Floor acceleration demands in reinforced-concrete buildings with nominal torsion

Abstract

Accurately estimating floor acceleration demands is essential to ensure the seismic safety of secondary systems and building contents. The floor acceleration demands can vary significantly among different building types, reflecting a complex interplay of various influencing factors. The research work presented in this thesis investigates the impact of nominal torsion on floor acceleration demands in buildings. To achieve the aforementioned objective, 24 reinforced-concrete moment-resisting frame buildings are analyzed, 16 torsionally irregular and eight torsionally regular, exhibiting varying degrees of torsional irregularity and diverse plan shapes. These buildings are subjected to 14,400 linear and non-linear dynamic time history analyses using a suite of 20 far-field ground motion records, with and without considering rotational ground motions. It is observed that current seismic code provisions significantly underestimate the peak floor acceleration demands for rigid secondary systems for both elastic and moderately inelastic torsionally irregular buildings. Among the investigated buildings, the median peak floor acceleration demands are underestimated by up to 137%, at the flexible edge. Likewise, for flexible secondary systems, existing code provisions can substantially miscalculate the median total acceleration amplification, underestimating it by as much as 539%. Both the degree of torsional irregularity and inelasticity significantly influence the amplification of the peak floor acceleration and the total acceleration. The total acceleration amplification further depends on the tuning ratio between the building and the secondary system. The rotational ground motions have a notable influence on both peak floor acceleration and total acceleration amplification, especially at the stiff edge (up to 53% and 54%, respectively), regardless of the extent of torsional irregularity and level of inelasticity (ductility demand) in the building. The relationships between torsional amplification factors and torsional irregularity indices are examined for rigid, flexible, and very flexible secondary systems. For rigid and flexible secondary systems, torsional amplification factors are observed to be well-correlated with floor displacement-based torsional irregularity indices. In contrast, for very flexible secondary systems, these factors tend to remain almost constant and independent of the characteristics of both the building and the secondary systems. Novel predictive equations are proposed to estimate torsional amplification factors for buildings with and without nominal torsion. These proposals are validated against multiple application examples, demonstrating that the proposed equations, when used alongside the latest code provisions, provide more accurate estimates of floor acceleration demands in buildings, also accounting for the effects of rotational ground motions.