In order to study the vibration characteristics of reinforced concrete (RC) structures during fire and to develop the method of damage assessment after fire, 4 simply-supported RC beams (L1~L4) were designed. Firstly, the finite element model of the simply-supported beams (L1~L4) were calibrated by picking up the modal information before exposure to fire. Then fire tests were conducted on L1~L4 for 60min, 90min, 120min and 150min respectively. During the fires, the structural modal information were collected, and the attenuation formula of frequency was obtained by fitting the test results. In order to study the residual stiffness and bearing capacity of L1~L4 after the fire, the dynamic tests and bearing capacity tests were carried out. Finally, based on the modal information of specimens after the fire, by taking equivalent explosion time as the damage index, the support vector machine intelligent algorithm was used to predict the damage degree of the RC beams. Based on the Standard for Appraisal of Building Structure After Exposure to Fire (CECS252-2009), a comprehensive grading of damage index for simply-supported RC beams exposure to fire was established, and the damage index grading of L1~L4 were evaluated at the end.
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Incidents such as fires, explosions, and impacts may cause progressive collapse of building. Since the 20th century, as terrorist activities in the world have been increasing year by year, experts and scholars around the world have begun to pay attention to the formulation of building collapse prevention codes and structural progressive collapse resistance. At present, Concrete filled steel tube column (CFST)-steel beam joints with outer ring plate have been widely used in engineering examples. However, outer ring plates occupy a large amount of building space due to welding on the outer wall of steel tubes, which affect the function of buildings, and CFST-steel beam joints with through-center construct, that is, the steel beams are wholly or partially passed through the core concretes and the steel tubes, which greatly save the use space of the buildings. In order to study the mechanism of CFST-steel beam joints with through-center construct under progressive collapse conditions, ABAQUS finite element software was used to establish five CFST-steel beam joints with through-center construct and one fully welded joint models. The resistance mechanism, deformation modes and internal forces of the joint were investigated,and the capability of progressive collapse was evaluated. The results showed that the failure mode of CFST-steel beam joints with through-center construct can be divided into two types, the collapse failure mode at the beam and the collapse failure mode at the column. The collapse failure mode at the column has better ductility and bearing capacity, but with instability. The vertical bearing capacity of the joints is mainly provided by the bending mechanism and the catenary mechanism. The bending resistance mechanism provides the early resistance and the catenary mechanism determines the ultimate bearing capacity at the later stage. The new CFST column-steel beam joint with through-center structure has good capability of progressive collapse resistance, and its index of progressive collapse resistance of joint is higher than that of the other joints with collapse failure mode at the beam. |