Lateral restraints against LTB may be discrete or continuous, full or elastic, and may concern the lateral displacement, the torsional rotation, the lateral flexural rotation and the warping. LTBeam applies to straight single- or multi-span beams, under simple bending about their strong axis, with doubly - or mono-symmetrical cross-sections about this plane of bending. It generates suddenly and simultaneously a lateral bending deformation and a longitudinal torsional deformation along the unrestrained length of beam. The lateral torsional buckling phenomenon (LTB) appears when the bending action reaches a critical level. It has been developed by CTICM (Centre Technique Industriel de la Construction Metallique - France) in the frame of an european research project partially funded by the European Community for Steel and Coal (ECSC Project No 7210-PR183 : "Lateral torsional buckling of steel and composite beams" - 1999-2002). LTBeam is a software to be used for the Design of Steel Structures. SCI – Steel Construction Institute, Berkshire UK.LTBeam is a software which deals with the elastic "Lateral Torsional Buckling of Beams" under bending action about their major axis. (2011): Stability of Steel Beams and Columns (In Accordance with the Eurocodes and UK National Annex). Therefore the section is not okay to resist lateral torsional buckling on the primary beam. Χ LT is the reduction factor for lateral torsional buckling. W y = W pl,y for Class 1 or 2 cross-sections In determining W y, no account need be taken for fastener holes at the beam ends. Where W y is the section modulus appropriate for the classification of the cross-section, as given below.
The design buckling resistance of a laterally unrestrained beam (or segment of beam) should be taken as The design requirement is that M Ed must be shown to be less than M b,Rd, and checks should be carried out on all unrestrained segments of beams (between the points where lateral restraint exists). The design bending moment is denoted by M Ed (bending moment design effect), and the lateral-torsional buckling resistance by M b,Rd (design buckling resistance moment). In general, the bracing system assumed to provide effective lateral restraint must be capable of resisting an equivalent stabilising force q d (defined in clause 5.3.3(2) of EC3), the value of which depends on the flexibility of the bracing system. Beams supporting concrete slabs (Garner, 2011)
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