Diagonal shear in thin-webbed reinforced concrete beams: Fibre and stirrup roles at shear collapse

The combination of steel or polymeric fibers with stirrups in reinforced concrete (RC) shear-sensitive elements has been recognized as an efficient means of resisting shear, but so far few models have attempted to describe fiber-stirrup interaction. A recently proposed limit-analysis model for the description of the ultimate behaviour of thin-webbed RC and prestressed concrete (PC) beams subjected to shear is extended here to the study of the role of fibers. A stabilized shear-crack pattern is assumed, with the stirrups at or beyond the yielding threshold, and the end sections of the concrete struts at the onset of failure in shear compression. The shear cracks, either limited to the web (PC beams) or extended to the lower flange (RC beams), are modelled with non-uniform opening and slip distributions, with mode 1 prevailing at the crack tip and mixed mode along the cracks. The fibers are introduced as fully kinked, randomly oriented, randomly embedded, linear connectors across the shear cracks, and their contribution to shear resistance is analysed in three different types of fibers (crimped and hooked-end or smooth steel fibers, and polyacrylonitrilic (PAN) fibers), different fiber contents, normal and high-strength concretes. In combination with stirrups, the role of crimped fibers and PAN fibers is found to be quite substantial. A comparison with other limit analysis models and 22 fiber-reinforced concrete tested beams is also presented.