EXPERIMENTAL STUDY ON INTERNAL FORCE VARIATION OF STEEL-CONCRETE COMPOSITE BEAM UNDER FIRE

In practical engineering, most steel-concrete composite beams have axial constraints, which cause the composite beam to produce catenary action to continue bearing more external loads at large deformations. To study the mechanism of catenary action of steel-concrete composite beam under fire, two full-scale tests of steel-concrete composite beams were carried out to obtain the internal force variations with the time. The test designation, loading scheme and measurement contents are presented in this paper. The experimental phenomena and failure characteristics are also described. The test results show that the temperature gradient distribution of the composite beam section changes greatly under fire, and so it is easy to generate additional bending moment, which has an adverse effect on the ultimate bearing capacity of the composite beam. The failure mode of the composite beam is mainly the formation of obvious plastic hinge at the negative bending moment area at the beam ends, and the overall lateral instability occurs on the tensioned lower flange of the steel beam at the bottom of the beam, which is contrary to the common phenomenon that overall unstability of a steel beam is usually attributed to the compression flange buckling of a steel beam. The load ratio is one of the important parameters affecting the fire resistance of the composite beam. Through the actual measurement, the changing process of the internal force of the composite beam in fire is obtained. It is found that a larger load value leads to more obvious catenary action of the composite beam at the same time under the same conditions.