Serviceability of floor systems with wood I-joists and concrete topping : CMHC External Research Program project : final report / by Y.H. Chui and I. Smith. : NH17-366/1997E-PDF

This project studies the serviceability behaviour of a form of floor construction comprising wood I-joists, wood-based sub-floor and a concrete topping, which has become increasingly popular in recent years in multi-family residential buildings. Despite its increased use very little is known with regards to how the addition of concrete topping influences deflection and vibration performance of wood floor systems. This project is intended to provide some understanding of this area, and generate preliminary design and construction guidelines. The project was conducted in three phases. In Phase 1 a series of composite beams comprising a single wood I-joist, a narrow strip of wood-based sheathing and a concrete topping were tested. These beams were tested before and after the addition of concrete topping. This phase studied primarily the influence of joist size and use of light shear connectors (double-headed nails) between concrete and sheathing material on beam deflection and vibration properties. The results showed that the addition of concrete topping increased the stiffness and lowered the first natural frequency of a beam. In addition, the use of light shear connectors did not produce any noticeable increase in beam stiffness. These test data were supplemented by results from computer modeling of beam behaviour in Phase 2. The computer model was first verified by test data from Phase 1 to ensure the accuracy of its output. It was then used to study the influence of connection stiffness between concrete and sheathing, concrete width (i.e. joist spacing in a floor) and concrete thickness of beam behaviour. It was found that beam behaviour was not sensitive to connection stiffness for most practical situations. While any increase in concrete width and thickness within the practical range led to amoderate increase in beam stiffness, it could cause a substantial reduction in first natural frequency of the beam. This reduction in natural frequency may cause vibration problems in floor systems since humans are more sensitive to low frequency than high frequency vibrations. Phase 3 involved testing of a full-size floor system which was constructed using one of the joist sizes tested in Phase 1. The objective was to evaluate if some of the findings in Phases 1 and 2 were still applicable to a two-way floor system. The test data showed that a substantial increase in system stiffness was achieved by the addition of a 38mm thick concrete topping compared with the bare floor. However, the beam analysis result with respect to the large reduction in natural frequency was evident in floor test measurements. In addition, it was found that the shrinkage of concrete caused the topping to deform as a "dish" which led to localised low frequency vibration especially near the edges of the floor. Based on this study it is recommended that: 1. Shear connectors be used to provide some form of connection between the sub-floor and concrete to minimize potential dishing of concrete during drying. 2. Natural frequencies of floor systems with an estimated service loading, be checked at the design stage to ensure that they are above the human sensitive range of 4 to 8 Hz. 3. The substantial contribution of concrete topping to the floor system stiffness be properly utilized at the design stage using an appropriate calculation procedure.

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