Seismic Performance Evaluation of Mid-Rise RC Buildings Considering Soil–Structure Interaction and Foundation Effects

Abstract

This study evaluates the seismic performance of an existing seven-story reinforced concrete parking garage (85 m × 50 m plan) that was not originally designed for seismic loads. The structure, featuring flat slabs, ordinary shear walls, and a circular vehicular ramp, is currently supported on bearing piles. Three-dimensional finite-element time-history analyses were conducted in Plaxis 3D using the local Halabja 2017 Mw 7.3 acceleration record. Parametric investigations compared the existing pile system against alternative raft foundations with plan dimensions of 1.1B, 1.5B, and 2B, where B = 50 m on both the actual four-layer site soil and an idealized soft-soil profile; a fourteen-story variant was also analyzed to examine height effects. Results show that the 14-story model on soft soil with the smallest raft (1.1B) exhibited the most critical response: roof displacement reached 26.5 cm, inter-storey drift peaked at 1.61 %, and the fundamental period lengthened to 2.46 s. Conversely, increasing the raft size to 2B reduced displacement and drift toward the fixed-base values (e.g., 7-storey site-soil displacement dropped from 7.0 cm.at 1.1B to 6.8 cm at 2B versus 6.5 cm fixed-base). Base shear followed an inverse trend, decreasing with soil flexibility but recovering toward the fixed-base level as raft stiffness increased. It is recommended that raft foundations be sized at least 1.5B–2B for mid-rise RC structures on deformable soils to limit SSI-induced period lengthening and drift amplification; explicit 3D SSI analysis should be mandated for seismic assessment of existing non-seismic buildings in seismically active regions such as Iraq.