Drift Concentration of a Three-Story Special Concentrically Braced Frame with Strongback under Earthquake Loading

In conventional special concentrically braced frame (SCBF) structures, the buckling of the braces leads to severe reduction in system strength and stiffness. Therefore soft story mechanisms followed by large permanent deformation are commonly observed in SCBF structures. The strongback system using additional structural components along the height of the building to resist local deformation is able to improve the distribution of the drift. This research conducted case studies to investigate the effects of strength and stiffness of the strongback system on the behavior of typical three-story SCBF system. The primary variables to be investigated are stiffness factor α (strongback stiffness/SCBF stiffness) and strength factor β (strongback strength/SCBF strength). We conducted nonlinear dynamic analyses to evaluate the effectiveness of α and β on structural demand parameters including maximum drift ratio and drift concentration factor (DCF). Analyses results show that although strongback system with excessively high α and β will reduce maximum drift ratio and DCF of SCBF systems, it is noneconomic. On the other hand, strongback systems with low α and β (α<0.0048 and β<0.054) have only little effects on improving structural behavior. The case studies suggested that 0.0096≤α≤0.0168 and 0.081≤β≤0.134 accounting for both efficiency and economics can be used in the design of the selected SCBF systems.