Influence of Vertical Extension on Seismic Drift Demand of Non-Engineered Reinforced Concrete Buildings Using Incremental Dynamic Analysis

Rapid urbanization in the Kathmandu Valley has led to widespread vertical extension of low-rise non-engineered reinforced concrete (RC) buildings. Many low-rise, non-engineered buildings initially constructed with two to three storeys are subsequently extended five to seven storeys without adequate structural assessment, redesign or regulatory approval. This study evaluates and compares the seismic vulnerability of 3, 5 and 7 storey RC frame buildings with identical plan configuration but varying elevation using Incremental Dynamic Analysis (IDA). Three-dimensional to RC frame buildings were modeled in SAP2000 v18 and subjected to eleven far-field ground motion records (Mw > 6.5) selected, Nonlinear time-history analyses were performed incrementally with maximum inter-storey drift ratio (IDR) monitored as the primary response parameter. Collapse limits were defined as 4% IDR for the three-storey buildings and 2.67% IDR for the five- and seven-storey buildings. The results revealed a non-linear relationship between building height and seismic performance. The three-storey building demonstrated the highest collapse capacity (0.60g PGA) and largest ductility margin (0.42g), whereas the seven-storey building exhibited the lowest collapse capacity (0.35g PGA) and smallest ductility margin (0.23g), representing a 42% reduction in collapse capacity. The five-storey building showed intermediate behavior, with a 21.6% reduction in collapse capacity compared with the three-storey building. Although the seven-storey building had delayed the onset of slight damage (0.12g PGA) due to its increased flexibility, it exhibited the lowest resistance to complete collapse.