Abstract

ABSTRACT
Seismic Fragility Assessment of Pre- and Post-Retrofit 1980s Concentrically Braced Frame Office Buildings in Moderate Seismic Zones
Ming Zheng Wang
In the past decades, concentrically braced frames (CBFs) have been frequently employed as earthquake resistant systems due to their high stiffness and straightforward design. Over a time line ended in 2000, the existing office building stock designed in the interval 1980-1989 in Quebec is about 25% of non-residential buildings. Among these buildings, more than 50% are low- and middle-rise steel structures with a CBF system. Since 1980, both the National Building code of Canada (NBCC) and Steel design standard (CSA/S16) have continually evolved, while lower interest was allocated in the development of guidelines for seismic assessment and retrofit of existing buildings located in seismic zones.
This study focuses on the seismic fragility assessment of pre- and post-retrofit fictitious low-rise (3-storey) and middle-rise (6-storey) CBFs office buildings located in Montreal (Qc.) and designed according to the NBCC 1980 and CSA/S16.1-78 standard provisions. Using the performance based design approach and incremental dynamic analysis (IDA), the IDA curves computed as pairs of earthquake intensity measure parameter (e.g. S(Ta)g) versus the engineering demand parameter (e.g. peak interstorey drift) were obtained. The analytical fragility curves of studied buildings have a lognormal distribution and are defined as a function of spectral acceleration (Sa). Both aleatoric and epistemic uncertainties were considered.
The seismic force resisting system of the studied fictitious office buildings consists of four chevron CBFs in the E-W direction and two times two adjacent CBFs with diagonal tension/compression braces in the N-S direction. Each brace-to-frame connection consists of a gusset plate welded to the slotted end part of the HSS brace. First, the seismic assessment of these buildings was conducted through equivalent static force procedure, as well as linear dynamic analysis and it was concluded that the ratio between the demanded factored base shear Vf-10 (NBCC 2010) and Vf-80 (NBCC 1980) is 2.2 for the low-rise buildings and 1.5 for the middle-rise buildings. In addition, it was found that several brace-to-frame connections, several beams and especially the middle column of CBFs with tension/compression diagonal braces do not possess sufficient strength and the retrofit action is required.
According to ASCE/SEI 41-13 the Rehabilitation objective is a combination of a Target building performance level, an Earthquake hazard level and an assigned Objective rehabilitation class which is Basic Safety (BSO) for office buildings, while the target building performance levels (PL) are Life Safety and Collapse Prevention. To reach the BSO, a conventional retrofit strategy implying the replacement of brace-to-frame connections, strengthening of CBFs beams and column cross-sections especially at lower floors was applied. In this light, three building performance levels: Immediate Occupancy, Life Safety and Collapse Prevention, with the associated earthquake intensity level (50%/50 yrs., 10%/50 yrs. and 2%/50 yrs) and three damage states (Light, Moderate, and Severe) were considered. The aforementioned performance levels were defined on each IDA curve resulted from nonlinear time-history analysis using OpenSees. The recorded engineering demand parameters were the interstorey drift and the residual interstorey drift, while the intensity measure parameter was Sa(T1)g. In this study, the light damage is associated with the elastic response of the building, the moderate damage with the attainment of 0.5%hs residual interstorey drift and severe damage when the building reaches a maximum interstorey drift of 2%hs.
Because the existing building structures are non-ductile and brittle failure of brace-to-frame connections is expected due to shearing of welds, there is no a gradual transition between damage states. In this regard, the Severe damage state occurs suddenly after the structure response exceeds the elastic limit characterized by Light damage. For the post-retrofit buildings, the probabilities for Light damage are relatively larger compared with those for Moderate and Severe damage. In addition, the probability for all retrofitted buildings (E-W) to sustain Severe damage is 20% for the 3-storey and 30% for the 6-storey. A similar probability of Severe damage was obtained for the CBF with diagonal braces located in the N-S direction.