On 12 May 2008, a magnitude 7.9/8.0 earthquake struck China, approximately 80 km west of Chengdu in the Sichuan (Wenchuan) province and 1550 km southwest of Beijing. This event occurred on one of the faults that run along the base of the Longmenshan Mountains marking the boundary of the Tibetan plateau. The rupture of the fault extended over 200 km and exceeded 6 m on the surface. The fatalities exceeded 69000 and millions were injured or left homeless. This area had previously been considered a moderate earthquake zone by the Chinese Building Code, and hence, the level of damage was not anticipated. Many buildings and bridges collapsed or sustained severe damage. Schools and hospitals were especially vulnerable and many collapsed. Several factors contributed to the unprecedented level of devastation. For the collapsed buildings, the lack of ductility, the absence of a well-defined load path, and the building irregularity were primary contributors. Many schools used a non-ductile masonry-concrete framing with hollow precast decks. Residential unreinforced masonry bearing wall buildings also fared poorly and many of them collapsed. Non-ductile reinforced concrete framed buildings performed slightly better but sustained significant damage. For concrete framed buildings, the infill walls were terminated at the first floor introducing weak story at the ground level. Captive column failure was also common resulting from attachment of partial height infill walls to concrete columns. The observed types of damage have previously been witnessed in many parts of the world in past earthquakes. Fortunately, robust, simply implemental, and cost-effective retrofit methodologies have been developed to alleviate such failures. Both conventional and innovative retrofits options are available. The authors were some of the first foreign structural engineers to reach the area and survey the damage. Their observations and recommendations for future mitigations are presented here.
H. Kit Miyamoto, President & CEO
Amir S.J. Gilani, Structural Specialist
Miyamoto International, Inc., West Sacramento, CA