Description

Site and regulatory constraints shaped this reinforced concrete building in the Philippines before any analysis began. A sloping lot cut off from the road by a creek required a single-span access bridge, aviation height limits under the Civil Aviation Authority of the Philippines (CAAP) capped the structure at three above-ground stories, two basement levels, and a roof deck, and the resulting step-back left the basements exposed on three sides.

The three-dimensional model was built in MIDAS Gen under the National Structural Code of the Philippines (NSCP) 2015 and ACI 318M-14, with beam end releases where secondary beams were not intended to transfer moment into the supporting girders, a rigid diaphragm assumption based on the continuity of the reinforced concrete floor system, and fixed supports at the column bases. The step-back triggered a vertical geometric irregularity check, which returned 255% against a 130% limit — a value treated as a correct description of an irregular building rather than a modeling error to be removed.

Response spectrum analysis was selected because NSCP 2015 requires a dynamic lateral force procedure for vertically irregular structures, and because the exposed basements participate in the lateral response rather than behaving as buried levels. Modal participating mass reached 98%, and the mode shapes showed translation coupled with up to 47.8% rotation, indicating an offset between the center of mass and the center of rigidity. Story drift ratios passed the 0.025 limit, but the maximum-to-average drift ratio exceeded the 1.2 limit — a torsional irregularity the drift ratio check alone does not reveal. The increased torsional demand was carried into the design of the critical columns under NSCP 2015 and UBC 1997 accidental torsion provisions.