Experimental Study on the Flexural Behavior of Steel-UHPC Composite Beams with Waffle Slab in Negative Moment Regions

To investigate the flexural behavior of steel-ultra-high performance concrete (UHPC) composite beams with waffle slab in negative moment regions, this study considered the influence of the slab to rib height ratio, reinforcement ratio, and adopting uplift-restricted and slip-permitted connections on the failure mode, crack development, and flexural capacity of the steel composite beams. A static loading test was conducted on four composite beam specimens subjected to a negative moment, by single-point loading at the mid-span. Based on the simplified plastic theory and the fact that the tensile stress distribution of the UHPC tensile zone is equivalent to a uniform stress distribution, a method is proposed to calculate the ultimate flexural capacity of steel-UHPC composite beams in negative moment regions. The results show that under the action of a negative moment, the failure mode of steel-UHPC composite beams with waffle slab presents typical bending failure characteristics. In the ultimate state, the longitudinal reinforcements in the waffle slab and the top flange of the steel beam yield under tension, and the bottom flange of the steel beam locally buckles under compression. The mid-span of the waffle slab passes through by the main crack, and the remaining thin cracks are intensely distributed. When the total height of the waffle slab is 90 mm and the slab to rib height ratio is reduced from 1:1 to 1:2, crack development is more severe in the waffle slab, and the cracking load and initial stiffness of the specimen slightly reduce, but the bearing capacity hardly reduces. By increasing the reinforcement ratio of the waffle slab by 1.05%, the ultimate load and stiffness of the specimen increase by 7.7% and 18.4%, respectively, and the main crack width of the waffle slab is better controlled. Adopting uplift-restricted and slip-permitted connections inhibits the development of main cracks in the serviceability states to a certain extent, but the ultimate load, stiffness, and ductility of the specimen reduce by 6.9%, 9.6%, and 19.7%, respectively. Finally, theoretical models were developed to calculate the ultimate flexural capacity of steel-UHPC composite beams with waffle slab under negative moments. According to the theoretical calculation method proposed in this study, the calculated value is slightly less than the experimental value, and the relative error is within 10%. © 2021, Editorial Department of China Journal of Highway and Transport. All right reserved.