Abstract
As part of a continuing study on effects of humans on loading and dynamic response of footbridges, a steel frame walkway has been the subject of studies on the effects of multiple pedestrians with respect to loading and response mitigation. Following finite element modeling and experimental modal analysis to identify the low frequency vibration modes likely to be excited by normal walking, the variation of response with pedestrian density and of system damping and natural frequency with occupancy by stationary pedestrians were both studied. The potentially mitigating effect of stationary occupants is still not well understood and the study included direct measurement of damping forces and absorbed energy using a force plate. The various tests showed that energy dissipation measured directly was consistent with the observed change in damping, that vertical and lateral response both varied approximately with square root of number of pedestrians, and that the simple model of a human as a single mass-spring-damper system may need to be refined to fit observed changes in modal parameters with a crowd of humans present. Modal parameter changes with moving pedestrians were small compared to those with stationary pedestrians indicating that within limits, modal parameters for the empty structure could be used in analysis.
Highlights
Structural design normally focuses on ultimate limit states, with passing attention to serviceability through limiting deflections
The frequency and damping estimates from the frequency response functions (FRFs) circle fit were repeatable for the low response levels, the most reliable estimates of frequency and damping are generally obtained from the single-mode free vibration decay, which can track non-linearity through amplitude dependence of these parameters
The study has provided the opportunity to address a number of contentious issues relating to vibration serviceability of footbridges
Summary
Structural design normally focuses on ultimate limit states, with passing attention to serviceability through limiting deflections. This paper describes a comprehensive study on an unusual pedestrian bridge It is a steel framed elevated walkway that is cantilevered at one end, it is relatively light, the vertical mode frequency is quite low, at 5 Hz, and damping is quite low. It is heavily used by students between classes, frequently empty, accessible and appears to be straightforward to model analytically. As well as estimating from a finite element model and from the modal constant derived by hammer testing, a force plate was used to measure directly contact force from a jumping student which, by fitting to a model with known frequency and damping, provided probably the most reliable estimate. It was found that the changes in modal parameters detected for a similar number of moving pedestrians were smaller and comparable with the level of accuracy of the parameter identification
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