COMPOSITE FLOOR SYSTEMS - A MATURE DESIGN OPTION

The design of composite structures in Canada can be described as being a mature design option. Many aspects of composite structural interaction between steel beams and concrete have been studied and reported in Canada as early as 1922. Since the early 1930s, composite bridges and buildings have been used in this country. Currently, composite steel-concrete floor systems are used in almost all multi-storey steel framed buildings, as well as for floor framing systems in tube-in-tube concrete structures. The purpose of this paper is to focus on those aspects of composite design in Canada which have contributed to the success of the system. Where appropriate, attention is drawn to the research work that has been required to substantiate new and innovative techniques. Designers of composite structures elsewhere in the world may benefit from the lessons which have been learned from the use of this design option in Canada. The features of composite floor framing utilizing composite deck-slabs in multi-storey buildings that have made it economically attractive include access to alternative structural systems, efficiency on longer spans, improved integration of structure with mechanical systems, and superior flatness of floors with minimal deflections under both superimposed dead and live loads. Composite floor framing systems commonly used include conventional beam-girder systems, composite steel trusses and the stub-girder system. Each of these systems is evaluated and discussed, including the concomitant deck-slab system, and slab reinforcing requirements. The quality of structural concrete, especially the shrinkage and creep characteristics that affect either structural performance, serviceability, or both, are noted. The problem areas of composite design and construction are also addressed. These include the tendency for slab cracking over primary support girders and around columns, the need for understanding of beam deflections during construction and control of same by pre-cambering, and particularly, appropriate shoring techniques for stub-girders. It is instructive to look at some examples of comparative costs. Those embodied in this paper include examples of cost savings using composite floor framing compared with conventional non-composite systems.