Potential for Using Tubular Sections in Open Web Steel Joists

Open web steel joists provide for efficient support of roof and floor systems. During their erection, and before any bridging is in place, they are often highly susceptible to lateral-torsional buckling (LTB) because they are quite slender out-of-plane. Given that closed shapes, such as tubes, can in general provide excellent torsional resistance, this paper will present a study that investigated the impact of using such shapes within steel joists. Although torsional resistance may have little impact on the capacity of members with smaller unbraced lengths, it was hypothesized that it could have significant potential when large unbraced length conditions exist. Although the Steel Joist Institute currently specifies only the use angles and double angles to construct joists, the alternative use of hollow structural sections (HSS) was investigated. Employing the finite element analysis programs Strand7 and MASTAN2, the structural behavior of several joist configurations employing HSS shapes were compared with a standard 32LH06 joist comprised of angles and double angles. The results of the analyses indicate that constructing the chords of these trusses from hollow structural shapes can significantly increase their elastic LTB strength. Most importantly, this study shows that using HSS chords can provide the required torsional stability of unbraced joists, and thereby eliminate bridging provided for erection and wind uplift conditions.

Learning Objectives:
Show how increasing the torsional stiffness of the chords of open web steel joists can significantly increase their lateral torsional buckling capacity in both braced and unbraced conditions.

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