Research on stiffness degradation of continuous steel - Concrete composite beams after concrete cracking

To investigate degradation rules of the flexural stiffness and stress distribution in negative bending regions of continuous steel-concrete composite beams after concrete cracking, a statistical analysis of crack morphology and distribution on the steel-concrete composite beams was carried out, and a concrete crack model in negative bending regions was proposed based on the crack morphology of concrete slabs. The comparison results show that the proposed crack model considering the effect of the uncracked concrete between adjacent cracks can better simulate the stiffness degradation after concrete cracking. The equivalent stiffness of the continuous steel-concrete composite beam exhibits a monotonously decreasing S-shaped degradation pattern during the loading process. When close to the yielding of the steel beam, the flexural stiffness of the composite beam decreases until less than 30% of the initial stiffness. The stress in the upper longitudinal bars is about 50-100 MPa higher than that in the lower longitudinal bars due to a higher tensile height. In addition, the tensile stress in the upper longitudinal reinforcing bars at the cracked section is 11.3% higher than that at the uncracked section. In the ultimate limit state, the yielding length of the steel beam occupies about 1/4 of the calculated length of the composite beam.