Evolution of Composite Floor Systems in the United Statesby C. Dale Buckner,
Document Type: Proceeding Paper
Part of: Structures 2001: A Structural Engineering Odyssey
Composite floor systems, which combine structural steel beams with a cast-in-place concrete slab, have grown in popularity in the U.S. during the past 40 years. In most early applications, removable wood forms were suspended from the floor beams. Shear connectors we re shop welded to the top flange of the beams, and a solid reinforced concrete slab was cast. Floor beams were typically spaced at 2 to 2.5 m on centers, and maximum beam spans were on the order of 8 m. Temporary shores were often used to limit the stresses and deflections under the weight of fresh concrete, Members were designed to achieve full composite action between the steel section and concrete slab using an Allowable Stress Design (ASD) procedure. The economical choice for structural steel in the 1960s had a yield strength of 225 MPa. Many changes have occurred over the years that affect the design and construction of composite floors. A consequence of these changes is that steel sections are significantly thinner and more flexible than those selected 40 years ago, which can make construction more challenging. In particular, problems sometimes occur with the installation of headed studs, with sagging or twisting of members, and with shear connectors and rebars protruding from the finished slab. Difficulties achieving a level floor finish are cited frequently as causing conflicts among designers, builders, and owners. A survey by the ASCE Committee on Composite Construction during 1997 indicated that construction-related problems in composite floors are widespread and costly. The Committee believes, however, that these problems can be prevented by adherence to current specifications and through better communication between designers and builders. As a result of this survey, the Committee prepared a state-of-practice report on construction considerations for composite floors (at the time of this writing, the report has been approved by the ASCE Committee on Composite Construction, and is awaiting final approval for publication in the ASCE Journal of Structural Engineering). The report provides an overview of current U.S. construction practices, summarizes appropriate literature, and provides guidance to help prevent construction problems in floor systems.
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