Vibration Serviceability and Dynamic Modeling of Cold-Formed Steel Floor Systems
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The use of cold-formed steel as a framework for floor systems in multi-story buildings and single occupancy residences is becoming an increasingly popular alternative to traditional materials and techniques. Builders and designers have recognized that the high strength-to-weight ratio provided by the cross-section of cold-formed steel members permits lighter structures and longer spans. The longer spans and lighter structures associated with cold-formed steel floor systems can result in vibration serviceability issues if proper design considerations are not made. Providing sufficient damping within the structure is the most effective way to ensure that occupants are comfortable under typical residential and office service loads. The modern, open-concept interior has open floor plans with few partitions and long spans, which result in inherently low structural damping. Cold-formed steel floor systems also have less mass than traditional floor systems, which will increase the amplitude of acceleration response. The vibration problems that may be present in cold-formed steel floor systems, like any other floor system, can be addressed if proper consideration is given by designers. Traditional design approaches for vibration serviceability have proven inadequate, and there are no current methods available to designers for calculating the response of cold-formed steel floor systems. In order to design a floor system to properly address occupant comfort, consideration must be given for the type of dynamic loading, resonance, dynamic response, and stiffness of the floor system. The objective of this thesis is to improve the understanding of the dynamic characteristics of cold-formed steel floor systems, and recommend an adequate model for predicting the dynamic response and modal properties of floor systems, in order to aid the design process. This thesis presents the results of an extensive laboratory and field study on the vibration of cold-formed steel floor systems. Floor systems built with cold-formed steel TreadyReady® joists and subfloor assemblies containing OSB, FORTACRETE®, sound reduction board, cold-formed steel deck, and LEVELROCK® topping were examined. Previous research has presented the observed influence of construction details on the modal properties of the laboratory floor systems tested. This thesis discusses the influence of different details on the transverse stiffness of the floor systems. It was found that effectively restrained strongbacks, and cold-formed steel deck subfloor assemblies provided significant increases in transverse stiffness. Based on the analysis of the field testing data, recommended design damping ratios are provided for floor systems constructed with the materials investigated in this study. Floor response that can be compared to serviceability criteria is presented. The peak RMS acceleration from walking excitation was found to be within the acceptable range for the ISO criterion based on residential occupancy, and the static deflection from a 1 kN point load was found to be within the acceptable range of Onysko’s criterion. An adequate design criterion for vibration requires a limiting value, and a means of estimating floor response for comparison. The AISC, ATC, and Smith, Chui, and Hu Orthotropic Plate design methods were evaluated by comparing predicted frequency against measured frequency for the test floors. The ATC and Smith, Chui, and Hu Orthotropic Plate methods were evaluated by comparing predicted deflection against measured deflection for the test floors. The ATC method is recommended as the best method for calculating floor response based on current publications. A design procedure is recommended for cold-formed steel floor systems, using the ATC design guide. The ATC acceleration criterion for walking excitation must be met for floors with fundamental frequencies of less than 15 Hz, and the ATC static deflection criterion must be met for all floors. Proposed modifications to the ATC method to improve the design of cold-formed steel floors include: adopting the recommended design damping ratios from this thesis; adopting the frequency-weighted ISO limiting acceleration and, obtaining several coefficients and empirical expressions that are relevant to cold-formed steel floors from further testing. Recommendations for improving the floor testing procedures at the University of Waterloo are given.