Torsional Amplification Factors for Floor Response Spectra in Buildings with Nominal Torsion

This study investigates the impact of nominal building torsion on characteristics of floor acceleration response spectra. To achieve this objective, a set of 16 reinforced‐concrete moment‐resisting frame buildings exhibiting varying degrees of torsional irregularity and diverse plan shapes is analyzed under different earthquake excitations. The study conducts a total of 3200 linear and nonlinear time history analyses. These analyses are performed across five levels of strength ratios in the buildings, using a suite of 20 far‐field ground motions. The torsional floor acceleration amplification factor—defined as the ratio of 5%‐damped floor spectral ordinate at the flexible or stiff edge to that at the center of rigidity—is evaluated as a function of the tuning ratio. The relationships between the peak torsional floor acceleration amplification factors and the building's torsional irregularity index are examined for rigid, flexible, and very flexible secondary systems. The torsional floor acceleration amplification factors for rigid and flexible secondary systems depend on building characteristics, namely the extent of torsional irregularity, the strength ratio (level of inelasticity), and the tuning ratio, whereas very flexible secondary systems remain insensitive to them. A strong correlation exists between the torsional floor acceleration amplification factors of rigid and flexible secondary systems and their corresponding building's elastic floor displacement‐based torsional irregularity index. Novel equations are developed for predicting torsional floor acceleration amplification factors in buildings with torsional irregularities. When integrated with the latest provisions of ASCE 7, these equations provide accurate estimates of floor acceleration demands, thereby improving seismic design of acceleration‐sensitive secondary systems in torsionally irregular buildings.