Improving The Earthquake Resilience Of Existing Multi Storey Concentrically Braced Frames Office Buildings In Moderate To High Seismic Zones

In the past decades, concentrically braced frames (CBF) have been frequently employed as earthquake resistant systems for low- and middle-rise buildings. Their configuration and straightforward design made them appealing for engineers. It is noted that building structures designed and built in Canada prior to 1970 were not proportioned to carry seismic loads, while those constructed between 1970 and 1985 were designed to withstand lower seismic forces than those required by the current code. As a consequence, these buildings are characterized by lack of seismic resilience and therefore are vulnerable to earthquakes. Herein, the building’s resilience is defined as the capability of a system to maintain a level of functionality in the aftermath of an earthquake event and is characterized by the performance metrics such as fragility, loss, and recovery functions. To quantify the seismic resilience of existing office buildings, a methodology was proposed and exemplified in a case study comprising of 3- and 6-storey fictitious CBF office buildings located in Quebec City and Vancouver. These buildings were designed in accordance with the requirements of the 1980 edition of the National Building Code and CSA/S16.1-M78 standard. It is noted that before 1985, Quebec City and Vancouver were located in the same seismic zone (the seismic demand was identical) and the Vancouver buildings were selected for comparison purpose. The proposed seismic resilience methodology consists in selecting the Rehabilitation Objective Class and the associated performance levels corresponding to earthquake hazard levels (e.g. 2%/50 yrs., 10%/50 yrs. and 50%/50 yrs.). To achieve this step, nonlinear dynamic time-history analyses are required and fragility curves computed for different hazard levels for both existing and retrofitted 3- and 6-storey fictitious buildings were generated from the Incremental Dynamic Analysis curves (IDA). Both aleatoric and epistemic uncertainties were considered. The loss estimation model is a function of system’s components deficiency determined by the use of performance limit thresholds for different damage states. In addition, functionality curves computed for different hazard levels using an exponential recovery model are also shown. The seismic assessment process was done according to performance based design principles and nonlinear time-history analysis by means of IDA using the OpenSees framework (Open System for Earthquake Engineering Simulation). Herein, all studied buildings were assessed against the current code demand. Based on the results, the buildings located in Quebec City and Vancouver show deficiencies at the level of structural members, especially the buildings located in Vancouver. Moreover, all brace-to-frame connections had insufficient strength and showed failure due to shearing of the welds. As reported from IDA curves, all existing buildings experienced collapse when subjected to ground motion intensities in agreement with the current code demand and a retrofit action was required. To respond to the Rehabilitation Objective Class defined as Basic Safety by the ASCE/SEI 41-13 provisions, the selected rehabilitation strategy consisted in local strengthening of system’s components (e.g. cover plating steel columns or beams and gusset plate replacement). According to the results, when the 3- and 6-storey retrofitted buildings located in Quebec City were subjected to ground motions scaled to the current code demand, their functionality was higher than 86.67% and 75.7%, respectively. Conversely, for the Vancouver buildings, besides the retrofit action, it is suggested to double the number of CBFs in order to pass the current code requirements. In conclusion, the proposed retrofit scheme for Quebec City buildings was able to improve the building performance and implicitly its earthquake resilience.