Numerical analysis of the influence of end restraints on the fire resistance of continuous reinforced concrete beams and slabs

by Limin Lu

Institution: Ghent University
Year: 2015
Keywords: Technology and Engineering
Record ID: 1074940
Full text PDF: http://hdl.handle.net/1854/LU-5941958


Fire resistance research of underground structures and structures above ground shows that the underground structures are always subjected to more severe fire disasters. Fire accidents in underground structures experience higher and longer temperature histories. From the structural side of view, the most obvious difference between the underground ones and the ones above ground is the restraint system at the boundaries. Underground structures are subjected to restraint forces which primarily result from the interaction with the surrounding soil. Although the effects of high temperature on the response of concrete beams and slabs have been widely studied in recent years, a literature review indicates that the fire behaviour of single members has been the main focus of most of the research projects. Especially from the experimental side, due to the limitations of the commonly available fire test set-ups, very few fire tests include restrained beams and slabs. However, restraints may have a significant influence on the flexural behaviour of reinforced concrete (RC) members at elevated temperatures. The existing research on the effect of restraints is always assuming a constant restraint moment or an axial force during the whole fire exposure process which is not in agreement with the real conditions. Due to the decrease of the member’s stiffness while exposed to fire, the restraint stiffness will increase with heating time. This will lead to an increase in restraint forces and a redistribution of the bending moment along the member. Hence, the investigation of the effect of restraints based on temperature-dependent restraint stiffness needs further attention. Since the fire simulation of restrained members is always very complex, finite element packages have to be applied. However, finite element methods are always time consuming, and the process is not clear since all the computing is performed by the computer “in a black box”. Moreover, some of the programs are not so user friendly and do not allow to define material models freely. In order to dispose of a user friendly research method for both structures above ground and underground structures, a new simplified numerical model was developed to simulate the fire performance of reinforced RC members related to bending. Parametric studies including the influence of implicit and explicit transient creep strain models and the influence of restraints on fire performance of RC members have been carried out with the proposed method. The numerical model is based on the temperature field of the cross-section and the mechanical properties of concrete and reinforcing steel at elevated temperatures. A multiiteration process is carried out in order to obtain force equilibrium, and an equivalent bending stiffness is defined for the structural analysis. A moment-curvature (M-k) relationship can be generated in the sectional analysis process for a given axial force. The curvature caused by thermal deformations as well as the ultimate moment of the RC member at elevated temperatures can…