Graphical poly-arch model for the ultimate limit state verification of RC deep beams under hanging loads

Abstract

The structural behavior of RC deep beams under hanging loads differs from that of conventional top-loaded beams. The hanging loads that are applied at the bottom of beams impose a remaining arch action and an intersecting tension field below such indirect load-carrying mechanism can be recognized, which are not dealt well with existing design approaches. Correspondingly, a graphical poly-arch model is developed for the ultimate limit state verification of RC deep beams under hanging loads, in which the function of interior inclined ties linking the arch rib and the longitudinal tie is acknowledged. Based on the connection between graphic statics and strut-and-tie models, the graphical analysis of proposed poly-arch models using reciprocal diagrams is also presented. After that, the proposed graphical poly-arch model is examined by two case studies, in which the predictions have a fairly good agreement with test results with different span-length-to-depth ratios. Furthermore, the advantages and limitations of the graphical poly-arch model are discussed. As an upgrade over existing strut-and-tie models, it can be seen that the graphical poly-arch model is highly valuable for organizing the load paths within RC deep beams under hanging loads and provides a clear mechanical explanation of how the inclined ties and the longitudinal ties interact, and what failure patterns govern.