Transverse Shear Capacity of Deep Composite Slabs

Abstract

For calculation of the resistance of a composite slab against the transverse shear force, the Eurocode 4 (composite structures) simply refers to the calculation procedures of the Eurocode 2 (concrete structures). It is assumed that the composite slab consists out of a consecutive range of concrete ribs in its width direction, which are solely responsible for resisting the transverse shear force. To calculate the transverse shear capacity of these concrete ribs, an empirical formula is used that was originally derived for regular reinforced concrete beams (without stirrups). However, the concrete ribs of the composite slab are created by the profile of the steel deck, making that each concrete rib is accompanied by two steel webs on the sides. According to the Eurocode 3 (steel structures), these webs of the steel deck have their own transverse shear capacity, which is neglected by the current design approach defined in the Eurocode 4. Besides, the interaction between the steel deck and the concrete may lead to an even higher transverse shear capacity of the composite slab. In this thesis, the aforementioned two aspects, which are currently overlooked by the design principle of the Eurocode 4, are further studied by means of non-linear finite element modelling.

The validation of this empirical formula of the Eurocode 2 for calculating the transverse shear capacity of the concrete ribs is the first point of interest. From the finite element analysis (FEA) of the concrete section of ComFlor 210, it is concluded that the prediction of the transverse shear capacity by the Eurocode 2 is unnecessarily conservative. The study suggests to use the mean width of the concrete rib (b0) in calculation, instead of the minimum width in the tensile area of the concrete rib (bw), as an improvement to the method of the Eurocode 2.

In the next stage, the contribution of the steel deck to the transverse shear capacity of the composite slab is studied. The exact bonding properties between the steel deck and the concrete (at the interface) were not clear when the finite element model was developed, so some assumptions had to be made. When assuming that the steel deck can’t separate from the concrete and the relative slip is restrained in longitudinal direction by the embossments, an increase of 131.6% in transverse shear capacity is found. Because of the assumed interface properties, the steel deck contributes to the total transverse shear capacity in the following ways: it resists a part of the transverse shear force in its webs; it acts as reinforcement to the concrete like a longitudinal rebar; it acts as reinforcement to the concrete like stirrups. However, whether this stirrup-functioning of the steel deck’s webs is representative for the actual transverse shear behaviour of deep composite slabs is being questioned, because it relies on the assumption of no separation at the interface. Therefore, a second FEA of ComFlor 210 is executed in which the interaction between the steel deck and the concrete is neglectable. Still, an increase of 51.4% in transverse shear capacity is found, which can be considered as a lower bound value.

At last, from the FEA results of this thesis, it can indeed be concluded that the current Eurocode 4 provides a unnecessarily conservative calculation method for the transverse shear capacity of ComFlor 210. However, using a simple engineering model that adds up the partial resistances of the concrete ribs and the steel deck’s webs, gives a better prediction while still being safe. For the partial resistance of the concrete ribs, the empirical formula of the Eurocode 2 is used, but this parameter bw is substituted by b0 as already mentioned in the foregoing. For the partial resistance of the steel deck’s webs, the procedures of the Eurocode 3 are followed.