Eurocode - Member Design Handbook

CSC Fastrak ™

Structural steelwork analysis and design

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HANDBOOK EUROCODE MEMBER DESIGN

Eurocode Member Design Handbook page 2

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Disclaimer

Disclaimer

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CSC (UK) Ltd does not accept any liability whatsoever for loss or damage arising from any errors which might be contained in the documentation, text or operation of the programs supplied. It shall be the responsibility of the customer (and not CSC) •

to check the documentation, text and operation of the programs supplied,

•

to ensure that the person operating the programs or supervising their operation is suitably qualified and experienced,

•

to ensure that program operation is carried out in accordance with the user manuals,

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Table of Contents

Eurocode Member Design Handbook Chapter 1

Introduction

Chapter 2

Basic Principles .

Chapter 3

Simple Beam

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. . Definitions . . . . Convention for member axes . Deflection checks . . . Error messages . . .

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. . . . . . . . . . . . . . . Lateral torsional buckling checks . Deflection checks . . . Web Openings . . . . Design Properties . . . . Size Constraints . . . .

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12 12 12 12 12 13 17 17 17 18 18 18 18 18 19 19 19 21 21 21 22 23 24 25 26 26 27

. . . . . . . . . . . Construction stage restraint conditions . Loading . . . . . . Construction stage design checks . . Composite stage design checks . .

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28 28 28 28 29 29 33 34 35 35 36 36 36 38 38

. . . . Steel sections . Web openings .

. . . . . . Restraint conditions . Applied loading . . Design checks . . Theory and Assumptions Analysis method . Design method . . Section classification . Shear checks . . Bending checks . .

. . . . . . . . . . . . . . . . . . . . Sections for Study (in Fastrak Building Designer) . Sections for Study (in Simple Beam) . . . Deflection . . . . . . . . Simple Beam Input (in Fastrak Building Designer) . Simple Beam Input (when run as a standalone program) Designing a beam . . . . . . Checking a beam . . . . . . Introduction. Scope . . Beam .

Chapter 4

Composite Beam

. . . . . . Profiled metal decking . Concrete slab . . . Shear connectors . . Reinforcement . . . Fibre Reinforced Concrete .

Introduction. Scope . . Beam .

. . . Steel sections . Web openings .


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Table of Contents

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Theory and Assumptions Analysis method . Design method . Construction stage . Composite stage . Web Openings . Design Aspects . .

. . . . . . . Use of Design Properties to Control Section Selection . Effective width calculations . . . . . Application of NCCI PN002 to Partial Shear Connection . Layout of Studs . . . . . . . Non-composite design within Composite Beam . . Automatic transverse shear reinforcement design . . Composite Beam Input (in Fastrak Building Designer) . Composite Beam Input (when run as a standalone program) Designing a beam . . . . . . . Checking a beam . . . . . . .

Chapter 5

General Beam .

Chapter 6

General Column

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. . Introduction . . . Scope . . . . Limitations and Assumptions Limitations . . . Assumptions . . . Analysis . . . . Building Modeller object . General Beam . . . Ultimate Limit State – Strength Classification . . . Shear Capacity . . Moment Capacity . . Axial Capacity . . . Cross-section Capacity . Ultimate Limit State – Buckling Compression buckling . Lateral Torsional Buckling . Combined buckling . . Design Control . . Member End Fixity and Supports

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. . . . . . . Imposed Load Reductions . Splices . . . . Design Forces . . .

Introduction Scope .

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39 39 39 39 40 47 50 50 54 55 56 63 64 66 68 68 69

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page 6

Table of Contents

Design Checks .

. . Building Modeller Object . Ultimate Limit State – Strength Classification . . . Axial Capacity . . . Shear Capacity . . . Moment Capacity . .

. . . . . . . . . . . . . . . . . . . . . . . . Combined Bending and Axial Capacity . . . Ultimate Limit State – Buckling . . . . Compression buckling . . . . . Lateral Torsional Buckling . . . . . Combined Buckling . . . . . . Serviceability limit state. . . . . . General Column Input (in Fastrak Building Designer)

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94 94 94 94 94 94 95 95 95 95 95 95 95 95 96 96

. Introduction. . . . . Why would you want to refine the original design? . Interaction Effects . . . . . . How to Access Design Refinement . . . . Simple Beam - Check Mode. . . . . . Simple Beam - Design Mode. . . . . Composite Beam - Check Mode . . . . Composite Beam - Design Mode . . . . General Column - Check Mode . . . . General Column - Design Mode . . . . Effective Use of Order Files in Refined Design . . Design Pass 1 . . . . . . . Design Pass 2 . . . . . . . Design Pass 3 . . . . . . .

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Analysis

Chapter 7

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Braces .

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. . . Steel sections . End Connections . Applied loading . . Design Forces . . Design checks . . Theory and Assumptions Analysis method . Design method . . Classification . . Axial Tension . . Axial Compression .

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Chapter 8

Refining Member Designs

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Chapter 9

References .

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Compression Buckling

Brace Input .


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Chapter 1 : Introduction


Eurocode Member Design Handbook

Chapter 1

Introduction Fastrak Building Designer designs steel members, composite members and connections to a range of international codes. This handbook specifically describes the design methods applied in the software when the BS EN 1993-1-1:2005(Ref. 1) and BS EN 1994-1-1:2004(Ref. 4) codes are selected. Within the remainder of this handbook BS EN 1993-1-1:2005 and BS EN 1994-1-1:2004 are referred to as EC3 and EC4 respectively. Unless explicitly noted otherwise, all clauses, figures and tables referred to are from EC3; apart from the Composite Beam section, within which references are to EC4 unless stated. A brief description of the contents follows: Basic Principles — (Chapter 2) terminology and basic principles common to each of the design applications. Simple Beam — (Chapter 3) ”non-composite steel beam with pinned ends designed for gravity loads acting through the web” Composite Beam — (Chapter 4) ”composite steel beam with pinned ends designed for gravity loads acting through the web” General Beam — (Chapter 5) ”non-composite steel beam designed as a beam/column” General Column — (Chapter 6) ”steel column designed as a beam/column” Braces — (Chapter 7) ”steel members with pinned ends designed for axial loads only” Refining Member Designs — (Chapter 8) advice to assist you in extracting individual members into each design application for more detailed assessment. References — (Chapter 9)


Chapter 2

Chapter 2 : Basic Principles

Basic Principles

Definitions Commonly applied Fastrak terms are defined below:

Attributes When a member is first created it’s properties (steel grade, maximum section depth etc.) are taken from the attribute set that is currently active. Once a member has been placed it’s properties can be edited as required. Ensuring the attribute set is correct before placement ensures the minimum amount of member editing.

Design Mode Within Fastrak Building Designer you can access the member design routines automatically for every member in the building model to choose the smallest section from a list of sections (referred to in the program as an order file).

Check Mode Alternatively you can access the member design routines to check the section size already assigned by you to each member, to determine whether they are able to carry the applied loading.

Interactive Design Within Fastrak Building Designer you can also extract key members from the model into the appropriate design program for further investigation in either Design Mode or Check Mode, providing you with still greater control over the design: • to enable multiple order files to be considered at the same time to determine a list of alternative sections, all of which can withstand the applied loading.

• to adjust the initial design manually without having to re-design the whole building. Any change to the section size or steel grade can then be passed back to the building model, but only affects the individual beam extracted. If the changes are to be applied to other beams also, you would need to update the building model separately and then re-design it. For further details see “Refining Member Designs”

Order Files Each order file is a list of section sizes of a given type arranged in the sequence in which they will be tried during the design. Undesirable sections can be excluded if required. Caution

If you exclude sections from an order file they will remain excluded for all designs until you decide to include them again.

The following terms are relevant when using Fastrak to design to the Eurocodes.



National Annex (NA) Safety factors in the Eurocodes are recommended values and may be altered by the national annex of each member state. Fastrak Building Designer currently has following EC3 national annex options are available: • EC3 (base)

• EC3 UK NA • EC3 Irish NA You can select the desired National Annex as appropriate, in which case the nationally determined parameters are automatically applied, or if you choose EC3 (base), the Eurocode recommended values are applied.

Nationally Determined Parameters (NDP’s) NDP’s are choices of values, classes or alternative methods contained in a National Annex that can be applied in place of the base Eurocode.

Partial Factors for Buildings The partial factors M for buildings as described in Clause 6.1(1) should be applied to the various characteristic values of resistance as follows:

•resistance of cross-sections irrespective of class: M1 •resistance of members to instability assessed by member checks: M1 •resistance of cross-sections in tension to fracture: M2 Depending on your choice of National Annex the above partial factors for buildings are set as follows:

Factor

EC Base value

UK Value

Irish Value

M0

1.00

1.00

1.00

M1

1.00

1.00

1.00

M2

1.25

1.10*

1.25

NOTE - for connection design BS EN1991-1-8 - M2 = 1.25



Convention for member axes The sign convention for member axes when designing to Eurocodes is as shown below.

Section axes - (x is into the page along the centroidal axis of the member).

Deflection checks Fastrak Building Designer calculates both relative and absolute deflections. Relative deflections measure the internal displacement occurring within the length of the member and take no account of the support settlements or rotations, whereas absolute deflections are concerned with deflection of the structure as a whole. The absolute deflections are the ones displayed in the structure deflection graphics. The difference between relative and absolute deflections is illustrated in the cantilever beam example below.

Relative Deflection

Absolute Deflection

Relative deflections are given in the member analysis results graphics and are the ones used in the member design.



Error messages As you define member data, Fastrak Building Designer continually checks to ensure that the data is valid. If a particular value is not valid, then it will be shown using a colour of your choice in the dialog (default red). If a value is not recommended, then a different colour will be used in the dialog, (default orange for ‘warning’). If you allow the cursor to rest over the error or warning field you will see a tip telling you the acceptable range of input. Until all the information within the dialog is valid (but not free of warnings) you will not be able to save the dialog since OK will be dimmed. Although checking in this way prevents you from defining invalid data there are some cases where particular errors occur that cannot be trapped - for instance where an error occurs due to inconsistencies that have arisen between information covered on different dialogs. In these cases when you attempt to perform a design you will see an error message indicating that data is not suitable for the design to proceed. Each message is self-explanatory. You should take a careful note of the error message and then change the member data to correct the problem. If there are other problems with the design, then you will see a series of warning messages in the results viewer. You should take note of any such warnings and take the action that you deem appropriate. Engineering tips are also available in the results viewer which may give you useful information about the assumptions or approach adopted for the particular calculation or about a particular recommendation of good practice with which we recommend that you comply.


Chapter 3

Chapter 3 : Simple Beam

Simple Beam

Introduction SIMPLE BEAM - “non-composite steel beam with pinned ends designed for gravity loads acting through the web” The Simple Beam design application allows you to analyse and design a structural steel beam which may have incoming beams providing restraint, and which may or may not be continuously restrained over any length between restraints. Simple Beam can determine the size of member which can carry the forces and moments resulting from the applied loading. Alternatively you can specify the member size and Simple Beam will then determine whether it is able to carry the previously mentioned forces and moments and satisfy the deflection requirements. Unless explicitly stated all calculations in Simple Beam are in accordance with the relevant sections of EC3(Ref. 1) and the associated UK(Ref. 2) or Irish(Ref. 3) National Annex.

Scope The scope of the Simple Beam application is as follows:

Beam The beam is designed for gravity loads acting through the shear centre in the plane of the web. Minor axis bending, uplift loads and axial loads are not considered. Note

If either minor axis bending, uplift loads, point moments or axial loads exist which exceed a limit below which they can be ignored, a warning will be given in the beam design summary.

Steel sections Simple Beam can handle design for an international range of steel sections for many different countries. Plated sections can also be checked. The following section types are available:

• I (including rolled, ASB, SFB, plated), C, RHS, SHS Where: I rolled = UKB, UKC, UB, UC, RSJ, IPE, HE, HD, IPN C = RSC, PFC, UAP, UPN RHS = RHS, Euro RHS SHS = SHS, Euro SHS



Web openings If you need to provide access for services, etc., then you can add openings to a designed beam and Simple Beam can then check these for you. You can define rectangular or circular openings and these can be stiffened on one, or on both sides. General guidance on size and positioning of openings is given in Table 2.1 of the SCI Publication P355(Ref. 8) and repeated below:

Parameter

Limit Circular Opening

Rectangular Opening

<= 0.8h

<= 0.7h

>= tf +30 mm

>= 0.1h

As above

As above and >= 0.1lo if unstiffened

Max. ratio of depth of Tees: hb/ht

<= 3

<=2

hb/ht

> = 0.5

>= 1

Max. unstiffened opening length, lo

-

<= 1.5ho high shear*

-

<= 2.5ho low shear

Max. stiffened opening length, lo

-


-

<= 4ho low shear

Min. width of web post: - Low shear regions

>= 0.3ho

>= 0.5lo

-High shear regions

>= 0.4ho

>= lo

Max. depth of opening: Min. depth of Tee, Min. depth of Top Tee:

Corner radius of rectangular openings:

-

ro >= 2tw but ro >= 15 mm >= lo

Min. width of end post, se:

>= 0.5ho

Min. horizontal distance to point load: - no stiffeners

>= 0.5h

>= h

- with stiffeners

>= 0.25ho

>= 0.5ho

and >= h

* A high shear region is where the design shear force is greater than half the maximum value of design shear force acting on the beam.



Symbols used in the above table: h = overall depth of steel section ho = depth of opening [diameter for circular openings] ht = overall depth of upper Tee [including flange] hb = overall depth of lower Tee [including flange] lo =(clear) length of opening [diameter for circular openings] se = width of end post [minimum clear distance between opening and support] tf = thickness of flange tw = thickness of web ro = corner radius of opening In addition, the following fundamental geometric requirements must be satisfied. do <= 0.8*h for circular openings do <= 0.7*h for rectangular openings do < 2 * (doc - tt - rt) do < 2 * (h - doc - tb - rb) d2 < doc - do/2 - tt- ts/2 d2 < h - tb- doc - do - ts/2 lo < 2 * Lc lo < 2 * (L - Lc) Ls < 2 * Lc Ls < 2 * (L - Lc) where dt=the depth of the web of the upper tee section measured from the underside of the top flange doc = the distance to the centre line of the opening from the top of the steel section d2 = the distance from the edge of the opening to the centre line of the stiffener ts = thickness of stiffener [constrained to be the same top and bottom] tt = the thickness of the top flange of the steel section



tb = the thickness of the bottom flange of the steel section rt = root radius at the top of the steel section rb = root radius at the bottom of the steel section Lc = the distance to the centre line of the opening from the left hand support L = the span of the beam You cannot currently automatically design sections with web openings, you must perform the design first to get a section size, and then add and check the openings. This gives you complete control of the design process, since you can add appropriate and cost effective levels of stiffening if required, or can choose a different beam with a stronger web in order to reduce or remove any stiffening requirement. Web openings can be added to a beam by a 'Quick-layout' process or manually. The 'Quick-layout' process, which is activated using the check box on the Web Openings dialog page, adds web openings which meet the geometric and proximity recommendations given in Table 2.1 of SCI Publication P355. The openings so created are the maximum depth spaced at the minimum centres recommended for the beam section size. Web openings can be defined manually in two ways from the Web Openings dialog page. With the Quick-layout check box unchecked, the ‘Add’ button adds a new line to the web openings grid to allow the geometric properties of the web opening to be defined, or alternatively, use of the ‘Add...’ button opens the Web Opening Details dialog page which gives access to more help and guidance when defining the opening. Both methods make use of 'Warning' and 'Invalid' text for data entry checks [the default colours being orange and red respectively] to provide assistance as the opening parameters are defined. On the Web Opening Details dialog page, the Centre button will position the opening on the beam centre whilst the Auto button will position the opening to meet the spacing recommendations given in P355. Also on this page tool tips give information on the recommended values for all the opening parameters. As web openings are defined, they are immediately visible in the diagram on the Web Openings dialog page. This diagram displays the results of the geometric and proximity checks that are carried out on the opening parameters using 'Warning' and 'Invalid' display colours to highlight those areas that are outside the recommended limits.



A typical display is shown below:

The areas that are subjected to the checks are end posts, web posts, web opening dimensions and tee dimensions. Using the above example, it can be deduced that:

• • • • •

The left hand end post is less than the recommended limiting value WO #1 diameter is within the recommended limiting values Internal Web Post #2 is within the recommended limiting values WO #2 dimensions are outside the recommended limiting values Internal Web Post #3 is less than the recommended limiting value but quite close to the limit. As the web post dimension reduces, the left and right triangles overlap to a greater degree at their apexes.

• WO #3 dimensions are invalid and must be adjusted to progress the definition of the opening.

• • • •

Internal Web Post #4 is within the recommended limiting values WO #4 dimensions are within the recommended limiting values Internal Web Post #5 is within the recommended limiting values WO #5 dimensions are within the recommended limiting values but the dimensions of the tee(s) are not.

This display helps you to decide whether to make any adjustments to the opening parameters before their design is checked. You should bear in mind that the checks carried out at this stage are geometric checks only and compliance with recommended limits is no guarantee that the opening will pass the subsequent engineering design checks.



Note

Dimensional checks. The program does not check that openings are positioned in the best position (between 1/5 and 1/3 length for udl’s and in a low shear zone for point loads). This is because for anything other than simple loading the best position becomes a question of engineering judgment or is pre-defined by the service runs.

Note

Adjustment to deflections. The calculated deflections are adjusted to allow for the web openings. See: ”Web Openings”in the Theory and Assumptions section. .

Restraint conditions If you need to check the lateral torsional buckling of the beam you can define the effective length by: • specifying the factors that you want to use for the lengths between restraints,

• or, you can enter the effective length of the sub-beam directly by entering a value (in m), You can position additional restraints at any point along the beam as required. You can also specify that any length (or lengths) of the beam should be taken as being fully restrained against lateral torsional buckling, independent of the restraint conditions for the adjacent length(s).

Applied loading You can specify a wide range of applied loading for the simple condition: • uniform distributed loads (over the whole or part of the beam),

• point loads, • varying distributed loads (over the whole or part of the beam), • trapezoidal loads. All loads must be positive since the beam is considered as simply supported and no negative moment effects are accommodated.

Design checks When you use Simple Beam to design or check a beam the following conditions are examined in accordance with EC3: • section classification (Table 5.2),

• shear capacity (Clause 6.2.6 (1)), • web shear buckling (Clause 6.2.6 (6)), • moment capacity: • Equation 6.13 for the low shear condition, • Equation 6.29 for the high shear condition, • lateral torsional buckling resistance (Clause 6.3.2.3), • web openings, • dead, imposed and total load deflection check.



Theory and Assumptions This section describes the theory used in the development of Simple Beam and the major assumptions that have been made, particularly with respect to interpretation of EC3.

Analysis method Simple Beam uses a simple analysis of a statically determinate beam to determine the forces and moments to be resisted by the beam.

Design method The design methods employed to determine the adequacy of the section for each condition are those consistent with EC3 unless specifically noted otherwise.

Section classification Cross-section classification for flexure is determined using Table 5.2. The classification of the section must be Class 1, Class 2 or Class 3. Sections which are classified as Class 4 are beyond the scope of Simple Beam. Implementation of the below clauses is as follows: • Classification is determined using 5.5.2 (6) and not 5.5.2 (7).

• 5.5.2 (9) is not implemented as clause (10) asks for the full classification to be used for buckling resistance.

• 5.5.2 (11) is not implemented. • 5.5.2 (12) is not implemented. A brief study of UK rolled UBs and UCs showed that flange induced buckling in normal rolled sections is not a concern.

Shear checks Major axis shear checks are performed at the point of maximum moment, the point of maximum shear, the position of application of each point load, and at all other ‘points of interest’ along the beam. Major axis shear — is determined in accordance with clause 6.2.6 (1). Where the applied shear force exceeds 50% of the capacity of the section, the high shear condition applies to the bending moment capacity checks (see below). Web Shear buckling — the 6.2.6 (6) limit is checked and if exceeded a warning is given. The warning indicates that additional calculations to EN 1993-1-5 are not carried out. The following points should be noted: • No account is taken of fastener holes in the flange or web - see 6.2.6 (7)

• Shear is not combined with torsion and thus the resistance is not reduced as per 6.2.6 (8)



Bending checks Major axis bending checks are performed at the point of maximum moment, the point of maximum shear, the position of application of each point load, as well as all other ‘points of interest’ along the beam. Bending moment capacity — for low shear this is calculated to equation 6.13 for class 1 and 2 cross sections and equation 6.14 for class 3 cross sections. In the high shear case equation 6.29 is used for class 1 and 2 cross sections and equation 6.14 for class 3 cross sections. Where the high shear condition applies, the moment capacity calculation is made less complicated by conservatively adopting a simplified shear area.

Lateral torsional buckling checks Lateral torsional buckling checks are required when any length is not continuously restrained. Simple Beam allows you to switch off these checks by specifying that the entire length between the supports is continuously restrained against lateral torsional buckling. If you use this option you must be able to provide justification that the beam is adequately restrained against lateral torsional buckling. When the checks are required you can position restraints at any point within the length of the main beam and can set the effective length of each sub-beam (the portion of the beam between one restraint and the next) either by giving factors to apply to the physical length of the beam, or by entering the effective length that you want to use. Any individual segment can be continuously restrained in which case no LTB check will be carried out over that segment. Each sub-beam which is not defined as being continuously restrained is checked in accordance with clause 6.3.2.3. Effective lengths The value of effective length factor is entirely the choice of the engineer. The default value is 1.0. There is no specific factor for destabilizing loads - so you will have to adjust the 'normal' effective length factor to allow for such effects.

Deflection checks Simple Beam calculates relative deflections. (see ”Deflection checks”in the Basic Principles chapter of this handbook.) The ‘Service Factor’ (default 1.0), specified against each load case in the combination is applied when calculating the deflections; the following deflections are available:

• dead load deflections, • imposed load deflections, • total load deflection i.e. the sum of the previous items. Deflection limits can be specified to each of the above, as a fraction of the span, or as an absolute limit, (or both).



Web Openings The deflection of a beam with web openings will be greater than that of the same beam without openings. This is due to two effects, • the reduction in the beam inertia at the positions of openings due to primary bending of the beam,

• the local deformations at the openings due to Vierendeel effects. This has two components - that due to shear deformation and that due to local bending of the upper and lower tee sections at the opening. The primary bending deflection is established by 'discretising' the member and using a numerical integration technique based on 'Engineer's Bending Theory' - M/I = E/R = /y. In this way the discrete elements that incorporate all or part of an opening will contribute more to the total deflection. The component of deflection due to the local deformations around the opening is established using a similar process to that used for cellular beams which is in turn based on the method for castellated beams given in the SCI publication, “Design of castellated beams. For use with BS 5950 and BS 449". The method works by applying a 'unit point load' at the position where the deflection is required and using a 'virtual work technique to estimate the deflection at that position. For each opening, the deflection due to shear deformation, s, and that due to local bending, bt, is calculated for the upper and lower tee sections at the opening. These are summed for all openings and added to the result at the desired position from the numerical integration of primary bending deflection. Note that in the original source document on castellated sections, there are two additional components to the deflection. These are due to bending and shear deformation of the web post. For castellated beams and cellular beams where the openings are very close together these effects are important and can be significant. For normal beams the openings are likely to be placed a reasonable distance apart. Thus in many cases these two effects will not be significant. They are not calculated for such beams but in the event that the openings are placed close together a warning is given. This will indicate that these effects on the deflection of the beam are not taken into account. This warning is issued when, so < 2.5 * do for rectangular openings so <1.5 * o for circular openings Where so=the clear length of web between adjacent openings do=the depth of a rectangular opening taken as the larger if the adjacent openings differ o=the diameter of a circular opening



Web Openings Circular Openings as an Equivalent Rectangle Each circular opening is replaced by equivalent rectangular opening, the dimensions of this equivalent rectangle for use in all subsequent calculations are: do'= 0.9*opening diameter lo = 0.45*opening diameter Properties of Tee Sections When web openings have been added, the properties of the tee sections above and below each opening are calculated in accordance with Section 3.3.1 of SCI P355(Ref. 8) and Appendix B of the joint CIRIA/SCI Publication P068(Ref. 9). The bending moment resistance is calculated separately for each of the four corners of each opening. Design Checks The following calculations are performed where required for web openings:

• • • • • • • • •

Axial resistance of tee sections Classification of section at opening Vertical shear resistance Vierendeel bending resistance Web post horizontal shear resistance Web post bending resistance Web post buckling resistance Lateral torsional buckling Deflections

Design Properties The Design Properties button provides a means by which you can both speed up the design process and control the design more precisely. Note

When you extract a beam from a Fastrak Building Designer model into Simple Beam for further investigation, Design Properties are accessed via the Design Wizard icon.

Size Constraints Size Constraints are only applicable when in Design Mode. They allow you to ensure that the sections that Simple Beam proposes match any particular size constraints you may have.



Sections for Study (in Fastrak Building Designer) This feature is only applicable when running the program in Design Mode. On the left of the page is a list of available order files, only one of which can be selected. The sections contained within the chosen order file appear in the Section Designation list on the right of the page. Only checked sections within this list are considered during the design process.

The design process commences by starting with the smallest section in the chosen order file. Any section that fails any of the design conditions is rejected and the design process is then repeated for the next available section in the list. On completion of the design process, the first satisfactory section from the Section Designation list is assigned to the beam. Caution

Limiting the choice of sections by unchecking a section within an order file is a global change that affects ALL projects, (not just the currently open one). It is typically used therefore to eliminate unavailable or non-preferred sections from the design process. If design requirements for an individual beam require section sizes to be constrained, (due to, for example depth restrictions), then the choice of sections should be limited instead by using Size Constraints, (as these only affect the current beam).

Note

It is possible to create additional order files using a text editor. If you require to do so, please contact your local CSC Technical Support Team for guidance.



Sections for Study (in Simple Beam) When you extract a beam from a Fastrak Building Designer model into Simple Beam for further investigation, a benefit of doing so is that several order files can be considered at the same time. If a check is placed against an order file the sections contained within it appear in the Section Designation list on the right of the page. Only checked sections within this list are considered during the design process.

Typically, you would uncheck those order files that are unlikely to be appropriate for simple beam design, Doing so speeds up the solution. The design process commences by starting with the smallest section in each order file. Any section that fails any of the design conditions is rejected and the design process is then repeated for the next available section in the list. On completion of the design, all the satisfactory sections from the Section Designation list are displayed and the results for each of these can be examined before one of the sections is assigned to the beam. Caution

Limiting the choice of sections by either unchecking an order file or an individual section is a global change that affects ALL projects, (not just the currently open one). It is typically used therefore to eliminate unavailable or non-preferred sections from the design process. If design requirements for an individual beam require section sizes to be constrained, (due to, for example depth restrictions), then the choice of sections should be limited instead by using Size Constraints, (as these only affect the current beam).

Note

It is possible to create additional order files using a text editor. If you require to do so, please contact your local CSC Technical Support Team for guidance.



Deflection The Deflections page allows you to control the amount of deflection by applying either a relative or absolute limit to the deflection under different loading conditions.

A typical application of these settings might be: • to apply the relative span/360 limit for imposed load deflection, to meet code requirements,

• possibly, to apply an absolute limit to the total load deflection to ensure the overall deflection is not too large.



Simple Beam Input (in Fastrak Building Designer) In order to create a simple beam within Fastrak Building Designer, you will first need to define an appropriate set of simple beam attributes. Listed below is the typical procedure for defining these attributes. Items in brackets [] are optional.

Step

Dialog

Page

Instructions

1

none

none

Create a new Beam Attribute Set.

2

Attribute Set

General

Give the Attribute Set a Title

3

Attribute Set

Design

Choose Simple construction type

4

Attribute Set

Design

Check the Automatic Design box if Design Beam Mode is required, else leave it unchecked to work in Check Beam Mode

5

Attribute Set

Design

[Check the Gravity Only Design box* if required]

6

Attribute Set

Design

Click the Design Properties button

7

Beam Design Properties

Size Constraints

[Define the Beam Constraints: • max and min beam size]

8


Sections for Study

If in Design Beam Mode choose the Order File

9


Deflection

Define and apply deflection limits • [dead] • imposed • [total]

10

Attribute Set

Alignment

[No changes are applicable for simple beams]

11

Attribute Set

Type

[Check the Fully Restrained box if required]

12

Attribute Set

Supports

For simple beams, simple connections are required at both ends. For cantilevers, one end must be fully fixed and the other must be free.

13

Attribute Set

Size

Choose the steel grade and, if in Check Beam Mode choose the section size

14

Attribute Set

Restraints

Define the restraint details. Note, this page is not visible if the beam is fully restrained.

*In order to speed the design process a distinction is made between those combinations consisting of gravity loads only and those which contain some components acting laterally (e.g. notional loads and wind loads). Setting simple beams to be designed for gravity loads only can significantly reduce the design time.



Simple Beam Input (when run as a standalone program) Design and check mode input procedures are listed below. Items in brackets [] are optional

Designing a beam Step

Icon

Instructions

15

Launch Simple Beam,

16

Create a new project giving the project name [and other project details],

17

Choose the type of beam as either a Simple Beam or a Cantilever Beam [and give the beam reference details],

18

Set Simple Beam into design beam mode,

19

Define the properties for the beam: • grade; • span.

20

Give the details of the beam restraints.

21

Define the loadcases that apply to the simple beam.

22

Incorporate the loadcases into a series of design combinations,

23

[Make any Design Wizard settings that you want to use to control the design.]

24

Perform the design

25

From the list of suitable sections preview the results for the more desirable sections and then choose the one that you would like to use,

26

Add in any web openings that you need to allow access for services etc.

27

Check the beam with the web openings. [Stiffen the web openings if necessary, or increase the size of the beam until the beam with openings is satisfactory.]

28

Specify the content of the report [and print it].

29

Save the project to disk.



Checking a beam In the typical procedure below items in brackets [] are optional.

Step

Icon

Instructions

1

Launch Simple Beam,

2


3

Choose the type of beam as either a Simple Beam or a Cantilever Beam [and give the beam reference details],

4

Set Simple Beam into check beam mode,

5

Define the properties for the beam: • section size, • grade, • span,

6

Add in any web openings that you need to allow access for services etc.

7

Give the details of the beam restraints.

8

Define the loadcases that apply to the simple beam.

9


10


11

Perform the check, (including any web openings),

12

[Stiffen the web openings if necessary, or increase the size of the beam until the beam with openings is satisfactory.]

13


14



Chapter 4

Chapter 4 : Composite Beam

Composite Beam

Introduction COMPOSITE BEAM - “composite steel beam with pinned ends designed for gravity loads acting through the web” The Composite Beam design application allows you to analyse and design a structural steel beam acting compositely with a concrete slab created using profile steel decking. Composite Beam can determine the size of member which: • acting alone is able to carry the forces and moments resulting from the Construction Stage,

• acting compositely with the slab using profile steel decking (with full or partial interaction) is able to carry the forces and moments at the Ultimate Limit State,

• acting compositely with the slab using profile steel decking (with full or partial interaction) is able to provide acceptable deflections, service stresses and natural frequency at the Serviceability Limit State. Alternatively you may give the size of a beam and Composite Beam will then determine whether it is able to carry the previously mentioned forces and moments and satisfy the Serviceability Limit State. An auto-layout feature can be used for stud placement which caters for both uniform and non-uniform layouts. The construction stage calculations are performed in accordance with the relevant sections of EC3(Ref. 1) and the associated UK(Ref. 2) or Irish(Ref. 3) National Annex. The composite stage design adopts a limit state approach consistent with the design parameters for simple and continuous composite beams as specified in EC4(Ref. 4) and the associated UK(Ref. 5) or Irish National Annex. Unless explicitly noted otherwise, all clauses, figures and tables referred to are from EC4.

Scope The scope of Composite Beam is described in this section:

Beam The beam must be a simply supported, single span unpropped structural steel beam. The following are beyond scope: • continuous or fixed ended composite beams,

• composite sections formed from hollow rolled sections, • composite sections where the concrete slab bears on the bottom flange.



The beam is designed for gravity loads acting through the web only. Minor axis bending and axial loads are not considered. Note If either minor axis bending or axial loads exist which exceed a limit below which they can be ignored, a warning is given in the beam design summary.

Steel sections Composite Beam can handle design for an international range of steel I/H-sections for many different countries and also for many specific manufacturers. Plated sections can also be checked. The following rolled section types are available:

• UKB, UKC, UB, UC, RSJ, IPE, HE, HD, IPN If required the section can be precambered to counteract the effects of dead load on the deflection of the beam.

Web openings If you need to provide access for services, etc., then you can add openings to a designed beam and Composite Beam can then check these for you. You can define rectangular or circular openings and these can be stiffened on one, or on both sides. General guidance on size and positioning of openings is given in Table 2.1 of the SCI Publication P355(Ref. 8) and repeated below:

Parameter


Rectangular Opening

<= 0.8h

<= 0.7h

>= tf +30 mm

>= 0.1h

As above

As above and >= 0.1lo if unstiffened

Max. ratio of depth of Tees: hb/ht

<= 3

<=2

hb/ht

> = 0.5

>= 1

Max. unstiffened opening length, lo

-


-

<= 2.5ho low shear

Max. stiffened opening length, lo

-


-

<= 4ho low shear

Max. depth of opening: Min. depth of Tee, Min. depth of Top Tee:


Parameter



Rectangular Opening

Min. width of web post: - Low shear regions

>= 0.3ho

>= 0.5lo

-High shear regions

>= 0.4ho

>= lo

Corner radius of rectangular openings:

-

ro >= 2tw but ro >= 15 mm >= lo

Min. width of end post, se:

>= 0.5ho

Min. horizontal distance to point load: - no stiffeners

>= 0.5h

>= h

- with stiffeners

>= 0.25ho

>= 0.5ho

and >= h

* A high shear region is where the design shear force is greater than half the maximum value of design shear force acting on the beam.

Symbols used in the above table: h = overall depth of steel section ho = depth of opening [diameter for circular openings] ht = overall depth of upper Tee [including flange] hb = overall depth of lower Tee [including flange] lo =(clear) length of opening [diameter for circular openings] se = width of end post [minimum clear distance between opening and support] tf = thickness of flange tw = thickness of web ro = corner radius of opening In addition, the following fundamental geometric requirements must be satisfied. do <= 0.8*h for circular openings do <= 0.7*h for rectangular openings do < 2 * (doc - tt - rt) do < 2 * (h - doc - tb - rb)



d2 < doc - do/2 - tt- ts/2 d2 < h - tb- doc - do - ts/2 lo < 2 * Lc lo < 2 * (L - Lc) Ls < 2 * L c Ls < 2 * (L - Lc) where dt=the depth of the web of the upper tee section measured from the underside of the top flange doc = the distance to the centre line of the opening from the top of the steel section d2 = the distance from the edge of the opening to the centre line of the stiffener ts = thickness of stiffener [constrained to be the same top and bottom] tt = the thickness of the top flange of the steel section tb = the thickness of the bottom flange of the steel section rt = root radius at the top of the steel section rb = root radius at the bottom of the steel section Lc = the distance to the centre line of the opening from the left hand support L = the span of the beam You cannot currently automatically design sections with web openings, you must perform the design first to get a section size, and then add and check the openings. This gives you complete control of the design process, since you can add appropriate and cost effective levels of stiffening if required, or can choose a different beam with a stronger web in order to reduce or remove any stiffening requirement. Web openings can be added to a beam by a 'Quick-layout' process or manually. The 'Quick-layout' process, which is activated using the check box on the Web Openings dialog page, adds web openings which meet the geometric and proximity recommendations given in Table 2.1 of SCI Publication P355. The openings so created are the maximum depth spaced at the minimum centres recommended for the beam section size. Web openings can be defined manually in two ways from the Web Openings dialog page. With the Quick-layout check box unchecked, the ‘Add’ button adds a new line to the web openings grid to allow the geometric properties of the web opening to be defined, or alternatively, use of the ‘Add...’ button opens the Web Opening Details dialog page which gives access to more help



and guidance when defining the opening. Both methods make use of 'Warning' and 'Invalid' text for data entry checks [the default colours being orange and red respectively] to provide assistance as the opening parameters are defined. On the Web Opening Details dialog page, the Centre button will position the opening on the beam centre whilst the Auto button will position the opening to meet the spacing recommendations given in P355. Also on this page tool tips give information on the recommended values for all the opening parameters. As web openings are defined, they are immediately visible in the diagram on the Web Openings dialog page. This diagram displays the results of the geometric and proximity checks that are carried out on the opening parameters using 'Warning' and 'Invalid' display colours to highlight those areas that are outside the recommended limits. A typical display is shown below:

The areas that are subjected to the checks are end posts, web posts, web opening dimensions and tee dimensions. Using the above example, it can be deduced that:

• • • • •

The left hand end post is less than the recommended limiting value WO #1 diameter is within the recommended limiting values Internal Web Post #2 is within the recommended limiting values WO #2 dimensions are outside the recommended limiting values Internal Web Post #3 is less than the recommended limiting value but quite close to the limit. As the web post dimension reduces, the left and right triangles overlap to a greater degree at their apexes.

• WO #3 dimensions are invalid and must be adjusted to progress the definition of the opening.

• Internal Web Post #4 is within the recommended limiting values



• WO #4 dimensions are within the recommended limiting values • Internal Web Post #5 is within the recommended limiting values • WO #5 dimensions are within the recommended limiting values but the dimensions of the tee(s) are not. This display helps you to decide whether to make any adjustments to the opening parameters before their design is checked. You should bear in mind that the checks carried out at this stage are geometric checks only and compliance with recommended limits is no guarantee that the opening will pass the subsequent engineering design checks. Note

Dimensional checks. The program does not check that openings are positioned in the best position (between 1/5 and 1/3 length for udl’s and in a low shear zone for point loads). This is because for anything other than simple loading the best position becomes a question of engineering judgment or is pre-defined by the service runs.

Note

Adjustment to deflections. The calculated deflections at both construction stage and composite stage are adjusted to allow for the web openings. See: ’Web Openings’in the Theory and Assumptions section.

Profiled metal decking A wide range of profiled steel decking from all current UK manufacturers and some international ones is included. You may define the profiled metal decking to span at any angle between 0° (parallel) and 90° (perpendicular) to the direction of span of the steel beam. You can also specify the attachment of the decking for parallel, perpendicular and angled conditions. Where you specify that the direction of span of the profiled metal decking to that of the steel beam is >=45°, then Composite Beam assumes it is not necessary to check the beam for lateral torsional buckling during construction stage. Where you specify that the direction of span of the profiled metal decking to that of the steel beam is 45°, then you are given the opportunity to check the steel beam for lateral torsional buckling at the construction stage. Note

This check is not mandatory in all instances. For a particular profile, gauge and fixing condition etc. you might be able to prove that the profiled metal decking is able to provide a sufficient restraining action to the steel beam until the concrete hardens. If this is so, then you can specify that the whole beam (or a part of it) is continuously restrained. Where you request to check the beam for lateral torsional buckling during construction then this is carried out in accordance with the requirements of EC3.

Where you specify that the direction of span of the profiled metal decking and that of the steel beam are parallel, you again have the same opportunity to either check the steel beam for lateral torsional buckling at the construction stage, or to set it as continuously restrained.



Longitudinal shear and decking The factors that influence the longitudinal shear capacity of your composite beam are: • concrete strength, slab depth and slab width – you can not change these independently for the longitudinal shear check, since they apply equally to the entire composite beam design,

• the attachment (or lack of attachment) of the decking - not applicable for parallel decks, • the areas of Transverse and Other reinforcement which you provide in your beam. Attachment of decking There are six separate cases which are detailed in the following table:

Beam Type

Decking angle Perpendicular Comment

Internal

Default setting • Discontinuous but effectively attached

“Discontinuous and not effectively attached” would be a more onerous condition than the default.

Parallel

• not applicable.

Angled

• Discontinuous but effectively attached

Comment

The comments for perpendicular and parallel decking angles above apply to the angled condition.

Perpendicular Comment Edge

• Discontinuous not effectively attached.

In this case you are expected to manually detail the provision of U-bars in accordance with SCI P333.

Parallel

• Not effectively attached.

Angled

• Not effectively attached.

Concrete slab You can define concrete slabs in both normal and lightweight concrete provided that you comply with the following constraints: • the slab depth must be between 90 and 500 mm,

• Normal concrete range C20/25 - C60/75 - See Clause 3.1(2), • Lightweight concrete range LC20/22 - LC60/66 - See Clause 3.1(2), • Minimum density for lightweight concrete 1750 kg/m3 - see Clause 6.6.3.1(1). Concrete properties are obtained by reference to EN 1992-1-1, 3.1 for normal concrete and EN 1992-1-1, 11.3 for lightweight concrete. If normal concrete is specified, you are required to specify the type of aggregate used as this influences the value of Elastic Modulus.



The default concrete densities are as follows: • normal concrete, wet - 2600 kg/m3,

• normal concrete, dry - 2500 kg/m3, • lightweight concrete, wet - 2150 kg/m3, • lightweight concrete, dry - 2050 kg/m3, Note

The default densities above allow for 0.5kN/m3 reinforcement; the wet densities also allow 1kN/m3 for water. The dry density of unreinforced concrete is taken from BS EN 1991-1-1 Annex A.

Shear connectors In the Eurocode version of Composite Beam, only 19 mm diameter studs with 100 and 125 nominal height (95 and 120 as welded height) are offered. These do not have a given capacity as their resistance is derived. Studs may be positioned in a wide range of patterns.

Reinforcement Since the profile metal decking can be perpendicular, parallel or at any other angle to the supporting beam the following assumptions have been made: • Transverse reinforcement is provided specifically for longitudinal shear,

• if you use single bars they are always assumed to be at 90° to the span of the beam, • if you use mesh then it is assumed to be laid so that the main bars1 are at 90° to the span of the beam.

• Other reinforcement is provided for other reasons, • if you use single bars they are always assumed to be laid in the direction that is parallel to the trough of the profile metal decking.

• if you use mesh then it is assumed to be laid such that the main bars(1) are always parallel to the trough. The following reinforcement choices are available: • high yield steel, H

• Mesh, A • Mesh, B • Mesh, C The reinforcement you specify is assumed to be placed at a position in the depth of the slab where it is able to contribute to the longitudinal shear resistance. Note

The modulus of elasticity, Es is taken to be the same as for structural steel i.e. 210,000 N/mm2.

Footnotes 1. These are the bars that are referred to as longitudinal wires in BS 4483: 1998 Table 1.



Fibre Reinforced Concrete In the Eurocode version of Composite Beam the option to use fibre reinforcement is not yet available.

Construction stage restraint conditions If you do need to check the lateral torsional buckling of the beam during construction (in the case where the profiled metal decking is unable to provide an acceptable level of restraint) you can define the effective length by: • specifying the factors that you want to use for the lengths between restraints,

• or, you can enter the effective length of the sub-beam directly by entering a value (in m), You can position additional restraints at any point along the beam as required. You can also specify that any length (or lengths) of the beam should be taken as being fully restrained against lateral torsional buckling, independent of the restraint conditions for the adjacent length(s).

Loading You may specify a wide range of applied loading including: • uniform distributed loads (over the whole or part of the beam),

• point loads, • varying distributed loads (over the whole or part of the beam), • trapezoidal loads. All loads must be positive since the beam is considered as simply supported and no negative moment effects are accommodated. Construction stage loading You define these loads into one or more loadcases as required. The Slab wet loadcase is reserved for the self weight of the wet concrete in the slab. If working within a Fastrak Building Designer model, clicking the Automatic Loading check box enables this to be automatically calculated based on the wet density of concrete1 and the area of slab supported. An allowance for ponding can optionally be included. If you uncheck Automatic Loading, or if you are using Composite Beam as a standalone application, the Slab wet loadcase is initially empty - it is therefore important that you edit this loadcase and define directly the load in the beam due to the self weight of the wet concrete. If you do not do this then you effectively would be designing the beam on the assumption that it is propped at construction stage. It is usual to define a loadcase for Imposed construction loads in order to account for heaping of the wet concrete etc.

Footnotes 1. You will have defined this value on the Floor Construction page.



Having created the loadcases to be used at construction stage, you then include them, together with the appropriate factors in the dedicated Construction stage design combination. You can include or exclude the self-weight of the beam from this combination and you can define the load factors that apply to the self weight and to each loadcase in the combination. Note

You should include the Slab wet loadcase in the Construction stage combination, it can not be placed in any other combination since it’s loads relate to the slab in its wet state. Conversely, you can not include the Slab dry loadcase in the Construction stage combination, since it’s loads relate to the slab in its dry state. The loads in the Construction stage combination should relate to the slab in its wet state and any other loads that may be imposed during construction.

Note

You are required to determine the effective lengths to be used in lateral torsional buckling (if such a check is appropriate). All loads within a Construction Stage combination are considered as 'non-destabilizing' - thus you must adjust the effective lengths if any of your loads are 'destabilizing'.

Tip

If you give any additional construction stage loadcases a suitable title you will be able to identify them easily when you are creating the Construction stage combination.

Composite stage loading You define the composite stage loads into one or more loadcases which you then include, together with the appropriate factors in the design combinations you create. You can include or exclude the self-weight of the steel beam from any combination and you can define the load factors that apply to the beam self weight and to each loadcase in the combination. The Slab dry loadcase is reserved for the self weight of the dry concrete in the slab. If working within a Fastrak Building Designer model, clicking the Automatic Loading check box enables this to be automatically calculated based on the dry density of concrete1 and the area of slab supported. An allowance for ponding can optionally be included. If you uncheck Automatic Loading, or if you are using Composite Beam as a standalone application, the Slab dry loadcase is initially empty - it is therefore important that you edit this loadcase and define directly the load in the beam due to the self weight of the dry concrete. For each other loadcase you create you specify the type of loads it contains – Dead, Imposed or Wind. For each load that you add to an Imposed loadcase you can specify the percentage of the load which is to be considered as acting long-term (and by inference that which acts only on a short-term basis). All loads in Dead loadcases are considered to be entirely long-term while those in Wind loadcases are considered entirely short-term.

Footnotes 1. You will have defined this value on the Floor Construction page



Construction stage design checks When you use Composite Beam to design or check a beam for the construction stage (the beam is acting alone before composite action is achieved) the following conditions are examined in accordance with EC3: • section classification (EC3 Table 5.2),

• major axis shear capacity (EC3 Clause 6.2.6 (1)), • web shear buckling (EC3 Clause 6.2.6 (6)), • moment capacity: • EC3 Equation 6.13 for the low shear condition, • EC3 Equation 6.29 for the high shear condition, • lateral torsional buckling resistance (EC3 Clause 6.3.2.3), Note

This condition is only checked in those cases where the profile decking does not provide adequate restraint to the beam,

• web openings, • construction stage total load deflection check. Composite stage design checks When you use Composite Beam to design or check a beam for the composite stage (the beam and concrete act together, with shear interaction being achieved by appropriate shear connectors) the following Ultimate Limit State and Serviceability Limit State conditions are examined in accordance with EC4, unless specifically noted otherwise. Ultimate Limit State Checks

• section classification - the classification system defined in EC3 Clause 5.5.2 applies to cross-sections of composite beams,

• vertical shear capacity in accordance with EC3 Clause 6.2.6, • longitudinal shear capacity allowing for the profiled metal decking, transverse reinforcement and other reinforcement which has been defined,

• number of shear connectors required (EC4 Clause 6.6.1.3 (5)) between the point of maximum moment and the end of the beam, or from and between the positions of significant point loads,

• moment capacity, • web openings. Serviceability Limit State Checks • service stresses - although there is no requirement to check these in EC4 for buildings (EC4 Clause 7.2.2), concrete and steel top/bottom flange stresses are calculated but only reported if the stress limit is exceeded.

• deflections, • self-weight, • SLAB loadcase, • dead load,



• imposed load, • total deflections, • natural frequency check.

Theory and Assumptions This section describes the theory used in the development of Composite Beam and the major assumptions that have been made, particularly with respect to interpretation of both EC3 and EC4. A basic knowledge of EC3 and the design methods for composite beams in EC4 is assumed.

Analysis method Composite Beam uses a simple analysis of a statically determinate beam to determine the forces, moments and so on, to be resisted by the beam under the Construction stage, at the Serviceability Limit State and at the Ultimate Limit State.

Design method The design methods employed to determine the adequacy of the section for each condition are those consistent with EC4 unless specifically noted otherwise.

Construction stage Composite Beam performs all checks for this condition in accordance with EC3. Section classification Cross-section classification is determined using EC3 Table 5.2. At construction stage the classification of the section must be Class 1, Class 2 or Class 3. Sections which are classified as Class 4 are beyond the scope of Composite Beam. Note

Clause 5.5.2 (6) is implemented, not the alternative 5.5.2 (7). Clause 5.5.2 (11) is not implemented Clause 5.5.2 (12) is not implemented

Member strength checks Member strength checks are performed at the point of maximum moment, the point of maximum shear, the position of application of each point load, and at all other ‘points of interest’ along the beam. Shear capacity — is determined in accordance with EC3 Clause 6.2.6 (1). Where the applied shear force exceeds 50% of the capacity of the section, the high shear condition applies to the bending moment capacity checks (see below). The following points should be noted: • No account is taken of fastener holes in the flange or web - see EC3 6.2.6 (7)

• Shear is not combined with torsion and thus the resistance is not reduced as per EC3 6.2.6(8)



Web Shear buckling — the EC3 Clause 6.2.6 (6) limit is checked and if exceeded a warning is given. The warning indicates that additional calculations to EN 1993-1-5 are not carried out. Bending moment capacity — for low shear this is calculated to EC3 Equation 6.13. In the high shear case Equation 6.29 is used. Where the high shear condition applies, the moment capacity calculation is made less complicated by conservatively adopting a simplified shear area. Lateral torsional buckling checks Composite Beam allows you to switch off lateral torsional buckling checks by specifying that the entire length between the supports is continuously restrained. If you use this option you must be able to provide justification that the beam is adequately restrained against lateral torsional buckling during construction. When the checks are required you can position restraints at any point within the length of the main beam and can set the effective length of each sub-beam (the portion of the beam between one restraint and the next) either by giving factors to apply to the physical length of the beam, or by entering the effective length that you want to use. Each sub-beam which is not defined as being continuously restrained is checked in accordance with EC3 Clause 6.3.2.3. Deflection checks Composite Beam calculates relative deflections. (see ’Deflection checks’ in the Basic Principles chapter of this handbook.) The following deflections are calculated for the loads specified in the construction stage load combination: • the dead load deflections i.e. those due to the beam self weight, the Slab Wet loads and any other included dead loads,

• the imposed load deflections i.e. those due to construction live loads, • the total load deflection i.e. the sum of the previous items. The loads are taken as acting on the steel beam alone. The ‘Service Factor’ (default 1.0), specified against each load case in the construction combination is applied when calculating the above deflections. If requested by the user, the total load deflection is compared with either a span-over limit or an absolute value The initial default limit is span/200.

Composite stage Composite Beam performs all checks for the composite stage condition in accordance with EC4 unless specifically noted otherwise. Equivalent steel section - Ultimate limit state (ULS) An equivalent steel section is determined for use in the composite stage calculations by removing the root radii whilst maintaining the full area of the section. This approach reduces the number of change points in the calculations while maintaining optimum section properties.



Section classification (ULS) Composite Beam classifies the section in accordance with the requirements of EC3, 5.5.2 except where specifically modified by those of EC4. A composite section is classified according to the highest (least favourable) class of its steel elements in compression. The compression flange and the web are therefore both classified and the least favourable is taken as that for the whole section. Flanges of any class that are fully attached to a concrete flange are assumed to be Class 1. The requirements for maximum stud spacing according to Clause 6.6.5.5 (2) are checked and you are warned if these are not satisfied. There are a small number of sections which fail to meet Class 2 at the composite stage. Although EC4 covers the design of such members they are not allowed in this release of Composite Beam.



Member strength checks (ULS) It is assumed that there are no loads or support conditions that require the web to be checked for transverse force. (Clause 6.5) Member strength checks are performed at the point of maximum moment, the point of maximum shear, the position of application of each point load, and at all other points of interest along the beam. Shear Capacity (Vertical) — The resistance to vertical shear, VRd, is taken as the resistance of the structural steel section, Vpl,a,Rd. The contribution of the concrete slab is neglected in this calculation. The shear check is performed in accordance with EC3, 6.2.6. Moment Capacity — for full shear connection the plastic resistance moment is determined in accordance with Clause 6.2.1.2. For the partial shear connection Clause 6.2.1.3 is adopted. In these calculations the steel section is idealised to one without a root radius so that the position of the plastic neutral axis of the composite section can be determined correctly as it moves from the flange into the web. Where the vertical shear force, VEd, exceeds half the shear resistance, VRd, a (1- ) factor is applied to reduce the design strength of the web - as per Clause 6.2.2.4. Shear Capacity (Longitudinal) — the design condition to be checked is: vEd  vRd where vEd = design longitudinal shear stress vRd = design longitudinal shear strength (resistance) vEd is evaluated at all relevant locations along the beam and the maximum value adopted. vRd is evaluated taking account of the deck continuity, it’s orientation and the provided reinforcement. This approach uses the ‘truss analogy’ from EC2. (See Figure 6.7 of EC2). In these calculations, two planes are assumed for an internal beam, and one for an edge beam. Only the concrete above the deck is used in the calculations. The values of vRd based on the concrete ‘strut’ and the reinforcement ‘tie’ are calculated. The final value of vRd adopted is then taken as the minimum of these two values. The angle of the strut is minimised to minimise the required amount of reinforcement - this angle must lie between 26.5 and 45 degrees.



In the calculations of vRd the areas used for the reinforcement are as shown in the following table.

Decking angle

Reinforcement type

Area used

transverse

that of the single bars defined or for mesh the area of the main wiresa

other

that of the single bars defined or for mesh the area of the main wires(a)

transverse

that of the single bars defined or for mesh the area of the main wires(a)

perpendicular

parallel other

single bars have no contribution, for mesh the area of the minor wiresb

a. These are the bars that are referred to as longitudinal wires in BS 4483: 1998 Table 1. b. These are the bars that are referred to as transverse wires in BS 4483: 1998 Table 1.

If the decking spans at some intermediate angle (r) between these two extremes then the program calculates: • the longitudinal shear resistance as if the sheeting were perpendicular, vRd,perp,

• the longitudinal shear resistance as if the sheeting were parallel, vRd,par, • then the modified longitudinal shear resistance is calculated from these using the relationship, vRd,perpsin2(r) + vRd,parcos2(r). Minimum area of transverse reinforcement The minimum area of transverse reinforcement is checked in accordance with Clause 6.6.6.3. Shear connectors (ULS) Dimensional Requirements — Various limitations on the use of studs are given in the code.



The following conditions in particular are drawn to your attention:

Parameter Spacing

Rule Ductile connectors may be spaced uniformly over length between critical cross-sections if:

Clause/Comment 6.6.1.3(3) - not checked

- All critical cross-sections are Class 1 or 2 - The degree of shear connection,  is within the range given by 6.6.1.2 and - the plastic resistance moment of the composite section does not exceed 2.5 times the plastic resistance moment of the steel member alone. Edge Distance

eD >= 20 mm

6.6.5.6(2) - not checked

eD  9*tf*sqrt(235/fy)

6.6.5.5(2) - applies if bare steel beam flange is Class 3 or 4 - not checked

Location

If can't be located in centre of trough, place alternately either side of the trough throughout span


Cover

The value from EC2 Table 4.4 less 5mm, or 20mm whichever is the greater.


The program does not check that the calculated stud layout can be fitted in the rib of the deck. Design resistance of the shear connectors — For ribs parallel to the beam the design resistance is determined in accordance with Clause 6.6.4.1. The reduction factor, kl is obtained from Equation 6.22. For ribs perpendicular to the beam, Clause 6.6.4.2 is adopted. The reduction factor, kt is obtained from Equation 6.23. The factor kt should not be taken greater than the appropriate value of kt,max from the following table;

No of Stud Connectors per rib nr = 1

nr = 2

Thickness of sheet, t mm

Studs with d  20 mm and welded through profiled steel sheeting, kt,max

Profiled sheeting with holes and studs with d=19 or 22 mm, kt,max

 1.0

0.85

0.75

> 1.0

1.00

0.75

 1.0

0.70

0.60

> 1.0

0.80

0.60



Note

Only the first column of values of kt.max is used from the above table since the technique of leaving holes in the deck so that studs can be welded directly to the beam is not used.

For cases where the ribs run at an angle, r the reduction factor is calculated as: kt*sin2r + kl*cos2r Stud optimization is a useful facility since there is often some over conservatism in a design due to the discrete changes in the size of the section. If you choose the option to optimise the shear studs, then Composite Beam will progressively reduce the number of studs either until the minimum number of studs to resist the applied moment is found, until the minimum allowable interaction ratio is reached or until the minimum spacing requirements are reached. This results in partial shear connection. The program can also automatically layout groups of 1 or 2 studs with constraints that you specify. For details refer to ’Layout of Studs’ in the Design Aspects section of this chapter. The degree of shear connection is checked at the point of maximum bending moment or the position of a point load if at that position the maximum utilisation ratio occurs. Note

During the selection process, in auto design mode point load positions are taken to be ‘significant’ (i.e. considered as positions at which the maximum utilisation could occur) if they provide more than 10% of the total shear on the beam. For the final configuration and for check mode all point load positions are checked.

To determine if the degree of shear connection is acceptable Composite Beam applies the following rules: • If the degree of shear connection at the point of maximum moment is less than the minimum permissible shear connection, then this generates a FAIL status,

• If the point of maximum utilisation ratio occurs at a point that is not the maximum moment position and the degree of shear connection is less than the minimum permissible shear connection, then this generates a WARNING status,

• If the degree of shear connection at any other point load is less than the minimum permissible shear connection, then this does not affect the status in any way. Lateral torsional buckling checks (ULS) The concrete slab is assumed to be laterally stable and hence there is no requirement to check lateral torsional buckling at the composite stage. (Clause 6.4.1). Section properties - serviceability limit state (SLS) A value of the short term elastic (secant) modulus, Ecm is defaulted in Composite Beam for the selected grade of concrete. The long term elastic modulus is determined by dividing the short term value by a user defined factor - default 3.0. The elastic section properties of the composite section are then calculated using these values as appropriate (see the table below). This approach is used as a substitute for the approach given in EC4 Equation 5.6 in which a knowledge of the creep coefficient, t, and the creep multiplier, L is required. It is envisaged that you will make use of EN 1992-1-1(Ref. 6) when establishing the appropriate value for the factor.



EN 1994-1-1, Clause 7.3.1.(8) states that the effect on deflection due to curvature imposed by restrained drying shrinkage may be neglected when the ratio of the span to the overall beam depth is not greater than 20. This relates to normal weight concrete. Fastrak makes no specific allowance for shrinkage curvature but does provide you with a Warning when the span to overall depth exceeds 20 irrespective of whether the concrete is normal weight or lightweight. Where you consider allowance should be made, it is suggested that you include this as part of the 'factor' described above. Composite Beam calculates the deflection for the beam based on the following properties:

Loadcase Type

Properties used

self-weight

bare beam

Slab Dry

bare beam

Dead

composite properties calculated using the long term elastic modulus

Live

composite properties calculated using the effective elastic modulus appropriate to the long term load percentage for each load. The deflections for all loads in the loadcase are calculated using the principle of superposition.

Wind

composite properties calculated using the short term elastic modulus

Total loads

these are calculated from the individual loadcase loads as detailed above again using the principle of superposition

Deflection checks (SLS) Composite Beam calculates relative deflections. (see ’Deflection checks’ in the Basic Principles chapter of this handbook.) The composite stage deflections are calculated in one of two ways depending upon the previous and expected future load history:

• the deflections due to all loads in the Slab Dry loadcase and the self-weight of the beam are calculated based on the inertia of the steel beam alone (these deflections are not modified for the effects of partial interaction). Note

It is the Slab Dry deflection alone which is compared with the limit, if any, specified for the Slab loadcase deflection.

• the deflections for all loads in the other loadcases of the Design Combination will be based on the inertia of the composite section allowing for the proportions of the particular load that are long or short term (see above). When necessary these will be modified to include the effects of partial interaction.



Note

Composite Beam reports the deflection due to imposed loads alone (allowing for long and short term effects). It also reports the deflection for the SLAB loadcase, as this is useful for pre-cambering the beam. The beam Self-weight, Dead and Total deflections are also given to allow you to be sure that no component of the deflection is excessive.

Stress checks (SLS) There is no requirement to check service stresses in EC4 for buildings (Clause 7.2.2). However, since the deflection calculations are based on elastic analysis then at service loads it is logical to ensure that there is no plasticity at this load level. Composite Beam calculates the worst stresses in the extreme fibres of the steel and the concrete at serviceability limit state for each load taking into account the proportion which is long term and that which is short term. These stresses are then summed algebraically. Factors of 1.00 are used on each loadcase in the design combination (you cannot amend these). The stress checks assume that full interaction exists between the steel and the concrete at serviceability state. The stresses are not reported unless the stress limit is exceeded, in which case a warning message is displayed. Natural frequency checks (SLS) Composite Beam calculates the approximate natural frequency of the beam based on the simplified formula published in the Design Guide on the vibration of floors(Ref. 7) which states that 18Natural frequency = -----

where  is the maximum static instantaneous deflection that would occur under a load equivalent to the effects of self-weight, dead loading and 10% of the characteristic imposed loading, based upon the composite inertia (using the short term elastic modulus) but not modified for the effects of partial interaction. Cracking of concrete (SLS) In Clause 7.4.1(4) simply supported beams in unpropped construction require a minimum amount of longitudinal reinforcement over an internal support. This is not checked by Composite Beam as it is considered a detailing requirement.

Web Openings Circular Openings as an Equivalent Rectangle Each circular opening is replaced by equivalent rectangular opening, the dimensions of this equivalent rectangle for use in all subsequent calculations are: do'= 0.9*opening diameter lo = 0.45*opening diameter



Properties of Tee Sections When web openings have been added, the properties of the tee sections above and below each opening are calculated in accordance with Section 3.3.1 of SCI P355(Ref. 8) and Appendix B of the joint CIRIA/SCI Publication P068(Ref. 9). The bending moment resistance is calculated separately for each of the four corners of each opening. Design at Construction stage The following calculations are performed where required for web openings:

• • • • • • • • •

Axial resistance of tee sections Classification of section at opening Vertical shear resistance Vierendeel bending resistance Web post horizontal shear resistance Web post bending resistance Web post buckling resistance Lateral torsional buckling Deflections

Design at Composite stage The following calculations are performed where required for web openings:

• • • • • • • • • • •

Axial resistance of concrete flange Vertical shear resistance of the concrete flange Global bending action - axial load resistance Classification of section at opening Vertical shear resuistance Moment transferred by local composite action Vierendeel bending resistance Web post horizontal shear resistance Web post bending resistance Web post buckling resistance Deflections

Deflections The deflection of a beam with web openings will be greater than that of the same beam without openings. This is due to two effects, • the reduction in the beam inertia at the positions of openings due to primary bending of the beam,



• the local deformations at the openings due to Vierendeel effects. This has two components - that due to shear deformation and that due to local bending of the upper and lower tee sections at the opening. The primary bending deflection is established by 'discretising' the member and using a numerical integration technique based on 'Engineer's Bending Theory' - M/I = E/R = /y. In this way the discrete elements that incorporate all or part of an opening will contribute more to the total deflection. The component of deflection due to the local deformations around the opening is established using a similar process to that used for cellular beams which is in turn based on the method for castellated beams given in the SCI publication, “Design of castellated beams. For use with BS 5950 and BS 449". The method works by applying a 'unit point load' at the position where the deflection is required and using a 'virtual work technique to estimate the deflection at that position. For each opening, the deflection due to shear deformation, s, and that due to local bending, bt, is calculated for the upper and lower tee sections at the opening. These are summed for all openings and added to the result at the desired position from the numerical integration of primary bending deflection. Note that in the original source document on castellated sections, there are two additional components to the deflection. These are due to bending and shear deformation of the web post. For castellated beams and cellular beams where the openings are very close together these effects are important and can be significant. For normal beams the openings are likely to be placed a reasonable distance apart. Thus in many cases these two effects will not be significant. They are not calculated for such beams but in the event that the openings are placed close together a warning is given. This will indicate that these effects on the deflection of the beam are not taken into account. This warning is issued when, so < 2.5 * do for rectangular openings so <1.5 * o for circular openings Where so=the clear length of web between adjacent openings do=the depth of a rectangular opening taken as the larger if the adjacent openings differ o=the diameter of a circular opening



Design Aspects This section aims to explain how the program can be used effectively with regard to the following design situations:

Use of Design Properties to Control Section Selection The Design Properties button provides a means by which you can both speed up the design process and control the design more precisely. Note

When you extract a beam from a Fastrak Building Designer model into Composite Beam for further investigation, Design Properties are accessed via the Design Wizard icon.

Size Constraints Size Constraints are only applicable when running the program in Design Mode. They allow you to ensure that the sections that Composite Beam proposes match any particular size constraints you may have. For instance for a composite beam you may want to ensure a minimum flange width of 150mm. If so you would simply enter this value as the Minimum width, and Composite Beam Design would not consider sections with flanges less than this width for the design of this beam.

Optimize shear connection If you choose the option to optimize the shear studs, then Composite Beam will progressively reduce the number of studs either until the minimum number of studs to resist the applied moment is found, until the minimum allowable interaction ratio is reached or until the minimum spacing requirements are reached. This results in partial shear connection. For further details of stud optimization and how the partial interaction rules are applied see ’Shear connectors (ULS)’ in Theory and Assumptions.



Sections for Study (in Fastrak Building Designer) This feature is only applicable when running the program in Design Mode. On the left of the page is a list of available order files, only one of which can be selected. The sections contained within the chosen order file appear in the Section Designation list on the right of the page. Only checked sections within this list are considered during the design process.

The design process commences by starting with the smallest section in the chosen order file. Any section that fails any of the design conditions is rejected and the design process is then repeated for the next available section in the list. On completion of the design process, the first satisfactory section from the Section Designation list is assigned to the beam. Caution

Limiting the choice of sections by unchecking a section within an order file is a global change that affects ALL projects, (not just the currently open one). It is typically used therefore to eliminate unavailable or non-preferred sections from the design process. If design requirements for an individual beam require section sizes to be constrained, (due to, for example depth restrictions), then the choice of sections should be limited instead by using Size Constraints, (as these only affect the current beam).

Note

It is possible to create additional order files using a text editor. If you require to do so, please contact your local CSC Technical Support Team for guidance on how to proceed.



Sections for Study (in Composite Beam) When you extract a beam from a Fastrak Building Designer model into Composite Beam for further investigation, a benefit of doing so is that several order files can be considered at the same time. If a check is placed against an order file the sections contained within it appear in the Section Designation list on the right of the page. Only checked sections within this list are considered during the design process.

Typically, you would uncheck those order files that are unlikely to be appropriate for composite beam design. Doing so speeds up the solution. The design process commences by starting with the smallest section in each order file. Any section that fails any of the design conditions is rejected and the design process is then repeated for the next available section in the list. On completion of the design, all the satisfactory sections from the Section Designation list are displayed and the results for each of these can be examined before one of the sections is assigned to the beam. Caution

Limiting the choice of sections by either unchecking an order file or an individual section is a global change that affects ALL projects, (not just the currently open one). It is typically used therefore to eliminate unavailable or non-preferred sections from the design process. If design requirements for an individual beam require section sizes to be constrained, (due to, for example depth restrictions), then the choice of sections should be limited instead by using Size Constraints, (as these only affect the current beam).

Note




Deflection It is often found that serviceability criteria control the design of normal composite beams. This is because they are usually designed to be as shallow as possible for a given span. The Deflections page allows you to control the amount of deflection by applying either a relative or absolute limit to the deflection under different loading conditions.

A typical application of these settings might be: • not to apply any deflection limit to the SLAB loads, as this deflection can be handled through camber,

• to apply the relative span/360 limit for imposed load deflection, to meet code requirements,

• possibly, to apply an absolute limit to the total load deflection to ensure the overall deflection is not too large. Camber Camber is primarily used to counteract the effects of dead load on the deflection of the beam. This is particularly useful in long span composite construction where the self-weight of the concrete is cambered out. It also ensures little, if any, concrete over pour occurs when placing the concrete.

The amount of camber can be specified either: • As a value

• As a proportion of span • As a proportion of dead load deflection In the latter case, if 100% of the dead load deflection is cambered out, it is also possible to include a proportion of the live load deflection if required. A lower limit can be set below which the calculated camber is not applied, this ensures that impractical levels of camber are not specified.



Effective width calculations Checking the effective width used in the design (in Fastrak Building Designer) Fastrak Building Designer will calculate the effective width of the compression flange, beff, for each composite beam as per Clause 5.4.1.2 of EC4. It is taken as the smaller of: • Secondary beams: the spacing of the beams, or beam span/4

• Primary beams (conservatively): 80% of the spacing of beams, or beam span/4 • Edge beams: half of above values, as appropriate, plus any projection of the slab beyond the centreline of the beam. These effective breadths are used in both strength and serviceability calculations. Although the program calculates beff, it is your responsibility to accept the calculated figure or alternatively to adjust it. Engineering judgement may sometimes be required. For example consider the beam highlighted below:

The program calculates the effective width as the sum of: • to the right of the beam, beff(right) = beam span/8

• to the left of the beam, beff(left) = one half of the shortest distance to the centreline of the adjacent diagonal beam To the left of the beam, some engineers might prefer to use one half of the mean distance to the adjacent beam. To do so you would need to manually adjust the calculated value via the Floor Construction page of the Beam Properties. Note

The calculation of the effective width to the left of the beam is only carried out if the angled beam lies within the tolerance on rectilinearity set within the Building Designer Design Options/Composite. The default tolerance is 15 degrees. If in the above example the angled beam were at say 20 degrees, you would then be prompted to enter the effective width manually - (unless of course you chose to relax the tolerance to something greater than 20 degrees).



If modifications have been made to the floor layout you can get Fastrak Building Designer to recalculate effective widths, either for selected beams only, or for all beams in the model. Pick Recalculate Effective Widths... from the Building menu to do this. Checking the effective width used in the design (in Composite Beam) When Composite Beam is run as a standalone program it does not know anything about the slab spans and so the limitations with regard the spacing of the beams are not implemented. Note The correct effective width based on beam span or distance to adjacent beams is passed through when you extract a beam from a from Fastrak Building Designer model into Composite Beam for further investigation.

Application of NCCI PN002 to Partial Shear Connection New requirements have been added to align EC4 with Amendment 1 to BS 5950-3.1:1990 published in April 2010. See documents PN001 and PN002 on www.steel-ncci.org. Stud strength is reduced (in some cases) to allow use of mesh above the stud head and the NCCI allows the use of the better partial interaction rules for ‘higher ductility’ studs. See also AD343 in NSC March 2010. A new Apply NCCI PN002 check box is available on the Studs -Strength page for the EC4 UK NA ‘Design Code’. These new features come with a number of requirements that are explicit in BS 5950-3.1: 1990 + A1 2010 and are explained in the above NCCI. It should be noted that to obtain the benefits of this NCCI,

• for all deck types and orientation the design live load (q qk) is limited to 9 kN/m2 • for all deck types and orientation the beam should be ‘unpropped’ at the construction stage (this is a general assumption in Fastrak for all composite beams).

• for perpendicular trapezoidal decks the studs should be placed on the ‘favourable’ side or in the central position.

• for perpendicular trapezoidal decks the reinforcement is assumed to be above the head of the stud. Consequently, a reduction is made to the stud resistance in accordance with NCCI PN001. Note that PN002 allows the use of improved partial interaction formulae for general use (not just linked to higher ductility studs) when the construction is ‘unpropped’. Fastrak only deals with unpropped construction and so this improvement can be applied. The gains are not that significant but are felt to be worthwhile, for example at 15m span the current EC4 formula gives the minimum partial interaction as 0.7 whereas these new formulae would give 0.63. For perpendicular trapezoidal decks the stud resistance is always reduced in the case of two studs per trough. There are circumstances where this may be conservative.



Layout of Studs Studs - Strength page You can allow group sizes of 1 or 2 studs - any group sizes that you don't want to be considered can be excluded.

For example, if you do not set up groups with 2 studs, then in auto-design the program will only try to achieve a successful design with a maximum of 1 stud in a group. For each group that you allow you must enter the Distance to nearer side of rib (e) and you also specify the pattern to be adopted (e.g. along the beam, across the beam, or staggered). Note

It is up to you to check that a particular pattern fits within the confines of the rib and beam flange since Fastrak will draw it (and use it in design) anyway.

Connectors-Layout page. The overall layout of studs is controlled from here. When running in Design Mode you may not want to specify the stud layout at the start of the design process. To work in this way check Auto-layout to have the program automatically control how the stud design will proceed. When the beam is subsequently designed Auto-layout invokes an automatic calculation of the required number of studs, which is optimized to provide an efficient design. Note

'Auto layout' can actually be checked regardless of whether you are auto designing the beam size or checking it. The combination of 'Check' design with 'Auto layout' of studs can be used to assist you to rationalise your designs e.g. to force a beam to be the same size as others in the building but have Fastrak determine the most efficient layout of studs

You may choose to perform the initial design with Auto-layout checked and then refine the spacing with Auto-layout unchecked if the spacing is not exactly as you require. This may arise if for instance the theoretical design needs to be marginally adjusted for practical reasons on site.

Auto-layout for Perpendicular decks For perpendicular decks, the Auto-layout dialog provides two options for laying out the studs as shown below:



Uniform — forces placement in ribs at the same uniform spacing along the whole length of the beam.

Whether the stud groups are placed in every rib (as shown above), alternate ribs, or every third rib etc. can be controlled by adjusting the limits you set for Minimum group spacing ( ) x rib and Maximum group spacing ( ) x rib. The number of studs in each group will be the same along the whole length of the beam, this number can be controlled by adjusting the limits you set on the Studs - Strength page. Example

If you set Minimum group spacing 2 x rib and Maximum group spacing 3 x rib, then the program will only attempt to achieve a solution with studs placed in alternate ribs, or studs placed in every third rib. It will not consider a solution in which studs are placed in every rib. Additionally, if on the Studs - Strength page, you have allowed groups of 1 stud and 2 studs; then if 1 stud per group proves to be insufficient the program will then consider 2 studs per group.

Non-uniform — If optimization has been checked (see ’Use of Design Properties to Control Section Selection’) studs are placed at suitable rib intervals (every rib, alternate ribs, every third rib etc.), in order to achieve sufficient interaction without falling below the minimum allowed by the code. If optimization has not been checked, studs are placed at suitable rib intervals in order to achieve 100% interaction. Knowing the number of studs necessary to achieve the required level of interaction, it is possible that placement at a given rib interval could result in a shortfall; the program will attempt to accommodate this by working in from the ends, (as shown in the example below). If every rib is occupied and there is still a shortfall, the remainder are 'doubled-up', by working in from the ends once more.

In this example the point of maximum moment occurs one third of the way along the span, this results in an asymmetric layout. If you prefer to avoid such arrangements you can check the box Adjust layout to ensure symmetrical about centreline. A redesign would then result in the symmetric layout shown below.

For both Uniform and Non-uniform layouts, if the minimum level of interaction can not be achieved this is indicated on the design summary thus: “Not able to design stud layout”.



Auto-layout for Parallel decks For parallel decks, the Auto-layout again provides Uniform and Non-uniform layout options, but the way these work is slightly different.

Uniform — forces placement at a uniform spacing along the whole length of the beam. The spacing adopted will be within the limits you set for Minimum group spacing distance and Maximum group spacing distance. If the point of maximum moment does not occur at mid span, the resulting layout will still be symmetric.

The number of studs in each group will be the same along the whole length of the beam, this number can be controlled by adjusting the limits you set on the Studs - Strength page. Non-uniform — If optimization has been checked (see ’Use of Design Properties to Control Section Selection’) studs are placed at a suitable spacing in order to achieve sufficient interaction without falling below the minimum allowed by the code. If optimization has not been checked, studs are placed at a suitable spacing in order to achieve 100% interaction. If the point of maximum moment does not occur at mid span, the resulting non-uniform layout can be asymmetric as shown below.

For both Uniform and Non-uniform layouts, if the minimum level of interaction can not be achieved this is indicated on the design summary thus: “Not able to design stud layout”. Manual Stud Layout You may prefer to manually define/adjust the group spacing along the beam. This can be achieved by unchecking Auto layout. Caution

If you specify the stud spacing manually, then it is most important to note: • the resulting design may not be the optimal design possible for the beam, or • composite design may not be possible for the stud spacing which you have specified.



To generate groups of studs at regular intervals along the whole beam use the Quick layout facility. Alternatively, if you require to explicitly define the stud layout to be adopted for discrete lengths along the beam use the Layout table.

Manual layout for Perpendicular decks For perpendicular decks, the dialog for manual layouts is as shown:

To use Quick layout, proceed in one of two ways: • Choose to position groups in either every rib, or alternate ribs, then specify the number of studs required in the group and click Generate.

• Alternatively: specify the total number of studs, then when you generate, if the number specified is greater than the number of ribs, one will be placed in every rib and the remainder will be 'doubled-up' in the ribs at each end starting from the supports. Similarly if the number specified is less than the number of ribs, but greater than the number of alternate ribs, one will be placed in every alternate rib and the remainder will be placed in the empty ribs. Limits of 600mm or 4 x overall slab depth, (whichever is less), are considered. To use the Layout table: • For each segment you should define the following parameters: No. of studs in length and No. of studs in group; Group spacing x rib.



• Your input for these parameters is used to automatically determine Distance end 2 - this latter parameter can not be adjusted directly, hence it is greyed out.

Note

To make it easier to visualize the effect of adjusting any of the above parameters, you can right click on the diagram below the table to switch from a zoomed in view to a display of the entire span. (right clicking once more returns to the zoomed in view).

• If required click Insert to divide the beam into additional segments. (Similarly Delete will remove segments). You can then specify a different stud layout for each segment.

• We would advise that having entered No. of studs in length, group and spacing and ignoring Distance ends 1 and 2 you click Update, this will automatically fill in the missing fields.

• You can also use the diagram below the table to change the number of studs in individual ribs. Simply hover the cursor over the rib until a hand icon appears as shown below. Click in the rib and the number of studs will increase and the table will be updated accordingly.



Manual layout for Parallel decks For parallel decks, the dialog for manual layouts is as shown:

To use Quick layout, proceed in one of two ways: • Choose to position groups at a set repeat distance, then specify the number of studs required in the group and click Generate.

• Alternatively: specify the total number of studs, then click Generate - the program calculates the repeat distance automatically, subject to the code limits. To use the Layout table: • The preferred method is to check the box to Define by no. of studs, in which case you can adjust the No. of studs in length and No. of studs in group. Alternatively you could leave this unchecked and then adjust No. of studs in group and Group spacing dist.

• If required click Insert to divide the beam into additional segments. (Similarly Delete will remove segments). You can then specify Distance end 1 for each new segment and it’s own stud layout.


Note


To make it easier to visualize the effect of adjusting any of the above parameters, you can right click on the diagram below the table to switch from a zoomed in view to a display of the entire span. (right clicking once more returns to the zoomed in view).



Non-composite design within Composite Beam Typically, at the outset you will know which beams are to be simple (non-composite) and which are to be composite and you will have specified the construction type accordingly. However, circumstances can arise in which a beam initially intended to be composite proves to be ineffective. Examples might be: • very small beams,

• beams with a significant point load close to a support, • beams where the deck is at a shallow angle to the beam, hence the stud spacing is impractical,

• beams where, for a variety of reasons, it is not possible to provide an adequate number of studs, and

• edge beams, where the advantages of composite design (e.g. reduced depth) are not so clear Where Composite Beam is unable to find a section size which works compositely, you can ask for a non-composite design for the same loading. You will find that this facility is particularly useful when you are using Composite Beam as a stand-alone application, or when you extract a key beam from a Fastrak Building Designer model into Composite Beam for further investigation. To invoke non-composite design in Fastrak Building Designer Edit the properties of the beam (right-click the beam and then pick the Edit option from the context menu that appears), and on the Design page ensure that Treat as non-composite is checked.

To invoke non-composite design in Composite Beam Pick Beam/Non Composite from the main menu, or click the non-composite icon ( ) from the Beam toolbar.



Automatic transverse shear reinforcement design It is possible to automatically design the amount of transverse shear reinforcement for each beam. This is achieved in Fastrak Building Designer by checking the Autoselect option, which is on the Reinforcement tab of the Composite Beam Properties, (or, if running Composite Beam directly, on the Reinforcement tab of the Floor Construction page as shown below:)

The auto-selected bars can be tied into the stud group spacing as shown above. Alternatively, the spacing can be controlled directly by the user. Irrespective of the method adopted, the user still needs to have control over the design. This is achieved in Fastrak Building Designer by clicking on the Design Properties button and then the Reinforcement tab (or, if running Composite Beam directly, via the Design Wizard). Note

You can only design transverse shear reinforcement automatically when you are designing a beam. If you are checking a beam, then you must specify the transverse shear reinforcement that you will provide, and then check out this arrangement.

Bar spacing as a multiple of stud spacing. When the option Bar spacing as a multiple of stud spacing is checked, the Reinforcement tab provides the user with control on the bar size and the multiples of stud spacing.

These can be used to achieve a selection of say, 12mm diameter bars at 2 times the stud spacing, with a slightly greater area than a less preferable 16mm diameter bars at 4 times the stud spacing.



Controlling the bar spacing directly. When the option Bar spacing as a multiple of stud spacing is not checked, the Reinforcement tab provides the user with direct control on the bar size and the bar spacing.



Composite Beam Input (in Fastrak Building Designer) In order to create a composite beam within Fastrak Building Designer, you will need to define an appropriate set of composite beam attributes. Listed below is the typical procedure for defining these attributes. Items in brackets [] are optional.

Step

Dialog

Page

Instructions

1

none

none

Create a new Beam Attribute Set.

2

Attribute Set

General


3

Attribute Set

Design

Choose Composite construction type

4

Attribute Set

Design


5

Attribute Set

Design

[Check the Gravity Only Design box* if required]

6

Attribute Set

Design


7


Size Constraints

[Define the Beam Constraints: • max and min beam size • optimize shear connection]

8


Sections for Study


9


Deflection

Define and apply deflection limits • [construction] • [slab dry] • imposed • [total]

10


Camber

[Apply camber]

11


Natural Frequency

[Define a limit for the natural frequency]

12

Attribute Set

Alignment, Type, Supports

[No changes are applicable to these attributes for composite beams]

13

Attribute Set

Size

Choose the steel grade and, if in Check Beam Mode choose the section size

14

Attribute Set

Type

Define construction stage bracing details

15

Attribute Set

Reinforcement

[Define continuity reinforcement over the beam]



Step

Dialog

Page

Instructions (Continued)

16

Attribute Set

Connectors

Choose the type of stud layout • Standard • Non-Standard

17

Attribute Set

Studs-Layout

Define stud length, material strength and stud layout

18

Attribute Set

Group Spacing

Define the stud spacing, or if in Design Beam Mode, you can choose to let the program position the studs automatically

*In order to speed the design process a distinction is made between those combinations consisting of gravity loads only and those which contain some components acting laterally (e.g. EHFs and wind loads). Setting composite beams to be designed for gravity loads only can significantly reduce the design time.



Composite Beam Input (when run as a standalone program) The design and check mode input procedures are listed below. Items in brackets [] are optional

Designing a beam In the typical procedure below items in brackets [] are optional.

Step

Icon

Instructions

1

Launch Composite Beam,

2


3

Choose the edge condition for the beam [and give the beam reference details],

4

Set Composite Beam into design beam mode,

5

Define the properties for the beam: • grade, • span,

6

Give the details of the floor construction: • floor construction type: • profiled metal decking • precast concrete slabs • profiled metal deck details or Bison precast concrete slab details (including slab information) • slab details (for profiled metal deck only) • reinforcement details (transverse and any other reinforcement present in the slab for profiled metal deck, transverse to beam only for Bison precast concrete slabs), • Define the shear connector type • shear studs • Hilti connectors • Define the shear connector layout, • construction stage restraint details (for profiled metal deck spanning parallel to or at less than 45° to direction of span of beam and for precast concrete slabs at your request),

7

Define the loadcases that apply to the beam including self-weight, construction stage loadcases and composite stage loadcases.



Step

Icon


8


9


10

Perform the design

11


12

Check the beam with the web openings. [Stiffen the web openings if necessary, or increase the size of the beam until the beam with openings is satisfactory.]

13


14


Checking a beam In the typical procedure below items in brackets [] are optional.

Step

Icon

Instructions

1

Launch Composite Beam,

2


3

Choose the edge condition for the beam [and give the beam reference details],

4

Set Composite Beam into check beam mode,

5

Define the properties for the beam: • section size, • grade, • span,


Step


Icon


6

Give the details of the floor construction: • floor construction type: • profiled metal decking • precast concrete slabs • profiled metal deck details or Bison precast concrete slab details (including slab information) • slab details (for profiled metal deck only) • reinforcement details (transverse and any other reinforcement present in the slab for profiled metal deck, transverse to beam only for Bison precast concrete slabs), • Define the shear connector type • shear studs • Hilti connectors • Define the shear connector layout, • construction stage restraint details (for profiled metal deck spanning parallel to or at less than 45° to direction of span of beam and for precast concrete slabs at your request),

7

Define the loadcases that apply to the beam including self-weight, construction stage loadcases and composite stage loadcases.

8


9


10

Perform the check,

11


12


Chapter 5 : General Beam

Chapter 5


General Beam

Introduction GENERAL BEAM - “non-composite steel beam designed as a beam/ column” The General Beam design application allows you to analyse and design a structural steel beam or cantilever which may have incoming beams providing restraint and which may or may not be continuously restrained over any length between restraints. In addition to major axis bending, it also considers minor axis bending and axial loads. Unless explicitly stated all calculations in General Beam are in accordance with the relevant sections of EC3(Ref. 1) and the associated UK(Ref. 2) or Irish(Ref. 3) National Annex.

Scope In its simplest form a general beam can be a single member between supports to which it is pinned. It is distinguished from a standard simple beam primarily by the loading it has to resist. It can also be a continuous beam consisting of multiple members that do not, with the exception of the remote ends, transfer moment to the rest of the structure. General beams that share load with columns form part of a rigid moment resisting frame. The design of general beams is carried out for doubly symmetric rolled and plated sections. Fabsec beams can be specified but cannot be designed within Fastrak Building Designer. Web openings are not permitted. General beams can be connected to supports or to the supporting structure in a number of ways. For the meaning and implementation of the various choices see “Member End Fixity and Supports”. The options are subtly different depending upon whether the general beam is defined in Fastrak Building Designer or is defined within General Beam directly. Conditions of restraint can be defined in- and out-of-plane for compression buckling and top and bottom flange for lateral torsional buckling (LTB). It is upon these that the buckling checks are based. When the general beam is an object in Fastrak Building Designer the design forces for strength and buckling checks are obtained from analysis of the member using the start forces for the member. These are obtained from the solver results. There can be a difference between the start forces from Fastrak Building Designer (analysis of the entire structure) and those obtained within General Beam (analysis of a limited model). Within General Beam a full range of loading is available, from which loadcases and design combinations can be created.



General beams can be transferred from Fastrak Building Designer to General Beam. When a general beam has been transferred from Fastrak Building Designer in this way its loads and loadcases are editable. However any changes to these will invalidate the start and end forces obtained from the building model. To cater for this, if any load or loadcase is modified, a design in General Beam will reanalyse all the beam’s loadcases. Editing of the design combinations does not require reanalysis since the start and end forces are obtained by superposition. A full range of strength and buckling checks are available. As mentioned above the buckling lengths are based on the restraints along the member. The effective lengths to use in the checks depend on: • the type of restraint particularly at supports,

• whether the loads or one component of the loads is destabilizing, • whether the frame is sway or non-sway in one or both directions – this has little effect on beam design. In all cases, General Beam sets the default effective length to 1.0L, it does not attempt to adjust the effective length (between supports for example) in any way. You are expected to adjust the effective length factor (up or down) as necessary. Any strut or LTB effective length can take the type ‘Continuous’ to indicate that it is continuously restrained over that length. Note

There is no specific factor for destabilizing loads - you are expected to adjust the 'normal' effective length factor to allow for such effects.

Each span of a continuous beam can be of different section size, type and grade. The entire beam can be set to automatic design or check design.




Limitations and Assumptions Limitations The following limitations apply: • composite beams are excluded,

• continuous general beams (more than one span) must be co-linear in the plane of the web within a small tolerance (sloping in elevation is allowed),

• only doubly symmetric prismatic sections (that is rolled or plated I- and H-sections) and doubly symmetric hollow sections (i.e. SHS, RHS and CHS) are included. Other section types are excluded,

• Fabsec beams (with or without openings) are excluded, • sections with unequal flanges are excluded. This includes plated section beams that have unequal flanges, Slimflor beams and asymmetric Slimflor beams,

• web openings are excluded, • there can be a difference in analysis results between those from Fastrak Building Designer (analysis of the entire structure) and those when run in stand-alone (analysis of a limited model),

• there is no automatic generation of pattern loads either in the stand-alone or in Fastrak Building Designer.

Assumptions All supports are considered to provide torsional restraint, that is lateral restraint to both flanges. This cannot be changed. It is assumed that a beam that is continuous through the web of a supporting beam or column together with its substantial moment resisting end plate connections is able to provide such restraint. If, at the support, the beam oversails the supporting beam or column then the detail is assumed to be such that the bottom flange of the general beam is well connected to the supporting member and, as a minimum, has torsional stiffeners provided at the support. In the Fastrak Building Designer model, when not at supports, coincident restraints to both flanges are assumed when one or more members frame into the web of the general beam at a particular position and the cardinal point of the centre-line model of the general beam lies in the web. Otherwise, only a top flange or bottom flange restraint is assumed. Should you judge the actual restraint provided by the in-coming members to be different from to what has been assumed, you have the flexibility to edit the restraints as required. Intermediate lateral restraints to the top or bottom flange are assumed to be capable of transferring the restraining forces back to an appropriate system of bracing or suitably rigid part of the structure. It is assumed that you will make a rational and ‘correct’ choice for the effective lengths between restraints for both LTB and compression buckling. The default value for the effective length factor of 1.0 may be neither correct nor safe.



Analysis Building Modeller object The member end forces for each unfactored loadcase are obtained by submitting the whole model from the Fastrak Building Designer to the solver. For a general beam in General Beam an appropriate sub-model is sent to the solver. The results from the Fastrak Building Designer and those from General Beam may not be exactly the same due to (potential) differences inherent in using the full- and sub-model.

General Beam The capacity or resistance is only calculated when an applied force exists about the relevant axis that is greater than the “ignore forces below” value you have specified.

Ultimate Limit State – Strength The strength checks relate to a particular point on the member and are carried out at regular intervals along the member and at ‘points of interest’.

Classification General — The classification of the cross section is in accordance with EC3 Cl. 5.5 Table 5.2 A general beam can be classified as: • Plastic Class = 1

• Compact • Semi-compact • Slender

Class = 2 Class = 3 Class = 4

Class 4 sections are unacceptable in General Beam and are either failed in check mode or rejected in design mode. Implementation of the below clauses is as follows: • Classification is determined using 5.5.2 (6) and not 5.5.2 (7).


• 5.5.2 (11) is not implemented. • 5.5.2 (12) is not implemented. The note at the end of Cl. 5.5.2 is not implemented. A brief study by CSC of UK rolled UBs and UCs showed that flange induced buckling in normal rolled sections is not a concern. No study was undertaken for plated sections. Hollow sections — The classification rules for SHS and RHS relate to “hot-finished hollow sections” only (“cold-formed hollow sections” are not included in this release).




Shear Capacity Major and minor axis shear — checks are performed according to clause 6.2.6 (1) for the absolute value of shear force normal to each axis at the point under consideration. Web Shear buckling — the 6.2.6 (6) limit is checked and if exceeded a warning is given. The warning indicates that additional calculations to EN 1993-1-5 are not carried out. The following points should be noted: • No account is taken of fastener holes in the flange or web - see 6.2.6 (7)

• Shear is not combined with torsion and thus the resistance is not reduced as per 6.2.6 (8) Moment Capacity Major and minor axis bending checks are performed in accordance with equation 6.9. Major axis bending — For the low shear case the calculation uses equation 6.13 for class 1 and 2 cross sections and equation 6.14 for class 3 cross sections. In the high shear case equation 6.29 is used for class 1 and 2 cross sections and equation 6.14 for class 3 cross sections. Where the high shear condition applies, the moment capacity calculation is made less complicated by conservatively adopting a simplified shear area. Minor axis bending — For the low shear case the calculation uses equation 6.13 for class 1 and 2 cross sections and equation 6.14 for class 3 cross sections. High shear in the minor axis is beyond the current program scope. Note Fastener holes in the flange or web are not accounted for in the calculations.

Axial Capacity Axial Tension — checks are performed according to equation 6.5 Implementation of the below clauses is as follows: • Cl 6.2.3 (3) - is not considered

• Cl 6.2.3 (4) - is not considered • Cl 6.2.3 (5) - is not considered • Eqn 6.7 is not considered for general beams. Axial Compression — checks are performed according to equation 6.9

Cross-section Capacity The cross-section capacity check covers the interaction of axial load and bending. Class 1 and 2 cross sections — equation 6.41 is applied. Note that in these calculations the combined effects of axial load and bending are assessed - clause 6.2.9 (4) is not considered Also note that the current "reduced plastic moduli" approach that is used in the published tables is adopted and not the approximate method given in 6.2.9.1(5). The latter is less conservative than the current approach at low levels of 'n'. Class 3 cross sections — equation 6.42 is applied. Note Axial and bending interaction checks are beyond the current program scope if coexistent high shear is present in the major axis.



Ultimate Limit State – Buckling Note

Classification for buckling checks - For rolled I sections, RHS and SHS classification varies along the member length due to the section forces changing along the member length - for combined buckling, the worst classification of the whole member should be used. In theory it should be the worst classification in the segment length considered for buckling. However, the segment lengths for lateral torsional buckling, minor axis strut buckling and major axis strut buckling can all be different. It is simpler and conservative therefore to use the worst classification in the entire member length.

Compression buckling General beams must be checked to ensure adequate resistance to buckling about both the major and minor axes and they must also be checked in the torsional mode over an associated buckling length. Since the axial force can vary throughout the beam and the buckling lengths in the two planes do not necessarily coincide, all buckling modes must be checked. There may be circumstances where it would not be safe to assume that the Combined Buckling check will always govern (see below). Effective lengths — In all cases General Beam sets the default effective length to 1.0L, it does not attempt to adjust the effective length in any way. Different values can apply in the major and minor axis. It is your responsibility to adjust the value from 1.0 where you believe it to be justified. Note It is assumed that you will make a rational and ‘correct’ choice for the effective lengths between restraints. The default value for the effective length factor of 1.0L may be neither correct nor safe. Coincident restraint points in the major and minor axis define the torsional and torsional flexural buckling effective length factor (this is assumed to be 1.0 and can not be changed). As an alternative to providing the effective length factor you can enter a value (in m) which is the actual effective length i.e. takes into account both the factor and the system length. Any strut buckling effective length can take the type ‘Continuous’ to indicate that it is continuously restrained over that length. There is no facility for specifying torsional, or torsional flexural buckling effective lengths as ‘Continuous’. There is no guidance in EC3 on the values to be used for effective length factors for beam-columns. Compression resistance — The relevant buckling resistances are calculated from Eqn 6.47. These consist of the flexural buckling resistance about both the major and minor axis i.e. Nb,y,Rd and Nb,z,Rd over the buckling lengths Lyy and Lzz and where required the buckling resistance in the torsional or flexural-torsional modes, Nb,x,Rd. The elastic critical buckling load, Ncr for flexural buckling about major and minor axes is taken from standard texts. The elastic critical buckling loads for Torsional, Ncr.T and for Torsional Flexural buckling, Ncr.TF are taken from the NCCI “Critical axial load for torsional and torsional flexural buckling modes” available free to download at www.steel-ncci.co.uk.




All section types are checked for flexural buckling. It is only hollow sections that do not need to be checked for torsional and torsional-flexural buckling.

Lateral Torsional Buckling Lateral torsional buckling checks are required between supports for all lengths that are not continuously restrained. General Beam allows you to switch off these checks by specifying that the entire length between the supports is continuously restrained against lateral torsional buckling. If you use this option you must be able to provide justification that the beam is adequately restrained against lateral torsional buckling. When the checks are required you can position restraints at any point within the length of the main beam and can set the effective length of each sub-beam (the portion of the beam between one restraint and the next) either by giving factors to apply to the physical length of the beam, or by entering the effective length that you want to use. Any individual segment can be continuously restrained in which case no LTB check will be carried out if the ‘check length’ is limited to that one segment. Each ‘check length’ which is not defined as being continuously restrained for its whole length is checked in accordance with clause 6.3.2.3. The formula for elastic critical buckling moment, Mcr is taken from standard texts. The moment factor C1 that is part of the standard formula has been derived analytically. LTB does not need to be checked for the following sections, • circular and square hollow sections,

• equal and unequal flanged I/H sections loaded in the minor axis only. Effective lengths — The value of effective length factor is entirely the choice of the engineer. The default value is 1.0. There is no specific factor for destabilizing loads - so you will have to adjust the 'normal' effective length factor to allow for such effects.

Combined buckling Combined buckling in General Beam is limited to doubly symmetric sections (I, H, CHS, SHS, RHS). In the context of combined buckling general beams are assumed to be dominated by moment with axial force. Restraints are treated as follows: • Each span is assumed to be fully supported at its ends (i.e LTB, y-y and z-z restraint) - this can not be changed.

• Tension flange restraints are ignored. • Coincident top and bottom flange restraints are considered as 'torsional' restraints i.e. as good as the supports. For each span of the general beam, the design process is driven from the standpoint of the individual LTB lengths i.e. the LTB lengths and the y-y lengths that are associated with each LTB length and the z-z lengths associated with the y-y length. Thus a 'hierarchy' is formed see the “Design Control” section below for details. Both Equ. 6.61 and Equ. 6.62 are evaluated recognizing that the LTB check is carried out for both the top flange and the bottom flange.



Effective lengths — In all cases General Beam sets the default effective length to 1.0L, it does not attempt to adjust the effective length in any way. Different values can apply in the major and minor axis. It is your responsibility to adjust the value from 1.0 where you believe it to be justified. Note It is assumed that you will make a rational and ‘correct’ choice for the effective lengths between restraints. The default value for the effective length factor of 1.0L may be neither correct nor safe. Combined buckling resistance — Equations 6.61 and 6.62 are used to determine the combined buckling resistance. With regard to these equations the following should be noted: • The ‘k’ factors used in these equations are determined from Annex B only.

• The note to Table B.3 that Cm should be limited to 0.9 is not applied (to general beams). Caution

Equations 6.61 and 6.62 are limited to doubly symmetric sections and do not consider torsional or torsional flexural buckling. Should either of these buckling modes govern the compression buckling check, you should consider very carefully whether the calculations provided by Fastrak for combined buckling can be considered valid.

Design Control Principles — There are multiple check lengths to deal with (LTB, y-y buckling and z-z buckling) all of which can be contained within or overlapped by their associated lengths. Consequently, a 'hierarchy' of checks is defined. In the approach taken the LTB segment length is taken as the driver and the other lengths whether overlapping or contained by this segment are mapped to it. Design Example — The following example illustrates how the checks are applied to I- and H-sections with equal flanges. 1.0

3.5

5.1

3.5

3.6

5.1

torsional restraint or support top flange and z-z restraint y-y restraint

The beam (span) is 10.2 m long and has torsional restraints at each end. The top flange is restrained out-of-plane at 0.7m, 4.2m and 6.6 m – these provide restraint to the top flange for LTB and to the beam as a whole for out-of-plane strut buckling. The bottom flange has one




restraint at mid-span and this restrains the bottom flange for LTB and the beam as a whole for in-plane strut buckling. (This is probably difficult to achieve in practice but is useful for illustration purposes.) Note that the top flange LTB restraints and z-z restraints are coincident in this example but will not always be coincident. General Beam identifies the following lengths and checks. (in this example all the effective length factors are assumed to be 1.0 for simplicity.)

LTB Segment

In-plane strut segment

Out-of-plane strut segment

length (m)

length (m)

6.61

0 – 5.1

0 – 0.7

6.62

0 – 5.1

0 – 0.7

6.61

0 – 5.1

0.7- 4.2

6.62

0 – 5.1

0.7- 4.2

6.61

0 – 5.1

4.2 – 6.6

6.62

0 – 5.1

4.2 - 6.6

6.61

5.1 - 10.2

4.2 - 6.6

6.62

5.1 - 10.2

4.2 - 6.6

6.61

5.1 - 10.2

6.6 - 10.2

6.62

5.1 -10.2

6.6 - 10.2

6.61

0 – 5.1

0 – 0.7

6.62

0 – 5.1

0 – 0.7

6.61

0 – 5.1

0.7- 4.2

6.62

0 – 5.1

0.7- 4.2

6.61

0 – 5.1

4.2 - 6.6

6.62

0 – 5.1

4.2 - 6.6

6.61

5.1 - 10.2

4.2 - 6.6

6.62

5.1 - 10.2

4.2 - 6.6

6.61

5.1 - 10.2

6.6 - 10.2

6.62

5.1 - 10.2

6.6 - 10.2

Equation

length (m) Top Flange 0 – 4.2 (first restraint ignored since top flange is in tension at this point)

Top Flange 4.2 – 6.6

Top Flange 6.6 - 10.2

Bottom Flange 0 – 10.2



Member End Fixity and Supports In order to provide a robust design model, the fixity at member ends and the associated supporting structure or supports to ground must be compatible with the type of connection, base and foundation that is to be used.

General Beam Stand-alone Internal supports are designated as Continuous and you cannot edit these. At the remote ends of the beam there are a number of options for the combined end fixity and support conditions. These are given below: • Free end — as in a cantilever,

• Simple connection — pinned to the support or supporting member. This means pinned about the major and minor axes of the section but fixed torsionally,

• Moment connection — major axis moment connection, and pinned about the minor axis. This option requires the size and length to the point of contraflexure of the columns above and below the connection,

• Fully fixed — encastré, all degrees of freedom fixed. Fastrak Building Designer End fixity in continuous beams — Whilst in the stand-alone program member end fixity and supports are dealt with as one entity, in the Fastrak Building Designer supports are a separate issue and hence are dealt with separately below. All internal connections are considered Continuous – if a pin were to be introduced at an internal position then there would be two beams, hence you cannot edit this setting. At the remote ends of the beam there are a number of options for the end fixity depending upon to what the end of the beam is connected. These are: • If not connected to a beam or column or to a supplementary support –

• Free end (default!) • If connected to an existing member – • Simple connection (default) • Moment connection • If connected to a Supplementary Support – • Simple connection (default) • Fully fixed. The interpretation of these descriptions in relation to being pinned about a particular axis is the same as in General Beam Stand-alone. Moment connections to supporting beams at the remote ends of general beams are prevented. Similarly, for such connections to the web of an I/H section column or to the face of a hollow section column. If you attempt to use such a connection General Beam issues a warning message. This is to draw your attention to the difficulty and cost of making such a connection and, perhaps more importantly, to the possibility that such a joint will not behave as fully rigid.




General Beam Input (in Fastrak Building Designer) In order to create a general beam within Fastrak Building Designer, you will need to define an appropriate set of attributes. Listed below is the typical procedure for defining these attributes. Items in brackets [] are optional.

Step

Dialog

Page

Instructions

1

none

none

Create a new Beam Attribute Set

2

Attribute Set

General


3

Attribute Set

Design

Choose General construction type

4

Attribute Set

Design


5

Attribute Set

Design


6


Size Constraints

[Define the Beam Constraints: • max and min beam size]

7


Sections for Study


8


Deflection

Define and apply deflection limits • [dead] • live • [total]

9

Attribute Set

Alignment

[Change the rotation/alignment of the beam as required]

10

Attribute Set

Beam

Choose the steel grade and, if in Check Beam Mode choose the section size for each span

11

Attribute Set

Supports

Define the supports required at the remote ends of the beam.

12

Attribute Set

Braced (LTB)

Specify if the beam top flange is continuously braced.



General Beam Input (when run as a standalone program) The input procedures for running the program in either design or check mode are listed in the tables below.

Designing a beam Items in brackets [] are optional

Step

Icon

Instructions

1

Launch General Beam

2

Create a new project giving the project name [and other project details]

3

Give the beam reference details

4

Set General Beam into design beam mode

5

Define the properties for the beam: • number of spans, span lengths, section types and section grades; • end support conditions.

6

Give the details of the beam restraints for lateral-torsional- and strut-buckling

7

Define the loadcases that apply to the beam

8

Incorporate the loadcases into a series of design combinations

9

[Make any Design Wizard settings that you want to use to control the design]

10

Perform the design

11

From the list of suitable sections preview the results for the more desirable sections and then choose the one that you would like to use

12

Check the beam

13

Specify the content of the report [and print it]

14

Save the project to disk




Checking a beam Items in brackets [] are optional.

Step

Icon

Instructions

1

Launch General Beam

2

Create a new project giving the project name [and other project details]

3

Give the beam reference details

4

Set General Beam into check beam mode

5

Define the properties for the beam: • number of spans, span lengths, section sizes and section grades; • end support conditions.

6

Give the details of the beam restraints

7

Define the loadcases that apply to the beam

8

Incorporate the loadcases into a series of design combinations

9

[Make any Design Wizard settings that you want to use to control the design]

10

Perform the check

11

Specify the content of the report [and print it]

12

Save the project to disk


Chapter 6

Chapter 6 : General Column

General Column

Introduction GENERAL COLUMN - “steel column designed as a beam/column” The General Column design application allows you to analyse and design a structural steel column which can have moment or simple connections with incoming members, and which can have fixity applied at the base. The column can have incoming beams which may also be capable of providing restraint, and may have splices along its length at which the section size may vary. You are responsible for designing the splices appropriately. The (subtle) difference between a general column (i.e. a column acting as a beam-column) and a general beam (i.e. a beam acting as a beam-column) is the predominance of axial force in the former. A general column can be designated as a ‘simple column’ in order to utilise simple construction techniques. In all cases you are responsible for setting the effective lengths to be used appropriate to the provided restraint conditions. All defaults are set to 1.0L Unless explicitly stated all calculations in General Column are in accordance with the relevant sections of EC3(Ref. 1) and the associated UK(Ref. 2) or Irish(Ref. 3) National Annex.

Scope In its simplest form a general column can be a single member between construction levels that are designated as floors. More typically a general column will be continuous past one or more floor levels, the whole forming one single entity typically from base to roof. General columns that share moments with general beams form part of a rigid moment resisting frame.

General Limitations The following general limitation applies: • there is no automatic generation of pattern loads. Sloping General Columns The following additional limitations apply: • the web of each stack of a sloping column must lie in the same plane,

• sloping general columns are limited to having either their web, or flanges in a vertical plane.

• eccentricity moments are not taken into account in design, • there is no imposed load reduction.



Steel sections The design of general columns is carried out for rolled I-sections, RHS, SHS and CHS only. Where: I rolled = UKB, UKC, UB, UC, RSJ, IPE, HE, HD, IPN RHS = RHS, Euro RHS SHS = SHS, Euro SHS CHS = CHS, Euro CHS Concrete filled hollow sections, plated sections and sections with unequal flanges are excluded.

Simple Columns A general column can be designated as a ‘simple column’ - in which case it is automatically pinned above floor level in the analysis model in all situations, (i.e irrespective of whether it is in a braced bay or not). A simple column should not have any applied loading in its length. Simple columns are subject to axial forces and moments due to eccentricity of beam reactions. The above limitations prevent moments due to frame action or due to member loading.

End Releases The top and bottom of each stack in the general column can be either pinned or fixed which means that the member is pinned about both the major and minor axes in the first case, or fixed about both axes in the second.

Imposed Load Reductions The floors that define the stacks can be designated either as ‘to be’ or ‘not to be’ included in the determination of the imposed load reductions. This feature enables what appears to be a roof to be counted as a floor, or conversely allows a mezzanine floor to be excluded from the number of floors considered for any particular general column. Also, floors can be designated to not have their imposed loads reduced, for example if they are storage or plant floors. In this case the full loading on that floor will be used in determining the reactions onto the column. The moments from fixed ended beams framing into a column are never reduced.

Splices Splices are allowed at floor levels only and must be placed at changes of angle between two adjacent stacks and at changes of section size or type. A validation error will result if this is not the case. The splice can be given an offset from the floor level - the default of 500mm is considered not to be structurally significant. You must detail the splice to resist the applied forces and moments. The detail should provide continuity of stiffness and strength. Splices given considerable offset should take account of the P- moment at the position as well as the forces from the analysis.



Each lift (length between splices) of a general column can be of different section size and grade. Different section types within the same column are not allowed due to the particularly complex design routines that general columns require. You are responsible for guaranteeing that the splice detail ensures that the assumptions in the analysis model are achieved and that any difference in the size of section between lifts can be accommodated practically. The entire column can be set to automatic design or check design.

Design Forces Design forces are obtained directly from Fastrak Building Designer.

Design Checks A full range of strength and buckling checks are carried out. Details of the checks performed follow. Note A sway assessment is also performed. This can optionally be de-activated for those columns for which it would be inappropriate, by unchecking the Alpha Crit Check box on the Column Properties dialog.

Analysis Building Modeller Object The member end forces for each unfactored loadcase are obtained by submitting the whole model to the Fastrak Building Designer solver. There is a particular difficulty with general columns in that there may exist both ‘real’ moments and eccentricity moments from beam end reactions. The effect of the eccentricity moments can reduce those real moments from frame action that are design critical. However, there are cases where this is not true and so the eccentricity moments are included to prevent over- or under-design due to their presence. In essence, General Column checks strength and buckling against the maximum moment due to the algebraic sum of real and eccentricity moments in two directions. General Column determines the uniform moment factors for use in the buckling interaction equations: • firstly from the profile of real moments: these factors are applied only to the real moments,

• secondly from the profile of the ‘combined’ real and eccentricity moments: these factors are applied to the ‘combined’ moments. Note that the eccentricity moments only apply at the ends of the stack and not at intermediate positions.

Ultimate Limit State – Strength Strength checks relate to a particular point on the member and are carried out at 5th points and ‘points of interest’. 1 Footnotes 1. ‘Points of interest’ are such positions as maximum moment, maximum axial etc.



Classification The classification of the cross section is in accordance with Table 5.2. General columns can be classified as:

• • • •

Plastic

Class = 1

Compact

Class = 2

Semi-compact

Class = 3

Slender

Class = 4

Class 4 sections are not allowed. Implementation of the below clauses is as follows: • Classification is determined using 5.5.2 (6) and not 5.5.2 (7).


• 5.5.2 (11) is not implemented. • 5.5.2 (12) is not implemented. A brief study of UK rolled UBs and UCs showed that flange induced buckling in normal rolled sections is not a concern.

Axial Capacity The axial tension and compression capacity checks are performed according to Clause 6.2.3 and Clause 6.2.4 respectively. The following points should be noted: • Cl 6.2.3 (3) - is not considered

• Cl 6.2.3 (4) - is not considered • Cl 6.2.3 (5) - is not considered Shear Capacity The shear check is performed at the point under consideration according to Clause 6.2.6(1): • for the absolute value of shear force normal to the y-y axis, Vy,Ed, and

• for the absolute value of shear force normal to the z-z axis, Vz,Ed The following points should be noted: • No account is taken of fastener holes in the flange or web - see 6.2.6 (7)

• Shear is not combined with torsion and thus the resistance is not reduced as per 6.2.6 (8) Shear buckling — When the web slenderness exceeds 72 shear buckling can occur in rolled sections. There are very few standard rolled sections that breach this limit. General Column will warn you if this limit is exceeded, but will not carry out any shear buckling checks.



Moment Capacity The moment capacity check is performed at the point under consideration according to Clause 6.2.5(1): • for the moment about the y-y axis, My,Ed, and

• for the moment about the z-z axis, Mz,Ed The moment capacity can be influenced by the magnitude of the shear force (‘low shear’ and ‘high shear’ conditions). Where the high shear condition applies, the moment capacity calculation is made less complicated by conservatively adopting a simplified shear area. The maximum absolute shear to either side of a point of interest is used to determine the moment capacity for that direction. High shear condition about y-y axis — The treatment of high shear is axis dependent. In this release for CHS, if high shear is present, the moment capacity check about the y-y axis is Beyond Scope. High shear condition about z-z axis — For rolled sections in this release, if high shear is present normal to the z-z axis then the moment capacity check about the z-z axis is Beyond Scope. For hollow sections, there is greater potential for the section to be used to resist the principal moments in its minor axis. Of course for CHS and SHS there is no major or minor axis and so preventing high shear arbitrarily on one of the two principal axes does not make sense. Nevertheless, if high shear is present normal to the z-z axis then in this release the moment capacity about the z-z axis is not calculated, the check is Beyond Scope. If high shear is present in one axis or both axes and axial load is also present, the moment capacity check is given a Beyond Scope status. If high shear and moment is present in both axes and there is no axial load (“biaxial bending”) the moment capacity check is given a Beyond Scope status.

Combined Bending and Axial Capacity The cross-section capacity check covers the interaction of axial load and bending to Clause 6.2.9 appropriate to the type (for example – doubly symmetric) and classification of the section. If high shear is present in one axis or both axes and axial load is also present, the cross-section capacity check is given a Beyond Scope status. If high shear and moment is present in both axes and there is no axial load (“biaxial bending”) the cross-section capacity check is given a Beyond Scope status. The following additional points should be noted: • the combined effects of axial load and bending are assessed and clause 6.2.9 (4) is not considered.



• the current ‘reduced plastic moduli’ approach in the published tables is used and not the approximate method given in 6.2.9.1(5). The latter is less conservative than the current approach at low levels of 'n'.

Ultimate Limit State – Buckling Note

Classification for buckling checks For rolled I sections, RHS and SHS classification varies along the member length due to the section forces changing along the member length - for combined buckling, the worst classification of the whole member (column stack) should be used. In theory it should be the worst classification in the segment length considered for buckling. However, the segment lengths for lateral torsional buckling, minor axis strut buckling and major axis strut buckling can all be different. It is simpler and conservative therefore to use the worst classification in the entire member length (column stack).

Compression buckling General columns must be checked to ensure adequate resistance to buckling about both the major and minor axes and they must also be checked in the torsional mode over an associated buckling length. Since the axial force can vary throughout the column and the buckling lengths in the two planes do not necessarily coincide, all buckling modes must be checked. There may be circumstances where it would not be safe to assume that the Combined Buckling check will always govern (see below). Restraints — Restraints to strut buckling are determined from the incoming members described within Fastrak Building Designer. The buckling checks are based on these.

Restraining members framing into either Face A or C will provide restraint to major axis strut buckling. Members framing into either Face B or D will provide restraint to minor axis strut buckling. Fastrak Building Designer determines the strut buckling restraints but you can override these. Note

The program assumes that any member framing into the major or minor axis of the column provides restraint against strut buckling in the appropriate plane. If you believe that a certain restraint in a particular direction is not effective then you can either override the restraint or adjust the effective length to suit – to 2.0L for example.



Torsional and torsional flexural buckling restraint is only provided at points restrained coincidentally against major and minor axis strut buckling. Note

Provided a level is restrained coincidentally against major and minor axis strut buckling, the program assumes that any member framing into the appropriate faces provides restraint against torsional and torsional flexural buckling at that level. There are a number of practical conditions that could result in torsional restraint not being provided at floor levels. At construction levels this is even more possible given the likely type of incoming member and its associated type of connection. You must consider the type of connection between the incoming members and the column since these can have a significant influence on the ability of the member to provide restraint to one, none or both column flanges. For example, consider a long fin plate connection for beams framing into the column web where the beam stops outside the column flange tips to ease detailing. The fin plate is very slender and the beam end is remote from the column flanges such that it may not be able to provide any restraint to torsional or torsional flexural buckling. The fact that a slab is usually present may mitigate this. You are expected to override the ineffective restraint.

General Column always assumes full restraint at the base and at the roof level when carrying out buckling design checks – you are warned on validation if your restraint settings do not reflect this. Restraints are considered effective on a particular plane providing they are within ±45° to the local coordinate axis system. Effective lengths — In all cases General Column sets the default effective length to 1.0L, it does not attempt to adjust the effective length in any way. You are expected to adjust the strut buckling effective length factor (up or down) as necessary. Different values can apply in the major and minor axis. Note

It is assumed that you will make a rational and ‘correct’ choice for the effective lengths between restraints. The default value for the effective length factor of 1.0L may be neither correct nor safe.

As an alternative to providing the effective length factor you can enter a value (in m) which is the actual effective length i.e. takes into account both the factor and the system length. The torsional and torsional flexural buckling effective length factor (1.0L) can not be changed. Any strut buckling effective length can take the type ‘Continuous’ to indicate that it is continuously restrained over that length. There is no facility for specifying torsional, or torsional flexural buckling effective lengths as ‘Continuous’. There is no guidance in EC3 on the values to be used for effective length factors for beam-columns. For general columns - The minimum theoretical value of effective length factor is 0.5 and the maximum is infinity for columns in rigid moment resisting (RMR) frames. Practical values for simple columns are in the range 0.7 to 2.0 (see For simple columns below). In theory, values less than 1.0 can be chosen for non-sway frames or for sway frames in which the effects of sway are taken into account using either the amplified forces method or P-Delta analysis. However, EC3 states that when second-order effects are included in this



way then the design “may be based on a buckling length equal to the system length” i.e. an effective length factor of 1.0. The program default of 1.0 matches this requirement but allows you flexibility for special situations. One such situation might be in RMR frames where the principal moments due to frame action preventing sway are in one plane of the frame. There will often be little or no moment out-of-plane and so, if using the amplified forces method, the amplification of these moments has little effect on the overall design. Nevertheless the stability out-of-plane can still be compromised by the lack of restraint due to sway sensitivity in that direction. In such cases a value of greater then 1.0 (or substantially greater) may be required. Similarly, in simple construction where only eccentricity moments exist, it is only the brace forces that 'attract' any amplification. Thus for the column themselves the reduced restraining effect of a sway sensitive structure may require effective length factors greater than 1.0. For Simple columns - There is no concept of simple columns in EC3 and hence no information on effective lengths either. However, reference can be made to 'NCCI' on the subject of simple construction but none of this includes the clear guidance on effective lengths of simple columns that was included as Table 22 in BS 5950-1: 2000. Again the program defaults the effective length factor to 1.0 Compression resistance — The relevant buckling resistances are all calculated from Equation 6.47. These consist of the flexural buckling resistance about both the major and minor axis i.e. Nb,y,Rd and Nb,z,Rd over the buckling lengths Lyy and Lzz and where required the buckling resistance in the torsional or flexural-torsional modes, Nb,x,Rd. All section types are checked for flexural buckling. It is only hollow sections that do not need to be checked for torsional and torsional-flexural buckling.

Lateral Torsional Buckling Effective lengths — The value of effective length factor is entirely your choice. The default value is 1.0 and is editable for flanges A & C. Any individual segment (for either flange) can be 'continuously restrained' in which case no lateral torsional buckling (LTB) check is carried out for that flange over that segment. For a level to be treated as torsional restraint it must have both A and C restraint and also be restrained for compression buckling in both the major and minor axis. There is no specific factor for destabilizing loads - you can however adjust the 'normal' effective length factor to allow for such effects. Lateral torsional buckling resistance — The LTB resistance is calculated from Equation 6.55. LTB does not need to be checked for the following sections, • circular and square hollow sections,

• equal and unequal flanged I/H sections loaded in the minor axis only.



Combined Buckling The column must be restrained laterally in two directions and torsionally, at the top and bottom of the 'design length'. This equates to LTB restraint to faces A and C and restraint to major and minor axis compression buckling all being coincident. A design length is allowed to have intermediate restraint and if the restraint requirements are not met at a particular floor then the design length does not have to be between adjacent floors. Thus a stack can 'jump' floors or sheeting rails can be attached. It is assumed that the restraints for compression buckling are fully capable of forcing the buckled shape. Hence, the compression buckling resistance is based on the restrained lengths whilst the LTB resistance ignores the intermediate restraint and hence is based on the full design length. Note It is conservative to ignore the intermediate restraints in this latter case. Loading within the design length is allowed. Effective lengths — Effective lengths for flexural (i.e. strut major and strut minor) and lateral torsional buckling are as described in the appropriate section above. Combined buckling resistance — The combined buckling resistance is checked in accordance with Equations 6.61 and 6.62. Both equations are evaluated at the ends of the design length and, except for simple columns, at the position of maximum moment, if that lies elsewhere. Eccentricity moments due to beam end reactions are added to the ‘real’ moments due to frame action: • in the first case the uniform moment factors are calculated from the real moments and applied to the real moments. Eccentricity moments are only added if they are more critical.

• in the second case all moments are ‘combined’ and all uniform moment factors are based on the combined moments and applied to them. Caution

Equations 6.61 and 6.62 are limited to doubly symmetric sections and do not consider torsional or torsional flexural buckling. Should either of these buckling modes govern the compression buckling check, you should consider very carefully whether the calculations provided by Fastrak for combined buckling can be considered valid.

Serviceability limit state The column is assessed for sway and the following values are reported for each stack: • Sway X (mm) and critx

• Sway Y (mm) and crity • Sway X-Y (mm) Depending on the reported crit the column is classified as Sway or Non sway accordingly. Note A sway assessment is only performed for the column if the Alpha Crit Check box is checked on the Column Properties dialog. If very short columns exist in the building model these can distort the overall sway classification for the building. For this reason you may apply engineering judgement to uncheck the Alpha Crit Check box for those columns for which a sway assessment would be inappropriate.



General Column Input (in Fastrak Building Designer) In order to create a general column within Fastrak Building Designer, you will need to define an appropriate set of attributes. Listed below is the typical procedure for defining these attributes. Items in brackets [] are optional.

Step

Dialog

Page

Instructions

1

none

none

Create a new Columns Attribute Set.

2

Attribute Set

General


3

Attribute Set

Design

Choose General construction type

4

Attribute Set

Design

Check the Automatic Design box if Design Column Mode is required, else leave it unchecked to work in Check Column Mode

5

Attribute Set

Design

[Check the Gravity Only Design box* and Simple box if required]

6

Attribute Set

Design


7

Column Design Properties

Size Constraints

[Define any Column Size Constraints: • max and min section size]

8

Column Design Properties

Sections for Study

If in Design Column Mode choose the Order File

9

Attribute Set

Alignment

The angle and alignment can be set here.

10

Attribute Set

Floors

The number of floors and any splice locations can be set here.

11

Attribute Set

Releases

Set the end conditions (pinned or fixed) here.

12

Attribute Set

Restraints (LTB)

The degree of LTB restraint provided in each direction at each floor level. can be set here.

13

Attribute Set

Restraints (Comp)

The degree of Comp restraint provided in each direction at each floor level. can be set here.

14

Attribute Set

Eccentricities

Beam end reactions are applied at the offset specified from the column face.

15

Attribute Set

Size

Choose the steel grade and if in Check Column Mode choose the section size

*In order to speed the design process a distinction is made between those combinations consisting of gravity loads only and those which contain some components acting laterally (e.g. EHFs and wind loads). Setting columns to be designed for gravity loads only can significantly reduce the design time.


Chapter 7

Chapter 7 : Braces

Braces

Introduction BRACE - “steel member with pinned ends designed for axial loads only” This chapter describes Fastrak’s Brace design application. This allows you to analyse and design a member with pinned end connections for axial compression and tension. Unless explicitly stated all brace calculations are performed in accordance with the relevant sections of BS EN 1993-1-1:2005 (herein abbreviated to EC3) and the associated UK or Irish National Annex.

Scope The scope of the Brace design application is as follows:

Steel sections The design of braces is carried out for rolled I-sections, C, T, RHS, SHS, CHS, A, Double As, and Flat. Where: I rolled = UKB, UKC, UB, UC, RSJ, IPE, HE, HD, IPN C = RSC, PFC, UAP, UPN T = STB, STC RHS = RHS, Euro RHS SHS = SHS, Euro SHS CHS = CHS, Euro CHS A = RSA (equal and unequal), Euro equal angles and unequal angles Double As = 2xRSA (equal, unequal long and short leg back to back), Euro double angles (equal, unequal long and short leg back to back)

End Connections Braces can only be connected to supports or to the supporting structure via pinned connections. A torsional release can be applied at one end if required. If the brace connects into a beam (e.g. an A brace) an axial end release can be specified at one end to prevent vertical load from the beam being carried by the brace.

Applied loading The following points should be noted: • Loads for the brace are derived from the building model.

• Element loads can not be applied directly to the brace itself. • Imposed load reductions are not applied. • Moments due to self weight loading are ignored.

Chapter 7 : Braces


Design Forces The design forces for strength checks are obtained from an analysis of the entire structure. Braces can be subject to axial compression or tension, but will not be subject to major and minor axis bending.

Design checks The brace can be set to automatic design or check design. Axial capacity and buckling checks are carried out as required, details of the checks performed are given in the Theory and Assumptions section that follows.

Theory and Assumptions This section describes the theory and the major assumptions that have been made for brace design, particularly with respect to interpretation of EC3 and the selected National Annex. A basic knowledge of the design methods for braces in accordance with the design code is assumed.

Analysis method An elastic analysis is used to determine the forces and moments to be resisted by the brace.

Design method The design methods employed to determine the adequacy of the section for each condition are those consistent with EC3, unless specifically noted otherwise.

Classification No classification is required for braces in tension. Braces in compression are classified according to Table 5.2 as either: Class 1, Class 2, Class 3 or Class 4. Class 4 sections are not allowed.

Axial Tension An axial tension capacity check is performed according to Clause 6.2.3.(1) The following points should be noted: • Cl 6.2.3 (3) - is not considered

• Cl 6.2.3 (4) - is not considered • Cl 6.2.3 (5) - is not considered Axial Compression An axial compression capacity check is performed according Clause 6.2.4.(1)


Chapter 7 : Braces

Compression Buckling If axial compression exists, the member is also assessed according to Clause 6.3.1.1(1) for flexural buckling resistance about both the major and minor axis i.e. Nb,y,Rd and Nb,z,Rd over the buckling lengths Lyy and Lzz and where required the torsional, or flexural-torsional buckling resistance, Nb,x,Rd. All section types are checked for flexural buckling. It is only hollow sections that do not need to be checked for torsional and torsional-flexural buckling. Different effective length factors can be applied for flexural buckling in the major and minor axis. The default effective length is 1.0L in both cases. You are expected to adjust the effective length factor (up or down) as necessary. The torsional and torsional flexural buckling effective length factor (1.0L) can not be changed.

Brace Input In order to create a brace within Fastrak Building Designer, you will need to define an appropriate set of attributes. Listed below is the typical procedure for defining these attributes. Items in brackets [] are optional.

Step

Dialog

Page

Instructions

1

none

none

Create a new Brace Attribute Set.

2

Attribute Set

General


3

Attribute Set

Design

Check the Automatic Design box if Design Mode is required, else leave it unchecked to work in Check Mode

4

Attribute Set

Design


5

Brace Design Properties

Size Constraints

[Define the Size Constraints: • max and min size]

6

Brace Design Properties

Sections for Study

If in Design Mode choose the Order File

7

Attribute Set

Alignment

[Change the rotation/alignment of the brace as required]

8

Attribute Set

Size

Choose the steel grade and, if in Check Mode choose the section size

9

Attribute Set

Releases

[Define any vertical or torsional releases as required]

10

Attribute Set

Compression

[Adjust effective length factors as required]

•

Chapter 8 : Refining Member Designs

Chapter 8


Refining Member Designs

Introduction Having carried out an initial design of your model you may then require to investigate key members in more detail. This is possible for any member that has been analysed by using Fastrak Building Designer’s design refinement capabilities.

Why would you want to refine the original design? Using Fastrak Building Designer interactively provides you with still greater control over the design of an individual member: • to enable multiple order files to be considered at the same time to determine a list of alternative sections, all of which can withstand the applied loading.

• to determine a suitable alternative to the initially designed section size without having to re-design the whole building. The new section size (and steel grade if required) can then be passed back to the building model, only the individual member will be updated. If the changes are to be applied to other members also, you would need to update the building model separately and then re-design it.

• to consider the possible effects on the member of more far reaching changes. For example the loading could be adjusted, or the restraint information modified. The resulting design can then be output, however, these wider changes can not be passed back to the building model.

Interaction Effects Because Simple Beams and Composite Beams are by definition pin ended in Fastrak Building Designer, interaction effects are of no concern. This is not the case for General Beams and General Columns - these member types can also be investigated interactively, however, they always interact with other members in the structure. This interaction is demonstrated in the two examples given in the Building Designer Handbook: • “Continuous Beam Example”, and

• “Braces Carry Gravity Loads Example” When you export elements to General Beam and General Column you lock in the interaction effects. If you do not change anything you can check the same beam or column and see the same results. However, you can also run an automatic design in which case the locked in interaction effects do not change. What this can mean in practice is that you may select a different section (larger or smaller), interactively. This section may seem to work satisfactorily when designed in isolation. You can then return this amended section size to the main model (where you will have to re-analyse and check your model). It is quite possible that the section which appeared to work when



designed in isolation will fail when checked after re-analysis of the full model. It is also entirely possible that other members in your model may fail (or have more capacity in hand) since the distribution of forces will be affected by the different section size which you picked.

How to Access Design Refinement The following sections illustrate how to use each of the Fastrak Building Designer design applications in either Check Mode or Design Mode to refine the design.

Simple Beam - Check Mode. Listed below is the typical procedure for checking a simple beam. Caution

Step

Shaded items in the below table can not be returned to the building model. You are advised not to make these changes interactively if your intention is to make the change permanent. Instead you should make the required change(s) directly to the building model.

Icon

Instructions

1

Right click on the beam to be investigated and launch Simple Beam,

2

Set Simple Beam into Check Beam Mode,

3

Modify the properties for the beam as required: • section size, • grade,

4

Modify the beam span,

5

Modify the details of the beam restraints.

6

Modify the loadcases that apply to the simple beam.

7

Modify the design combinations,

8

Make any Design Wizard settings that you want to use to control the design,

9

Perform the check,

10


11

Return the section size and grade to the building model.



Simple Beam - Design Mode Listed below is the typical procedure for interactively designing a simple beam. Shaded items in the below table can not be returned to the building model. You are advised not to make these changes interactively if your intention is to make the change permanent. Instead you should make the required change(s) directly to the building model.

Step

Icon

Instructions

1

Right click on the beam to be investigated and launch Simple Beam,

2

Set Simple Beam into Design Beam Mode,

3

Modify the steel grade if required,

4


5

Modify the details of the beam restraints.

6

Modify the loadcases that apply to the simple beam.

7


8


9

Perform the design

10


11


12




Composite Beam - Check Mode Listed below is the typical procedure for interactively checking a composite beam. Caution

Step


Icon

Instructions

1

Right click on the beam to be investigated and launch Composite Beam,

2

Set Composite Beam into Check Beam Mode,

3

Modify the properties for the beam as required: • section size, • grade,

4


5

Modify the details of the floor construction: • steel deck • slab details • reinforcement details (continuity and any other reinforcement present in the slab, • shear stud size, layout, spacing • construction stage restraint details (if applicable)

6

Modify the loadcases that apply to the beam.

7


8


9

Perform the check,

10


11




Composite Beam - Design Mode Listed below is the typical procedure for interactively designing a composite beam. Caution

Step


Icon

Instructions

1

Right click on the beam to be investigated and launch Composite Beam,

2

Set Composite Beam into Design Beam Mode,

3


4


5

Modify the details of the floor construction: • steel deck • slab details • reinforcement details (continuity and any other reinforcement present in the slab, • shear stud size, layout, spacing • construction stage restraint details (if applicable)

6


7


8


9

Perform the design

10


11


12




General Column - Check Mode Listed below is the typical procedure for interactively checking a general column. Caution

Step


Icon

Instructions

1

Right click on the column to be investigated and launch General Column,

2

Set General Column into Check Column Mode,

3

Modify the size and grade for the column as required,

4

Modify the other column properties: • add additional floors if required; • releases; • eccentricities,

5

Modify restraint details,

6

Modify the loadcases that apply to the column.

7


8


9

Perform the check,

10


11




General Column - Design Mode Listed below is the typical procedure for interactively designing a general column. Shaded items in the below table can not be returned to the building model. You are advised not to make these changes interactively if your intention is to make the change permanent. Instead you should make the required change(s) directly to the building model.

Step

Icon

Instructions

1

Right click on the column to be investigated and launch General Column,

2

Set General Column into Design Column Mode,

3


4

Modify the other column properties: • add additional floors if required; • releases; • eccentricities,

5

Modify restraint details,

6


7


8


9

Perform the design

10


11


12


Effective Use of Order Files in Refined Design This section aims to explain how intelligent use of the Design Wizard can reduce design time and make the design process more effective. The following situations are investigated: • limiting the range of Sections for Study to reduce design time and prevent selection of non preferred sections,



• applying Size Constraints to further reduce design time, To investigate the above an example model can be defined in Fastrak Building Designer and a beam then extracted into Simple Beam. Let’s take a simple example of a 9 m span spine beam with 6 m span secondary beams at third points.

The floor loading is:

Condition

Value

giving point load at 3 m and 6 m of

Dry Slab

2.0 kN/m2

36kN

Services

1.0 kN/m2

18kN

Live load

5.0 kN/m2

90kN

Design Pass 1 If you input this model and then run a design you will find that Simple Beam shows a dialog of acceptable sections. If no one has tailored the sections that Simple Beam investigates, then the list will appear as below.

If you move down the list of Available files, you will see all the Section Designations that can carry the applied loading. These are only the ones that pass the design, Simple Beam has tried all the sections in each of the Available files, to determine the acceptable ones. You may have



noticed the different section designations in the progress bar as the design ran. However checking all these sections comes at a price, the more sections there are to investigate, the longer the design takes. Click Cancel to return to the Beam Definition window. Note

Clicking Cancel leaves the program in Design Beam Mode. Clicking OK would have flipped the program in to Check Beam Mode with the highlighted acceptable section becoming the check section.

Simple Beam allows you to choose just the sections you want to include for the design through its Design Wizard.

Design Pass 2 Sections for Study Remove the tick against all the Available files whose section types you don’t want to investigate, and Simple Beam won’t look at any of these sections during the design process. If you remove the tick against all the Available files other than UBBeamOrder.Eur, and then re-perform the design you will find a significant increase in speed as Simple Beam only investigates the universal beams.



Furthermore Simple Beam investigates the sections in the order that they appear in the Section Designation list. If you scroll down many of the lists, you will find that there is a point at which larger sections give way to smaller ones again.

We have ordered the Section Designation list based on our many years experience of the industry, the sections at the top of the list are the ones we know you prefer to use, whilst those at the bottom are those which you use less frequently if at all. By default all the Section Designations are ticked, but you might want to remove the ticks against some or all of the non-preferred sections. In doing so, you are controlling the design, making Simple Beam look at just the section designations you are likely to accept, and in the process speeding up the design itself. Simple Beam maintains the Sections for Study settings that you make, until you choose to change them again. It is therefore worthwhile taking the time to tailor the list so that Simple Beam picks sections of which you are likely to approve during its designs. Note


Design Pass 3 You may also have other constraints specific to this particular project, for instance you may need to restrict the use of sections with flanges less than a certain width for erection purposes, or you may need to set a limit on the maximum beam depth due to height constraints.

Size Constraints Such restrictions can be set via the Size Constraints tab, Simple Beam will then only consider section designations that fall within these limits (once again speeding up the design).


Note


Size Constraints settings are only applied to the current project, unlike changes made to the Sections for Study which are applied to all projects.

For the third design pass the minimum width of beam will be set to 150mm. With this constraint applied all narrower sections are excluded, Simple Beam has to check far fewer sections and the design is almost instantaneous.

By default the acceptable sections are listed in a descending order of the capacity ratio. Full design results are available for each section on the list. To see the results, highlight a section and click Preview. This will take you to the Design Summary for the chosen section. Closing the Design Summary allows you to preview the results for further sections if required. When you have decided on the most appropriate section, highlight it from the list and click OK.


Chapter 9

Chapter 9 : References

References 1. British Standards Institution. BS EN 1993-1-1:2005. Eurocode 3: Design of steel structures – Part 1-1: General rules and rules for buildings. BSI 2005. 2. British Standards Institution. NA to BS EN 1993-1-1:2005. UK National Annex to Eurocode 3: Design of steel structures – Part 1-1: General rules and rules for buildings. BSI 2005. 3. National Standards Authority of Ireland. I.S EN 1993-1-1 National Annex. Irish National Annex (informative) to Eurocode 3: Design of steel structures – Part 1-1: General rules and rules for buildings. NSAI 2007. 4. British Standards Institution. BS EN 1994-1-1:2004. Eurocode 4: Design of composite steel and concrete structures - Part 1-1: General rules and rules for buildings. BSI 2005. 5. British Standards Institution. NA to BS EN 1994-1-1:2004. UK National Annex to Eurocode 4: Design of composite steel and concrete structures - Part 1-1: General rules and rules for buildings. BSI 2005. 6. British Standards Institution. BS EN 1992-1-1:2004. Eurocode 2: Design of concrete structures. General rules and rules for buildings. BSI 2004. 7. The Steel Construction Institute. Publication 076. Design Guide on the Vibration of Floors. SCI 1989. 8. The Steel Construction Institute. Publication P355. Design of Composite Beams with Large Web Openings. SCI 2011. 9. The Steel Construction Institute. Publication 068. Design for openings in the webs of composite beams. SCI 1987.

Eurocode - Member Design Handbook

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