Seismic Vulnerability Assessment and Development of Loss Functions for Steel Structures with Khorjini Connections with Capacity Spectrum Method

The present paper focuses on the development of fragility curves and on the seismic vulnerability assessment of existing steel structural systems with a certain type of semi-rigid connection known as Khorjini (Saddle) connections. A significant number of such building type suffered from extensive damage to total collapse in 1990 Manjil (Northern Iran) M 7.4 earthquake. In this study, several variants of two-dimensional structural models with such connections including concentrical braced frames only, infilled frames only, and a combination of both, have been investigated. Three- and five-story models have been considered as they are the most common structures with Saddle connections in Iran. Development of fragility curves have been completed based on HAZUS methodology. The methodology benefits from a damage estimation approach based on the capacity spectrum method. Previous studies have proved that the methodology adopted in HAZUS documents is applicable in risk assessment studies. The methodology requires an inelastic demand spectrum as well as the capacity curve of the structure presented in Acceleration-Displacement Response Spectrum (ADRS) format. The former is derived by reducing elastic design spectral ordinates to account for hysteretic damping, while the latter is obtained from nonlinear static (pushover) analysis of the nonlinear model of the structural system. For the design spectrum, the NEHRP acceleration spectra at two levels, 2% probability of exceedance in 50 years (level 1) and 10% probability of exceedance in 50 years (level 2) were utilized. Nonlinear models of the buildings are constructed in OpenSees platform utilizing nonlinear beam-column elements with their plasticity behavior concentrated at both ends of the elements (zero length nonlinear rotational springs). Saddle connections are modeled using two torsional springs to model their rotational constraining effects at beam-columns joints. Moment-rotation responses of such springs were calibrated to match available experimental results with similar connection details considered in this study and in relevant literatures. Moreover, all infill panels are modeled using equivalent compression struts based on the current acceptable approaches found in reference literature. Performance points of the structures are found by intersecting the demand spectrum with the capacity curve of the structure. The final output of the HAZUS methodology is the fragility curve of the structure, which depicts the probability of exceedance of certain levels of drift-based performance levels according to peak ground acceleration (PGA) values. Nevertheless, uncertainties are incorporated in the fragility curves development process. Results show that the probability of exceedance of Immediate Occupancy (IO), Life Safety (LS) and Collapse Prevention (CP) performance levels in 3-story models subjected to the level 1 earthquake (475 return period) are, respectively, 50%, 23% and 12%; while the corresponding values for the level 2 earthquake (2475 values) are 87%, 70% and 56%, respectively. For the 5-story models, the probabilities are 64%, 29% and 16% for the level 1 earthquake and 96%, 81% and 67% for level 2 earthquake. Due to such high levels of vulnerabilities, seismic retrofitting of such existing buildings in the city of Tehran is absolutely essential.