Efficient design of a dynamically sensitive footbridge

A new slender footbridge was recently erected across the Amsterdam Rhine Canal near Muiden in The Netherlands, parallel to the existing road bridge. The superstructure comprises a continuous steel box girder with a total length of 318m and is supported by two intermediate piers. The main span length is 162m with a variable section height of 4.0 to 5.0m. The dynamic behaviour has proven to be the most challenging aspect during the design process. This paper aims to give insight into the design process, the approach of dynamic response measurements and will elaborate on how is dealt with the discrepancy between theoretical response analyses and actual behaviour of the bridge.

The slenderness of the deck structure (ratio of span length to section height) is relatively high and for that reason the bridge proved to be sensitive for wind and huma n induced vibrations. To meet the design requirements regarding dynamic response, the application of tuned mass dampers was foreseen. The dynamic response analysis identified five critical natural frequencies that did not meet the specific allowable accele ration requirements. Seven bespoke tuned mass dampers were designed and installed before the bridge was erected. The tuned mass dampers were designed and installed to enable alteration of ist damping characteristics according to the actual dynamic behaviou r of the bridge. In situ dynamic response measurements took place after the bridge was erected to determine the natural frequencies, mod e shapes and damping values. The results of the in situ measurements proved to be valuable for retrospective study of th e chosen design principles in relation to the actual behaviour of the bridge