SEISMIC PERFORMANCE OF STIFFNESS IRREGULAR MOMENT RESISTING FRAME DESIGNED FOR HIGH STRENGTH CONCRETE

Abstract

Damage observed after previous earthquakes indicates that a large number of existing buildings are vulnerable to seismic hazard. The damage in the structures generally initiates at location of the structural weak planes present in the building systems. These weaknesses trigger further structural deterioration which leads to the structural collapse. The weaknesses often occur due to presence of the structural irregularities in stiffness, strength and mass in a building system. Buildings with irregular configurations demonstrated more vulnerability in the past during earthquakes. It has been observed after previous earthquakes that the buildings having irregularities have higher seismic demands. The structural irregularity can be broadly classified as plan and vertical irregularities. The focus of the present study is to assess the performances of soft storey at first storey level (stiffness irregular) buildings designed for normal strength concrete (NSC) and high strength concrete(HSC). A building is said to be stiffness Irregular (Soft Story) when there is a story in which the lateral stiffness is less than 70% of that in the story above or less than 80% of the average stiffness of the three stories above. Accordingly in this study the average stiffness of first storey are 68.75 % and 69.14% of the storey above it for NSC-SI and HSC-SI respectively. For the purpose of this study four 12- Storey moment resisting frames (MRF) building were designed and detailed using CSI ETABS v18.1 according to new Ethiopian building code ES EN 1998-1 2015 guidelines. The study program grouped into two: One with regular configuration designed for both NSC and HSC and the other group as soft storey at first storey level designed for both NSC and HSC. Following the design and detailing, Seismic performance assessment were conducted using nonlinear static pushover analysis and Incremental dynamic analysis by Sesimostruct (2018) software. Static pushover analysis indicates that the ultimate base shear capacity is increased by 2.88% for HSC-R, decreased by 20.37% and 13.55% for NSC-SI and HSC-SI respectively when compared to NSC-R. The value of roof displacement at ultimate base shear capacity is reduced by 13.92%, 5.65% and 14.11% for HSC-R, NSC-SI and HSC-SI respectively compared to NSC R. Similarly, the results from IDA shows the relative increase in ultimate base shear capacity by 6.65% for HSC-R, 23.97% decrease for NSC-SI and 7.018% decrease for HSC-SI compared to NSC-R. Roof drift at ultimate base shear capacity is decreased by 8.71% for HSC-R, 1.68% increase for NSC-SI and 13.8% increase for HSC-SI in comparison to NSC-R. The study is extended to the seismic vulnerability analysis using the result from IDA analysis as the input for seismic fragility analysis. These curves depict probability of exceeding limit state capabilities identified as immediate occupancy (IO), life safety and collapse prevention limit states under different levels of seismic intensity. The fragility analysis indicates that the NSC-R moment resisting frame shows 90.01%, 77.73% and 43.39% probability of exceeding the IO, SD and NC performance levels respectively. For HSC-R, it was observed that 84.54%, 81.53% and 59.29% probability of exceedance of IO, SD, and NC performance level of exceedance. Regarding the stiffness irregular (soft-story) MRF, the results indicates that 96.34%, 85.04% and 53.72% of probability of exceedance of IO, SD and NC respectively whereas, for HSC-SI the probability of exceedance of IO, SD and NC were 92.92%, 91.12% and 77.37% respectively. The results from both pushover analyses and Incremental Dynamic analysis shows that collapse shear capacity increase slightly over wide ranges of concrete strength. Increase in concrete strength does not improve seismic performance to desired level in seismic zone as structural member fail early prior reaching their full capacity due brittleness nature of the concrete. Fragility analysis further confirms that the frames designed for high strength (HSC) perform generally well at immediate occupancy (IO) performance level whereas, frames designed for normal strength concrete (NSC) are relatively better at Significant damage (SD) and near collapse (NC) for both regular and stiffness irregular (Soft-storey) MRF.