Seismic Performance of High Rise Flat Slab Structure with Reinforced Concrete Outriggers in Seismically Active Regions: A Case Study

This study investigates the seismic performance of a 20-storey reinforced concrete office building with dimensions of 30m x 48.75m and a storey height of 3.40m. The building’s lateral stability is ensured by shear walls, outriggers, and perimeter belt trusses on the 20th floor. Gravity loads are transferred through a flat slab system supported by perimeter edge beams, with the entire structure resting on a pile-supported mat foundation. The structural elements are cast in-situ using C40 concrete. Two cases were analyzed: Case 1 without outriggers and Case 2 with reinforced concrete outriggers. Finite Element (FE) software ETABS was used for global analysis, adhering to the National Structural Code of the Philippines (NSCP) 2015. The study comprises three phases: development and analysis of the building model, determination of seismic response, and evaluation of seismic response. Results indicate that the inclusion of outriggers significantly improves the building’s seismic response, reducing the fundamental period from 3.322s (Case 1) to 2.929s (Case 2). The study concludes that reinforced concrete outriggers are an effective and practical solution for enhancing the seismic performance of high-rise buildings.