Study On The Seismic Behavior Of The Strengthened Concrete Shear Wall After Fire

In recent years, with the increasing number of high-rise buildings, the structure of reinforced concrete (RC) shear wall has been widely used because of its satisfactory seismic behavior. But at the same time the fire hazards in these high-rise buildings are of frequent occurrence, such as that took place in the teacher’s apartments on Jiaozhou road in Shanghai, a painful lesson. Fire can extremely weaken the capacity of structures and cause significant damage. However if we conduct some reasonable repair, these damaged structures can still work. Hence, it is necessary for practical engineering to study the strengthening method of the RC shear wall after fire and also its seismic behavior after strengthening.Quasi-static tests of four shear wall were performed. Four working conditions are considered:1, under normal temperature; 2, after fire without strengthening; 3, after fire strengthened by angle steel; 4, after fire strengthened by reinforced polymer mortar. A comprehensive analysis was presented on the failure mode, capaciy, ductility, energy dissipation and stiffness degradation of these four tested shear walls. The test results show:1, the capacity and ductility of RC shear walls after fire decrease evidently and brittle failure mode was observed; 2, Strengthened by angle steel, the RC shear walls after fire behave increasing ductility whose hysteretic loop was plumper and energy-dissipating better than that under normal temperature without strengthening; 3, Strengthend by polymer mortar, the capacity and initial stiffness of the shear wall after fire was restored to the level of the normal-temperature condition. Its ductility and energy-dissipating ability was upgraded compared to that without strengthening, but inferior to that under normal temperature.FE package, ABAQUS 6.10, was employed to simulate the temperature field of the concrete shear wall under fire. The calculated results were verified compared to the experimental observations. The results indicate that there are notable temperature gradient along the thickness of the shear walls’ section while the temperature distributes evenly along the direction of width and height. The calculation of temperature field established the foundation of subsequent mechanical analysis.Second development of ABAQUS 6.10 was implemented to incorporate constitutive models not available in ABAQUS through user-subroutine (UMAT). Based on the Modified Compression Field Method (MCFT), a new RC plane stress element was built which was further improved to be able to considering the degradation of the re-bars and concrete’s behavior after fire, and also the strengthening function of the bonded angle steel and reinforced polymer mortar. This established UMAT subroutine can predict, with accuracy, the load-displacement curves, the stress and strain at any place of the shear wall before approaching the peak. Moreover, it is time-saving and more efficient than the traditional solid model with discrete truss element representing re-bars. Based on the existing "Standard for building structural assessment after fire" and the developed UMAT, a simplified assessment was proposed from which the engineers can easily know the degradation level of a shear wall after fire just according to the parameter of the shear wall and the fire load. A newly grading method was also provided based on the coefficient of the degradation. The study shows that with respect to a certain width, the key factor influencing the degradation coefficent is fire duration and axial load ratio. The presented table can be refered to calculate this coefficent.Based on the developed UMAT, a parameter analysis was performed to quantify the strengthening effect of the method of sticking angle steel and casting reinforced polymer mortar on different shear wall with various degradation degree. For reference, a calculation table was proposed to predict the minimum steel ratio when adopting sticking angle steel as strengthening and the most economical re-bar ratio when employing polymer mortar. At last, On the basis of the available code, a series of simplified calculation equations were proposed to predict the capacity of the shear wall strengthened by bonded angle steel and that by reinforced polymer mortar after fire.