Effect of tension bracing on the collapse mechanism of steel moment frames

The nonlinear response of Tension Braced Moment Resisting Frames (TBMRF) under lateral loading with regards to their sequential failure mechanisms and ductility is studied. Both distributed and lumped plasticity finite element approaches are considered and parametric studies are carried out to evaluate the sequence of element failures in various single and multi-storey TBMRFs. Three possible failure mechanisms representing different specific characteristics of TBMRF systems are presented and their differences are outlined. Also, several multi-storey dual-system TBMRFs are analyzed and the results are utilized to set up an analytical model which can appropriately differentiate and evaluate the load carrying contribution of brace and frame elements. Finally, an empirical formula and design curves to calculate frame-brace contribution ratios in different performance levels are proposed.