The design of a member for bending consists of five design checks. These check the section capacity of the member about the major and minor axes, the shear capacity about both axes and the member, and the buckling, capacity about the major axis.
When performing a bending check it is necessary to specify how lateral-torsional buckling of the member is resisted. Restraint could be provided by other members, purlins, girts or by other structural elements that are not modelled in Multiframe such as concrete slabs. Steel Designer provides three methods of specifying how a member is restrained against lateral buckling. The user may specify
§ That the member is fully restrained against lateral buckling in which case no lateral buckling checks will be performed, or
§ The location and type of lateral and torsional restraints applied to the member in which case Steel Designer will appropriately divide the member into a number of spans and consider the capacity of each of these spans in determining the capacity of the member, or
§ The laterally unbraced length (Lb) and moment modification factor (mLT).
You may also need to specify a number of properties relating to the location and type of lateral restraints and the stiffener spacing along the member
To compute the buckling capacity of a member it is necessary to know the spacing of any lateral and torsional restraints (if any) along the member. The restraints could be provided by purlins, girts or other structural elements, which are not modelled in Multiframe. Steel Designer uses this information to determine the length of segments used in the design calculations for lateral torsional buckling. In Steel Designer, The restraint provided by a support is described by how it restraints the top and bottom flanges and how it restraints the cross-section of the member at that location against torsion.
Restraints must always be specified at the ends of the member. If no actual restraint exists at the end of a member then it should be specified as unrestrained. Lateral restraints at the ends of a member may also be specified as providing either full or partial restraint against rotation on plan. By default, the ends of a member will be assumed to be laterally restraint at both the top and bottom flange but provide no resistance to on plan rotation of the member. Torsional restraints at the ends of a member may be specified as unrestrained, fully restrained, partially restrained or frictionally restrained. Partial restraints inhibit the rotation of the cross section by the connection of the bottom flange to the supports while frictional restraints resist rotation of the member about its longitudinal axis by only the pressure of the bottom flange onto its supports (Refer to Table 13 of BS5950).
Intermediate restraints applied to the member may provide lateral and torsional restraint. No distinction is made for the on-plan rotational resistance that may be provided by lateral restraints.
The location and type of lateral restraints can be displayed in the Frame and Plot windows. The display of lateral restraints can be turned on or off via the Symbols Dialog which now contains options for displaying lateral restraints and labelling these restraints.
The restraints are draw as a short line in the plane of the major axis of the member. These lines extend each side of the member for a distance that is roughly the scale of a purlin or girt. Lateral restraints are also displayed in the rendered view of the frame in which they are draw to extend from each flange by approximately the size of a purlin. The restraints may be labelled using a one or two letters to indicate the type of restraint. Lateral are labelled using the following notation
U – Unrestrained
L – Lateral restraint
LR – Lateral restraint with full restraint against rotation on plan
LP – Lateral restraint with partial restraint against rotation on plan
Note that lateral restraints at the end of a member are draw slightly offset from the node so that restraints at the ends of connected members may be more readily distinguished.
Instead of specifying the position of lateral restraints it may be preferable to directly set the laterally unbraced length of the member. When doing this, it is also necessary to specify the bending coefficient (mLT) as this can no longer be automatically determined by Steel Designer. The design codes permit a conservative value of mLT =1.0 to be adopted which is the default value used by Steel Designer.
When checking or designing a member for bending, you may need to specify the spacing of any stiffeners along the web of the member. This affects the member’s susceptibility to buckling due to bending. If there are no transverse stiffeners, you should leave the stiffener spacing set to zero.
When checking or designing a member for bending, you may need to specify the load height position. This is used in determining the effective lengths of segments or sub-segments along the member.
To set the properties for bending
Ø Select the required members in the Frame window
Ø Choose Bending from the Design menu
If the member is fully braced against lateral torsion buckling
Ø Select the “Member is fully laterally restrained” option
or if the location of lateral bracing along the member is to be specified
Ø Select the “Position of Lateral Restraints” option
To add new restraint to the member
Ø Position the cursor with the table and click the Insert button to add a lateral restraint to the member.
Ø Select the position of each restraint
Ø Select the type of each lateral restraint from the combo provided in each cell.
or
Ø Click the Generate button to automatically generate a number of restraints.
To delete a restraint from the member
Ø Position the cursor within the table on the lateral restraint to be deleted and click the Delete button.
And then to display the list so segment defined by the restraints
Ø Click on the Segments tab
Ø For each segment choose the position of the load from popup menu
or if the unbraced length of the member if the be specified directly
Ø Select the “Unbraced Length” option
Ø Enter the unbraced length (le)
Ø Enter the moment modification factor coefficient (mLT) to be used in the design of this length of the member.
Ø If there are transverse stiffeners on the web, type in values for the stiffener spacing (s)
Ø Click OK
When the user selects to generate the lateral restraints from the Bending dialog, the Generate Lateral Restraints dialog is displayed. This dialog enables the user to generate lateral restraints at a specified spacing along the member.
Ø From the Bending dialog, click the Generate… button
The Generate Lateral Restrains dialog will appear allowing you to specify the restraints to be generated.
Ø Select the type of restraints to be used at the ends of the member
Ø Select the type of restraints to be used at intermediate points within the member
Ø Enter the offset length at which the first intermediate restraint will be positioned. Leave this field as zero if offset is the same as the spacing
Ø Enter the number and spacing between the intermediate restraints.
Ø Click OK
All lateral restraint applied to the member will now be regenerated and will replace all existing restraints.