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This paper aims to investigate into the reasons for the widespread column-end damages of reinforced concrete (RC) frame buildings caused by the 2008 China Wenchuan earthquake. A six-story office building is used as the prototype building, and ten RC frames under different combinations of Seismic Intensity and Seismic Category are designed for the prototype building in accordance with the Chinese codes. Two joints of the second floor of each frame are selected for calculating the column-to-beam flexural strength ratio (CBSR), and the effects of the following four factors on the CBSR during a strong earthquake are analyzed: (1) the detailing practice of beam longitudinal reinforcement adopted in China; (2) slab participation in beam flexure; (3) reduction of joint concrete strength; and (4) variation of column axial force. The numerical results of CBSR indicate that the code requirement for the “strong column-weak beam” design in China is not sufficient for the RC frames designed for a Seismic Intensity of 0.2 g or lower to avoid column-end damages during a strong earthquake. Suggestions are made for improving the “strong column-weak beam” design of RC frames in China.

This paper experimentally investigated the effects of axial load level on the residual strength and stiffness of concrete-filled steel tubular (CFT) stub columns which were heated and cooled down to room temperature under sustained axial load. Eight stub columns were axially loaded and heated to specified high temperatures in a purpose-built electric furnace. After the specimens cooled down to room temperature while the axial load was kept constant, the stub columns were loaded to failure. The test results show that not only the axial load level and the high temperature exposure have significant effects on the residual strength and stiffness of stub column, but the residual strength index and stiffness index of the fire-damaged CFT stub columns with pre-load are remarkably different from those without pre-load. From the test results, it is recommended that the sustained axial load effects and the fire cooling phase should be taken into consideration in assessing the fire-damaged CFT columns.

A computational model, in which the effects of high temperature on steel and concrete’s properties and the composite action and interfacial properties between steel tube and concrete core were considered, was developed using ABAQUS program. Based on a damage model of concrete at ambient condition and tested stress versus strain curves of fire-damaged concrete, a new damage model of concrete after exposure to high temperatures was developed to consider the influence of high temperatures on the damage of concrete. By introducing the damage model of fire-damaged concrete, the reasonable equivalent stress-strain relations of confined concrete and a modified steel tube-concrete interface model into the ABAQUS FE model, the mechanical behaviors of the fire-damaged CFT columns and connections were simulated precisely and verified by some relative test results.