A new procedure based on time domain indicators for optimal TMD tuning on footbridges

Abstract Structures subjected to excitations like human induced vibrations may produce large accelerations and serviceability limit state problems. Passive, semi-active and active vibration controls have been proposed as possible solutions to reduce the vibration level at civil structures such as bridges, multi-storey buildings or slender floor structures, among others [1]. It is known that Tuned Mass Dampers (TMD) mitigates the vibration response of a structure by increasing its damping through the application of inertial forces generated in response to the movement of the structure. Recently, different TMD implementations have been proposed in order to improve their tuning. In the case of structures with closely spaced natural frequencies the TMD design may not be obvious because Den Hartog’s theory [2] cannot not be applied due to coupling effects between modes of structures and their TMDs. In this work, alternative design techniques are applied for the case under study consisting on a simplified model of a footbridge with a main span 40m long. The first two modes are at 2.104Hz and 2.505Hz, both in the range prone to be excited by walking or jogging (90 to 180 bpm). Also the third one (at 3.18Hz) could be excited by runners. Once the finite element model is calibrated, mitigation devices (one or two TMDs) are proposed. Some standard analysis are carried out for a range of crossings at different paces and several simulations are carried out considering multiple scenarios in which the TMDs are attached to different locations. After studying several proposals, the best solutions are compared also in the frequency domain in order to extract some interesting and not obvious conclusions.