Numerical Assessment of the Seismic Performance of a Lead-Rubber Base-Isolated Low-Rise RC Frame under Site-Specific Earthquake Excitations

Quetta lies in one of Pakistan's most seismically active regions, where the Building Code of Pakistan (BCP-2007) assigns a design ground acceleration of 0.32 g and UBC-97 Seismic Zone 4 is adopted as the design basis. Despite this hazard, location-specific research on seismic isolation for the local building stock remains scarce. This study evaluates lead-rubber bearing (LRB) base isolation for a four-storey reinforced concrete (RC) special moment-resisting frame representative of local construction. Two identical three-dimensional ETABS models - one fixed-base, one LRB-isolated - were developed in accordance with UBC-97 and ACI 318. Two ground-motion records (Northridge-Reseda, 1994 and Tabas, Iran, 1978) were spectrum-matched to the UBC-97 Zone-4 design spectrum in SeismoMatch and applied in time-history analysis, with material nonlinearity confined to the bearings. Isolation lengthened the fundamental period from 0.799 s to 2.028 s (a ratio of 2.54) and reduced superstructure demand markedly: base shear by 49-70%, roof displacement by 74-86%, inter-storey drift by 90-95% and peak roof acceleration by 42-54%. The corresponding deformation was transferred to the isolation layer, where the bearing displacement reached 330-360 mm - the value governing the required seismic gap. LRB isolation is therefore a technically effective means of reducing seismic demand in low-rise RC frames in Quetta. These findings are conditioned on two ground-motion records, a linear-elastic superstructure and unidirectional excitation; larger record suites, inelastic superstructure modelling and bidirectional excitation are recommended before design-level application