomes of risk assessments. These method statements may be generic, covering the lifting of items detailed in the site Lifting Plan under a set of given circumstances or may be lift specific where the item to be lifted has not been included in the Lifting Plan or where the circumstances on site have changed. Case study - implications for changes to lifting plan during construction A tower crane was being used to lift factory built service pods into position on an office block project. All the pods were intended to be identical in weight, size and position of centre of gravity. Consequently the lifting of the pods had been incorporated into the site lifting plan and was covered by a generic risk assessment and method statement. This meant that providing nothing changed, the lifting of the pods could be carried out without a specific risk assessment and method statement for each lift. Halfway through the installation of the pods over a nine month period, the client asked for the pods to be upgraded to a higher specification for the top floors of the building. This upgrade resulted in an increase in the weight of the pods. When informed of this the appointed person arranged for a new risk assessment which showed that in one pod location in the building, the revised units were outside the rated capacity of the tower crane. This resulted in a revision of the lifting method for these few pods, utilising a large mobile crane. This increased project costs and delayed the project programme.
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4.1.6 Maintenance Tower cranes are complex pieces of machinery and as such require regular servicing, inspection and thorough examination if they are to operate safely and reliably. The arrangements for ensuring that this is carried out should be established at the time of procurement. Normally the tower crane supplier will undertake all repairs, servicing, inspections and thorough examination of the crane. It is however the responsibility of the Principal Contractor to ensure that this is carried out. This can be done by asking for sight of all records of these activities. Daily and weekly checks are generally carried out by the crane operator (see BS7121-5).
4.1.7 Re-configuration During the duration of the tower crane’s time on site, alterations to its configuration may be required. These may include:• Altering the length of the jib; • Increasing the height of the crane; • Adding ties to a previously freestanding crane; • Foundation check The management of these operations should be carried out in the same way as for initial erection. In some cases a different crane may have been supplied compared with that specified and all aspects of the temporary works, erection, use and dismantling of the crane should be checked to ensure they are satisfactory. Altering the height of a crane often includes the use of a climbing frame. This presents particular risks and should not be undertaken by inexperienced personnel and without detailed planning, including assessment of the specific hazards involved. Further guidance on this is given in BS 7121 Code of Practice for the safe use of cranes – Part5: Tower cranes.
4.1.8 Dismantling When dismantling the tower crane at the end of its time on site, it should not be automatically assumed that the risk assessment and method statement used at initial erection can be used without alteration. Many factors may well have changed such as: • The configuration of the crane; • The presence of new structures; • The presence of new services, both overhead and underground; • Access to the site; • Standing areas and foundations for mobile cranes. These could require a complete reappraisal of the method of dismantling. To avoid this, significant thought should always be given to the constraints on dismantling during initial planning. Small changes at that stage can often lead to significant savings in time and money when the crane is dismantled.
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Case study - importance of early planning for dismantling A tower crane had been erected at low level on a congested commercial property development. The crane had been climbed as the building progressed and finished up at 63 metres under the hook. Given the congested site, the position and capacity of adjacent tower cranes, the only way of dismantling the crane was to use a 500 tonne mobile crane rigged with 47m of main boom and a 42m luffing fly jib. At first sight this appeared to be a simple operation, but identification of the hazards and evaluation of the risks involved highlighted a number of problems: • • • • •
The only place to stand and rig the mobile crane was on a road with a sheet piled edge and limited load bearing capacity, alongside a waterway; Access to the rigging position was across the roof slab of a basement area that could not accept the wheel loads of the mobile crane in transport configuration; The clearance between the building edge and the mobile’s main boom was less than 2m; Limiting wind speed for the mobile in that configuration was 9 m/s (20 mph); The mast of the tower crane to be dismantled was only 15m from an existing railway line.
Through careful planning the dismantling was completed without incident but at considerable cost to the project. Greater consideration of the dismantling requirements at the initial planning stage and the adjustment of the position and capacity of an adjacent tower crane might well have enabled it to be used in the dismantling of the crane in question, saving a considerable amount of time, disruption on site and money.
4.2
Communication
Good communication between all parties involved in the safe installation, operation and maintenance of tower cranes on construction sites is essential. It helps to avoid errors and provides advance warning of potential problems and not only increases safety but also aids the smooth running of a project, helping to keep to both programme and budget. As the one person involved at all stages of the tower crane project, the person appointed by the Principal Contractor to take responsibility for the safe installation and use of the tower crane should coordinate and oversee the communication between all the parties involved. A tower crane project will generate a large quantity of records which should be reviewed, collated and stored by the appointed person. Table 4.1 shows the typical communication requirements for the life cycle of a tower crane on a project. Communication operates at two levels:• Formal communication – The process and outcomes of planning for the procurement, installation, operation, maintenance and dismantling of a tower crane. • Informal communication – The direct giving and receiving of instructions and feedback between personnel involved in the physical tasks of installation, use and maintenance of a tower crane. The coordination of formal communication should be the responsibility of one person, normally the appointed person, to ensure that all parties involved at each stage of the tower crane project are provided with adequate information and are informed of all changes which may affect their role in the project. Communication should be in writing or by e-mail. Effective informal communication will be the responsibility of all involved in the execution of the physical work of the tower crane project, again coordinated by the appointed person. He should ensure
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that all personnel are fully briefed on their duties and tasks, and are in possession of adequate written instructions where this is required. He should also ensure that adequate communication systems are in place between: - personnel on the ground, personnel on the tower crane structure and the operators of any other cranes involved (Figure 14.). This is normally achieved using hand held VHF/UHF radios and arrangements should be made to ensure that good communication is maintained at all times. These might include choosing a unique frequency to avoid interference from other sites, ensuring that batteries are fully charged and that spare handsets are available in case of breakdown. Personnel using radios should be trained in the correct procedure to ensure that a radio failure does not lead to a dangerous situation. For example:- a slinger using a radio should continuously instruct the operator to lower a load, e.g. by saying “Lower-lower-lower...”, and failure of this continuous instruction from the slinger indicates that the operator should halt all crane movements. Further advice on radio and hand signal communication is given in BS 7121 Code of Practice for the safe use of cranes – Part 5: Tower cranes.
Figure 18 Clear communication between operator and the ground is required (Courtesy HTC Plant Ltd)
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Table 4.1 Project Stage
Initial Planning
Design
Procurement
Erection
Operation
Maintenance
Alteration
Dismantling
Typical Communication Requirements Parties Involved
Essential Information
Appointed Person. Crane Layout Designer. Permanent Works Designer. Planner. Temporary Works Designer. Potential Suppliers. Geotechnical Engineer. Appointed Person. Permanent Works Designer. Temporary Works Designer. Geotechnical Engineers. Potential Suppliers. Appointed Person. Potential Suppliers. Buyer. Appointed Person. Temporary Works Designer. Supplier’s Representative. Erection Supervisor. Erection Team. Mobile Crane Driver. Competent Person (Thorough Examination).
Schedule of lifts. Site plan. General arrangement drawings. Geotechnical reports.
Crane co-ordinator Appointed Person. Crane Team:Crane Supervisors. Crane Operators. Slinger / Signallers. Construction Management. Trade Contractors. Sub Contractors. Appointed Person. Supplier’s Representative. Maintenance personnel. Construction Management. Appointed Person. Temporary Works Designer. Supplier’s Representative. Erection Supervisor. Erection Team. Mobile Crane Driver. Competent Person (Thorough Examination). Appointed Person. Temporary Works Designer. Supplier’s Representative.
Site specific crane specification Tower crane foundation loads Tower crane tie loads (if required) Mobile crane foundation and axle loads. Site specific crane specification. Hire offer. Hire contract. Job specific risk assessment. Job specific method statement. Schedule of responsibilities – site. Schedule of responsibilities – supplier. Mobile crane foundation and axle loads. Erection Team briefing. Instructions via radio between Supervisor, Erection Team and Mobile Crane Driver. Verbal feedback from Erection Team. Thorough examination report. Lift plan. Generic risk assessments. Generic method statements. Lift specific risk assessments. Lift specific method statements. Daily/Weekly schedule of lifts. Instructions via radio between Supervisor, Slinger / Signallers and Tower Crane Driver. Daily Crane Team meeting. Records of problems and actions taken. Maintenance schedule Generic risk assessments Generic method statements Task specific risk assessments Task specific method statements Reports of thorough examinations Job specific risk assessment. Job specific method statement. Schedule of responsibilities – site. Schedule of responsibilities – supplier. Erection Team briefing. Instructions via radio between Supervisor, Erection Team and Mobile Crane Driver. Verbal feedback from Erection Team. Thorough examination report. Job specific risk assessment. Job specific method statement. Schedule of responsibilities – site.
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Erection Supervisor. Erection Team. Mobile Crane Driver.
4.3
Schedule of responsibilities – supplier. Mobile crane foundation and axle loads. Erection Team briefing. Instructions via radio between Supervisor, Erection Team and Mobile Crane Driver. Feedback from Erection Team.
Operational issues
This section deals with specific practical issues that may be encountered during the installation, operation and maintenance of tower cranes on construction sites. General guidance on the day to day operation of tower cranes can be found in BS 7121 Code of Practice for the safe use of cranes – Part 5: Tower cranes.
4.3.1 Monitoring wind speed Local weather forecasts, short and long range, should be used to aid the detailed planning, erection, reconfiguration, operation and dismantling of the crane. Wind speed indicators The assessment of wind speed is necessary for determining the safe operating conditions for cranes. This may be carried out by means of a wind speed indicator or monitoring device. These should be sited so that they indicate the uninterrupted wind conditions approaching from all wind directions and are not be affected by nearby buildings or other obstructions. Where an indicator is sited on a crane it should be positioned so that it is unaffected by the local flow generated by the crane; this usually means it should be on a mast sufficiently tall to allow it to measure the uninterrupted air flow. Often sensing components are placed on the top section of the mast above the cab ( Figure 19.) Devices which measure the wind speed and direction are called anemometers, those that measure and record wind speed and direction are called anemographs. Devices which only measure wind direction are called wind vanes. It will be important for the crane driver or appointed person to be able to interpret the results from the wind speed measurement device in order to judge whether crane operation can continue. Because gusts are so transient, they are not necessarily the best quantity to measure when monitoring wind speeds. Just because one large gust has been measured does not mean that the next or subsequent gusts will be as large. The mean wind speed gives a more consistent indicator of the general wind speed. This must be related to the operational requirements for the crane (whether the maximum mean wind speed or maximum gust speed over a given length of time as defined by the crane operating requirements).
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Figure 19 Wind speed assessment and its location (Courtesy HTC Plant Ltd). Operational strategies For the safe operation of tower cranes it is normal to specify an in-service wind speed which the crane is designed to withstand under normal operating conditions. In practice, this may represent an upper bound in that the load being lifted may only be controllable at lower wind speeds than the nominal maximum. The crane driver is able to determine whether the load can be manoeuvred safely and must be permitted to cease operations if, in his opinion, this is not the case. An operational strategy should be employed to ensure that lifting operations cease in the event that the measured wind speed approaches the maximum in-service wind speed. It will be important that: • the driver can correctly interpret and understand the relevance of the wind measurement output to the operating conditions for the crane, • the device should have a display in clear sight of the crane driver. It should be maintained in good working order and should be calibrated regularly such that the readings can be related to the operational requirements of the crane, • the reference wind measurement is at the height of the cab where the maximum wind speed generally is. However, if the measurement is made at a different height then corrections for height should be made. Wind loads on a crane (or any other structure) are generated by the wind pressure, not the wind speed. Wind pressure is proportional to the square of the wind speed. If the wind speed doubles the wind pressure increases by a factor of four times. It is for this reason that warning levels for safe operational wind speeds should ideally be based on the square of the wind speed to give adequate margin.
4.3.2 Verticality Tower Cranes are designed to have their towers erected on site within specified limits of verticality (as specified in the crane information). Erecting the crane with the tower outside these limits can have four negative effects: The foundation loads and loads on the crane structure will be higher than those considered by the manufacturer, due to increased moments. (b) The crane hook may not reach the planned radius preventing the lifting of items to planned locations. (c) On a tied crane the ties may not fit easily between the tower and supporting structure if the centreline of the tower deviates significantly from the predicted vertical from which the tie (a)
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attachment points have been set out. To fit the ties, considerable static deflection of the tower may be required. This will put additional forces of unknown magnitude into both ties members and the supporting structure. (d) When carrying out climbing operations an out-of-vertical tower may overload the climbing frame and prevent the crane top from being effectively balanced. It is therefore important that verticality checks are carried out during erection of the crane and also during any alteration of the tower. These should be carried out directly on the tower rather than by checking the level of the base, particularly on tied cranes and those with a significant freestanding height. Verticality checks can be made using an accurate three vial spirit level against a vertical member of a tower section. This may however lead to inaccuracies due to local deformation of the vertical member and checks are best carried out using accurate surveying instruments operated by experienced personnel. Tower crane towers will deflect significantly in normal use and it is therefore essential that the crane top above the slew ring is balanced about the centre of the tower before verticality checks are carried out. The crane manufacturer should be asked to provide a suitable load radius combination for this purpose. Wind forces will also influence mast deflection and checks should be carried out in minimum wind conditions.
4.3.3 Foundation checks The foundation of a tower crane should be constructed in accordance with the checked details issued by the foundation designer. Concrete should be poured and vibrated in accordance with good site practice and the manufacturers instructions for the placement of fixings should be adhered to. If anomalies arise, they should be referred back to the foundation designer for resolution. All foundations should be thoroughly inspected during construction (e.g. to check the formation, reinforcement and any cast-in items) and before the tower crane superstructure is erected, and then monitored at intervals to ensure that no undue settlement or distress is occurring during the operation of the tower crane. Confirmation that the concrete has reached the required strength may be needed before the crane is erected, this should be carried out by testing. The exact details and frequency of the inspections required will depend on the type of foundation involved. It is recommended that both the foundation designer and the crane supplier be consulted when determining the details, especially where a non-standard foundation solution has been adopted. The appointed person should ensure that a suitable inspection regime is specified and followed. Having determined the scope of the inspections required, arrangements should be made to clearly define the personnel who will carry them out. Any demarcations between the main contractor, subcontractor, tower crane supplier, etc., should be clearly defined and agreed. Case study - Importance of communicating to the site team A construction company with a well developed Temporary Works management system won a contract where a tower crane was judged to be the solution to the materials handing requirement. The company procedure required that each site had a Temporary Works Coordinator (TWC) and a person was nominated in the approved manner. The TWC obtained the Tower Crane data, established the location on site and collected the relevant soil investigation reports. This information was passed to the company's Technical Services Dept who produced a design for a reinforced concrete foundation which included a cast-in steel frame for the mast of the Tower Crane to be attached to. Design checks were made and the calculation signed off and sent to site with drawings, bar bending schedules and a minimum concrete cube 2 strength (40 N/mm ) that was to be achieved.
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The Tower crane base was constructed and the concrete cured for 28 days followed by the TC erection using a mobile crane and load test performed. All appeared to be well until one member of the site team noticed that the concrete poured in to the base was not to the correct specification and only 75% of the required concrete crushing strength may be available (30 2 N/mm )! The site team were concerned and reported the error in the concrete grade to the Technical 2 Services Dept. stating that they had cube crushing histories from past pours of the 30 N/mm 2 concrete mix where results in the order of 40 N/mm had been obtained. Uncrushed concrete cubes from the TC base existed and the site team proposed that to mitigate the risk of foundation failure there was to be no use of the TC until the cubes had been crushed and the results appraised by the Technical Services Dept. Unfortunately the worst design load case for the TC was in the out-of-service condition! The site team had not appreciated this to be the case when proposing the risk mitigation strategy. Fortunately the cube crushing results showed that concrete met the required crushing strength, and in the few days needed for the tests the weather was mild and the full out-ofservice wind speed was never approached. The existence of concrete cubes from the TC base meant that the strength could be confirmed and it was fortunate that the weather was clement. The site personnel in this case did not understand the worst TC load case was when the crane was out of service. The TC erection method statement should have required the value of the concrete compressive strength to have been assessed to be sufficient before erection. Pre-erection Inspection The following inspections should be carried out before a tower crane can be erected on the foundation. An example of a suitable certificate is shown in Figure 10. Table 4.2
Pre-erection inspection checklist
All foundations
Cast-in Items (foundation anchors etc.) Reinforced concrete
Piles
Steelwork
Rails
Certification of details in accordance with foundation designer's drawings. Base set of monitoring readings (see Table 4.3) Level, plumb and to tolerance. Record of concrete mix and placement date, cube tests where carried out, to ensure concrete of correct grade and sufficient maturity. Results of pile tests; Confirmation that design has sufficient reinforcement bond length into pile cap and pile to take tension where applicable. Steel correct grade; Bolts tight (check if particular torque required); Weld quality (NDT results if required). Bedding properly compacted; Sleepers of sound quality and rail clips securely fixed; Rail centres and levels to correct tolerance; Limit ramps and end stops correctly positioned and firmly fixed; Rails earthed. 38
Regular Monitoring
Regular monitoring of the foundation systems should be carried out in addition to the inspection of the crane structure and machinery. A checklist is shown in Table 4.3. Table 4.3
Foundation monitoring checklist
All Foundations Reinforced Concrete Steelwork Rails Ground water level
Level checks. Inspection for cracking, especially around cast-in items. Bolts tight; Inspection for cracks in welds. Rail centres and level; Limit ramps and end stops correctly positioned and firmly fixed. Level check if foundation stability would be impaired by water rise.
The Appointed Person should produce a standard form to record the results of the agreed monitoring regime. Frequency During the first week of crane operation, it is recommended that level checks and condition inspections be carried out daily. Thereafter, visual inspection should be carried out weekly, with a more detailed examination of critical items such as bolts and welds at three monthly intervals. Level checks for rail mounted tower cranes should continue at weekly intervals. For cranes mounted on reinforced concrete foundations the frequency of level checks may be reduced to monthly for the first three months and then three-monthly thereafter, so long as the settlement is within projected limits. Rail-mounted static cranes founded directly on reinforced concrete foundations may be classified as mounted on reinforced concrete foundations. The frequency of inspections and/or level checks should be increased if any adverse tendencies are noted, or if adjacent works are liable to compromise the stability of foundations. Records The following records should be kept on site: • • •
Pre-erection inspection and levels; Monitoring records (condition inspection and level checks); Records of any remedial actions taken following review of the inspection records.
Certification An example of a certificate used in connection with the design and inspection of tower crane foundations is shown in Figure 10. The exact format will be dependent on the type of foundation involved and other specific site requirements such as Quality Assurance Procedures, client requirements, contractual arrangements, etc.
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Deformation of Foundations The requirements for verticality of the mast will limit the deformation of the foundations that is permitted. In many cases these will provide the serviceability limits for the foundation. The verticality limits quoted by various manufacturers are different and may range from 1/500 to 1/1000.
4.3.4 Maintenance and Thorough Examination The inspection, thorough examination, testing and maintenance of all cranes and lifting accessories must be carried out in accordance with the requirements of the Lifting Operations and Lifting Equipment Regulations 1998 (LOLER). Detailed guidance on the maintenance and thorough examination of tower cranes is given in BS 7121 Code of Practice for the safe use of cranes – Part 5 Tower cranes. If any inspection, thorough examination or test shows that a crane cannot be used safely, the crane must not be used until the faults are rectified. A system for the recording, reporting and checking of minor defects must be in place.
4.3.5 Daily Checks At the beginning of each shift or working day, the crane should be checked by the operator to ensure that it is in a fit condition to start work; operators should have been trained to carry out this task.
4.3.6 Weekly Inspections Once a week the crane should be inspected to ensure that no damage or undue wear has occurred and that all safety systems are functioning correctly. This inspection is normally carried out by the operator if he has been assessed as competent to carry out this task. The results of the weekly inspection should be recorded in a “Records of Inspection” book such as that published by the Construction Confederation or an approved equivalent.
4.3.7 Maintenance Tower cranes must be maintained in accordance with the manufacturer’s instructions at intervals which take into account the intensity of use, operating environment, variety of operations and the consequence of malfunction or failure. Maintenance should only be carried out by personnel who are both familiar with the equipment and competent to carry out the work. Sufficient time should be allowed in the site programme for maintenance to be carried out effectively and the appointed person should ensure that a safe system of work is in place before maintenance starts. All maintenance activities should be recorded in a permanent maintenance log.
4.3.8 Thorough Examinations All cranes must be thoroughly examined by a competent person before being taken into use for the first time, after any substantial alteration or repair, or in the case of a tower crane, after each erection on site or alteration of configuration. Cranes should also be thoroughly examined at intervals not exceeding twelve months if the crane is NOT used to lift persons. Where a crane IS used to lift persons then the interval between thorough examinations must not exceed six months. The results of the examination, including the results of tests on any Rated Capacity Limiter/Indicators, should be reported and recorded in a suitable form, which contains at least the information required by Schedule 1 of LOLER.
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4.3.9 Overload Testing Tower cranes should be overload tested by a competent person before being taken into use for the first time, after any substantial alteration or repair, and after each erection on site. Cranes should also be overload tested at intervals not exceeding four years. Testing should be: • carried out according to the manufacturer’s instructions • carried out up to a load in excess of the tolerance of the rated capacity indicator. The results of the test and subsequent thorough examination should be reported and recorded in a suitable form that contains at least the information required by Schedule 1 of LOLER. This should be made available at subsequent thorough examinations.
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Appendices
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APPENDIX 1 A1.1
Loading and structural elements
INFORMATION SUPPLIED
Manufacturers or crane suppliers provide information relating to the loading applied to foundations and ties which can be used for structural and geotechnical design. Loading is calculated according to the prevailing standards, and the information will therefore vary with the age of the crane, the crane type and its supplier. In some cases only the worst loading case will be supplied. The load changes during erection, in service and out-of-service will have been considered by the crane manufacturer in the crane design. Designs for standard ties (including height, distance between ties, tie forces), fixing angles and ballast are generally included as are instructions for out-of-service positions.
A1.2
WIND
The wind speed itself has a great impact on the loading calculated from it, as the dynamic wind pressure is proportional to the square of the wind speed. However, the calculation of applied loads caused by the wind also relies on pressure and area coefficients that take account of the way in which the wind acts on the tower crane structure. This calculation is carried out by the manufacturer in determining the loading values supplied with the crane. It will be important that the wind speed and coefficients used in the calculations are consistent, and this is best carried out by the crane manufacturer. Wind speeds vary across the UK and the EU, this variability may need to be taken into account when calculating loading for tower crane foundation and tie design; loads may need to be recalculated on the basis of a higher wind speed. Designs prior to 2005 are likely to have been carried out to FEM 1.001 (sometimes to DIN1508), after which EN13001 and EN14439 are anticipated to be applied. Even so, when these EN standards are applied, wind speed assessment will still be carried out to the FEM rules as these are referenced. This is in contrast to the structural eurocodes approach, where the 'standard condition' of zone C or D (into which most of mainland Europe falls) can be compared with other parts of Europe for which different zones are applicable. Where wind speeds fall outside this standard condition (likely to be similar to the FEM rules), the special conditions can be found using the EN wind maps and the manufacturer asked to recalculate design loading under these conditions. The wind speed selected should be consistent with the other methodology for calculating wind loads (see next section). Loading supplied will generally be unfactored, and the appropriate safety or partial factors should be applied in the geotechnical and structural design. It is therefore relevant to note the different ways in which wind speed is mapped and loading calculated across Europe, for each code.
Code
Storm wind speed and notes
FEM 1.001 Rules for the design of hoisting appliances. Booklet 2
Wind speeds vary between 36m/s at ground level to 46m/s at or above 100m
Most cranes in use in 2005 will have been designed to FEM 1.001. BSEN13001-2 Crane safety general design. Part 2: load effects.
Uses wind speed map that places UK in C/D/E categories
BS6399-2:1997 Loading for
Uses wind speed based on hourly mean with 2 terrain categories,
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buildings. Code of practice for wind loads
country and town. Only wind pressure coefficients given.
EN1991-1-4 Actions on structures, wind actions
Wind speed map of EU is based on 10 minute mean and has 5 terrain categories.
Because no one method for the calculation of wind pressures and force coefficient was agreed across all member states a number of clauses allow alternative methods and there are a number of informative annexes. The use of these and the procedures are confirmed in the relevant National Annex.
A standard procedure for the calculation of wind loads applies to those structures whose structural properties do not make them susceptible to dynamic excitation. A detailed procedure is given that applies to those structures which are likely to be susceptible to dynamic excitation and which fall outside of the scope of the standard procedure.
When considering wind loads on static building structures there are many similarities between EN1991-1-4 and BS6399-2, but there are also a number of major differences. For example the Eurocode uses different methods and procedures for calculating the wind loads, e.g. instead of using gust wind speeds, which are familiar to UK designers, the Eurocode works in terms of peak velocity pressures. Therefore, where in the UK we might currently consider a design wind speed of, say, 40m/s, the Eurocode would use a peak wind velocity pressure of 980 Pascals.
In general the EN approach gives lower wind speeds but higher pressures than the equivalent BS6399.
There can also be changes of wind speed depending on siting of crane. If the site is likely to be either in an unusual location (near the coast, close to an escarpment) or in a highly congested site it may be prudent to ask the crane supplier for advice.
A1.3
DESIGN FOR FATIGUE
A number of structural elements may need to be designed with fatigue in mind. Re-useable ties, and connections may be required to resist variable forces over a period of time. For other elements, the number of load cycles may be too low to warrant fatigue checks. Elements should either be: •
installed according to the manufacturers instructions (if proprietary ties and fixings are used)
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• •
checked against fatigue design in BS 5950 Structural use of steelwork in building and BS7608 Code of practice for fatigue design and assessment of steel structures checked against the fatigue loading actions given in BSEN 1991-4 . The methods prescribed by BSEN 1991-4 Actions induced by machinery are compatible with the provisions of EN13001-118 to facilitate the exchange of data with crane suppliers. Actions induced by cranes are classified as variable and accidental actions and are represented by various models. Guidance for the determination of the following load arrangements is provided • vertical loads from monorail hoist • blocks underslung from runway beams • horizontal loads from monorail hoist • blocks underslung from runway beams • vertical loads from overhead travelling cranes • horizontal loads from overhead travelling cranes • multiple crane action. When not supplied by the crane manufacturer, dynamic amplification factors are given for vertical loads and advice is included on treating wind actions for cranes located outside buildings. Fatigue damage equivalent loads are used to classify fatigue actions in relation to a load effect history parameter.
A1.4 TEMPORARY STRUCTURAL DESIGN (TIES AND ANCHORS) Tie and Fixing Design Tie forces are generally obtained from the crane manufacturer, although single tie installations are relatively simple to calculate (see Figure A1.1).
Fig. A1.1 – Single Tie Calculation
Multiple tie installations are much more complex and the manufacturer will need to know the following information in order to calculate the tie loads:-
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• • • • • •
Crane model Jib length Proposed height under hook Proposed number of ties Proposed tie heights above base level Maximum out-of-service wind speed (see Section 3.1)
The manufacturer will then supply a force acting on the centre of the tower at each tie level, together with a moment (if applicable). This has to be resolved into reaction forces at the points of attachment of the tie legs to the building or supporting structure. A typical arrangement is shown at Figure A1.2.
Figure A1.2 – Typical Tie Loads and Reaction Forces
It may not be possible to adopt ideally positioned tie fixing points on the adjacent structure, and alternative tie arrangements have to be used. Whilst this allows flexibility in the positioning of the crane and ties, the effect of close fixing points or acute tie leg angles is to significantly increase the loads in both the tie legs and attachment points to the adjacent structure. This can bring further problems in that the structure to which the crane is being tied may well not be able to accept point loads of such magnitude without substantial and costly reinforcement. Such reinforcement of the supporting structure is also likely to incur delays to the construction programme. To avoid such problems it is essential that tower crane tying is considered at the earliest possible stage in the planning process. The method of permanent works construction may well have a significant effect on tie design. The current trend for office blocks of using a slip formed concrete core with steel framing to support floors and walls often means that a tower crane has to be tied twice at the same level. Initially using long ties supported by the core and then substituting shorter ties attached to the walls or floor slabs, once the steel frame is in position, to allow internal construction to proceed. Another consideration when positioning ties is their vertical location on the structure. There is frequently a requirement for cladding to be installed on the façade of a building whilst the tower crane ties are still in place. On some cladding systems it is possible to leave out panels for later installation but this is not always possible and it is of great advantage if the tie fixings can be
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positioned to allow the tie legs to pass through openings such as windows or breaks in the cladding. Unfortunately this may involve raising the tie fixing above floor slab level, which may in turn put an unacceptably high moment into the slab, necessitating the provision of a temporary column between adjacent floors to take out the moment. The positioning of ties in both the horizontal and vertical planes can be quite complex, involving reconciliation of the often conflicting requirements and limitations of the crane manufacturer, building designer and construction method. Providing effective solutions often requires creative thinking by the tie designer and effective liaison with the tower crane supplier, temporary works designer and building designer. Minor alterations to the design of the structure at an early stage to accommodate tie loads are often more cost effective and efficient than adding additional temporary bracing as the crane arrives on site.
A multi-story office block was being constructed using a slipformed concrete core with steel framing forming the support for floors and curtain walling. Tower cranes were used on either side of the building to service the construction of the steel frame. These cranes were climbed as the construction of the core progressed and once they had reached their maximum freestanding height of 60m they were tied to the alreadyconstructed building. However due to the nature of the building construction and the magnitude to the tie forces it was not possible to tie to the steel frame at the face of the building and substantial horizontal steel bracing had to be installed to take the tie loads from the face back to the core of the building. This considerably increased the cost of tying the cranes and led to delays in the construction programme which could have been avoided by effective consultation at the early planning stages of the project.
Formatted: Bullets and Numbering
Design and Manufacture of Bespoke Tie Components Where a proprietary tie system supplied by the tower crane manufacturer is not being used bespoke ties will generally have to be designed and manufactured. The design should be carried out by a competent engineer with experience in the design of steel structures. The design should be carried out to a suitable design standard such as BS2573: 1983 Rules for the design of cranes Part 1 Specification for classification, stress calculations and design criteria for structures. It should be born in mind that the tie components will be subject to cyclic loading and that the fatigue strength may well need to be considered. The ends of the pin jointed tie legs are usually attached to the supporting structure by a fabricated bracket through which the joint pin passes. Design of the interface between the bracket and the supporting structure should be agreed between the tie designer and the permanent works designer. Fixing may be by several means such as welding or bolting to the permanent steelwork or embedment in concrete structures, or by fastening using mechanical or chemical anchors into concrete. It is important that such fixings are designed correctly with an adequate factor of safety. As tower crane ties are safety critical components and their failure may well result in catastrophic collapse of the crane it is essential that the tie components are manufactured according to the designer’s specification and that the workmanship is of sufficient quality to ensure the integrity of the tie system. Designers should ensure that sufficient information is given on detail drawings. This should include:• • • •
Material specifications Welding process Filler materials Pre-heat (if required)
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• • • • • •
Weld fit up Dimensional tolerances Inspection requirements during and after manufacture (including NDT) Fit of screw threads (particularly important to avoid fretting on adjustable tie legs) Assembly instructions Protective finish. Formatted: Bullets and Numbering
Tie Installation The installation of tower crane ties should be considered in the overall planning for the erection of the crane (See 4.1.4). Particular problems that are likely to be encountered are those of access for personnel carrying out installation of tie components. As part of the planning process a risk assessment should be carried out to evaluate the risk of falling, whether that risk can be eliminated and if it cannot (as is likely to be the case) the control measures that are required to minimise that risk. The Work at Height Regulations 2005 specify that collective measures (e.g. guard rails) should be given priority over personal protection measures (e.g. safety harnesses). If it is necessary to adopt personal protection measures guidance can be found in BS 8437:2005 - Code of practice for selection, use and maintenance of personal fall protection systems and equipment for use in the workplace. When installing ties, measures should be taken to ensure that the crane tower remains vertical within the limits set by the manufacturer. To this end, adjustable tie legs provide a ready means of plumbing the tower after tie installation. Tower verticality is especially important during climbing as any substantial deviation from the vertical can affect the hook radius specified for balance by the manufacturer. Once ties have been installed and before the crane is put back into service the crane, ties and attachments to the supporting structure should be subjected to a thorough examination by a competent person (See section 4).
Foundation elements Anchors and other structural elements that connect the tower crane to the foundations must be properly designed. Most tower cranes are supplied with proprietary anchor fixings.
A1.5
PERMANENT WORKS DESIGN
The tie leg arrangement shown in Fig. A1.2 provides relatively low reactions and tie leg forces. Frequently however, it is not possible to adopt such advantageous tie fixing points on the adjacent structure, and tie arrangements such as those shown in Fig. A1.3 have to be used. Whilst this allows flexibility in the positioning of the crane and ties, the effect of close fixing points or acute tie leg angles is to significantly increase the loads in both the tie legs and attachment points to the adjacent structure. In some cases these can be in the order of 500kN. This can bring further problem in that the structure to which the crane is being tied may well not be able to accept point loads of such magnitude without substantial and costly reinforcement. Such reinforcement of the supporting structure is also likely to incur delays to the construction programme. To avoid such problems it is essential that tower crane tying is considered at the earliest possible stage in the planning process. The method of construction may well have a significant effect on tie design. The current trend for office blocks of using a slip formed concrete core with steel framing to support floors and walls often means that a tower crane has to be tied twice at the same level. Initially using long ties supported by the core and then substituting shorter ties attached to the walls or floor slabs, once the steel frame is in position, to allow internal construction to proceed.
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Another consideration when positioning ties is their vertical location on the structure. There is frequently a requirement for cladding to be installed on the façade of a building whilst the tower crane ties are still in place. On some cladding systems it is possible to leave out panels for later installation but this is not always possible and it is of great advantage if the tie fixings can be positioned to allow the tie legs to pass through openings such as windows or breaks in the cladding. Unfortunately this may involve raising the tie fixing above floor slab level, which may in turn put an unacceptably high moment into the slab, necessitating the provision of a temporary column between adjacent floors to take out the moment. The positioning of ties in both the horizontal and vertical planes can be quite complex involving reconciliation of the often conflicting requirements and limitations of the crane manufacturer, building designer and construction method. Providing effective solutions often requires creative thinking by the tie designer and effective liaison with the tower crane supplier, temporary works designer and building designer. Minor alterations to the design of the structure at an early stage to accommodate tie loads are often more cost effective and efficient than adding additional temporary bracing as the crane arrives on site.
Fig. A1.3 – Alternative Tie Layouts
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APPENDIX 2 A2.1
Foundations
SITE INVESTIGATION AND DETERMINATION OF GROUND PARAMETERS
The site investigation information should enable an estimate to be made of the bearing capacity of the near surface ground and/or the capacity of piles. The SI (whether the SI for the main construction or specifically for the TC) should therefore provide information on: • soil descriptions • shear strengths (drained and undrained) • position of the water table and any drainage The information should be relevant to the location of the TC base and time of TC installation. Design parameters for the TC base must be maintained during installation and throughout the service life, so that activities which may compromise them should be avoided. If construction processes are likely to significantly alter the nature of the ground or properties, this needs to be taken into account. Such processes may occur if a deep basement, services or other foundations have been or will be excavated or if some form of ground treatment has taken place. The nature and extent of the SI should be determined and supervised by a competent person, who has been appraised of the requirements of the tower crane base design.
A2.2
DESIGN
The design of foundations to carry the complex loading applied by a tower crane requires experience consult a specialist. Whilst normal construction tolerances may be assumed to be accounted for in the partial factors applied, it will be prudent for the designer to specify the tolerance that the deign can withstand. For example, significant out-of-level in a gravity base may increase the maximum in-service moment.
A2.2.1
Principles
The TC base must provide stability and serviceability (limited deformation) over the life of the TC. In general UK geotechnical practice, whether for deep or shallow foundations, these two requirements have both been satisfied by ensuring a large factor of safety against overall failure. An explicit determination of deformation has in the past been rare. In this guide, two approaches are outlined for shallow foundations: – traditional design based on allowable bearing stresses – BSEN1997 compliant design The base must be designed to resist three key failure modes or limit states described by, respectively: – Loss of stability (equilibrium) – Failure of the soil (geotechnical capacity) – Failure of the structural foundation In some cases failure in or movement of the soil due to loading or stability considerations not imposed by the crane may also be an issue. The design of a piled base follows a similar pattern, but generally the tower crane base designer does not design the piles; the example shows how the information to be passed to the pile designer is obtained
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A2.2.2
Factors of safety
For each of the failure modes a minimum factor of safety is recommended in this report for each of the two approaches. The factors have been selected as representing current practice in accordance with British Standards or have been derived from the principles of BSEN1997-1, and have been based on current practice.
Stability (equilibrium) The same factor of safety has been used for both approaches, derived from BSEN 1997-1. The partial factors recommended by BSEN 1997-1 relate to the effects of the destabilising actions and the stabilising actions. Different partial factors relate to actions that are permanent or variable (P/V), favourable or unfavourable (FAV / UNF). Some manufacturers may require specific factors of safety to be applied to the stability of their cranes. The loading on the tower crane base can be considered as follows:
Loading applied by mass of crane. P, FAV
Moment - due to structure, hook load and wind P/V UNF
Horizontal load V, UNF
Load applied by mass of base P, FAV Figure A2.1
Loading applied to base by tower crane on cast-in anchors
Annex A2, BSEN 1997-1 recommends partial factors to be used in the calculation for the EQU limit state (overall stability). The factors are dependant on the type of loading applied - favourable, unfavourable, variable or permanent as shown in table A2.1. Table A2.1 Action Permanent Favourable Permanent Unfavourable Variable
EQU limit state applied to stability of tower crane gravity base Partial factor Tower crane base application recommended Moment, vertical and All load components horizontal loads specified specified 0.9 Vertical (WT) Weight of crane and base 1.1
Moment, horizontal (MA, H)
0
Vertical
Moment caused by crane weight and wind, horizontal wind load Actions caused by weights
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Favourable Variable Unfavourable
1.5
Moment, horizontal
Actions caused by winds
The stability calculation requires that the factored overturning moment due to the design actions (Mo) is less than the factored restoring moment (Ms): Mo x1.5 ≤ Ms x 0.9 i.e. Ms ≥ 1.67 Mo where the base is a square of side length L and depth D. When the partial factors associated with the effect of each action are combined (and neglecting the effect of any soil interaction in the stability) the minimum overall factor recommended is 1.67 (1.5 ÷0.9). Where all load components are specified separately, it should be possible to separate the variable and permanent elements of the unfavourable action and apply partial factors accordingly. In these cases the overall factor may be slightly lower than 1.67. Where a piled bases is used, the stability check requires a split of loads depending on whether the pile is in tension or compression. Table A2.2 Foundations for tower cranes on piled bases: values of partial factors for stability check.
For max pile load (o-o-s) Min pile load (o-o-s)
Vertical load Permanent unfavourable 1.1 Permanent favourable 0.9
Moment and horizontal load Variable unfavourable 1.5 Variable unfavourable 1.5
Geotechnical capacity In a traditional design, this would involve comparison of the calculated maximum stresses with allowable stress determined either from the SI or from BS 8004. In some cases the allowable stress may have been calculated from the ultimate bearing capacity with a large factor of safety applied to restrict settlements. In this guide, two approaches are described: – An allowable stress method in which the calculated soil stresses are compared with allowable bearing capacity provided by either the SI report or from BS 8004. – Design by calculation, in accordance with BSEN1997-1, in which both soil resistance and loads are factored by partial factors. The section on BSEN 1997-1 compliant design indicates the minimum partial factors to be used. Where design is carried out using an allowable bearing capacity approach, a factor of safety of 3 should provide the required margin against both ultimate and serviceability failure. The possibility of an increase in water level may need to be considered. This would reduce geotechnical capacity as well as require the use of a submerged density of concrete in the stability calculation. The design of piles for piled bases is often carried out by the piling contractor, who is supplied with the required working loads in tension and compression. It is recommended that the pile designer is instructed to ensure a minimum factor of safety of 2.5 for working compression loads compared with ultimate capacity. Where a pile test close to the location of the tower crane is available, the factor required for compressive capacity may be reduced to 2. A factor of 3 for the tensile capacity should apply in all cases. Tension piles will always require reinforcement that must be fully tied into the pile cap.
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Structural capacity The structural design of the foundation elements should be carried out according to current practice. All loading cases, using appropriate partial factors, should be examined to find the most onerous case. The worked examples 1-3 of reinforced concrete foundations that follow are carried out in accordance with BS8110. However, because the loads given by the manufacturers are often not fully broken down into dead, imposed and wind loads, care must be taken in choosing appropriate γf factors for the ultimate limit state (BS8110 Table 2.1). In particular, note that the wind load envisaged in Table 2.1 is the maximum predicted e.g. 1 in 50 year value, which corresponds to the out-of-service condition. The wind load included in the in-service loads is considerably lower but more likely to occur, as the wind speed is limited for the operation of the crane. It is considered inappropriate to use case 3 of Table 2.1 'dead and imposed and wind' - for the in-service condition. Rather, in-service should be treated as case 1, and out-of-service as case 2, giving values of γf as shown in Table A2.3. If only a single set of "worst case" loads is given, the higher set of partial factors should be used. Table A2.3 RC foundations for tower cranes on cast-in anchorages: values of γf for design to BS8110, if applied loads have not been broken down into dead, imposed and wind loads. BS8110 Table 2.1, case In-service (case 1) Out-of-service (case 2) γf γf Dead weight of Adverse 1.4 1.4 foundation Beneficial 1.0 1.0 Applied vertical Adverse 1.6 1.4 load Beneficial 1.0 1.0 Applied moment and horizontal load 1.6 1.4 Often, the depth required to accommodate the tower anchorages will mean that minimum reinforcement is adequate. Bar spacing may be determined by a minimum aperture from BS8110 and must take account of the need for safe walking across the reinforcement. Structural elements must be designed with sufficient capacity to mobilise the factor of safety required for stability. E.g. in a piled base, the top reinforcement must be able to mobilise the tension pile load required for stability. This may require a larger area of reinforcement than calculated from the working load case. Where any element of the base (e.g. pile caps) utilises part of the permanent works the design must be approved by the permanent works designer.
A2.2.3
Foundation construction issues
The required concrete strength for the design, at the time the tower crane is to be erected, may determine the concrete specification. It may be critically important that concrete cubes have been taken from the construction of the foundation as these may need to be tested if a problem arises. Dimensional constraints such as: • minimum pile spacing for construction tolerance and pile capacity, • minimum depths based on fixing anchors or bolts, • minimum beam widths for rails to accommodate rails and fixings, may impact on the size of the structural members selected. Placement of cast-in anchors: the first section of the tower3 is often connected to the anchors before placement to ensure accurate positioning of the anchors; sometimes a template is used. Anchors need to be supported e.g. with stools or concrete blocks, at the correct level whilst constructing reinforcement 3
Sometimes called the base section
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and pouring concrete. The thickness of blinding concrete beneath the base must be sufficient to support the weight of the anchors and (if used) the first section of tower during base construction.
A2.3
TRADITIONAL DESIGN OF TYPICAL BASES
Three worked examples of reinforced concrete bases are given: a pad (or gravity) base, a piled base for a tower crane on cast-in anchors; and parallel beams supporting a rail-mounted crane. These are intentionally simple examples, to illustrate the design principles. The pad and piled bases are square and concentric; the same principles can be applied to more complex bases. For the beams, a simple design method is shown that could be replaced by more sophisticated calculation methods. Reinforced concrete design in the examples is carried out to BS8110, because this reflects current practice at the time of publication of this guide. In time, this code will be superseded by BSEN 1992. The symbols used in worked examples 1-3 generally follow BS8110, and not the alternative nomenclature of BSEN 1992 or BSEN 1997.
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Symbol L s D d e V
Length of side of pad base or pile cap (ex 1&2) Length of ‘nominal pad base’ for rail-mounted bogie (ex 3) Spacing of piles Overall depth of base Effective depth (BS8110) Eccentricity Vertical load applied to base by tower crane; shear force due to ultimate design loads (BS8100); design ultimate value of concentrated load (BS8110) Horizontal load applied to base by tower crane
H Moment applied to base by tower crane MA Slewing torque applied to base by tower crane MT Overturning moment Mo Restoring moment Ms Design ultimate bending moment (BS8110) M Self weight of pad base or pile cap G Total vertical load on foundation W Horizontal load per pile due to horizontal load, H hH Horizontal load per pile due to slewing torque, MT hT Partial factor for load for RC design to BS8110 γf Abbreviations Centre line c/l In-service i-s Out-of-service o-o-s Site investigation SI
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EXAMPLE 1. Reinforced concrete square gravity base This is a simple example of a gravity base – a square base, with the mast concentric and in the same orientation as the base. The design principles can be applied to more complex bases – for example rectangular or other shapes, an eccentrically placed mast or oriented at an angle to the main axes of the base. The design is in 3 steps, corresponding to 3 possible modes of failure: a) stability: select size of base to achieve a factor of safety of ≥1.67 (See Section A2.2.2.) b) geotechnical capacity: check bearing pressure, using an “allowable stress” method (BS8004) with working (unfactored) loads c) structural design : reinforced concrete design to BS8110, using factored loads It should be noted that in due course BSEN 1992 and BSEN 1997 will supercede the current UK codes. Crane type: Liebherr 200HC, 49.6m under hook L Foundation loadings supplied: Load Unit Inservice (i-s) Moment, MA kNm 2100 Vert. load, V kN 710 Horizontal kN 47 load, H
Out-ofservice (o-o-s) 3680 680 82
L
V MA
H Cast-in length of anchors 1125mm (±15mm) Minimum depth of base: 1.5m Mast dimensions: 1.981 x 1.981m crs
G
O
D ≥1.5m
Site investigation report supplied allowable soil bearing pressure (i.e. allowable increase in bearing pressure) for shallow foundations as 175kN/m2 and the ground water level is 10m below base foundation level. By inspection, design for o-o-s loads.
a) Stability Factor of safety required for rotation about one edge ≥ 1.67 Take worst case, in this example the o-o-s loading. Determine base size to provide stability, i.e. for square base determine length L for minimum depth 1.5m: Destabilising moment, Mo
= MA + HxD = 3680 + 82x1.5 = 3803 kNm
Taking density of concrete, γconc = 24 kN/m3
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COMMENT If the ground water level is above the foundation level for the base, or is liable to be so e.g. if flooding is likely, then the submerged density of concrete should be used. Additionally the allowable soil bearing pressure may be reduced. Self weight of base, G = L x L x 1.5 x 24 = 36 L2 kN Vertical load from TC, V = 680 kN Stabilising moment, Ms = (G+V) x L/2 = (36L2 +680) L/2 = 18L3 + 340L kNm For overall FOS ≥1.67, Ms ≥ 1.67 x Mo i.e.
18L3 +340L ≥ 1.67 x 3803 = 6351 kNm
By iteration minimum base size, L = 6.2m Check: Self-weight of base, G = 6.2 x 6.2 x 1.5 x 24 = 1384 kN i.e. Total vertical load, W=V+G = 680 + 1384 = 2064 kN Stabilising moment, Ms = 2064 x 6.2 / 2 =6398 kNm Factor of safety against rotation about an edge = 6398 / 3803 = 1.68 (>1.67)
b) Geotechnical capacity Maximum bearing pressure occurs when the moment Mo occurs about a diagonal axis. Using Meyerhof's (1953)4 construction for the transformation of a non-uniform loading distribution to an equivalent uniform pressure over a reduced rectangular area5, e =ex=ey= Mo x sin45° / W = 3803 x 0.7071 / 2064 = 1.303m
Mo
ex O
Side length of equivalent square area = L - 2e = 6.2 - 2 x 1.303 = 3.594m
L-2ey
ey
L=6.2m 4
Meyerhof G G (1953). The bearing capacity of foundations under eccentric and inclined loads. Proceedings of 3rd International Conference on Soil Mechanics and Foundation engineering, Zurich, vol 1, pp 440-445.
5
The Meyerhof construction is not the only way in which to deal with such loads. For very stiff soils it may in some cases be necessary to use a non-uniform distribution.
57
Equivalent uniform pressure pM= W / (L-2e)2 = 2064 / 3.5942 = 160 kN/m2 Take density of soil to be 18kN/m3 Overburden pressure ≈ 1.5 x 18 = 27 kN/m2 ∴net bearing pressure under equivalent area = 160 – 27 = 133 kN/m2 This is less than the allowable increase in bearing pressure quoted in the SI so the base size required for stability also satisfies the requirements of geotechnical capacity. COMMENT If the required bearing pressure was too high the base size could be increased or an alternative foundation type chosen. For a traditional bearing capacity analysis the maximum bearing pressure calculated can be compared with either: – Allowable bearing capacity from SI or BS8004 – Bearing capacity calculated from standard formulae and measured parameters, using a factor of safety of 3 to calculate the allowable bearing pressure COMMENT Friction and adhesion beneath the base are generally assumed to be sufficient to resist the horizontal forces arising from horizontal load and slewing torque. Often a calculation check is not required, although this could be carried out. Occasionally adjacent structures are available to resist the horizontal forces, but passive resistance of the soil against the sides of the base should not be relied upon - the base may be constructed in formwork and backfilled or subsequent construction activity may require excavation close to the base (section 3.3).
c) Structural design COMMENT Concrete strength: The crane manufacturer may specify a minimum concrete strength. BS8110 recommends a minimum fcu=30N/mm2. Concrete of higher strength than this is often used. This may be because it is convenient to use one of the standard mixes for the site; or it may be in order to achieve sufficient early age strength (because the tower crane is often erected very soon after construction of the base). Whatever concrete is used, the strength assumed by the design must be reached before the tower crane is erected; loads during erection may be a worst case, and out-of-service loads could occur at any time. Note that higher strength concrete is more prone to shrinkage cracking, because of its higher cement content (see comment below about cracking adjacent to the anchors). Take concrete strength, fcu=30N/mm2 and steel, fy=460N/mm2
58
Bending and shear will be calculated per metre width of the base (the same in both directions because it is a square base with a centrally located mast). The worst case is required – i.e. maximum ground bearing pressure, which will occur when the overturning moment Mo occurs about a diagonal axis. Meyerhof’s equivalent uniform pressure distribution will be used as in the geotechnical design, but this time with the BS8110 partial factors (γf) applied (see Table A2.3). Both i-s and o-o-s will be calculated to find the worst case. In-service: Mo = MA + HxD = 2100 + 47x1.5 = 2171 kNm W = V + G = 710 + 1384 = 2094 kN For ULS Mu = γf x Mo = 1.6 x 2171 = 3474 kNm Wu = γf x W = 1.0 x 2094 = 2094 kN e = 3474 x sin45°/ 2094 = 1.173 m L - 2e = 6.2 – 2x1.173 = 3.854 m
2.552m
Equivalent uniform pressure pM= Wu / (L-2e)2 = 2094 / 3.8542 = 141 kN/m2 3.10m Out-of-service: Mo = MA + HxD = 3680 + 82x1.5 = 3803 kNm W = V + G = 680 + 1384 = 2064 kN For ULS Mu = γf x Mo = 1.4 x 3803 = 5324 kNm Wu = γf x W = 1.0 x 2064 = 2064 kN
0.99m 2.11m 36kN/m2 317kN/m22
e = 5324 x sin45°/ 2064 = 1.824 m L - 2e = 6.2 – 2x1.824 = 2.552 m Equivalent uniform pressure pM= Wu / (L-2e)2 = 2064 / 2.5522 = 317 kN/m2 ∴design for o-o-s case. Using γf =1.0, factored self weight of 1.5m deep base = 1.0 x 1.0 x 1.5 x 24 = 36 kN/m2 Critical section for design is at the face of the mast – take c/l of legs. Dimension from c/l of legs to edge of base: = 3.10 – 1.981/2 = 2.11 m Design ultimate moment, M = 317 x 2.112/2 – 36 x 2.112/2
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= (317 – 36) x 2.112/2 = 626 kNm/m Bottom reinforcement: Take bottom cover = 40mm and allow for T25 bars. Effective depth d = 1500 - 40 - 25 -25/2 = 1422 mm
(say 1420 mm)
Lever arm, z= d { 0.5 + √(0.25 - K/0.9)} K= M / bd2 fcu= 626 x 106 / 1000 x 14202 x 30 = 0.010 ∴z = d{0.5 + √(0.25-0.010/0.9)} = d x 0.99 so use z = 0.95 d Area of tension reinforcement As = M / 0.95 fy z = 626 x 106 / 0.95 x 460 x 1420 x 0.95 =1062 mm2/m Minimum reinforcement = 0.13% = 1000 x 1500 x 0.13 /100 = 1950 mm2/m ∴provide minimum reinforcement of 1950 mm2/m (e.g. 2-way T20 at 150 crs) Distribution of reinforcement (BS8110, 3.11.3.2) lc = ½ spacing between legs = 1.981/2 = 0.99 m c= width of leg (ignored), d = effective depth = 1.42 m ∴(3c/4 + 9d/4) = 9 x 1.42 / 4 = 3.195 m ∴ lc < (3c/4 + 9d/4) so reinforcement should be uniformly distributed. COMMENT Here 150mm centres has been chosen for safety reasons - steel fixers and concreters are likely to stand on both bottom and top mats of reinforcement. Top reinforcement: Under the maximum design moment for the base, positive ground bearing pressure acts under one corner of the base and there is zero pressure under the remainder – including all the base beyond the tension legs of the mast. Maximum hogging moment in the base is therefore due to self weight and occurs at the tension legs of the mast. Self weight is adverse and so γf =1.4 M = 1.4 x 36 x 2.112/2 = 112 kNm/m << design sagging moment above. ∴provide minimum top reinforcement of 1950 mm2/m (e.g. 2-way T20 at 150 crs) (As for bottom reinforcement, this should be uniformly distributed.)
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COMMENT In all bases with cast-in anchors, top reinforcing bars should be placed close to both sides of the anchors, in both directions. Bars should be displaced where necessary, and not cut. This is done to prevent the formation of cracks adjacent to the anchors, ensuring that the concrete provides full lateral support to the anchors, right up to the top level of the concrete.
Shear: Critical section is at the face of the mast – take c/l of legs. Shear force due to ultimate design loads, V = 317 x 2.11 – 36 x 2.11 = (317-36) x 2.11 = 593 kN/m width ∴nominal design shear stress, v = V/bvd = 593 x 103 / ( 1000 x 1420) = 0.42 N/mm2 < 0.8 √fcu = 4.38N/mm2 (shear enhancement applies) Check shear where enhancement does not apply, using simplified method (BS8110 3.4.5.10) – i.e. on vertical section at distance d from face of mast. Distance from this section to edge of base = 2.11 – 1.42 = 0.69 m ∴V = (317 – 36) x 0.69 = 194 kN/m width Design concrete stress, vc ≈ 0.23 N/mm2 (from BS8110, Table 3.8 using the formula given) v = V/bvd = 194 x 103 / ( 1000 x 1420) = 0.14 N/mm2 < vc ∴no shear reinforcement required. COMMENT: punching and pull-out shear In most cases, the cast-in anchors will have been designed and fabricated by the crane manufacturer. If the manufacturer’s recommendations regarding shear reinforcement are followed, punching and pull-out shear should be satisfactory. However, there will be cases where the mast legs are close to the edge of the base e.g. if the mast is offset from the centre of the base or oriented at an angle to the axes of the base. This can reduce the length of the punching shear perimeter (because only 2 or 3 sides, not 4, are available). In such cases punching and pull-out shear must be checked. For this, the details of the cast-in anchors will be required.
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FINAL BASE DESIGN 6.2m x 6.2m x 1.5m deep square reinforced concrete base. Design concrete strength, fcu=30N/mm2 and steel, fy=460N/mm2 Cast-in anchors to be supplied
Section through gravity base, showing main bars (omitted: any shear reinforcement specified by crane manufacturer or required for pullout or punching, chairs to support top mat) Bars displaced & positioned close to both sides of anchors (in both directions)
T20-150
1500mm
T20-150
T20-150 Supports for anchors (not hollow sections
T20-150
6200mm
Blinding thickened if necessary to support anchors
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EXAMPLE 2. Piled square base This is an example of a simple piled base – a square pile cap on a square 4-pile group, with piles capable of tension as well as compression. The mast is concentric and oriented in the same direction as the axes of the base. The principles can be applied to other arrangements – different numbers of piles in a symmetrical or non-symmetrical group, compression piles only, an eccentrically located mast or one oriented at an angle to the axes of the base, etc. The design is in 4 steps, corresponding to three possible modes of failure: (a) select size of base and calculate working pile loads (b) stability – calculate ultimate pile loads required to provide the required factor of safety (see A2.2.2) (c) structural design of pile cap (to BS8110) (d) list information to be provided to pile designer Crane type: Liebherr 200HC, 49.6m under hook Foundation loadings supplied: Load Unit Inservice (i-s) Moment, MA kNm 2100 Vert. load, V kN 710 Horizontal load, H kN 47 Slewing torque, MT kNm 290
s Out-ofservice (o-o-s) 3680 680 82 -
L 1.981m
L
Cast-in length of anchors 1125mm (±15mm) Minimum depth of base: 1.5m Mast dimensions: 1.981 x 1.981m crs Permanent works piles will be 600mm diameter in stiff clay ground conditions. This diameter will allow piles for the tower crane that can be designed to carry tension as well as compression. Working pile capacities of +1200kN (compression) and –400kN (tension) have been initially identified for the permanent works piles.
600mm diameter pile with 75mm cover to pile reinforcement
a) Base size and working pile loads (vertical and horizontal) Minimum spacing of piles is determined by ability to construct piles, and avoiding clashes between anchors and pile reinforcement:
75mm
i) Minimum pile spacing 3xdiameter, i.e. 3x600 = 1800mm 350mm 63
ii) To prevent clash between anchors and pile reinforcement: Mast size + anchor width + pile cage diameter, i.e. 1981 + 350 + (600-2x75) =2781mm ≈ 2.75m Therefore pile spacing, s should not be less than 2.75m c-c. Take worst case, in this example the o-o-s loading: Max pile loads occur when Mo is about diagonal axis Overturning moment, Mo = MA + HxD = 3680 + 82x1.5 = 3803 kNm Taking density of concrete, γconc = 24 kN/m3 Minimum pile cap weight, G = L2 x 1.5 x 24 = 36L2 taking the minimum depth D=1.5m Maximum pile working load Minimum pile working load
= W/4 + Mo / s √2 = (V+G) / 4 + Mo / s √2 = W/4 - Mo / s √2 = (V+G) / 4 - Mo / s √2
i.e. max and min working loads can be calculated from: (680 + 36 L2)/4 ± 3803 / s √2 Keeping pile cap as small as possible, L = s + 0.6 + 0.15 = s + 0.75 m To select base size, calculate pile loads for a range of pile spacing, s. s L Max working (V + G) /4 Mo / s √2 pile load m kN m kN kN 2.75 3.5 280 978 1258 3.25 4 314 827 1141 3.75 4.5 352 717 1069 4.25 5 395 633 1028 4.75 5.5 442 566 1008 Note, negative load indicates tension To match the pile loads with working capacities available on site (+1200kN and –400kN, see above), s=3.75m and L=4.5m have been chosen.
Min working pile load KN -698 -513 -365 -238 -124
s=3.75m Mo
i.e. the selected pile cap is 4.5 x 4.5 x 1.5m ∴self-weight of pile cap, G = 4.5x4.5x1.5x24 = 729kN and maximum pile working load = 1069kN minimum pile working load = -365kN (tension)
L=4.5m 1.981m
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Horizontal loads COMMENT: The horizontal load and slewing torque applied to the base by the crane must be transmitted to the ground. Passive resistance of the soil is unlikely to be reliable, especially if the pile cap is backfilled after casting against formwork. Occasionally adjacent structures will be available, but normally it must be assumed that the horizontal loads will be transmitted to the piles.
Horizontal pile loads due to slewing torque 1.875
Distance from centroid of pile group to each pile, r= 1.875 √2 = 2.65 m Let hT= horizontal load per pile due to slewing torque MT
hT MT
then 4hT x r = MT ∴hT = MT / 4r
r
Let hH= horizontal load per pile due to horizontal load H ∴hH = H/4 Max. total load occurs when ht and hh act in the same direction. This will act at the same time as vertical pile loads (compression or tension)
1.875
hT Horizontal pile loads due to MT
hH o-o-s loads: MT=0, H=82kN ∴horizontal load per pile, hH = H/4= 82/4 = 20.5kN i-s loads: MT=290kNm, H=47kN ∴horizontal load per pile = hT + hH= MT / 4r + H/4 = 290/(4 x 2.65) + 47/4 = 39.1kN i.e. max. horizontal pile loads are in service. Calculate concurrent i-s vertical pile loads: Mo = MA + H x D = 2100 + 47 x 1.5 = 2171 kNm W = V + G = 710 + 729 = 1439 kN
H
Horizontal pile loads due to H
Max/min i-s working pile loads = 1439/4 ± 2171/ (3.75√2) = 360 ± 409 = 769 or –49 kN
b) Determine ultimate pile loads for base stability Critical case for stability is overturning about the x-x (or y-y) axis, i.e. with Mo applied about the x-x (or y-y) axis. By inspection, o-o-s loads are worst case.
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G=4.5 x 4.5 x 1.5 x 24 = 729 kN W=V+G = 680+729 = 1409 kN Use partial factors according to BSEN 1997 equilibrium criteria (A2.2.2). Ultimate pile capacities required for stability are: Max load = 1.1 x W/4 + 1.5 x Mo/2s = 1.1 x 1409 /4 + 1.5 x 3803 / (3.75 x 2 ) = 387 + 761 = 1148 kN Min load = 0.9 x W/4 - 1.5 x Mo /2s = 0.9 x 1409 /4 - 1.5 x 3803 / (3.75 x 2 ) =317 – 761 = - 444 kN (i.e. tension) Ultimate compression stability load / working load = 1148 / 1069 = 1.07 Ultimate tension stability load / working load = 444 / 365 = 1.22 Normal pile design would use factors of safety of at least 2 (3 in tension), so piles designed for the working loads will provide adequate stability.
c) Structural design of reinforced concrete pile cap Design for maximum compression and tension pile loads, i.e. Mo applied about diagonal axis. Use the bending theory method i.e. consider pile cap as a beam spanning between 6
piles . Estimate the width available to be designed as a beam using the BS8110 recommendation relating to truss analogy method i.e. consider reinforcement within 1.5 times the pile diameter (1.5 x 600 = 900mm). 0.99m 1.26m ∴width of ‘beam’ = 375 + 900 = 1275mm 1.5m
(Note that alternatively a ‘beam’ width could be estimated from BS8110 clause 3.5.2.2 and Figure 3.6 but this formula isn’t strictly applicable because it is written for 1-way spanning slabs). By inspection, design for o-o-s loads.
0.885m 0.375m
Calculate pile loads using BS8110 partial factors (see Table A2.32). For compression pile loads, vertical loading is adverse so γf=1.4 for both vertical load and overturning moment. For tension pile loads, vertical loading is beneficial so γf=1.0 for vertical load and γf=1.4 for overturning moment. Max pile load
6
= 1.4 x W/4 + 1.4 x Mo / s√2 = 1.4 x 1409/4 + 1.4 x 3803 / 3.75√2
An alternative truss method is also shown in BS 8110.
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Min pile load
= = = =
493 + 1004 = 1497 kN 1.0 x W/4 - 1.4 x Mo / s√2 1.0 x 1409/4 - 1.4 x 3803 / 3.75√2 352 – 1004 = -652 kN
Use concrete strength, fcu=30N/mm2 and steel, fy=460N/mm2 i) Bottom reinforcement ‘Beam’ width (see above) = 1.275mm Self weight of beam, with γf=1.4, = 1.4 x 1.275 x 1.5 x 24 = 64 kN/m Max bending moment (at compressive leg of mast) M = 1497 x 0.885 – 64 x 1.262/2 = 1325 – 51 = 1274 kNm Effective depth, d ≈ 1500 – 75 – 20 – 20/2 = 1395 mm K = M/ (bd2fcu) = 1274 x 106 /(1275 x 13952 x 30) = 0.0171 ∴z = d{0.5 + √(0.25-k/0.9)} = d x 0.981 so take z = 0.95 d ∴As = M/ (0.95fyz) = 1274 x 106 / (0.95 x 460 x 0.95 x 1395) = 2200 mm2 (i.e. 2200/1.275 = 1725 mm2/m) minimum reinforcement = 0.13% = (0.13/100) x 1275 x 1500 = 2486 mm2 (i.e. 1950 mm2/m) ∴provide minimum reinforcement e.g. T20 at 150 crs ii)
Top reinforcement
Max design tension pile load = 652 kN In calculation of pile load, dead weight was factored x 0.9. As above take 'beam' width =1.275m ∴ Factored dead weight of pile cap = 0.9 x 1.275 x 1.5 x 24 = 41 kN/m ∴ Mmax = 652 x 0.885 + 41 x 1.262/ 2 = 577 + 33 = 610 kNm By comparison with bottom reinforcement, minimum reinforcement will be satisfactory e.g. T20 at 150 crs. Shear (see BS8110, clauses 3.11.4.3, 3.11.4.4 and 3.11.4.5) i) shear on vertical section across pile cap – critical section is 20% inside the face for shear enhancement, a strip of width 1.5 diameters each side of the pile can be utilised i.e. the same width as the ‘beam’ taken in designing bending reinforcement. Width =1.5 x 600 +375 = 1275mm
705mm = av
1.5m 120mm 67 885mm 375mm
180mm
Design for max pile load =1497kN (see above) Self-weight of pile cap is relatively small – ignored here. ∴shear force due to design ultimate load, V=1497kN design concrete shear stress, vc ≈ 0.23 N/mm2 (BS8110 Table 3.8 using the formula given) 20% of pile diameter = 0.2 x 600 = 120mm ∴distance from pile leg to shear plane, av= 885 – 180 = 705mm ∴shear enhancement factor = 2d/av = 2 x 1395/705 = 3.96 ∴enhanced vc = 3.96 x 0.23 = 0.91 N/mm2 Design shear stress, v=V/bvd = 1497 x 103/(1275 x 1395) = 0.84 N/mm2 (< enhanced vc) COMMENT av is defined in BS8110 as the distance from the face of the column to the critical shear section; however, in this example av has been taken from the c/l of the leg, to allow for some uncertainty in the effective area and depth of application of the load through the cast-in anchor.
75mm
375mm
∴length of one side of shear perimeter = (600-120) +75 = 480 +75 = 555mm ∴uo = 2x 555 = 1110mm
120 480
885mm
ii) punching shear around pile a) stress on perimeter uo: take the smallest perimeter, uo, to be 20% inside the face of the pile (see BS8110 Figure 3.23), i.e. 120mm inside face.
Assume cover to top reinforcement is 50mm; load is applied at cutoff level of pile i.e. 75mm above soffit of pile cap. ∴effective depth, d= 1500 – 75 – 50 – 20 – 20/2 = 1345mm
uo u 705mm
Design ultimate pile load, V=1497kN Maximum design shear stress, v=V/uod = 1497 x 103/ (1110 x 1345) = 1.00 N/mm2 < 0.8 √fcu = 4.38 N/mm2 b) stress on perimeter touching leg of mast ; for simplicity work to c/l of leg ∴shear perimeter, u= 2 x (885+35) = 2 x 1260 = 2520 mm Distance from edge of loaded area to shear perimeter, av=705mm (as above) ∴shear enhancement factor = 1.5d/av = 1.5 x 1345/705 = 2.86 design concrete shear stress, vc ≈ 0.23 N/mm2 (as above) ∴enhanced vc = 2.86 x 0.23 = 0.66 N/mm2
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Design shear stress, v=V/bvd = 1497 x 103/(2520 x 1345) = 0.44 N/mm2 (< 0.66, enhanced vc) COMMENT The calculation for punching shear around a pile, which has been carried out above, is not specified in BS8110. However, the peculiarities of a piled tower crane base make it important to check this case. Shear related to tension pile load – by inspection this is less onerous than the cases calculated for compression pile load. (iii) Punching and pull-out shear around legs of mast In most cases, the cast-in anchors will have been designed and fabricated by the crane manufacturer. If the manufacturer’s recommendations regarding shear reinforcement are followed, and if there is plenty of concrete between the mast legs and the sides of the pile cap, punching and pull-out shear should be satisfactory. However, piled bases are generally smaller than pad bases, so there will be more cases when punching and pullout shear around the legs should be checked e.g. if shear perimeters with only 3 sides are a possibility because a leg is close to the side of the pile cap. This is particularly likely if the mast is oriented at an angle to the main axes of the cap.
(d) Information for pile designer COMMENT: It can be argued that a full set of combinations of vertical and horizontal loads per pile should be listed, so that the pile designer can ensure that the worst combined case is covered. In practice, it is normal to give only the maximum loads. When, as in this example, there is a significant difference in the i-s and o-o-s maxima, both sets should be given. It is normal practice to give working pile loads to the pile designer. However, if the calculations show an ultimate pile load that would exceed the factor of safety for normal pile design (say 2-3 times the working load depending on compression or tension) then both should be given. Check whether ultimate pile loads need to be specified to pile designer (in addition to working loads): Compression: max working load = 1069 kN max ultimate load = 1497 kN ultimate / working = 1497 / 1069 = 1.4 Normal pile design uses a factor of safety of at least 2 in compression, so the ultimate compression load does not need to be specified for geotechnical design. Tension:
max working load = 365 kN max ultimate load = 652 kN ultimate / working = 652 / 365 = 1.8
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Normal pile design uses a factor of safety of at least 3 in tension, so the ultimate tension load does not need to be specified for geotechnical design. However, pile reinforcement MUST be sufficient to carry the ultimate load. The area of reinforcement calculated from the working load would not be sufficient (the pile designer would use γf of 1.4 or 1.6), so the ultimate tension load should be specified for the reinforcement design. Information for pile designer 4 no. 600mm dia piles, spacing 3.75m x 3.75m Working loads per pile: i-s o-o-s kN kN Max compression load 770 1070 Max tension load 50 365 Max horizontal load 39 21 Additional requirement: area of longitudinal reinforcement per pile must be sufficient to carry an ultimate tension load of 650kN.
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FINAL BASE DESIGN 4.5m x 4.5m x 1.5m deep square reinforced concrete pile cap with 4x600mm diameter piles at corners. Design concrete strength, fcu=30N/mm2 and steel, fy=460N/mm2 Cast-in anchors to be supplied
Section through pile cap, showing main bars (omitted: any shear reinforcement
specified by crane manufacturer; chairs to support top mat) 4500mm
1500mm
Bars displaced & positioned close to both sides of anchors (in both directions)
T20-150 T20-150
T20-150 T20-150 Supports for anchors
Blinding thickened if necessary to support anchors 1875mm
Pile reinforcement must have full bond in pile cap
1875mm
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EXAMPLE 3. RC beams for a rail mounted tower crane This example uses a simple method of designing beams to support rails; a short length of beam is designed as a ‘notional pad base’ supporting one bogie. The reinforcement arrangement is then provided throughout the full length of the beam. Note that the pad base would require only bottom reinforcement; but it is likely that some tension will be developed in the top of the beam (in the ‘span’ between the bogies). Therefore it is recommended that minimum reinforcement is provided in the top of the beams. In any case, top bars will be needed for fixing links. Alternatively, a more complex calculation method, such as a “beam on elastic foundation” can be used. With a rail mounted tower crane, stability is provided by kentledge on top of the chassis, so no stability calculation is required in the design of the foundation beams. The design therefore consists of 2 steps: a) Calculate the length of the ‘notional pad base’ for a selected beam width, using the allowable increase in bearing pressure7 and the working (i.e. unfactored) bogie load. This may require comparison of some trial sizes, including preliminary structural design. The initial choice of beam size may be influenced by the following considerations: • Minimum width and depth to accommodate rail fixings • Economical balance between concrete and reinforcement • Adequate depth for shear strength For this simple method, the ‘notional pad base’ should receive load from only one bogie; therefore the length of the ‘notional pad base' should be less than the bogie centres. b) Structural design of the beams using factored loads in accordance with BS8110. Crane type: Potain Topkit H20/14c, rail-mounted, travelling Unit Max bogie loadings
kN
In-service (i-s) 760
Out-of-service (o-o-s) 940
Dimensions: Rail centres 4.5m Bogie centres 4.5m Bogies: wheel centres 410mm Rail fixings at 300mm centres
7
In this example the allowable increase in bearing pressure has been quoted in the SI. Note that, for non-cohesive soils, bearing capacity is a function of foundation width.
72
Site investigation report supplied allowable increase in soil bearing pressure for shallow foundations as 200kN/m2 The foundation will consist of two parallel longitudinal beams, each supporting one rail, connected by RC ties at 4.5m centres. Preliminary calculations of steps (a) and (b) have indicated that a beam 1500mm wide x 450mm deep may be suitable in terms of bearing pressure and reinforcement. It is also large enough to accommodate the rail fixings. 4.5m Carry out full calculations for this beam width.
a) Calculate length of ‘notional pad base’ for beam 1500mm wide x450mm deep
1500mm Ties at 4.5m centres
4.5m
410mm m
410mm m
Max bogie load (o-o-s) = 940 kN Allowable increase in ground bearing pressure = 200 kN/m2 Allow for 1500 mm beams to be founded 300mm below ground surface.
450mm
length of beam for ‘notional pad base’
Taking soil density 18 kN/m3 ∴Weight of soil displaced = 18 x 0.3 = 5.4 kN/m2 ∴Gross bearing pressure available = 205.4 kN/m2
300mm
450mm
Self weight of beam = 0.45 x 24 = 10.8 kN/m2 Bearing pressure due to bogie load < 205.4 – 10.8 = 194.6 kN/m2 1500mm Length of beam required as ‘notional pad base’ = 940 /(194.6 x 1.5) = 3.22m This is less than the bogie centres (4.5m), so can be considered as a pad base supporting one bogie, in isolation from the adjacent bogie. Calculating length of beam required for i-s bogie load (760kN): Length of beam = 760/ (194.6x1.5) = 2.60m
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b) Structural design (to BS 8110) fy=460 N/mm2 Take fcu=30 N/mm2 For this temporary application 30mm cover is deemed sufficient (see BS 8110 Table 3.3). In this case manufacturers data has not given a split of loads for dead, imposed and wind loads, so use values of γf as recommended in Table A2.3:
i.e. factors used in this example are; in-service 1.6 1.4
Bogie load Dead weight of beam ∴design bogie loads:
i-s o-o-s
out-of-service 1.4 1.4
1.6 x 760 = 1216 kN 1.4 x 940 = 1316 kN
Length of beam and design bogie load are both greater for o-o-s case, so design for o-os as most severe. For RC design, self-weight of beam is balanced by ground bearing pressure in calculation of moments. ∴use net design ground bearing pressure= 1316 / (3.22 x 1.5) = 273 kN/m2 Cases to be calculated are longitudinal bending and shear, and transverse bending and
shear. 1316 kN
Longitudinal bending Max bending moment and shear force occur at centres of wheels. Design ultimate moment, M= 273 x 1.5 x 1.4052/2 = 404 kNm Allow for T10 links and T20 main reinforcement bars: ∴d = 450 – 30 –10 -20/2 = 400mm
1.405m
∴K = M/(bd2fcu) = 404 x 106 / (1500 x 4002 x 30) = 0.0561 ∴z = d [ 0.5 + √(0.25 – 0.0561/0.9)] = 0.93 d
0.41m
1.405m
3.22m notional pad base
∴As = M/(0.5fyz) = 404 x 106 /(0.95 x 460 x 0.93 x 400) = 2485 mm2 Use 8 - T20 (2510mm2)
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273 kN/m2
Top longitudinal reinforcement – provide minimum reinforcement i.e. 0.13% 1500 x 450 x 0.13/100 = 878mm2 Use 8 - T12 (905mm2) Longitudinal shear 273 kN/m2
Design shear force due to ultimate loads (at centres of wheels), V = 273 x 1.5 x 1.405= 575kN ∴vmax = 575 x 103 /(1500 x 400) = 0.96 N/mm2 (< 0.8 √fcu = 4.38)
0.4
1.005 1.405m
For enhanced shear strength close to point loads, the shear stress may be calculated at a distance d from the support (clause 3.4.5.10, BS8110) Shear force at a distance d=400mm from centreline of wheel: = 273 x 1.5 x 1.005 = 412 kN 100As/bvd = 100 x 2510 / 1500 x 400 = 0.418 ∴vc ≈ 0.5 N/mm2 (table 3.8, BS8110) v= V/bvd = 412 x 103 (/1500 x 400) = 0.687 N/mm2 ∴0.5vc < v < (vc + 0.4) ∴provide minimum links (applying BS8110 Table 3.7 for beams) Rail fixings are at 300mm centres so provide links at 300mm centres. Asv ≥ 0.4 bv sv / 0.95 fyv = 0.4 x 1500 x 300 / 0.95 x 460 = 412 mm2 (at 300mm centres) e.g. 6-T8 + 2-T10 (i.e. 3-T8+1-T10 links at 300mm crs which gives 459mm2) Transverse bending For simplicity, ignore width of rail; M = 273 x 0.752/2 = 76.8 kNm/m d = 450 – 30 – 10/2 = 415 mm 450mm 6
273 kN/m2
2
∴K= 76.8 x 10 / 1000 x 415 x 30 = 0.0149 ∴z = d [ 0.5 + √(0.25 – 0.0149/0.9)] = 0.98 d - use 0.95d ∴As = 76.8 x 106 /(0.95 x 460 x 0.95 x 415) = 446 mm2/m
0.75m
0.75m
Minimum reinforcement = 1000 x 450 x 0.13 / 100 = 585 mm2/m Arrangement of links will give one full width link (T8 or T10) at 300 crs; add 2 bobbed bars at 300 crs, to give total of 2-T8 + 1-T10 at 300 crs (598mm2/m)
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Transverse shear For simplicity, ignore width of rail; Max shear force (at centreline of rail) = 273 x 0.75 = 205 kN/m At centreline, vmax = 205 x 103 /(1000 x 415) = 0.49 N/mm2 (< 0.8 √fcu = 4.38)
273 kN/m2 0.335 0.415
Shear force at a distance d =415 mm from centreline: = 273 x 0.335 = 91.5 kN/m
0.75m
100As/bvd = 100 x 598 / 1000 x 415 = 0.14 ∴vc ≈ 0.34 N/mm2 (BS8110 Table 3.8) v= 91.5 x 103 /(1000 x 415) = 0.22 N/mm2
< vc
For transverse shear, we have a slab rather than a beam. As v
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Final Design 2x 1500x450mm beams, tied at 4.5m centres, cast at 300mm depth. Design concrete strength, fcu=30N/mm2 and steel, fy=460N/mm2 Cross-section showing possible arrangement of reinforcement: Note – reinforcement must be arranged so as to avoid rail fixings (especially if these are post-drilled). Nominal RC ties shown.
Rail fixings
RC ties at 4.5m c/c
150mm
8-T12 450mm
T8
T8
T8
T10 links in 4’s at 300mm crs
8-T20
2-T8 at 300 crs
1500mm
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A2.4
BASE DESIGN TO EUROCODE FOR GEOTECHNICAL DESIGN, BSEN 1997-1 AND 2
The current suite of BS Codes and Standards will, in due course, be almost entirely replaced by a system of Eurocodes and Standards (ENs ) published by BSI as BS ENs .
A2.4.1
Principles of EN 1997
The Eurocodes adopt, for all civil and building engineering materials and structures, a common design philosophy based on the use of limit states and partial factors; this is a substantial departure from much traditional British geotechnical design practice as embodied in BS Codes. BSEN 1990 defines limit states as “states beyond which the structure no longer fulfils the relevant design criteria”. While the adoption of the Eurocodes will become obligatory in all member states, the level of safety applicable in a state remains its concern. Therefore, such items as the value of partial (safety) factors are left to individual state determination through the ‘National Annex’, although indicated values are given in the main code. Eurocode 7 consists of two Parts: Part 1 (BSEN1997-1) - Geotechnical design – General rules and Part 2 (BSEN 1997-2) - Ground investigation and testing. BSEN1997-1 is not a detailed geotechnical design manual but is intended to provide a framework for design and for checking that a design will perform satisfactorily; that is, the structure will not reach a ‘limiting condition’ in prescribed ‘design situations’. The Code therefore provides, in outline, all the general requirements for conducting and checking design. It provides only limited assistance or information on how to perform design calculations. Understanding parameters and terminology Actions may be forces (loads applied to the structure or to the ground) and displacements (or accelerations) that are imposed by the ground on the structure, or by the structure on the ground. Actions may be permanent (e.g. self-weight of structures or ground), variable (e.g. imposed loads on building floors) or accidental (e.g. impact loads). Design values of actions (Fd) are calculated using the general equation:
Fd = γF x Frep where Frep γF
is the representative (or characteristic Fk) value of an action (or of the effect of an action); is the partial factor for an action (or γE, for the effect of an action).
A characteristic value as being “selected as a cautious estimate of the value affecting the occurrence of the limit state considered”.
A2.4.2
How to ensure designs comply with EN 1997
The limit states that are most likely to be appropriate for tower cranes are: – loss of equilibrium of the structure or the ground, considered as a rigid body, in which the strengths of structural materials and the ground are insignificant in providing resistance (EQU); – internal failure or excessive deformation of the structure or structural elements, including footings, piles, basement walls, etc, in which the strength of structural materials is significant in providing resistance (STR); – failure or excessive deformation of the ground, in which the strength of soil or rock is significant in providing resistance (GEO) e.g. bearing resistance of spread foundations or pile foundations.
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Design calculations in BSEN 1997-1 involve the following processes: – establishing actions; – establishing ground properties and ground resistances; – defining limits that must not be exceeded (e.g. bearing capacity or values of differential settlement); – setting up calculation models for the relevant ultimate and serviceability limit states; – showing in a calculation that these limits will not be exceeded. DESIGN APPROACHES - COMPARISON BETWEEN ALLOWABLE STRESS OR LIMIT STATE In ‘traditional’ design calculations such as those employed in BS 8004, satisfactory design was achieved by making sure that stresses in materials, e.g. in the soil beneath a foundation, were kept to ‘working’ levels; this was done by applying a ‘global safety factor’ in the design calculation. For example, in: E=R÷F (B1) where E is the disturbance or force acting; R is the resistance offered by the structure (e.g. the bearing pressure beneath the footing); F is a factor to make sure that E is ‘sufficiently’ less than R. The precise nature and magnitudes of the ingredients of E and R are not often clearly specified in current national geotechnical Codes. Traditionally, F has been a fairly large number, for example 3 for a simple strip footing and 2 – 3 for a pile. These factor values were found, from experience by testing and from the back-analysis of observations, firstly to prevent ‘failure’ and secondly to ensure that settlements under working loads remained acceptably small. In the global safety factor method, checking that both ultimate limit states (failure, ULS) and serviceability (SLS) requirements were met was performed in the one calculation. The Eurocode design philosophy, on the other hand, firstly clearly separates ULS from SLS and secondly deals rather more rigorously with the identification and treatment of the many uncertainties inherent in a design problem. It does this by: making a clear distinction between actions from the superstructure (and stipulated by BSEN 1991) and those from the ground; by separating the uncertainties in these actions, with the partial factors for structural actions coming from BSEN 1990 and for geotechnical actions from BSEN 1997-1. by separating the uncertainty in the reactions from the ground from that of the structural loads. The ‘traditional’, lumped factor approach merged all the uncertainty into one factor value, as well as providing an element for limiting the strength mobilized and thus the settlements. Since most of BSEN 1997-1 is concerned with checking the avoidance of a ULS, this rigor is applied to the uncertainties in the calculation of, for example, bearing capacity and ground strength rather than to settlement calculations for checking the avoidance of an SLS. In contrast to eqn. (B1), the basis of ULS design in BSEN 1997-1 lies in the following inequality: Ed ≤ Rd (B2) where Ed is the design value of the sum of the actions and of the effects of them; Rd is the design value of the corresponding resistance of the ground and/or structure. It is in determining the design values of these two terms that the most fundamental departure from ‘traditional’ British geotechnical design practice, at least as embodied in most BS Codes of Practice, occurs. This is because, in the Eurocodes, design values are calculated from characteristic values and partial factors.
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EXAMPLE 4. Reinforced concrete square gravity base This is a simple example of a gravity base – a square base, with the mast concentric and in the same orientation as the base - and follows Example 1. The example is used to illustrate the design principles from BSEN 1997, which can be applied to more complex bases. The example lists relevant clause numbers, equations and tables from BSEN 1997-1 and represents our interpretation of the code. BSEN 1997 requires checks against all possible modes of failure (ultimate limit states, ULS), using appropriate combinations of partial factors as well as a serviceability check. The design is in 5 steps, corresponding to 3 possible modes of failure: d) stability: determining size of base to satisfy the EQU limit state e) geotechnical capacity: The UK National Annex to BS EN 1997-1 will specify which Design Approach should be used in the UK. It is currently (2005) considered Design Approach 1 (DA1) is to be adopted for geotechnical design in the UK. For ULS, BSEN 1997-1 gives two ‘combinations’ that need to be considered in DA1; these in general relate to structural (STR) and ground-related (GEO) behaviour (Combination 1 and 2 respectively). The stages are: i) check combination 1 (STR) to resist bearing failure ii) check combination2 (GEO) to resist bearing failure iii) check combination 1 (STR) to resist sliding failure iv) check combination2 (GEO) to resist sliding failure v) check serviceability limit states (SLS) f) structural design to BSEN 1992: this example gives the loads that will need to be considered in the structural design Crane type: Liebherr 200HC, 49.6m under hook Foundation loadings supplied: Load Unit Inservice (i-s) Moment, MA kNm 2100 Vert. load, V kN 710 Lateral load, H kN 47
Out-ofservice (o-o-s) 3680 680 82
L
L
Cast-in length of anchors 1125mm (±15mm) Minimum depth of base: 1.5m Mast dimensions: 1.981 x 1.981m crs SI has supplied cu=100kPa for the clay soil on this site. By inspection, design for o-o-s loads.
V MA
H
G
O
D ≥1.5m
80
COMMENT Characteristic values should be cautious estimates of values of those parameters affecting the limit state. Whilst in general loading information will be supplied for the crane and location, characteristic geotechnical parameters must be assessed for each site by the designer. See clause 2.4.5.2. Determine characteristic parameters from information supplied: γsoil;k, characteristic unit weight of soil - 20kN/m³ γconc;k, characteristic unit weight of concrete - 24kN/m³ cu;k, characteristic undrained shear strength = 100kPa Characteristic permanent vertical crane load Vk = 680kN Characteristic variable horizontal load, Hk = 82kN Characteristic variable applied moment MA;k = 3680kNm COMMENT The horizontal load and moment components include the effect of wind and are therefore variable actions.
a) Stability Following Example 1 outlined earlier, overall Stability check against EQU ULS and considering moments about O. Comment: For BSEN 1997 calculations are made in terms of 'design' actions (loads and moments); these are derived from their characteristic values multiplied by a partial factor, the value of which depends on the design situation being considered, whether it is 'permanent or variable' and if it is 'favourable or unfavourable'. The values are tabulated in the code and, where appropriate, can be set by the UK National Annex. For stability, the vertical load Vk is a favourable permanent action and therefore the design value is: = γG;stab x Vk Vd = 0.9 x 680 Table A1 (favourable permanent) = 612 kN The characteristic weight of the base Gk is: Gk = L x L x 1.5 x γconc; k = L2 x 1.5 x 24 = 36 L2 kN This is also a favourable permanent action and so = γG;stab x G Table A1 (favourable permanent) Gd = 0.9 x 36 L2 = 32.4 x L2 kN The design stabilising moment is: Mstab;d = (Vd + Gd) x L/2
81
The characteristic destabilising moment from the moment and the horizontal load is: Mdestab;k = Mk + Hk x 1.5 This is an unfavourable variable action and so Mdestab;d = Mdestab;k x γQ;dst = Mdestab x 1.5 = (3680 + 82 x 1.5) x 1.5 = 5704.5 kNm
Table A1 (unfavourable variable)
So, for equilibrium: Mstab;d ≥ Mdestab;d ∴ 5704.5 = (612 + 32.4 x L2) x L/2 = 306 L + 16.2 L3
(to find minimum L)
Solving by iteration L = 6.2 gives 5758 kNm moment and so Mstab;d ≅ Mdestab;d
(b) geotechnical capacity Purpose: To check that the base size, L, selected above for overall stability meets the Bearing Resistance requirements (clause 6.5.2). Comment Note that for STR and GEO ULS different partial factors will be applied to the characteristic values and the design values of actions will change from the EQU ULS values. In all ultimate limit states we need to show that: Vd ≤ Rd
eqn 6.1
Where, Vd = Design value of the (unfavourable) load (action) Rd = Design value of resistance to load (action) Characteristic vertical load Loads (actions) are: Imposed load from the crane = 680 kN Weight of Foundation =(L x L x 1.5) x γconc;k = 6.2 x 6.2 x 1.5) x 24 = 1384kN Total Vertical Characteristic Permanent Load, Vk = 680 + 1384 = 2064 kN i)
STR ULS calculation for bearing capacity
Considering Combination 1 A1 “+” M1 “+” R1
clause 2.4.7.3.4.2
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COMMENT Subscript '1' is used in the following calculation to distinguish that the values are derived from combination '1'. A represents the partial factors for actions (γF) or the effect of actions (γE) M represents the partial factors for material (γM) R represents the partial factor for resistance (γR and "+" means 'used in combination'. In the ULS STR, uncertainty in the structural actions is dominant (factors A1 generally > 1.0 and factors M1 = 1.0) Applied Loads (Action A1) Design vertical load Vd1= γG x Vk = 1.35 x 2064 = 2786kN Design horizontal load Hd1= γQ × Hk = 1.5 × 82 = 123 kN Design moment Md1 = MA;k x γQ + Hd1 x 1.5 = 3680 x 1.5 + 123 x 1.5 = 5704kNm
Table A3 (permanent unfavourable)
Table A3 (variable unfavourable)
Table A3 (variable unfavourable)
Design soil strength (M1) Cud1= Cuk/γcu = 100/1.0 = 100kPa
Table A4 (undrained shear strength)
Comment The horizontal load and moment introduce an eccentric loading on the base. For calculation of allowable bearing capacity a revised width (L') is derived based on the method of Meyerhof G.G (1953). The bearing capacity of foundations under eccentric and inclined loads. 3rd ICSMFE. Zurich. Vol.1p.440-445. In this case the applied moment and horizontal load are used to calculate the reduced area, whilst the bearing capacity under inclined loads is taken account in the method given in BSEN 1997 Appendix D. Eccentricity e = Md1 x sin 45o/Vd1 = 5704 x sin 45o/2786 = 1.45 L′1 = L – 2e = 6.2 – 2 × 1.45 = 3.3 m Effective area of base: A′1 = 3.3 × 3.3 = 10.89 m2
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Using the analytical method to obtain a Design bearing resistance.
clause 6.5.2.2
R/A′1 = (π + 2) cud . sc . ic + q
eqn D.1
COMMENT The bc term is not included as the base is not inclined. The term in BS EN 1997-1 Annex D, ic, is required to take account of a horizontal load component which gives rise to an inclination of the total applied load.
1 H i = 1 + 1 − c1 2 A'×cu
D.3
1 123 i = 1 + 1 − c1 2 10.89 × 100
D.3
ic1 = 0.97 Sc1 = 1.2
for a square base D.3
Bearing Resistance Rd1 =10.89 ((π + 2) × 100 × 1.2 × 0.97 + 1.5 ×20) = 6844 kN Check
Rdl ≥ Vdl
eqn 6.1
6844 > 2786 therefore base ok for this ULS. ii)
Geo ULS calculation for bearing capacity
Combination 2. A2”+”M2”+”R1
2.4.7.3.4.2
COMMENT Subscript '2' is used in the following calculation to distinguish that the values are derived from combination '2'. The calculation follows the same steps as STR but uses different partial factors, such that the uncertainty lies in the soil parameters. Applied Loads (Action A2) Design vertical load Vd2= γG x Vk = 1.0 x 2064 = 2064 kN
Table A3 (permanent unfavourable)
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Design horizontal load Hd2= γQ × Hk = 1.3 × 82 = 106.6 kN Design moment Md2 = MA;k x γQ + Hd2 × 1.5 = 3680 x 1.3 + 106.6 x 1.5 = 4944 kNm
Table A3 (variable unfavourable)
Table A3 (variable unfavourable)
Design Strength (M2) Cud1= Cuk/γcu = 100/1.4 = 71.4 kPa
Table A4 (undrained shear strength)
Eccentricity e = Md2 x sin 45o/Vd2 = 4944 x sin 45o/2064 = 1.69 m L′2 = L – 2e = 6.2 – 2 × 1.69 = 2.82 m Effective area of base: A′1 = 2.82 × 2.82 = 7.95 m2 Using the analytical method to obtain a Design bearing resistance.
6.5.2.2
R/A′2 = (π + 2) cud . sc . ic + q
Eqn D.1.
ic 2 =
1 106.6 1 + 1 − 2 7.95 × 71.4
ic2 = 0.95 Sc2 = 1 + 0.2 = 1.2
D. 3
for a square base D. 3
Bearing Resistance Rd2 =7.95 ((π + 2) × 71.4 × 1.2 × 0.95 + 1.5 ×20) = 3565 kN Check
Rd2 ≥ Vd2
eqn 6.1
3565 > 2064 therefore base OK for this ULS.
iii)
STR ULS sliding resistance
clause 6.5.3.
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Design resistance, Rd = Ac. cu:d where cu:d is the relevant factored shear strength eqn 6.4.a Area of base under compressive load, Ac = A1’= 10.89 m2 Design soil shear strength cu;d1 = 100 kPa
see (i) see (i)
Shear resistance of base assuming shear of clay-concrete interface for interface: cu:d= α. cu;d1 For clay: concrete interface, in this example assume α = 0.5 α cu:d1 = 0.5 × 100 = 50 kPa ∴Sliding Resistance, Rd1 = 10.89 × 50 = 544.5 kN Design horizontal load, Hd1 = 123 kN Check
Rd1 ≥ Hd1 544.5 > 123
eqn 6.2
Base OK for this ULS. iv)
GEO ULS sliding resistance
clause 6.5.3.
Design resistance, Rd2 = Ac. cu:d
eqn 6.4.a
Area of base under compressive load, Ac = A2’= 7.95 m2
see (ii)
Design soil shear strength cu;d2 = 71.4 kPa Shear resistance of base assuming shear of clay-concrete interface cu:d= α. cu;d2 For clay: concrete interface, in this example assume α = 0.5 α cu:d2 = 0.5 × 71.4 = 35.7 kPa ∴Sliding Resistance, Rd2 = 7.95 × 35.7 = 284.2 kN Design horizontal load, Hd2 = 106.6 kN Check
Rd2 ≥ Hd2 284.2 > 106.6
eqn 6.2
Base OK for this ULS.
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COMMENT If the slewing torque combined with the horizontal in-service loads is significant, a check should be made against rotational sliding in the same way as shown here.
v)
Serviceability limit state (SLS)
COMMENT BSEN 1997-1 requires that both ultimate and serviceability (SLS) limit states be checked. A number of different ways in which this can be checked are given in the code, including: (a) Verification that the design values of actions are lower than that required to cause the SLS (Ed ≤ Cd (BSEN 1997-1 inequality 2.10)) (b) Verification that a sufficiently low fraction of the ground strength is mobilised so that deformations are within the SLS (BSEN 1997-1 clause 2.4.8(4)). This can only be carried out in situations where the value of deformation is not required and experience is available of similar structures in comparable ground conditions. For spread foundations, clause 6.6.2 (16) indicates that if the ratio of bearing capacity of the ground at its initial undrained shear strength to the loading applied at SLS is greater than 3 then explicit calculation is not necessary. It has been assumed in this example that the 'initial' shear strength is the characteristic value. Generally partial factors are set to 1.0 for such calculations. Characteristic loads (base size 6.2m square): Vk = 680 + 1384 = 2064 kN Hk = 82 kN Mk= 3680 + 82x1.5 = 3803 kNm Characteristic soil shear strength: cu;k =100 kPa Bearing resistance
clause 6.5.2.2
COMMENT As all partial factors have been set to 1, the design values in the equations below are numerically equal to the characteristic values. For clarity, subscripts 'k' have been used to illustrate this. Eccentricity e = Mk x sin 45o/Vk = 3803 x sin 45o/2064 = 1.3 m L′k = L – 2e = 6.2 – 2 × 1.3 = 3.6 m Effective area of base: A′k = 3.6 × 3.6 = 12.96 m2 Rk/A′k = (π + 2) cu;k . sc . ic + q
Eqn D.1.
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ic =
1 82 1 + 1 − 2 12.96 × 100
D.3
ick = 0.98 Sck = 1.2
for a square base D.3
Bearing Resistance, Rk =12.96 ((π + 2) × 100 × 1.2 × 0.98 + 1.5 ×20) = 8225 kN Check Rk / Vk = 8225 / 2064 = 4 As this is > 3 an explicit SLS check for bearing resistance is not required. Sliding Area of base under compressive load, A′k = 12.96 m2 Characteristic soil shear strength cuk = 100 kPa ∴Sliding Resistance assuming α=0.5 Rdk = 12.96 × 100 x 0.5 = 648 kN
clause 6.5.3
eqn 6.4a
Design horizontal load, Hdk = 82 kN Check Rdk / Hdk = 648 / 82 = 8 so an explicit check for deformation is not required!
(c) structural design COMMENT BSEN 1992 is complementary to BSEN 1997 and uses a similar partial factor approach. The worst combinations of loads calculated in the geotechnical design must be used in BSEN 1992 to carry out the reinforced concrete design. Because the partial factors in STR are applied to the loads, generally for this design situation it is this combination that is likely to be critical to the structural design. Loads to be used in BSEN 1992 Design STR case Hd1 Vd1 Bearing 2786 123
Md1 5704
GEO (for comparison only) Vd2 Hd2 1976 106.6
Md2 4944
Note that the structural design in this example has not been carried out - only the geotechnical load cases are given as structural design to BSEN 1992 is closely aligned to BS 8100.
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APPENDIX 3 -
Standards and timetables for change
Standard
Anticipated changes and timetable Will be superseded in due course
Referred to in new EN's
FEM 1.001: 1998 – Rules for the design of hoisting appliances – Booklets 1,2,3,4,5,7,8 & 9 FEM 1.004: 2000 – Recommendation for the calculation of wind loads on crane structures FEM 1.005: 2003 – Recommendations for the calculation of tower crane structures in out of service conditions BS EN 13001-1:2004 Crane safety. General design. General principles and requirements BS EN 13001-2:2004 Crane safety. General design. Load effects BS EN 13135-1:2003 Cranes. Safety. Design. Requirements for equipment. Electrotechnical equipment BS EN 13135-2:2004 Cranes. Safety. Design. Requirements for equipment. Non-electrotechnical equipment
Will be superseded in due course
Referred to in new EN's
All in effect in 2004
New standards to which mobile cranes are designed
prEN 14439 Cranes – Tower Cranes
Due in 2005
BS 7121 Part 5 Tower cranes
Revision due 2006. Replacing 1997 edition.
Refers to FEM 1.001 for all design. Significant overhaul of all clauses relating to operation.
DIN 15018 : 1984 - Cranes; steel structures - Parts 1, 2 & 3
BS8004 Code of practice for Foundations BS5930 Code of practice for site investigation BS1377 (9 parts) Method of tests for soils for engineering purposes
BSEN 1990 - 1998 Eurocodes for structural design
Expected to be in period of 'co-existence' until at least 2008
Comment
Currently cover site investigation and foundation design in the UK. Codes of practice giving guidance rather than standards, little coverage of temporary works design. For tower cranes these standards will govern design of structural ties and other elements as well as foundations. Related geotechnical testing and execution standards covering site practice that may become
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requirements before 2008. Other guidance
BS EN 13586:2004 Cranes. Access BS EN 12077-2:1999 Cranes safety. Requirements for health and safety. Limiting and indicating devices BS EN 14238:2004 Cranes. Manually controlled load manipulating devices BS EN 12077-2:1999 Cranes safety. Requirements for health and safety. Limiting and indicating devices BS ISO 4308-1:2003 Cranes and lifting appliances. Selection of wire ropes. General BS ISO 4309:2004 Cranes. Wire ropes. Care, maintenance, installation, examination & discard Add ISO 12482-1:1995, Cranes – Condition monitoring. BS7121 Safe use of cranes (in 14 parts) Part 1 General Part 2 Inspection, testing and examination Part 3 Mobile cranes Crane Stability on Site, 1996, Revised 2003. CIRIA C703. Construction Industry Research and Information Association (CIRIA). A voluntary code of practice for safe use of cranes in and around airports. 2004. Off-highway Plant and Equipment Research Centre (OPERC) .
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Glossary anemometer
a device that measures and records wind speed anemograph
a device that measures and records wind speed and direction appointed person person with the training, practical and theoretical knowledge and experience required to comply with Clause 3.3 of BS 7121: Part 1 : 1989 competent person a person who has such practical and theoretical knowledge and such experience of the crane and the equipment used in the lifting operation as is necessary to carry out the function to which the term relates in each particular context crane coordinator person who plans the sequence of operations of tower cranes on sites having more than one crane, to ensure that cranes, components and loads do not collide crane operator person who is operating the crane for the purpose of positioning loads or erection of the crane NOTE Sometimes referred to as “crane driver”. crane supervisor person who controls the lifting operation, and ensures that it is carried out in accordance with the appointed person’s safe system of work DIN German standards body. dead loads weights of the tower crane components EN standard issued by CEN, the European standards body. EQU equilibrium, or ‘overall stability’ limit state in BS EN 1997: geotechnical design. FEM 1.001 standards for crane design imposed loads weight of the load being lifted live loads wind loading prEN draft CEN standard. reconfiguration a change in the crane configuration when the crane has been installed. This may be raising the crane height or altering the jib length for example.
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signaller person responsible for directing the crane operator to ensure safe movement of the crane and load slinger person responsible for attaching and detaching the load to and from the crane and for correct selection and use of lifting accessories
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References BR470 Working platforms for tracked plant. CRC, 2003. BS 5950:2000 Structural use of steelwork in building BS 7121- 1:2000, Code of practice for the safe use of cranes – Part 1 General BS 7121-2:2003, Code of practice for the safe use of cranes – Part 2: Inspection, testing and examination. BS 7121–3:2000, Code of practice for the safe use of cranes – Part 3: Mobile cranes. BS 7121- 5:1997, Code of practice for the safe use of cranes – Part 5 Tower cranes BS7608 Code of practice for fatigue design and assessment of steel structures BS 8004: Code of practice for foundations. BS EN 12077-2:1999, Cranes safety. Requirements for health and safety. Limiting and indicating devices. DIN 15018 : 1984, Cranes; steel structures - Parts 1, 2 & 3 EN 1990-1998 Structural Eurocodes (in 9 parts) FEM 1.001: 1998 – Rules for the design of hoisting appliances – Booklets 1,2,3,4,5,7,8 & , European Handling Federation. PD 5304:2000, Safe use of machinery. prEN 14439, Cranes - Tower cranes. Crane Stability on Site, C703, 2003. CIRIA (formerly Special Publication 131, 1996). Construction Industry Research and Information Association; Code of practice for the safe use of lifting equipment. Lifting Equipment Engineers' Association; The Lifting Operations and Lifting Equipment Regulations 1998 (LOLER) The Provision and Use of Work Equipment Regulations 1998 (PUWER) Census of Fatal Occupational Injuries 1994-95. OSHA, see http://www.osha.gov/oshstats/ Safe use of lifting equipment. Approved Code of Practice and Guidance to LOLER. 1998. L113. HSE Books Safe use of work equipment. Approved Code of Practice and Guidance to PUWER. 1998. L22. HSE Books. Special Digest 5. Wind loads on unclad structures. BRE, Watford The Health and Safety at Work etc. Act 1974 The Supply of Machinery (Safety) Regulations The Construction (Design and Management) Regulations 1994 (As amended) 93
The Work at Height Regulations 2005 A voluntary code of practice for safe use of cranes in and around airports. 2004. Offhighway Plant and Equipment Research Centre (OPERC) . Cranes and Planes - A guide to Procedures for Operation of Cranes in the Vicinity of Aerodromes. Airport Operators Association 2003. (AOA).
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Bibliography BS 7262:1990, Specification for automatic safe load indicators. BS 8437:2005. Code of practice for selection, use and maintenance of personal fall protection systems and equipment for use in the workplace. prEN 14502-1, Cranes - Equipment for the lifting of persons - Part 1: Suspended baskets. prEN 14502-2, Cranes - Equipment for the lifting of persons - Part 2: Elevating control stations. HSE Guidance Note GS 6, Avoidance of danger from overhead power lines.1997. HSE Books. HSE Guidance Note GS 39, Training of crane drivers and slingers (withdrawn).1986. HSE Books. Recommendations for safe slinging. National Association of Port Employers and the General Council of British Shipping. Tower Crane Installation Training Programme. 2004. Construction Industry Training Board (CITB).
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