Vibration Comfort Level Design For Lightweight FRP Footbridges

FRP footbridges have many advantages over footbridges withconventional materials and they possess excellent application prospect.Nevertheless, the characteristics of light-weight and low stiffness of FRPfootbridge can easily lead to the excessive dynamic response, which is excitedby human-induced load, hence the pedestrians may feel uncomfortable.Therefore, for FRP footbridge, the human-induced vibration and comfort levelissue of pedestrians have prominent significance concerning vibration theoryand engineering application. In this paper,2FRP footbridges are designed andconstructed for engineering practice. To propose a suitable method forvibration comfort level design, a series of research work is conducted, whichare on quantification of pedestrian’s comfort level, human-bridge interactionvibration, comfort level design procedures and methods for suppressingvibration. The achievements are as follows.(1) Through a series of tests on mechanical properties of FRP beams, itis proved that FRP beams and FRP-concrete composite beams have sufficientloading-capacity and they basically fail because of excessive deformation. Atentative FRP cable-stayed footbridge is designed and in-situ tested, as well asthe suppressing vibration measure is analyzed. It is demonstrated that,although the natural frequency can meet the design requirements, distinctvibration still exists. Therefore, to reflect the comfort level of light-weightFRP footbridge, the vibration acceleration should be a more importantindicator than natural frequency for design.(2)21practical footbridges in Beijing are in-situ tested and aquestionnaire survey on the comfort level of pedestrians is conducted.Combining with the data of questionnaire survey, two pedestrians comfortlevel factors, which are quantitatively defined as felt fraction and comfortablefraction, are respectively mapped to natural frequency and maximal acceleration. It is proved that the two factors have weak correlations to thenatural frequency but have strong correlations to the maximal acceleration.Based on the analyzing results, the comfortable fraction-maximal accelerationcurve is fitted, which provides a quantification expression as well. These arefor footbridge performance-based vibration design on the basis of the maximalacceleration.(3) By selecting an appropriate human body model, the “internal humandriving force” is proposed for modification. With the modified human bodymodel, the coupling vibration equation of human-bridge system is derived toprecisely analyze the vibration response for light-weight footbridge.(4) With Monte-Carlo simulation, the comfort level design method basedon the human-bridge interaction analysis and comfortable fraction-maximalacceleration curve is proposed, the program for engineering application isdesigned as well.(5) On the basis of the comfort level design method, an actual FRPfootbridge is designed, analyzed and in-situ tested. Different methods forsuppressing vibration are compared and it is demonstrated that after paving alayer of concrete on the surface of the bridge, which is proposed in this paper,the effect of suppressing vibration is acceptable and this method is moreengineering-oriented.