Behaviour and Design of Composite Metal Deck Diaphragms Subjected to In-plane Shear Forces

Abstract

The primary function of suspended flooring systems in multi-storey buildings is to support gravity loads, and transfer these loads to other structural members, such as beams, walls and columns. Moreover, they play a key role in the distribution of lateral loads into the structural system and through to the foundations. Lateral loads caused by earthquake accelerations and deformations generate in-plane shear actions on these floors acting as diaphragms. Floor diaphragms transfer these actions to the lateral load-resisting systems, which then transfer them to the ground. Consequently, providing a dependable load path from the diaphragm to the lateral load-resisting system is essential, and should be well designed and constructed, as should the structural integrity of the floor diaphragm itself. Flooring comprising in situ lightly reinforced concrete slabs cast onto and acting compositely with light metal decking, known simply as composite metal (steel) deck floors, is one of the most commonly used floor systems in multi-storey buildings in New Zealand and worldwide. This popularity is due to their cost-effectiveness, the speed and efficiency of construction, and their demonstrated robustness under severe fire and earthquake actions. There are many parameters that influence the seismic behaviour of composite slabs, and researchers have addressed different failure modes of composite floors subjected to in-plane shear forces. However, the influence of the orientation of decking ribs and decking anchorage to the supporting steel beams on the strength and stiffness of the diaphragm to lateral load-resisting system has not been studied, and this needs to be understood for reliable diaphragm design of composite floor systems. This study determined the seismic shear behaviour of composite slabs, focussing on the interaction between these slabs acting as diaphragms and the lateral load-resisting systems and made some comparisons between demand and design capacity while providing key material to enable further work in that area. The first part of this project was numerically based (linear and nonlinear time history analysis) to quantify the accuracy of currently proposed provisions for establishing the diaphragm design actions. This showed that the current method for determining the diaphragm interface design shear is reasonable, predicting actions corresponding to the 80% level of peak demand generated from the appropriate time history analyses. The second part involved experimental testing of representative diaphragm components, with nine tests undertaken. These involved ribs oriented either perpendicular or parallel to the supporting beams, and at pseudo-static and seismic dynamic rates of testing. Using a novel pure shear test rig, three end-anchorage details have been examined. In the first floor slab, the deck rib orientation was parallel to the supporting beam. For the second and the third floor slab configurations, the deck rib orientation was perpendicular to the supporting beam. The second floor slab used 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 used the standard end-anchorage detail adopted in Europe, in which the decking continued over the secondary beam and the shear studs were welded through the decking. Three identical specimens of each detail were constructed and tested, the first monotonically and second and third cyclically. It was found that all three slabs had similar strength and stiffness, albeit with different failure modes. The first end-anchorage detail exhibited the most brittle behaviour, whereas the second specimen showed smoother post-peak behaviour and was the most ductile detail among the three. A comparison between the test results and existing design equations has been made and a new equation improved to define the ultimate and design shear strength of composite floor slab diaphragms.