Horizontal distribution of concentrated loads in deep composite slabs

Abstract

Composite steel-concrete floor slabs are a type of flooring that consists of a steel profiled deck, cast in-place concrete and steel reinforcement meshes in the concrete and steel reinforcement bars in the profiled ribs of the steel deck. Composite floors can roughly be separated into two types, those that use: shallow steel decks and deep steel decks. The deep profiled deck ComFlor210 is installed by hand and is then used as a work floor and as permanent form work for the casting of the concrete. The use of deep composite steel-concrete floor slabs is advantageous if aspects such as low self-weight, construction speed and floor height are important. The ComFlor210 composite floor is constructed by default with one reinforcement mesh, but if a large concentrated force is expected in the design than a second reinforcement mesh is added. However the effect that this added mesh has on the distribution of that concentrated force is not exactly known. The design of deep composite slabs also falls outside of the scope of Eurocode 4, design of composite structure, which leads to conservative assumptions and inefficient usage of materials. These factors lead to a need for more insight into the behaviour of composite slabs loaded by a concentrated load and in what manner this is effected by the addition of a second reinforcement mesh. An analytical was created in order to predict the horizontal distribution. In the analytical model it was chosen to only model a two dimensional strip over the width of the composite slab. Any three dimensional aspects occurring in the slabwere then incorporated into the two dimensional model. This was achieved by assuming the ribs of the composite slab as zero dimensional points which were supported by translation springs. The translation springs were modelling the ribs deflection and stiffness behaviour. The top part of the composite slab was modelled as a continuous beam spanning the entire width of the model. In order to incorporate the rotation resistance of the ribs, rotation springs were used to connect the ribs to the translation springs. For the analytical model accuracy, ease of use and a basis in mechanical behaviour were the aims. A finite elementmodel was also created in order to validate the analytical model and to analyse the behaviour of the composite slabs. Only a quarter of the slab was modelled by making use of the symmetry and only the areas of the steel deck effective in bending were included in the model. Finally laboratory experiments were also performed on life-sized test specimens. Three test specimens: one with two reinforcement meshes, one with one mesh and one without a mesh were tested. A concentrated load was applied in a non-destructive test at a quarter of the span length and a destructive test was performed at half the span length. The results of these tests were compared with the analytical and finite element model. The result of the comparison was that the accuracy goal of the analytical model could not be reached. This was because the translation springs in the model included both the distribution at the point of loading and the redistribution between that point and the supports. Therefore the distribution cannot be accurately estimated without further research and experimentation. The analytical model should therefore be revised in order to more easily assess its parameters. The stiffness of the supports is another aspect that was not taken into account into the model that could be included. The finite element model also did not have sufficient accuracy in order to use as a validation method, possibly due to the use of only the effective steel profile. Adding the whole steel deck might improve the accuracy. The experiment results yielded the conclusion that the second reinforcement mesh increases the distribution of the load in the horizontal direction significantly while loaded at mid span. The slab without mesh also could be loaded until high loads of 100 kN, but its load was far more concentrated than in the other slabs. Although initially at 10 kN all slabs had very similar distributions and until a load 60 kN the maximum difference in the middle rib was only 10% of the total applied load. It was also concluded that the steel deck was active in the transverse direction after the chemical steelconcrete bonding was lost, but the size of contribution depended on the number of reinforcement meshes. Suggestions for future research that were not included in this thesis include the horizontal and vertical shear capacity of the composite deck, the crack propagation in the slabs and the behaviour of the steel-concrete chemical bond.