Behaviour and design of composite metal deck diaphragms subjected to in-plane shear forces

Abstract

During an earthquake, the floors of a multistorey building are designed to function as rigid in-plane diaphragms. As such they are subjected to significant shear demand, especially at the interfaces between each floor and the seismic resisting system. They have to remain elastic to be a rigid diaphragm. However, the behaviour of composite floor slab diaphragm interfaces in the elastic and the inelastic range has not been previously researched, so designers are forced to be very conservative when determining the limits for elastic behaviour the different failure modes of composite floors subjected to in-plane shear forces have been determined by a number of researchers using pseudo-static testing. However, all previous experimental tests subjected the floors to a combination of shear and moment and did not represent the boundary conditions applying at the diaphragm interfaces with the seismic resisting system. This paper proposes a new experimental test setup in which the slabs being tested are subjected to near pure shear at the slab to supporting beam interface. Using the new experimental test setup, three composite floor slabs comprising a reinforced concrete slab on trapezoidal steel deck have been tested. In the first floor slab, the deck rib orientation is parallel to the supporting beam. For second and third floor slab configuration, the deck rib orientation is perpendicular to the supporting beam. The second floor slab uses the standard end anchorage details adopted in New Zealand, involving a solid rib of concrete surrounding the shear studs along the secondary beam. The third floor slab uses the standard end anchorage detail adopted in Europe, in which the decking runs a short distance over the secondary beam and the shear studs are welded through the decking.
It was found that all three slabs had similar strength and stiffness, albeit with different failure modes. The first slab exhibited the most brittle behaviour, with a sharp drop in load carrying capacity following attainment of peak load. The second and third slabs showed a more stable behaviour, with the New Zealand end anchorage details exhibiting a smoother post-peak behaviour and being the most ductile detail among these three end anchorage details. A comparison between the test results and existing design diaphragm interface shear capacity design equations has been made and a new equation is developed to better represent the in plane shear strength of composite floor slab diaphragms.