Investigation into the effective lengths of web compression elements in parallel chord trusses

[摘要] The Southern African Institute of Steel Construction expressed concern with regard to the unitdefinition of the effective length factor, K, stipulated for compressive elements of parallel chordtrusses in Clause 15 of SANS 10162-1:2005 - Limit state design of hot-rolled steelwork. Thesimplified method for truss design specified in the code assumes all compression members arepin-connected, which allows for greater design simplicity and reduces the amount of codeinterpretation required by the designer. In addition to this, Clause 15 requires the additionalreduction in resistance of the first web compression members by a factor of 0.85. However, thisapproach may be considered overly conservative and in current design practice the effectivelength factor is often reduced to less than 1.0.This research investigates the effective length factor of web compression members in parallelchord trusses, by means of investigative structural analyses of representative trusses usingANGELINE and Prokon analytical programs, and by designing, constructing and testing sixrepresentative trusses, using current design practices. A comparative study of a number ofdifferent countries' codified approaches to truss design is also included.The structural analyses revealed that in-plane buckling of the web compression members wasthe consistent mode of failure, however at a much greater applied load than the design loaddetermined according to SANS 10162-1:2005. Contrary to the expected mode of failure, all sixtests performed on the representative trusses exhibited elastic out-of-plane buckling, or strongaxisbuckling, of the web compression members, but still at a much greater applied load thanthe design load. The unexpected out-of-plane buckling of the web members is due to the inplanestiffness of the end connections used. In order to stay true to current design practice,gusset plates and longitudinal welds were used to join the web members to the chords. Thestiffness of the gusset plates therefore significantly reduced the effective length of the webcompression members in-plane, but did not reduce the effective length out-of-plane.Despite the unanticipated behaviour of the tests performed, certain conclusions can still bedrawn from the results. The unit definition of the effective length factor for in-plane buckling ofweb compression members is too conservative and a K factor of 0.8 is recommended. Inaddition to this an effective length factor for out-plane-buckling of web compression members of1.1 is recommended for trusses with welded connections. The necessity of the reduction inresistance of 0.85 of the first web compression members requires further investigation. Themost important conclusion to be drawn is that out-of-plane buckling of web compressionmembers can be the dominant failure mode, which is not taken into consideration in currentdesign practice.

[发布机构] Stellenbosch University