Instrumentation, monitoring and execution control of the new footbridge over Mondego river in Coimbra

Abstract

A new arch footbridge of unique and appealing geometry is being built in Coimbra. The distinctive feature of antisymmetrical development of both the arch and the deck along the longitudinal axis of the bridge is source to complex behaviour and pedestrian comfort issues. One of the main concerns to begin fabrication of the structural steelwork regarded the influence of construction stages on pre-camber design. Due to the time constraint of the project, it has been decided not to alter the pre-camber design and turn the focus into the construction process through analysis and monitoring. The bridge was built with resource to an embankment platform on the river, which allowed propping of the whole structure. As the foundation settlements that might be expected would compromise the geometry of the super-structure, hydraulic jacks were installed at the propping structure. A three-dimensional finite element model was built using both curved shell and beam elements and a full phased analysis was performed considering all the stages of the construction process. The results so obtained are the foundation for assessment of propping system and interpretation of the static monitoring measured values. The existence of several natural frequencies in the range of those excited by pedestrians showed clearly that the footbridge is prone to vibrate under pedestrian excitation, which meant the necessity of considering appropriate vibration control devices. The existence of design uncertainties associated with a very narrow frequency band of efficiency of the TMDs required several in-situ dynamic testing campaigns.