Experimental Study On Fire-resistance And Post-fire Seismic Behavior Of Concrete Shear Walls

In recent years, reinforced concrete (RC) shear walls are increasing used in tall buildings for efficiently resisting earthquake forces. Review of past research has found that the RC shear walls studied were mainly focused on the mechanical behaviors at room temperature. Yet, there is limited information in the literature on their performance under and after high temperatures. In the event of a fire, RC shear walls are often served as the fire-resistance wall in addition to load-bearing elements. Therefore, it is of practical significance to evaluate the behavior of RC shear walls with the combined action of fire exposure and load. On the other hand, fire damaged RC structures can be repaired properly to resume service if the fires are put out in time. To propose effective and economical repair plan, it is essential to investigate the post-fire behaviors of structural elements. Consequently, the research on post-fire seismic behavior of the RC shear wall has vital significance for fire damaged RC structures with shear walls. So, corresponding experimental and analytical evaluations for RC shear walls under fire condition as well as after fire exposure are made in this paper. Based on the project of Specialized Research Fund for the Doctoral Program of Higher Education—Fire response of RC shear wall and their post-fire seismic performance (No.200801410005), the following aspects are carried out in the present paper:1. A total of eleven mid-rise normal strength concrete (NSC) shear walls were exposed to fire on one side followed by a decay phase. The test conditions included varing levels of axial load, different reinforcement ratios and fire durations. The results show that:(1) the presence of axial load under fire and an increase of reinforcement can give rise to concrete spalling. (2) In the first 20 min of fire heating, the axial load intensity and reinforcement ratio have little influence on the deflection of the shear wall. In contrast, beyond 20 min of the heating, an increase of axial load and reinforcement ratio appears to reduce the deflection of shear walls.The deflection of the shear wall continues to increase slightly firstly and then decreases gradually after extinguishment. Moreover, the shear wall with higher axial load or more reinforcement exhibits lower deflection restoring capacity.(3) The three requirements of fire safety—structural adequacy, insulation,and integrity—are satisfied for the wall studied in this study.2. The thermal field of RC shear wall under fire was obtained using finite element software. The effects of thermal properties of concrete, convection coefficient and radiation factor of concrete for the exposed surface, temperature-time curve in the furnace and moisture content of concrete were also investingated by a developed parametric analysis. The results indicate that:(1) thermal conductivity and heat capacity of concrete given by the related documents have little influence on the temperature field. (2) The convection and radiation factors of concrete for the heated face, as well as the temperature-time values in the furnace have great influence on the temperature distribution. Moreover, the shorter the distance from the ponint to exposed side is, the bigger the effect of the parameters stated above on the temperature. (3) The calculated temperature can be higher if the effect of moisture content is ignored.3. Based on the strip method, a nonlinear analysis method was presented to simulate the response of RC shear walls under fire conditions. And a coupling constitutive relation of concrete at elevated temperature was adopted in this analysis. Then a corresponding computer program was developed. The mid-span deflection of the RC shear wall, various strain components as well as stresses at various locations of the mid-span section was calculated by this program. The results indicate that:(1) the thermal strain decreases non-linearly over the cross section from exposed to unexposed side. (2) The mechanical, transient and creep strain display parabola distribution over the section. And they are all compressive at the region near the suface of the wall. However, they show different characters at the center region of the section under various axial loads. (3) The stresses distribution over the section is accordant with the mechanical strain. Moreover, the compressive stresses at the unexposed side are much bigger than that at the exposed side. As increasing the axial load, the compressive stresses at the same position increase. (4) The appearance of cracks during the fire test is reasonably explained by the predictions of the strain and stress over the cross section. The cracks occurred on the exposed surface may be due to temperature changing. In contrast, the cracks occurred on the unexposed side can be caused by stresses due to the combined action of heating and axial load.4. Fifteen RC shear walls were tested under low cyclic loading. Prior to the cyclic test, eleven specimens were exposed to fire. The effects of fire duration, reinforcement ratio and axial load ratio on the seismic behavior were investigated. The experimental results show that: (1)the reference shear walls show the main diagonal crack characteristics at failure, while these characteristics become less noticeable and even disappear with an increase of fire duration time and the axial load. (2) Fire exposure decreases the ultimate resistance and ductility of RC shear walls, obviously drops down the energy dissipation and the initial stiffness. This deterioration can be slowed down by properly increasing reinforcement. (3) For all tested shear walls, the bearing capacity and stiffness increase, while the ductility and the energy dissipation decrease as the axial load increases within certain range. (4) The combined action of elevated temperatures and axial load results in more energy dissipation than the individual fire exposure. (5) Fire exposure causes an increase of shear deformation, especially for shear walls with less reinforcement or under higher axial load.5. Based on the Modified Compression Field Theory (MCFT), a section analysis model was presented to predict the behavior of the heated shear walls under combined shear and moment. Then a corresponding computer program is developed considering the effect of elevated temperatures on the concrete and steel properties. The shear strength, shear response curves and tension strain of the horizontal bar were calculated by this program. The validity of the proposed numerical model was established by comparing the numerical results with experimental data. Further, the effects of fire exposure and reinforcement ratio on the strain of the horizontal bar were analyzed by the computer program and the results agreed well with this experiment.