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Manual BS 5950-1: 2000 steel code check

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All information in this document is subject to modification without prior notice. No part or this manual may be reproduced, stored in a database or retrieval system or published, in any form or in any way, electronically, mechanically, by print, photo print, microfilm or any other means without prior written permission from the publisher. Scia is not responsible for any direct or indirect damage because of imperfections in the documentation and/or the software.

© Copyright 2008 Scia Group nv. All rights reserved.

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Table of contents Introduction .............................................................................................................................................. 4 

Buckling system in Scia Engineer ......................................................................................................... 5 

“Steel” tree service ................................................................................................................................. 7 

Design Parameters (Setup) .............................................................................................................. 7 

Setup for member check ................................................................................................................ 8 

Setup of relative deformation limits .............................................................................................. 10 

Setup of buckling defaults ............................................................................................................ 12 Steel member data .......................................................................................................................... 13 Additional lateral restraints ............................................................................................................ 14 Steel slenderness ............................................................................................................................ 14 Check ................................................................................................................................................ 15 

ULS design check ........................................................................................................................ 15 

SLS check .................................................................................................................................... 23 

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Introduction

This document discusses those parts of the user interface of Scia Engineer (v 2008) relevant to the BS 5950-1:2000 steel code checks.

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Buckling system in Scia Engineer

The buckling data for a member in Scia Engineer can be edited through the “Buckling and relative lengths” dialog accessed through the member properties dialog.

Scia Engineer detects whether a member is connected to other members when they satisfy the conditions mentioned below and assembles them into a buckling system which is displayed diagrammatically as shown above. Conditions for connecting members as a single buckling system:

1. For straight members, whether a member is connected to other members in a straight line.

2. For curved members, it depends on how the curve is defined. In general each curve will have an associated buckling system. For example, circular arcs formed by 3 nodes will come under a single buckling system with the number of parts set to 2.

For polyline (polyline by straight and arc) members, each polyline will have an associated buckling system The example above has two members forming a buckling system. Scia Engineer applies default directional restraints to the nodes which it infers from the presence of connected members. Restraints to buckling about the YY (major) section axis are shown on the left and restraints to ZZ (minor) axis buckling are shown on the right. The user can add or remove restraints to modify the default buckling system. The ZZ restraints so defined are applied to both flanges providing effective torsional restraint to the system axis. If restraints to one flange only are required these must be applied using the LTB restraints dialog and any duplicated node restraints removed. Select Edit > Buckling data to open the page shown below to review / modify the default restraints. The table has a line for each node in the system and restraints can be added or deleted by toggling Fixed to Free in the YY and ZZ fields for restraints in the Z and Y directions respectively. For any node restraint you can use the Sway YY and Sway ZZ fields to indicate the sway status of the restraint. By default the Base settings will be applied. The drop menu in the relevant field allows the sway condition to be changed to Yes or No.

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The effective length factors that can be edited from this page are ky – axial compression (buckling about member major axis) kz – axial compression (buckling about member minor axis) kLT – lateral torsional buckling The corresponding effective lengths (ly, lz, and lltb) are calculated as effective length factor times the system length.

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“Steel” tree service

This tree service will be enabled only when the job is calculated and analysis results are available.

Design Parameters (Setup)

The design parameters can be accessed through Main -> Steel -> Beams -> Setup tree option. The design parameters are grouped into 3 sets for steel – BS 5950-1:2000 code:-

1. Setup for member check 2. Setup of relative deformation 3. Setup of buckling defaults

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Setup for member check

Default sway type The user can choose the default sway type (condition) through this option and accordingly the buckling length factors will vary.

Buckling length ratios – ky, kz ky and kz are effective length factors for axial compression buckling about the major and minor axes respectively. Maximum k ratio – The calculated value of ky and kz is limited to this maximum value. Maximum slenderness – If the slenderness of the member exceeds this value, the program prints a warning in the output report. Second order buckling ratios - For 2nd order calculation, the buckling data as defined can be used, or the structure is considered as non-sway for all buckling data. This is because the analysis includes the second order effects and it is not necessary to allow for them by means of k factors.

Section check only If this option is ticked only the section check is carried out. No stability check is performed.

Calculation type of m, n

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These factors allow for the shape of the moment diagram in stability calculations. ‘m’ is the equivalent uniform moment factor and ‘n’ is the equivalent slenderness factor. These factors are necessary to perform the lateral-torsional buckling check and torsional buckling check. For the lateral-torsional buckling check, there are two methods for dealing with lateral-torsional buckling namely: The 'm approach' i.e. the 'equivalent uniform moment method' with n=1, see Table 18. This approach is used for any moment gradient in a uniform member. The 'n approach' i.e. the 'equivalent slenderness method' with m=1, see Annex B.2.5. This approach is used for any moment gradient in a non-uniform member. For the torsional buckling check also there are two methods for dealing with torsional buckling namely: The 'm approach' i.e. the 'equivalent uniform moment method' with n=1, see G.4.2.This approach is used for a linear moment gradient in a uniform member. The 'n approach' i.e. the 'equivalent slenderness method' with m=1, See G.4.3. This approach is used for a non-linear moment gradient in a uniform member or any moment gradient in a non uniform member. In any given situation, only one method will be admissible. The selection of these approaches is restricted to the relevant conditions as stated above. So, unlike BS 5950-1:1990, there is no user choice.

Numerical section classification A numerical section is one user-defined by its numerical section properties without reference to its shape and so normally used for non-standard sections. Numerical section classification is set to be class3 semi compact for the BS 5950-1:2000 code check and so there is no choice.

Compound members This option is applicable only for members consisting of paired sections. The user can specify:-

1. Compound type as laced or battened for the job. 2. Spacing between lacings or battens in metres

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Setup of relative deformation limits

The available member types in Scia Engineer are:

This dialog allows different deformation limits to be applied to different member types. Deflection

The user can define the deflection limit for each member type in terms of minimum span/deflection ratio as given below, Deflection limit = span / input specified in the set up of relative deformation. For example a beam member of 8.000 m span. Deflection limit = Span/200 = 8000/200 = 40 mm

Twist limit The user can define the twist limit for each member type. Utilisation calculation for twist limit:

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Utilisation = Actual twist / input of twist limit specified in the set up of relative deformation. Default limit = 20 mrad = 20/1000 radian = 0.02 radian.

Deflection with twist For each member type the user can specify the maximum combined deflection at an offset from the member axis due to flexural and torsional effects. This is also defined as a minimum span/deflection ratio. Maximum normal offset:

The maximum normal offset is the distance perpendicular to the member axis to the point at which the deflection is required eg, face of a supported wall. Calculation for actual deflection with twist: Actual deflection with twist = Flexural deflection + offset times angle of twist Calculation for deflection with twist limit: Deflection with twist limit = span / input specified in the set up of relative deformation.

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Setup of buckling defaults

Buckling systems relation The user can set bucking systems (buckling segments) relation as:-

1. ZZ =YY i.e. the user defines one set of buckling segments about YY which is common to ZZ and YY 2. ZZ = ZZ i.e. the user defines two different sets of buckling segments about ZZ and YY

In other words this user option allows the user to specify different set of restraints (system length) for compression buckling in major and minor direction.

Relative deformation system relationships

The user can set relative deflection systems (deflection segments) relationships as:-

1. def y = ZZ i.e. the user can assign buckling segment ZZ to deflection segment along y. 2. defy = YY i.e. the user can assign buckling segment YY to deflection segment along y. 3. defy = defyy i.e. the user can define new deflection segment along y.

Similarly:- 4. def z = ZZ i.e. the user can assign buckling segment ZZ to deflection segment along z. 5. def z = YY i.e. the user can assign buckling segment YY to deflection segment along z.

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6. def z = def y i.e. the user can define new deflection segment along y. 7. def z = def z i.e. the user can define new deflection segment along z. Effective length factor

Effective length factor can be calculated by Scia Engineer or it can be input by the user in terms of a factor or length.

Influence of load position The user can set the influence of load position as Normal, Destabilising or Stabilising

Steel member data

Steel member data can be used to override certain options from the Setup at the member level.

Compound member This option is applicable only for members consisting of paired sections. The user can specify:-

1. Compound type as laced or battened for the job. 2. Spacing between lacings or battens in metres

Field

Field – Position: Absolute / Relative: From begin (x): From end (x’) This option allows the user to specify elastic & section check options over a restricted length of a member. For finding the resultant utilisation for a member, Scia Engineer will not consider the strength and stability position (slice) results in the segments x and x’ (diagram below for x and x’). This can be useful at beam-column junctions.

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Additional lateral restraints

Additional lateral restraints may be applied at the top or bottom flange of the member to control buckling due to axial force and moment. A top flange restraint is defined by a positive position z and a bottom flange restraint by a negative position z.

Steel slenderness

In the Steel tree, selection of Steel slenderness causes the slenderness parameters to be displayed for selected members. The parameters may be chosen from the drop menu list:-

1. System length (lengths between restraints) in major (Ly) and minor (Lz) directions 2. Buckling ratio (effective length factor) in major (ky) and minor (kz) directions 3. Buckling length (effective length) in major (ly) and minor (lz) directions 4. Slenderness ratio for buckling about major (λy) and minor axis (λz) 5. System length and effective length for lateral torsional buckling

Note that it is necessary to select Refresh to display after choosing a new parameter.

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Note: Effective length factors calculated in Scia Engineer are code independent. Some approximate formulas are used which vary from sway to non-sway structures and crossing diagonals (cross bracing). Two special load cases are used for this purpose and the user has a choice to perform the calculation in 2 modes (linear or second order) for computation of these factors.

Check

There are two types of checks. They are:-

1. ULS design check 2. SLS design check.

ULS design check

This option is used to perform BS5950-1:2000 design check for all the selected members. Although the user may select just one member of a buckling system to be checked, the software will check the whole buckling system which includes the member so as to cover for any overlapping major and minor axis segments in the system. Check for polylines has to be done by splitting them into lines. Polylines are not directly supported in this release. To split a polyline select the same and use the split option available from ‘Modify’ menu (Modify > Poly line edit > Edit Poly line > Break into single curves). A warning message will be included in the result if you still try to design a polyline member. In the member stability check, buckling segment creation is done considering both sets of restraints from the ‘buckling data’ dialog box and the additional member restraints. Examples: Example1: Segments within a beam Beam B2 has an intermediate node with supports. Buckling data therefore shows 2 parts. In this case we will create 2 segments in major and 2 segments in minor direction

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In the same beam the user can edit the buckling data as shown below. For example to create 2 segments in normal direction and 1 segment in lateral direction we can uncheck zz at node 2 ignoring the restraint parallel to YY:-

Example 2: Continuous members forming one buckling segment

The above picture shows two beams B5 and B6. Since the beams are connected through a common node in a straight line a single buckling system applies to both (as shown below).

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Buckling data for the system is shown below.

In this case, the segments (both major and minor directions) extend from node 1 (N9) to 3 (N11). It would be meaningless (and unsafe) to perform the buckling check on just member B5 or B6. Case 3: Beam with additional lateral restraints

The diagram above shows two beams B7 and B8. Node N9 is fixed and node N11 is pinned. Buckling data for this system by default will show one part (Segment or Buckling length) between N9 and N11. This has been modified to provide lateral restraint at node N10 (point ‘d’) so that different values for effective length factor ‘ky’ and `kz’ [axial compression -buckling about minor and major axes] can be entered if appropriate as shown in the picture below. There are also additional restraints applied on both beams at positions ‘b’, ‘c’, ‘e’ and ‘f’.

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There will be one major axis (normal buckling) segment between nodes N9 and N11. For buckling about the minor axis (for lateral buckling) there will be 6 segments. a to b with kz = 0.8 b to c with kz = 0.8 c to d with kz = 0.8 d to e with kz = 1.0 e to f with kz = 1.0 f to g with kz = 1.0 Case 4: Beam with lateral restraints to the tension flange only (Annex G of BS5950-1:2000)

The diagram above shows the same beams as in case3 together with the major axis moment profile due to uniform applied load. Point b’ is the contraflexure position between lateral restraints b and c. As they are supported, nodes N9 and N11 are considered by default to be laterally restrained at top and bottom faces (i.e. torsional restraint). Intermediate lateral restraints are also applied to the top face only at b, c, e and f. Where a member has lateral restraints to one face/flange only, the program checks for buckling in the torsional mode in accordance with BS 5950-1 Annex G as described below. For the load combination illustrated, the restraint at position b is applied to the tension face and the compression face is not directly restrained. At the next restraint position c the moment has changed from hogging to sagging causing the restrained face to be in compression. The compression face at c is therefore directly restrained. The program therefore detects a potential torsional buckling length between a and c. For the remainder of the beam length consecutive restraints are directly applied to the compression flange and no more torsional buckling lengths are found for this load combination.

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ULS design check properties - The properties of the steel check are explained below.

Name – The name of the code used to perform the check. This should be BS 5950-1:2000 for SWCC link to work. Selection – Scia Engineer allows code check to be done on all or selected members. Steel code check is called iteratively depending upon the number of members on which the code check is to be performed.

All : All relevant members in the display (default) Current : Relevant members among the selected Advanced : A dialog is displayed by which we can add/delete the previously selected list of members

Named selection: relevant members that are grouped with a single name

Type of loads – Scia Engineer enables code check to be performed on a combination, load case or class.

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For the ULS design check, Scia Engineer does not list the SLS load combinations in the properties of steel check. Similarly for the SLS design check, Scia Engineer does not list the ULS load combinations in the properties of steel check. It is also permissible to perform a check on load cases and result classes. It is assumed that the users will use load cases and result classes according to BS5950-1:2000 (i.e. do not use both ultimate and serviceability load combinations in a result class) Filter – The set of members where the results are displayed may be specified by means of a filter. In other words, the selection of member to be evaluated may be filtered out by wildcard(search by typing), cross section, material, layer and fire duration

No – there is no filtering, Wildcard – the selection is given by the typed "wildcard expression", e.g. B*, BEAM1?, etc. Values – This menu controls which results are displayed graphically (local capacity/stability/member overall utilisation/fire check u-ratio) Extreme – From the code check results Scia Engineer has four options to sort the output display. The displayed value can be extreme for a member or section or local or for the structure.

These are options for displaying the results label in the graphical view. Global extreme – the label will appear for the member with maximum utilisation within the job.

Member extreme – the label will appear for each member marking the maximum utilisation for each member.

Local extreme – the label will appear for each member for every maximum and minimum u-ratio within the member.

Section extreme – Labels will appear for all available sections within a member

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Output – Three levels of output are possible in Scia Engineer. The details of output in each category for every check will be documented.

Drawing setup – Gives options to control the display (drawing style) on the screen. Brief – Brief single line output of utilisation values. Summary – Summary provides applied effect, capacity and utilisation for each check at the selected position. Detailed - The output contains the all the necessary data to verify the code check results at a position.

Section – Depending upon the section filter the results display will vary. ‘All’ outputs results at all sections where the check is performed. ‘End’ outputs results for end section only. ‘Input’ outputs results for user input sections only. In addition to the default section/position where results are output by Scia Engineer the user can view the results at a particular section/position of his/her interest within the member. In order to do so he/she should first create a section using ‘Section on member from the Structure menu and then choose ‘Input’ here.

In the set of actions the user can choose from the above options. Autodesign –

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In auto design, Scia Engineer computes the required section for the user based on certain criteria. The user is allowed to set a limit for the utilisation ratio and choose a parameter for design. For user defined sections the maximum, minimum and the step value (Amount by which the particular dimension is increased. i.e. design increment) are also input by the user.

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For standard sections the optimum section is selected from a list of standard sections based on height / cross section area / moment of inertia.

Split CSS –

“Split CSS is a function which automatically subdivides a group of members with a common section so that those members with unity check/utilisation less than a certain value are assigned a smaller common section. By default the unity value for the split is 0.25 but this can be changed by the user.

Unify CSS –

The option 'Unify css' works the other way around to split CSS. Imagine a set of members with different cross-sections. If it is required for all members to have the same cross-section, this option will be useful to automatically assign a common section which satisfies the design criteria for all. Here the resultant utilisation of each member will guide the user whether to unify or not.

Preview – Opens the preview (text output) window

SLS check

A check on the member normal and lateral deflection is performed by comparing the actual with the limits specified.