Reinforced concrete (RC), as one of the major building materials of modern architectural structures, will form nonuniform temperature fields after exposure to fire (Shao et al.
2003; Cheng
2007). The thermal stress and expansion (Zhao
2014) due to fire will lead to stress redistribution of overall structures and reduction of structural flexural capacity or even the local structural destabilization, which further induce the collapse of overall structures and bring about massive losses to the possessions and life of humans (Ban
2008). Thus, the temperature fields should be determined first for subsequent analysis of RC structures to understand the degradation in the mechanical properties and flexural capacity after exposure to fire, which are the keys to evaluate whether the RC structures can be used anymore (Tian
2002; Zeng
2006). So far, there is considerable research in the aspect of test and finite-element analysis (FEA). Fu et al. (
2015) studied how the fire surfaces of RC beams affected the temperature fields. Liao et al. (
2013) designed the shear tests and a FEA method was proposed to calculate the shear flexural capacity among one room temperature and seven post-fire RC beams, which has the accepted accuracy. Sun et al. (
2002) investigated the mechanical performances in the tensile region and the compressive region of beam sections at elevating temperature and after cooling. Shi and Guo (
2000) tested the structural performances of RC beams at high temperature and presented the temperature distributions of beam sections, a coupling temperature–stress constitutive law of concrete and the main performances at different surfaces after fire exposure. Lu et al. (
1993) studied the 1-, 2- and 3-surface fire exposure of 12 RC simply-supported beams, presented the bending moment–curvature relations in different temperature fields and computationally validated the load–deflection relations of RC beams. El-Hawary et al. (
1997) investigated the mechanical behavior of post-fire RC beams in the shear zone by the method of test. Parthasarathi et al. (
2019) carried out experimental research on 10 RC short columns at different temperatures, obtained the deflection, stiffness and variation of the specimen at high temperature, and compared the experimental results with the FEA results. The results were reasonably well. Kodur and Agrawal (
2017) studied the fire response of RC beams caused by temperature-induced bond degradation and an FEA was developed. Fu (
2016a) developed 3D FE modelling approach using ABAQUS. Fu (
2020) developed a machine learning algorithm for fire safety design. Eamon and Jensen (
2013) conducted a reliability analysis of RC columns subjected to fire, and various factors such as the fire type, load ratio, reinforcement ratio, concrete cover, concrete strength were considered in the analysis. Xu et al. (
2015) conducted a fire endurance test of 6 high strength steel RC continuous T-beams and confirmed the feasibility of designing high strength steel RC continuous T-beams using plastic analysis. Hassan et al. (
2019) designed an experiment to investigate the effect of different retrofitting techniques on the performance of concrete at different temperature on 31 beams. Chen et al. (
2009) carried out experimental research on the mechanical properties of 9 full-size concrete columns. The results showed that with the increase of fire time, the residual flexural capacity decreases. Mathew and Joseph (
2018) studied the flexural behavior of fly ash geopolymer concrete beams at different temperature. The results showed that the deformation characteristic of geopolymer concrete beams is similar to that of the reinforced cement concrete beams at ambient temperature, but big difference noticed at high temperature. Di Capua and Mari (
2007) proposed two models, the thermal model and the mechanical model, for calculating the RC beams exposed to fire, which were in good agreement with test results. Bratina et al. (
2003) explored the post-fire nonlinear structural analysis of RC beams. In theory, Xiang et al. (
2010) studied the calculation method of flexural capacity for fire-damaged RC bending members, Yu et al. (
2005a,
b) studied the effects of different factors on the mechanical properties of concrete exposed to high temperature and proposed the equations of stress–strain relation for various concrete after high temperature. Hu et al. (
2006) established estimation methods for the flexural capacity of slabs subjected to fire and axially loaded concrete square columns subjected to fire on four surfaces. Lie and Irwin (
1993) described a method for the calculation of the fire resistance of RC columns with rectangular cross sections. Jiang et al. (
2008) discussed the rectangle beam with single reinforcement, and proposed a method to calculate the residual capacity of the RC member.
Based on above research, it can be seen that, most scholars’ research primarily focusses on the constitutive relationship of materials during or after fire, as well as the response analysis and damage assessment, and the flexural capacity of theoretical calculation of the post-fire RC beams is barely investigated. In addition, the major factor affeting the behaviour of the RC beams in fire is not well investigated. The 500 °C isotherm method specified in Eurocode (BSI
2004) is the most widely used theoretical calculation method for the residual bearing capacity of RC beams after fire. The method is based on the assumption that concrete above 500 °C does not provide load-bearing capacity to members, while concrete within 500 °C is considered to maintain its initial values of strength and elastic modulus. However, this method is a rough calculation method cannot accurately evaluate the structures after fire. BS 5950: Part 8 (
2003) specifies a calculation method of the fire resistance of members in bending which is called ‘moment capacity method’ or called "section method". It is used primarily for steel sections but concrete members. Therefore, RC beams fire tests and subsequent bending test were performed in this paper, based on the test results and the existing moment capacity method, a new theoretical calculation model using improved section method to calculate the flexural capacity of the post-fire beams. The model first conducts heat transfer analysis to obtain the temperature field of the beam section, and then divides the beam section to small divisions and obtain the average temperature of each divisions. Finally, based on the material strength reduction formulas in fire, the flexural capacity can be accurately obtained for RC beam after fire. Compared with the 500 °C isotherm method, the advantage of this method is that it fully considers the changes of the temperature field and accurately considers the changes of the material strength caused by the temperature changes in each micro area, and the results obtained are more accurate. The accuracy of the theoretical calculation is validated through the comparison with simulation and test results, which offers a new way for the evaluation of post-fire RC structures. In addtion,a thermodynamic model of RC beams with three sides under fire was built using finite-element analysis (FEA) software ABAQUS. The FEA model was validated with the results of fire tests. Different factors were taken into account for further parametric studies in fire using the propsed FE model.