On 12 May 2008, a violent earthquake with a magnitude of 8.0 occurred in Wenchuan, Sichuan Province, China. The earthquake resulted in a large number of collapses and damages of buildings, and brought huge loss to the people's lives and properties. This paper gathers the building seismic damage data in major disaster area. The data are classified according to structural type, construction time and occupancies. It is learned that the strict implementation of seismic design codes is the prerequisite to guarantee the seismic capacity of building structures. For masonry structures, the design of structural system and earthquake-resistant measures should be emphasized and the construction quality should be ensured. For RC frame structures, the construction measure of the connections between enclosure and filler walls and the main structure should be carefully designed. Measures to ensure the strong column-weak beam damage mode should be revised and improved. Furthermore, methods to increase the integral aseismic capacity of building structures are discussed in the framework of system theory. It is shown that the safety margin of building structural systems can be divided into three levels, say fundamental, integral and unexpected safety margins. The overall seismic capacity and collapse prevention of building structures are mainly determined by the integral safety margin and unexpected safety margin. Lack of unexpected safety margin is one of the main reasons of the severe building damage in the Sichuan earthquake. The unexpected safety margin of a structural system mainly comes from its robustness, stability and firmness. Researches on the integral and unexpected safety margins of building structures are still very limited. Corresponding requirements and specifications in current design code for building structures are to be improved.