Design for punching of prestressed concrete slabs

Prestressing in flat slabs helps in controlling deformations and cracking under service loads and allows reducing the required slab thickness, leading thus to more slender structures and being therefore an economic solution for long spans. However, as a consequence of the limited thickness of these members, punching is typically governing at ultimate limit state. Investigations on the topic of punching shear strength have shown that the presence of prestress in flat slabs has a number of potential beneficial effects, namely the vertical component (force) carried by inclined tendons, the in-plane compression stresses and the bending moments developed near the supported region. The approach provided by codes of practice for punching design in presence of prestressing may however differ significantly. Some neglect the influence of the introduced bending moments due to prestressing and the sections at which deviation forces of the tendons are considered may be located at different distances from the edge of the supported region. In this paper, the influence of prestressing on the punching shear strength of members without shear reinforcement is investigated by using the fundamentals of the Critical Shear Crack Theory. On that basis, and accounting also for 65 tests available in the scientific literature, the suitability and accuracy of a number of design codes, such as Model Code 2010, Eurocode 2 and ACI 318-11, is investigated and compared.