Experimental study on crack propagation in negative bending moment region of steel-composite beam under fatigue load

The bending moment has an extreme value at the middle support in steel-composite beam structures, while the concrete plate is liable to crack owing to the large tensile stress. In this study, based on a typical steel-concrete composite beam bridge, fatigue tests were carried out on two double-layer steel-composite beams. The results demonstrated that a significant nonlinear stage appeared before the failure of the shear nail and confirmed that the stiffness decline caused by slip can be ignored under the fatigue load, and the composite beam can be regarded as a complete shear connection. As the fatigue loading cycles increased, the stiffness of the steel-composite beam exhibited a nonlinear downward trend, with the declining speed increased rapidly at approximately 80% of the fatigue life. It can be stated that the slip growth theory of CEB-FIP MC90 can be adopted to predict the growth of crack width of steel-composite beam under static load test, which usually suffers from large prediction error under fatigue test owing to the strong constraints of the steel flange and welded nails. The conclusions of the study provide a reference for understanding the crack extension mode in the negative bending moment region of the steel composite beam under fatigue load and effectively controlling the crack width, which ultimately achieves the purpose of optimizing the performance of bridges and prolonging the service life of bridges.