ABSTRACT
In design codes the effective lengths of truss chord in the case of out of truss plane buckling is assumed as distance between braces and braces are considered as stiff side supports (PN 1990). Bracing system required to provide lateral stability of beams, columns or trusses should carry additional equivalent load, as for example according the polish steel design code one can consider that the member is side supported when the side support is able to resist additional force F0 given by formula:
where: Nc is normal force in the compression chord, Ac cross section of the compression chord, fd is steel strength.The code PN(1990) defines that maximal displacement of side support should not exceed l1/200, where l1 denotes the side-support distance. In the case of a few members braced by the same bracing the side support should resist additional force Fm
According to Eurocode 3 (2001) a member can be considered as side-supported when equivalent distributed load q can be transferred to the fundament by horizontal bracing
In the above equation it is assumed that the member is bent in out of the plane direction and that the initial bow imperfection is
Another assumption is that the normal force is constant along the member length (Fig. 1).
