Steel Designer splits the compressive design of a member to LRFD into two design checks. You may choose to check the member capacity and/or the member’s slenderness about the major and minor axes.

When checking or designing members for compression, it is necessary to specify the effective length factors and unbraced lengths of the member.

To determine the critical buckling condition of a member, it is also 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, Lcx corresponding to the spacing of restraints preventing compression buckling about the x-x axis and Lcy corresponding to the spacing of restraints preventing compression buckling about the y-y axis.

To determine the critical buckling load for a member, it is necessary to enter an effective length to indicate the type of restraint on the ends of the member. The effective length is given by an effective length factor multiplied by the unbraced length of the member. The effective length may be different for buckling in the major and minor axis directions. The effective lengths are given by

Lx = Kx * Lcx ,    Ly = Ky * Lcy  and   Lz = Kz * Lcz 

Where Lcx and Lcy is the unbraced length of the member and Kx, Ky the two effective length factors for the major and minor axes respectively. Lcz is the unbraced length and Kz is the effective length factor of the member for torsional buckling.  The initial values of Kx, Ky and Kz are 1.0 and the initial values of Lcx, Lcy and Lcz are the length of the member.

In addition to checking the compressive capacity of the member, a design constraint will be applied to the member enforcing the slenderness of the member to be less than 200.

To set the properties for compression

Ø       Select the required members in the Frame window

Ø       Choose Compression… from the Design menu

 Either

Ø  Click on the icons for the end conditions in each direction or…

Ø  Type in values for Kx and Ky

Ø  Type in values for Lcx and Lcy

Ø  Type in values for Kz and Lcz

Ø  Click OK

If you choose a standard end condition, the recommended Kx and Ky values will be automatically entered for you.