In general, the design procedure incorporates the maximum out of straightness allowances permitted in EC5, Section 10, i.e. (1/300)£ for solid timber and (1 /500)£ for glued laminated timber or laminated veneer lumber, where I is the distance between the supports of the member.

The function of lateral bracing is to provide lateral support to a member in order to prevent it from moving laterally at the bracing position and by so doing increase the buckling strength of the member. The approach used for the design of bracing providing lateral support to single members in compression is to design each bracing member for the force and stiffness required to effectively hold the compression member in position at each bracing member location. Where several members or assemblies (e.g. roof trusses) have to be braced laterally and a bracing system is to be used, the system must be designed to withstand the lateral loading arising from the bracing members. The forces are determined on the assumption that each compression member will deform in a single wave sinusoidal mode, and to comply with the assumptions used in the analysis, the maximum deformation of the bracing system under this loading together with any other external loading (e.g. wind loading) the system supports must not exceed span/500.

The design procedure for members subjected to a direct compression force is different to that used for members subjected to a compression force caused by a bending moment. For members subjected to direct compression, the design force in the bracing is derived from the full value of the design compression force in the member, ignoring any compressive resistance provided by it in the unbraced condition. In the case of members subjected to bending moment, the unrestrained lateral torsional buckling strength is taken into account and the design force to be taken by the bracing is the design compressive force in the member less this resistance.

For a rectangular member of depth h subjected to a design moment, Md, with a lateral buckling instability factor kcrit derived in accordance with the procedures described in, the approximate solution used in EC5 to determine the design value of the compression force in the member, Nd, is:

0 0

Post a comment