Prestressing and the advent of high strength materials enable the construction of more slender floor slabs with lower values of natural frequency and damping. Under certain circumstances, the vibrations due to forcing frequencies of normal human activities can be annoying to the occupants.
Since the occupants are both the source and the sensor, the vibration cannot be isolated and must be controlled by the structural system. At present, there is a limited knowledge about the overall dynamic characteristics of such concrete slabs, including contributions from individual structural and non-structural components.
An extensive programme of modal testing on a slender one-way spanning 50% scaled post-tensioned concrete slab is described. Testing was performed using electromagnetic shaker, instrumented hammer, heeldrop, and walking excitations, to determine full floor dynamic characteristics. The tests investigated the effect on vibration performance of the level of prestress, and of various non-structural additions, including vibration absorbers and effect of occupants.
It was found that an increase in prestressing force increases natural frequency and decreases damping due to closing up of microcracks. A model is presented to reflect these changes in terms of effective second moment of area. Cantilever partition tests showed energy to dissipate by swaying, and full-height partitions were seen to act as line supports leading to a significant stiffening of the slab.
Analytical models are derived for both forms of partitions. Tests with false floors showed a significantly higher increase in slab damping when the floor panels rested on the pedestals, as opposed to being rigidly fixed to them. Although the addition of viscoelastic screed layers were not seen to have great effect in damping, an analytical model is derived which shows the advantages of using such layers. A TMD system was designed and installed on the floor, using plywood sheets, which led to a reduction in vibration response by as much as 80%. A theoretical model is derived to represent the TMD results and a design criterion is suggested.
Finally, the effect of human-structure interaction is investigated. An analytical model shows the natural frequency of the body to be 10.43Hz with a damping of 50%. Results are also reported of tests on a full-scale field slab, confirming some of the findings of the model slab experiments. Broadly, the results show that contrary to popular belief, merely the presence of non-structural components does not necessarily enhance the dynamic behaviour of the system. The design of these components and nature of their installation are important factors affecting their contribution to the overall floor vibrational behaviour.
Source: University of Oxford
Author: Shahram Falati