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The Appendix G: Stability Checks brief (Annex BB.3 - Buckling of components of building structures in BS EN 1993-1-1:2005) performs a check on the local torsional stability of the unrestrained compression flange. Since the purlins provide restraint only to the top flange, in portal members the zone of the rafter in hogging at the eaves, or where the wind the load causes uplift, the design must assess the stability of those lengths of the unrestrained bottom flange which is in compression.
For Masterframe models, the steel design module deals with the steel member design and the Appendix G/Annex BB3 checks separately. This differs from the MasterPort modules, which has a specific Axial with Moment and Appendix G combined brief. Therefore, for a Masterframe model, it is necessary, where restraints are provided to one flange only, to apply both the Axial with Moments and the Appendix G briefs to the applicable members of a model.
The Appendix G brief is applied to a member by selecting the Appendix G: Stability Checks from the Integrated Design. Once the Appendix G/Annex BB brief is applied to a member, the typical screen layout is shown below.
The Section AutoDesign tab contains options to amend the steel member section size, to make the member a compound member (which also provides options to provide plates to the member) and to web openings. Within the Section AutoDesign tab, there are also options to carry out an Autodesign on the selected member or on all members visible on screen.
For more detailed information on the AutoDesign area, see the Designing a Member section.
The layout of the Appendix-G Stability Check tab is shown below.
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C1, C2, C3 factors - Eurocode factors for modifying the bending moment equations. The factors are dependant on the shape of the bending moment diagram of the beam or part of the beam under consideration. The software will automatically determine the C1, C2 and C3 factors, values only need to be input in the Beam and Beam-Portion if the user wishes to overwrite the values.
zg - the zg factor modifies the bending moment equations to account for the height of the point of load application above the shear centre of the beam. The zg value is input in millimetres (mm) and a positive value represents a load applied above the shear centre. The zg input only needs to be used for destabilising or stabilising loads. With a default value of 0 the software takes the load to be applied at the shear centre of the member
a - the distance from the centroid of the member to the centroid of the restraint members.
Lx - length between restraints to the member major axis.
Kx-factor - effective length factor
Lambda limit - Slenderness limit. This was originally a BS 5950 check, but was dropped in later versions of the BS code and does not appear in the Eurocodes. However, it was opted to maintain this input in the steel design.
The type of stability checks carried out are controlled by selection of the options in the grey drop downs. These allow the user to control:
Stability analysis - the user can choose between an elastic stability analysis or plastic stability analysis, using the top drop down. With AutoSelect set, the software will use the appropriate check depending on whether an elastic or plastic analysis has been carried out.
Restraint position - the position of the lateral restraints can be defined, relative to the member Beta angle.
Bending and Lambda_lt factors - The 3rd drop down allows the user to control the tables used to determine the LTB factors ( determined using the British Standard method). Setting this drop down to 'Autoselect' means the software will automatically pick the appropriate table from BS 5950-1 for determining the lambda_Lt values
Top flange in Tension/Compression - allows the user to specify whether the top flange is in compression or tension.
The Torsional Restraints tab layout is as shown below.
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This tab allows the user to define the position of any torsional restrains which are used to provide out-of-plane stability to the unrestrained flange where it is in compression.
This area allows the user to define the positions of up to nine individual torsional restraints. The input value represents the length of the portion. Torsional restraints can be spaced using the Portion 1-9 inputs, if necessary. The start at option allows for up to three separate grouping of torsional restraints to be made in any single member. This allows restraints to be positioned in groups on the members to account for variation in the shape of the bending moment diagrams, such as can occur when portal rafters or roof beams are in an uplift condition due to wind load. When a Start at value is input, the portions will commence from that point.
The increment input can be used to assist in inputting portion lengths. With an increment value input, using the +/- arrows will increase the portion length by the increment amount.