The design of a member for bending is divided into four design checks. These check the flexural and shear capacity of the member about the major and minor axes. Each of these checks may consider one or more limit states depending upon the section and the actions within the member.
When performing a bending check it is necessary to specify how lateral 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.
The location and type of lateral 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.
The laterally unbraced length (Lb) and bending coefficient (Cb).
You may need to specify a number of properties relating to the location and type of lateral restraints and the stiffener spacing along the member
If the spacing of lateral restraints along the member is specified, Steel Designer uses this information to break the member up into a number of spans in order to determine lateral torsion buckling capacity of each span. In Steel Designer, these spans are known as segments.
Each lateral restraint specified by the user is assumed to provide bracing against lateral displacement of the critical flange and/or prevent twist of the cross section. At any cross section, the critical flange is the flange that, in the absence of any restraint at that cross section, would deflect the furthest during buckling of the member. In most members the critical flange will be the compression flange. However for a cantilevered member, the critical flange is the tension flange.
For each restraint located along a member, the user must specify the type of restraint. As this depends upon which flange is the critical flange, which is not know a priori, the user must specify the type of lateral restraint that would be present at a section if
§ The top flange was the critical flange, and
§ The bottom flange was the critical flange.
In LRFD no distinction is made between different types of lateral restraints. However, to be compatible with other design codes, Steel Designer allows for lateral restraints at a cross section to be classified as follows
§ Full Restraint –supports the cross section against lateral displacement of the critical flange and prevents twist of the cross section.
§ Partial Restraint – provides support against lateral displacement of the section at a point other than the critical flange and prevents twist of the cross section.
§ Lateral Restraint – resists lateral displacement of the critical flange only.
For the purpose of design in LRFD, each of these restraint types is consider adequate to provide lateral support to the cross section at which they are applied.
Lateral restraints must always be specified at the ends of the beam and so the minimum number of lateral restraints is two. If no restraint exists at the end of a member then it should be specified as unrestrained in which case the member would be regarded as a cantilever. The initial lateral restraints applied to the member are full restraints at each end for either of the flanges being the critical flange.
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 contains options for displaying and labelling lateral restraints. The restraints are drawn 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 (e.g. F - fixed, P – partial, L - lateral).
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 (Cb) as this can no longer be automatically determined by Steel Designer. LRFD permits a conservative value of Cb=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.
To set the properties for bending
Ø Select the required members in the Frame window
Ø Choose Bending from the Design menu
Ø Select the “Member is fully laterally restrained” option, or
Ø Select the “Position of Lateral Restraints” option, and then
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.
Ø Or select the “Unbraced Length” option, and then
Ø Enter the unbraced length (le)
Ø Enter the moment modification factor coefficient (am) to be used in the design of this length of the member.
Ø Choose the position of the load from popup menu
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 are a specified spacing along the member.
Ø From the Bending dialog, click the Generate… button
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 no offset is same as the spacing
Ø Enter the number and size of spacings for the intermediate restraints.
Ø Click OK
All lateral restraint applied to the member will now be regenerated and will replace all existing restraints.