Numerical Evaluation of Code Requirements and Nonlinear Performance of Torsionally Irregular Structures

Abstract

Structures with torsionally irregular configurations – those with non-coincident centers of mass, stiffness, and strength – are vulnerable to amplified seismic demands from twisting modes of response that localize deformation and damage, increasing the likelihood of failure or collapse during strong ground shaking. Despite decades of research, design provisions for torsional irregularity remain inconsistent across international codes and are often based on studies of reduced-order models that may not adequately capture the behavior of multi-story or spatially irregular systems. To address these perceived gaps, this dissertation leverages high-performance computing to investigate the design and behavior of torsionally irregular structures through three interconnected studies: (1) Minimizing superstructure twist in irregular bridges through optimization of structural parameters (2) Evaluation of design provisions in the New Zealand seismic design standard (NZS 1170.5:2004) for the seismic assessment of torsionally irregular buildings, and (3) Improving the seismic performance of torsionally irregular buildings using force-limiting diaphragm connections. The first study investigates geometrically irregular bridges using a validated finite element model of a previously tested reinforced concrete bridge. It evaluates three modification strategies: adjusting column effective heights, altering end fixity conditions, and redistributing superstructure mass, to reduce torsional response. Numerical and optimization-based studies showed that increasing the effective stiffness of columns by reducing their effective height was the most efficient strategy. The study also demonstrated that a small subset of hazard-consistent ground motions could capture the essential behavior required for optimization, providing a practical balance between computational efficiency and accuracy. The second study examines torsionally irregular buildings within the context of the New Zealand seismic design standard (NZS 1170.5:2004). Reinforced Concrete Shear Wall (RCSW) and Steel Special Moment Frame (SSMF) buildings were designed and analyzed using nonlinear time-history simulations of site-specific ground motions derived from the 2022 New Zealand National Seismic Hazard Model. A comparative analysis of the current code provisions and proposed updates by a working task group showed that the proposed updates substantially reduced maximum drift demands and collapse probabilities, especially for highly ductile SSMF systems, while penalizing designs with excessive torsional irregularity. The third study explores the potential of deformable Inertial Force-Limiting Connections (IFLC) to reduce seismic demands in irregular buildings. By replacing conventional rigid diaphragm-to-lateral system links with deformable connections designed to dissipate energy and limit force transfer, the study established rational benchmarks for connection stiffness and strength based on diaphragm design forces and system drift constraints. Results indicated that properly tuned IFLCs can reduce lateral force demands, making them a promising design option for improving the seismic resilience of irregular structures. Overall, the findings of this dissertation improve our understanding of torsionally irregular structural systems through code-level evaluations employing advanced numerical modeling techniques and the examination of innovative connection strategies through numerical optimization. The results provide a rational basis for updating national seismic design standards and highlight the potential of force-limiting methods as a next-generation seismic design tool for irregular structures.