Design for punching of prestressed concrete slabs

Abstract

Prestressing in flat slabs helps to control deformations and cracking under service loads and allows the required slab thickness to be reduced, thus leading to more slender structures and an economic solution for long spans. However, as a consequence of the limited thickness of these members, punching typically governs at the ultimate limit state. Studies of the punching shear strength have shown that the presence of prestressing 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. However, the approach provided by codes of practice for punching design in the presence of prestressing may differ significantly. Some codes neglect the influence of the bending moments introduced as a result of prestressing and the sections at which the deviation forces of the tendons are considered may be located at different distances from the edge of the supported region. The influence of prestressing on the punching shear strength of members without shear reinforcement is investigated in this paper by using the fundamentals of the critical shear crack theory. Using that as a basis, and also taking into account 65 tests available in the scientific literature, the suitability and accuracy of a number of design codes, e.g. Model Code 2010, Eurocode 2 and ACI 318‐11, are investigated and compared.

Mechanical Behavior of Widening Prestressed Concrete Deck Slabs under Concentrated Load

“…This indicates that the increase in the prestressing level from 0.5 to 1.0 MPa can improve the ultimate capacity (16.5%) more effectively than the increase in the prestressing level from 1.0 to 2.0 MPa (10.6%). This is in agreement with Hwang et al (2010) and Clement et al (2013) who concluded that the ultimate capacity is not increased proportionally with the initial prestressing level. The positive influence of the prestressing level tended to reduce with the increase in the initial prestressing level.…”

“…The most common failure mode is the punching shear. Many researchers (Higashiyama and Matsui 1998;Hamada et al 2008;Muttoni et al 2012;Clement et al 2013) have proposed the equation to predict the punching shear capacities. Significant work was also performed by Mander et al (2011) to investigate the full-depth precast concrete bridge deck cantilevers, which failed in a flexural and shear mixed failure at the panel-to-panel connection.…”

Background to fib Model Code 2010 shear provisions – part II: punching shear

“…The same approach has also been adopted in fib Model Code 2010 to account for the contribution of steel fibres to punching shear strength [26]. Further, application to prestressed slabs [27,28], post-installed shear reinforcement [29] or non-axis-symmetric slabs [30,31] can be found elsewhere.…”

Influence of moment redistribution and compressive membrane action on punching strength of flat slabs

“…With reference to the influence of an axial compression r n in the control perimeter, it can be taken into account by replacing the rotation w with a modified rotation w 0 = w + 45r n /E c P 0 as shown in [11]. The load-rotation curve may be calculated with several levels of approximation [4] and it may also include the influence of slab continuity [12], normal forces and applied moments [11,13]. The general approach of the Critical Shear Crack Theory can also be used to treat slabs with irregular column layout and uneven loading [14] as well as punching of rectangular columns [15].…”