Factors affecting the seismic collapse performance of realistic low‐rise braced frame buildings, including soil structure interaction and foundation flexibility

Abstract

Previous computational studies of short-period buildings have shown increasing collapse probabilities with decreasing building period; a trend that has not been observed in past earthquakes. In this study, collapse performances of twelve short-period steel special concentrically braced frame (SCBF) building archetypes were examined using dynamic analysis and the methods described in FEMA P-695 to investigate this gap between analytically predicted and historically observed collapse rates. The archetypes varied key design and modeling parameters that could influence the behavior. These parameters included number of stories, level of design seismicity, redundancy, the inclusion of soil-structure interaction (SSI) and foundation flexibility, and the removal of reserve moment frame resistance in modeling. Practicing engineers designed the archetypes resulting in realistic designs, including overstrength values representative of this building type. The calculated overstrength values were much higher than those found in previous studies, largely a result of the archetype building layouts, brace width-to-thickness ratio limitations, and the large differences between tension and compressive brace strength that occur when design demands are low. The higher overstrength values found in this study resulted in lower probabilities of collapse, relative to previous collapse studies and showed that probabilities of collapse decreased with decreasing period. Additionally, the probability of collapse increased significantly for the nonredundant archetype, but changed insignificantly for the archetype without reserve moment frame capacity. Inclusion of SSI and foundation flexibility resulted in a complete change in behavior where braces remained elastic and the braced frames rocked on their foundations, resulting in decreased collapse probabilities.

Stability assessment and development of design criteria for SCBF systems with in‐plane buckling braces

“…The approximate design methods, such as the Whitmore width method 6 to evaluate the yielding areas and Thornton's method 2 to evaluate the buckling strengths of the gusset plates/knife plates, are adopted in practice. 7 Figures 2A and B illustrate the process to evaluate Whitmore widths and the effective buckling lengths of the gusset plates and knife plates, respectively. The partial yielding of gusset plates and/or knife plates is usually observed at the onset of flexural buckling of the IPB system because of the non-uniform stress flow at the connection.…”

“…The evaluation of the state of stress in the gusset plates and knife plates under cyclic loading is not straightforward primarily because of varying degrees of end restraints and geometric properties of components. The approximate design methods, such as the Whitmore width method 6 to evaluate the yielding areas and Thornton's method 2 to evaluate the buckling strengths of the gusset plates/knife plates, are adopted in practice 7 . Figures 2A and B illustrate the process to evaluate Whitmore widths and the effective buckling lengths of the gusset plates and knife plates, respectively.…”