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 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

 and ,

where

L_cx and L_cy are the lengths of the member in x and y direction respectively,

K_x and K_y are the two effective length factors for the major and minor axes respectively. The initial values of K_x and K_y are 1.0.

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, L_cx corresponding to the spacing of restraints preventing compression buckling about the x-x axis and L_cy corresponding to the spacing of restraints preventing compression buckling about the y-y axis.

To set the properties for compression

Ø  Select the required members in the Frame window

Ø  Choose Compression… from the Design menu

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

Ø  Type in values for K_x and K_y

Ø  Type in values for L_cx and L_cy

Ø  Click OK

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

The initial values of L_cx and L_cy are the length of the member.