EFFECT OF TORSION ON FLOOR ACCELERATION DEMANDS OF NSCS ATTACHED TO RC BUILDINGS WITH DIFFERENT LATERAL LOAD RESISTING SYSTEMS

In this study, the acceleration demands on NSCs in regular and torsionally irregular six-story reinforced concrete (RC) buildings with moment-resisting frames (MRF) and shear walls (SW) as lateral load-resisting systems were investigated. Four building models, including two regular (R) and two torsionally irregular (IR) configurations as defined by ASCE 7-22, were analyzed using twelve earthquake ground motions calibrated to Montreal’s uniform hazard spectrum for site class C, with a 10% probability of exceedance in 50 years. The acceleration demands including the height factor (PFA/PGA), the floor response spectra (FRS), and the horizontal force factor (Sp), were determined through linear time-history analysis, with results compared to the provisions of NBCC 2020 and ASCE 7-22. The findings indicate that the RSW building exhibits a higher PFA/PGA ratio than the RMRF building across all floors with differences becoming more pronounced at the upper levels, reaching 95%. Torsional effects further amplify the increase in the PFA/PGA ratio in both structural systems, with maximum increases of 20% at the upper floors in the IRMRF building and 18% at the lower floors in the IRSW building. Additionally, the computed Sp factor increases by 30-35% in the IRMRF building and 40-60% in the IRSW building compared to their regular counterparts. A comparison with code provisions reveals that ASCE 7-22 and NBCC 2020 underestimate PFA/PGA in the IRMRF building by 35% and 8% at the second floor, respectively, while remaining within code limits at other floors. In contrast, the computed PFA/PGA ratio for the IRSW building exceed code predictions throughout most of the building height with maximum discrepancies of 20% at the rooftop. Furthermore, the obtained code-based force factor (Sp) values captured the demands in the RMRF, while they underestimated those in the IRMRF by up to 60% at the roof and 20% at the second floor. For SW buildings, both NBCC 2020 and ASCE 7-22 underestimated the demands in all cases. The Sp values of the RSW showed up to 100% exceedance at the top floor, while values in the IRSW building reached up to 200% at both the roof and second floor.