Dynamic Behaviour of Long-Span Timber Ribbed-Deck Floors
- Publication Type:
- Thesis
- Issue Date:
- 2019
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The development of engineered wood products and the environmental benefits of timber over conventional building materials has led to an increased interest in the use of timber for the construction of multi-storey buildings. Timber has a high strength-to-weight ratio making it structurally efficient for long-span floor applications (a common practice in commercial buildings). However, the low mass of such floors makes them more susceptible to walking-induced vibrations compared to heavier floors such as those made from concrete. In fact, when designing long-span timber floors, dynamic performance criteria tends to govern the design rather than strength. Unfortunately, there is a lack of specific vibration design guidance for long-span timber floors with much of the current criteria based on tests of short-span timber joist floors in residential applications. In addition, there is uncertainty as to how accurately other vibration design guides, mainly used for concrete and steel-concrete composite floors, predicts and assesses floor performance of long-span timber floors.
This thesis addresses this gap by investigating the dynamic behaviour of a long-span timber floor through both experimental and numerical methods. Impact hammer and walking tests with two subjects were performed on a 9 m span ribbed-deck floor which consists of a laminated veneer lumber (LVL) panel glued and screwed to three LVL web members, forming one cassette. The influence of various boundary conditions and cassette-to-cassette connections on the modal properties and floor response were explored. A numerical model of a single cassette, calibrated to measured results through model updating, was used to investigate three human walking load models including the deterministic modelling approach adopted in current vibration design guides. In addition, a numerical model representing the cassette-to-cassette connection was developed and updated using measured results of double cassette tests. These details were adopted in a multi-cassette floor model, based on the dimensions of a typical commercial building floor grid, to investigate the influence of common design parameters on modal properties and floor response.
One of the main findings from walking tests was that the floor exhibited neither a completely transient nor a completely resonant response, despite being classified as a ‘high-frequency’ floor. This assumption of floor behaviour resulted in inaccuracies in response prediction using current vibration design guides and it is proposed that a step-by-step load model which considers the stochastic nature of walking is more appropriate. This load model also provides the response time-history which allows an assessment of the duration of certain vibration amplitudes (through a cumulative distribution function) during the walking event to be considered in design.
Modal clustering was consistently observed, particularly for the first two or three modes, in the single and double cassette experiments as well as the multi-cassette numerical model. Furthermore, the multi-cassette model revealed that higher modes with low modal masses largely contributed to the floor response. This finding highlights that criterion which only considers the fundamental mode may not be adequate. In regards to design considerations which may benefit the floor response, damping was found to play a key role. This may be in the form of incorporating an elastomer (such as Sylomer®) at the support locations where experimental tests revealed that the damping ratio could increase from 1% to approximately 5% and 7% for the first and second modes, respectively. The findings from all investigations were used to provide guidance and commentary for a vibration design procedure, based on a finite element approach, suitable for long-span timber ribbed-deck floors which was presented in the form of a flow chart.
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