| Earthquakes and fires are natural disasters that seriously threaten the safety and reliability of buildings,and China is a country with frequent earthquakes.The secondary fires caused by earthquakes lead to huge property losses and casualties.Reinforced concrete beam in frame structure is one of the key components to ensure the overall effectiveness of the structure.The performance of frame beams will become more complex after they are subjected to earthquakes and secondary fires.Therefore,in order to verify the fire resistance of seismic damaged concrete constrainted-beam in secondary fires,it is necessary to carry out in-depth research.Firstly,based on a large number of shaking table tests and actual seismic damage data of concrete frame structures,the main geometric damage types of reinforced concrete beams damaged by earthquakes are summarized as cracks and concrete spalling.The geometric characteristic parameters corresponding to the seismic damage types are determined,and the calculation model of frame beam damaged by earthquakes is established.By numerical simulation and analysis with ABAQUS software,the temperature distribution law of seismic damaged concrete frame beam under three-sided fire conditions and the influence of geometric characteristic parameters such as cracks and spalling of protective layer on the temperature field of cross-section are obtained.Then,the quantitative relationship between the reduction coefficient of fire resistance limit and geometric characteristic parameters of seismic damage frame beam is obtained by the analysis of fire resistance limit of seismic damage reinforced concrete frame beam.Finally,the quantitative relationship among geometric characteristic parameters,fire resistance limit and reduction coefficient of high temperature bearing capacity of seismic damaged concrete frame beam is established.The main contents of this paper are as follows:(1)According to the existing seismic damage data and the experimental data of the research group,the main geometric damage types of reinforced concrete frame beam after earthquake are cracks and concrete spalling.The geometric characteristic parameters such as damage location and damage degree are determined.(2)The finite element software ABAQUS is used to establish the three-dimensional seismic damage concrete frame beam model.The effects of cracks,spalling of concrete cover and their coexistence on temperature field of seismic damaged concrete frame beam are obtained.When the crack exists alone,the temperature field distribution in the high temperature zone around the crack is parabolic,and the change of temperature field around the crack is significant with the increase of the depth of the crack.The larger the spalling thickness is,the higher the temperature of the concrete at the spalling site and the faster the heating rate is;the spalling length has less influence on the temperature field,and the influence of cracks on the temperature field is more sensitive than that of concrete spalling.When there are many cracks in the beam,the influence of cracks on the temperature field is much greater than that of the spalling.(3)The effects of geometrical damage parameters such as relative depth of cracks,thickness of concrete spalling and length of spalling on the fire resistance of reinforced concrete frame beam are studied.The quantitative relationship between the fire resistance limit reduction coefficient and the seismic damage geometric parameters is obtained.The influence of span-height ratio and load ratio on the fire resistance of the concrete confined beam is analyzed.The quantitative relationship between fire resistance limit and relative crack depth,span-height ratio and load ratio is established.(4)The influence of geometric characteristic parameters such as crack depth and spalling thickness of concrete cover on high temperature bending capacity of seismic damaged concrete frame beam is analyzed.According to the relationship curve between reduction coefficient of high temperature bearing capacity and geometric characteristic parameters of seismic damaged concrete constrained-beam,the quantitative relationship between reduction coefficient of high temperature bending capacity and geometric characteristic parameters of seismic damaged concrete frame beam is constructed. |
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