IMPACT OF VERTICAL IRREGULARITIES ON INTERNAL FORCES AND HORIZONTAL DISPLACEMENT IN HIGH-RISE BUILDING

Abstract

Vertical irregularities in tall buildings such as abrupt changes in mass, stiffness, or geometry along the height have a profound effect on seismic response. These discontinuities disrupt uniform lateral load paths, leading to amplified internal forces, story drift, and lateral displacements under earthquake loading. This paper investigates these effects using finite-element models of a 30‐story reinforced concrete high-rise in Islamabad (a seismic Zone 2B region) designed per ASCE 7-16 and the Building Code of Pakistan (BCP-2021). Five models were analyzed in ETABS: one regular (baseline) and four with vertical irregularities (mass, geometric, stiffness, and combined irregularity). Key metrics including story stiffness, base shear, shear forces, bending moments; axial loads, lateral drift, and displacement were compared. The study found that stiffness irregularity (e.g. a “soft story” or taller story at level 20) produced the largest increases in lateral displacement (about 5.3% higher roof drift than the regular building), while the combined-irregularity model led to pronounced story drift and stress concentrations at the irregular floor. In contrast, the mass-irregular model showed negligible or slightly reduced displacement, indicating that added mass at mid-height can dampen lateral motion. These results confirm that vertical irregularities significantly degrade seismic performance. To mitigate these effects, we recommend reinforcing irregular floors by enlarging member sections or adding shear walls, and conducting advanced nonlinear analysis during design. The findings provide guidance for engineers to enhance the resilience and safety of high-rise buildings in seismic zones.