| Open web steel joist (OWSJ) designs are optimized to provide high in-plane strength and stiffness, yet individual joists are weak and flexible in the out-of-plane (lateral) direction. They rely upon external lateral supports to maintain out-of-plane stability, especially along their slender compression chords. Under gravity loading, the attached deck diaphragm usually provides adequate lateral support to the joist's top compression chord and no additional bracing is necessary. However, due to their light weight, OWSJ roof systems often experience net uplift loading during windstorms, creating a stress reversal in each joist member. Of particular concern is the slender bottom chord which becomes compressed and must be laterally supported by strategically placed bracing members to prevent out-of-plane buckling. The steel industry has raised concerns about OWSJ net uplift behaviour and the adequacy of current design guidelines. This study investigates OWSJ stability and attempts to address these concerns.;Two 3-dimensional finite element techniques, eigenvalue and nonlinear buckling, were utilized to further study OWSJ stability behaviour. Both techniques were first validated by comparing analytical results with the experimental data and further parametric and bracing studies undertaken, identifying additional factors which influence stability. This information was then used to formulate an alternate 2-dimensional bottom chord finite element model which was verified using 2-dimensional finite element software. Finally, a manual technique using readily available spreadsheet software was developed and validated. Recommendations were made to address ambiguities and concerns with the Canadian Steel Standard, CAN/CSA S16-01, and areas of continued research were proposed.;An extensive experimental program using full-size joist specimens was undertaken to investigate OWSJ stability and bracing behaviour. A unique, automated testing apparatus was designed and constructed to simulate wind uplift loading. Experimental techniques were devised and a comprehensive testing regime implemented to study the stabilizing effects of joist span, depth, bottom chord moment of inertia, and bracing configuration. Additionally, bracing forces and stiffnesses were investigated at various locations. Eighteen full-size joist specimens were fabricated, 86 buckling experiments conducted and 403 bracing tests performed. |
