Modeling sensitivity analysis of concentrically braced frames under short and long duration ground motions

Special concentrically braced steel framed (SCBF) system is one of the popular lateral seismic resistant systems used for buildings. Modeling of SCBF involves several sources of epistemic uncertainties such as the low-cycle fatigue input parameters. Low-cycle fatigue induced fracture of braces is critical in controlling the seismic response and resiliency of SCBF structures. Thus, it is important to better quantify and investigate the influence of modeling parameters on the seismic response of SCBF systems under earthquake motions. An extensive analytical study is conducted using OpenSees to study the modeling sensitivity, and in turn, generate fragility curves for two simple configurations of one-story one-bay SCBF under two sets of ground motions that have varying duration. Different modeling parameters are treated deterministically as sources of uncertainties and Tornado Diagram Analysis is used to rank these sources of uncertainty in SCBF modeling. All the analysis results are integrated to develop two sets of fragility curves under short and long duration earthquakes. The results shows that the estimated median collapse capacity is 45% lower when using the long duration set, as compared to the short duration set and using the drift capacity as the seismic intensity measure. A reduction in the obtained drift capacity of about 7% is also observed on average for every 10 seconds increase in the ground motion significant duration increase, which reflects a premature failure than what is intended from design.