EACS 2016 paper - Simulation of the response of a lively footbridge under pedestrian loading with two tuned mass dampers for its two first modes (2.1Hz and 2.5Hz)
journal contributionposted on 2017-03-28, 15:16 authored by Norberto Ibán, Javier Castaño, Álvaro Magdaleno, Mariano Cacho, Alberto Fraile, Antolín Lorenzana
EACS 2016 Paper No. 182
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 . 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 the tuning of mechanical parameters. In the case of structures with spatially distributed and closely spaced natural frequencies, the TMD design may not be obvious because Den Hartog’s theory  may not be applied due to the existence of a coupling between the motions of the vibration modes of the structures and the used TMD’s . Alternative design techniques are applied for the case under study consisting on an arched bridge with a main span 40m long and several shorter access spans. The first two first modes are at 2.1Hz and 2.5Hz, both in the range prone to be excited by walking. Also the third one (at 3.18Hz) could be excited by runners.
For the simulation, firstly, a finite element model of the bridge is created in a commercial CAE software and static and modal response is numerically estimated. Then, experimental measurements using static loading test and ambient vibration tests are performed. Initial finite element model is adjusted to match with the static response by fitting some selected parameters. Modal parameters (natural frequencies, mode shapes and modal damping) are extracted and after that the current finite element model is updated. Once the numerical model is calibrated, TMDs are attached. The problem of finding the optimal location and tuning is not a simple one. For understanding the coupled response, several simulations are carried out, from the logical one (TMD located just in the middle of the main span and tuned at 2.1Hz) to others. The responses of the footbridge for different scenarios (depending on the number of TMDs installed and their position) are compared in order to extract some interesting conclusions.