Evaluating vibration serviceability using experimental modal testing

Predicting the dynamic response of pedestrian structures such as building floors and footbridges to human-induced activity is a complex problem that involves estimation of the dynamic properties of the structure and estimating its response to a loading that varies in intensity and location with respect to time. Even though the problem is complex, researchers have developed guides to aid designers in avoiding vibration serviceability issues by providing simple analytical tools for evaluating a proposed design. The strength of these guides lies in prevention of serviceability problems at the design stage, although they also serve as an informative measure when in-service floors or pedestrian bridges are reported to have excessive vibrations. Current methods of on-site serviceability evaluation typically involve heel-drop tests to determine natural frequencies and walking tests to record sinusoidal peak acceleration response, generally as response-only single channel measurements. A general understanding of the floor response is achieved this way by looking at the resulting acceleration traces and autospectra resulting from these unmeasured excitations. The most accurate method for estimating the dynamic properties of a structure is experimental modal testing to acquire accelerance frequency response functions (FRF). This paper proposes a method for evaluation of vibration serviceability using the mid-bay/span driving point accelerance FRFs of low-frequency (<9 Hz) pedestrian structures derived from experimental modal testing. The method proposes using an accelerance limit curve generated from a contemporary design guide to represent a tolerance limit of vibration serviceability. On-site evaluation can be performed with modal testing by comparing the peaks of a measured accelerance FRF with the accelerance limit curve. The method is demonstrated using a set of mid-bay driving point accelerance FRF measurements from an in-situ building floor and comparing with a widely recognized design guide-based accelerance limit curve.