Structural Evaluations of Reinforced Concrete Buildings Damaged by Chi-Chi Earthquake in Taiwan

At 1:47:12.6 a.m. on September 21, 1999, a 7.3 Richter scale earthquake struck Taiwan at its geographical center, Nan-Tou. The epicenter was located in a mountainous town called Chi-Chi and the focus was 6.9 km below. The maximum seismic accelerations of 9.87 m/s² east-west, 5.63 m/s³ north-south, and 5.20 m/s² vertical were measured at three different seismographic stations near the epicenter. The quake is the strongest one ever recorded in Taiwan. It moved mountains, cut off rivers, caused liquefaction, toppled buildings as well as bridges, cracked highways and concrete dams, and damaged many infrastructures. The quake caused approximately 2,400 deaths, 12,000 injuries, 18,000 totally/partially collapsed buildings, and estimated property loss of U.S. $10 billion. This paper presents postquake fact findings and structural evaluations of the collapsed buildings based on the Taiwan Building Codes. The shocking findings involve an inadequacy of current seismic code provisions, improper design details, faulty construction, and use of poor quality concrete. Most collapsed buildings were designed in accordance with the building codes but constructed with reinforcement inconsistent with the design details and with concrete that had strength of <50% of the code requirement 27.5 MPa. The faulty construction had resulted in the formation of strong-beam/weak-column systems in which the columns failed before any other structural elements. The information presented in this paper will be useful to engineers who design reinforced-concrete structures for earthquake-risk regions.