RECONNAISSANCE REPORT OF THE 2008 SICHUAN EARTHQUAKE,DAMAGE SURVEY OF BUILDINGS AND RETROFIT OPTIONS

摘要

On 12 May 2008, a magnitude 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 tectonically related 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 approached 70,000 and millions were injured or left homeless. Damage was estimated at billions of US $. This area had previously been considered a moderate earthquake zone by the Chinese Building Code, and hence, the level of damage was not anticipated. Thousands of buildings and many bridges collapsed or sustained severe damage. Schools and hospitals were especially vulnerable and many such buildings collapsed. Many 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 hybrid structural system comprised of masonry columns, concrete beams, and hollow precast decks. This system was responsible for a disproportionate number of collapsed buildings. Residential unreinforced masonry buildings also fared poorly and many of them collapsed. By comparison, non-ductile reinforced concrete framed buildings performed slightly better. Many of these buildings sustained significant damage, but did not collapse. For concrete framed buildings,the presence of masonry infills introduced additional failure modes. For many 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. Analytical tools, experimental data, and available knowledge provide the basis of the suggested retrofits with the objective of strengthening and adding ductility to the structure to protect vulnerable non-ductile components. 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 in this paper.