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 interact ion 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 frame d buildings, as well as for floor framing systems in tube-in-tube conc rete structures. The purpose of this paper is to focus on those aspect s of composite design in Canada which have contributed to the success of the system. Where appropriate, attention is drawn to the research w ork that has been required to substantiate new and innovative techniqu es. Designers of composite structures elsewhere in the world may benef it from the lessons which have been learned from the use of this desig n option in Canada. The features of composite floor framing utilizing composite deck-slabs in multi-storey buildings that have made it econo mically attractive include access to alternative structural systems, e fficiency on longer spans, improved integration of structure with mech anical systems, and superior flatness of floors with minimal deflectio ns under both superimposed dead and live loads. Composite floor framin g systems commonly used include conventional beam-girder systems, comp osite 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 concret e, especially the shrinkage and creep characteristics that affect eith er structural performance, serviceability, or both, are noted. The pro blem areas of composite design and construction are also addressed. Th ese include the tendency for slab cracking over primary support girder s and around columns, the need for understanding of beam deflections d uring construction and control of same by pre-cambering, and particula rly, appropriate shoring techniques for stub-girders. It is instructiv e to look at some examples of comparative costs. Those embodied in thi s paper include examples of cost savings using composite floor framing compared with conventional non-composite systems.