Steel Designer splits the compressive design of a member to BS5950 into three design checks. You may choose to check the section capacity and/or the member buckling capacities about the major and minor axes.

The section capacity check calculates the capacity of the members cross-section to carry the axial load and computes the capacity of the members as simply the gross area times the yield strength. This check is not explicitly defined in BS5950 as the capacity of the cross section will always be adequate if the member satisfies the member buckling checks.  However, this check has been provided within steel designer to help distinguish this type of failure mechanism in the design of the column.

To determine the buckling capacity for a column it is necessary to know the spacing of any bracing (if any) along the member.  This bracing could be provided by purlins, girts or other structural elements, which are not modelled in Multiframe. Some bracing may only restrain lateral deflection in one direction therefore it is necessary to enter unbraced lengths for both axes of the section.  In Steel Design the unbraced length of a member may be specified in either of the following ways;

By specifying a single unbraced length and effective length factor for buckling about each axis, or

By breaking the member into column segments and setting the effective length factor for each segment.  Each column segment is then designed separately for compression.

To determine the buckling load for a member the user may choose to specify a single unbraced length of the member for buckling about each principle axis.  It is also necessary to enter an effective length factor to indicate the type of restraint applied to the ends of the unbraced span of the column. These may be different for buckling in the major and minor axis directions.  The effective lengths for determining the buckling capacity of the member are given by

Lx=Kx*Lcx and Ly=Ky*Lcy

where Lcx and Lcy are the unbraced lengths of the member and Kx and Ky are the two effective length factors for the major and minor axes respectively.

The initial values of Lcx and Lcy are the length of the member and the initial values of Kx and Ky are 1.0.

A more sophisticated method for the design of a member for compression allows for the division of the member into a number of column segments.  These segments are defined by restraints that resist column buckling that are applied at intervals along the member.  In Steel Designer, restraints against buckling can be specified at joints along a design member.  These restraints are used to break the member into a number of column segments that may differ for the design of the member about its major and minor axis.  The effective length associated with each segment may also be specified to account for the restraint conditions at each ends of the segment.

When column segments are specified, the design of the member will be performed by considering the design of each segment separately. 

To set the properties for compression

Ø  Select the required members in the Frame window

Ø  Choose Compression… from the Design menu

If the unbraced lengths of the member are to be specified directly then

Ø  Select the Unbrace Length radio button.

Ø  Type in values for Kx and Ky

Ø  Type in values for Lcx and Lcy

Ø  Click OK

Otherwise if the design for compression is to be performed using column segments.

Ø  Select the Column Segments radio button.

The tabbed control in the dialog will become active. The first page in this table lists the location of joints along the members and indicates if they provide restraint against column bucking about either axis of the member.

Ø  Enter the restraints associated with each node.

The restraint information is used to build a list of column segments that span between the specified restraints.

Ø  Click on the Major Axis tab.

This displays a table of column segments that will be used for the design of the member for compression when considering buckling about the major axis.

Ø  Enter the effective length factor (K) for each segment.

Ø  Click on the Minor Axis tab and enter the effective length factors for the minor axis column segments.

Ø  Click OK.