The Mechanical Properties of Reinforced Concrete Beam-Column Joints Under the High-Temperature Effect of Fire

In order to study the deformation behavior of reinforced concrete frame beam-column joints under high-temperature fire and the variation of their mutual constraint with temperature, structural tests were conducted on the entire process of temperature rise and fall of reinforced concrete frame beams under high-temperature coupling load. Innovatively, based on structural testing, the Vulcan program was used to conduct numerical analysis of the fire response of the moment-angle relationship between beam-column nodes under constant load when the frame beam is subjected to high temperature alone or when both beams and columns are subjected to high temperature simultaneously. The results show that the rotational deformation and restrained bending moment of the beam-column joints in reinforced concrete frames increase first and then decrease with the increase of fire temperature, and the peak rotation angle of the beam-column joints has a lag effect compared to the peak bending moment. As the temperature continues to rise, the beam end angle begins to decrease after reaching its peak value, and the peak value of the beam end angle appears later than the constrained bending moment. Based on the thermal mechanical coupling test, a fire response calculation model for reinforced concrete beam-column joints was established using the Vulcan program; when the beam is subjected to high temperature alone and when the beam-column is subjected to high temperature simultaneously, the rotation angle shows an increasing trend within 80 min and 120 min, respectively, and the structural bearing capacity of the latter decreases more severely than the former. The bending moment and rotation angle of beams under high-temperature conditions alone and beams-columns under high-temperature conditions exhibit synchronous changes with the development of fire temperature within 60 min before fire exposure. After the fire exposure time is extended, the deformation of the rotation angle continues to increase, while the bending moment begins to decrease until it stabilizes. The structural response of beams and columns subjected to high temperature simultaneously is stronger than that of beams subjected to high temperature alone, and their peak moment appears earlier. The deformation and constraint force of beam-column joints during high-temperature fire are nonsynchronous.