The effects of raised access flooring on the vibrational performance of long-span concrete floors
There is a current trend towards ever more slender concrete floor structures, which is resulting in more frequent problems with their vibration serviceability. Predictive methods for vibration serviceability must consider not only the structures themselves, but also the non-structural elements which are attached to them, as these may have a significant effect on the dynamic characteristics of the floor structural system. As there has been very little past research in this area, this thesis describes an investigation into the effects of raised access floors on the vibration serviceability of long-span concrete floors. The development of a new modal testing facility based on electrodynamic shaker excitation, which was capable of producing high quality estimates of the modal properties of full-scale floor structures, is described. This was subsequently utilised to determine the modal properties of three full-scale floor structures, before and after the installation of various configurations of raised access floors. The response of these structures to controlled pedestrian excitation was also measured. Realistic finite element models of all structures were developed and updated using the results from the experimental work. These were subsequently utilised for investigation of the experimentally measured effects of the raised access floors. It was found that raised access floors had only minor effects on the modal properties of the long-span concrete floors. Reductions in natural frequencies due to the increased mass were, to some extent, offset by the slight increases in stiffness following the installation of the access floors. Modal damping ratios increased for some modes of vibration, but these changes were rather unpredictable and hence they were too unreliable to be used in design. The response of the structures under controlled pedestrian excitation reduced following the installation of various configurations of raised access floors. The reduction appeared to be greater for relatively deep access floors (500 - 600 mm) than for relatively shallow access floors (150 - 200 mm). Therefore, it is recommended that the effects of access floors may be included in vibration serviceability analyses by applying a reduction factor to predicted responses calculated by assuming a bare floor. The proposed reduction factors are 0.9 for access floors where the finished floor height is less than 500 mm and 0.8 for access floors where the finished floor height is 500 mm or greater.