Traffic-induced shifts in modal properties of bridges

A construction can be identified by vibration measurements, from which are derived the intrinsic properties of the structure like eigenfrequencies and modeshapes. Heavy structures like bridges, buildings and dams, having very low natural frequencies, are difficult to put into vibration. In this case ambient vibrations, caused by wind, traffic, microtremors, ... are generally used. One of the advantages is that normal operation is not disturbed by the measurement. Substantial damage will affect these properties: so their use for health monitoring is an interesting and intensively explored research topic. However, the measured values do not necessarily return the exact properties of the bridge. Under the influence of environmental factors, like e.g. temperature and humidity variations, the dynamic properties will undergo changes. In this paper the influence of traffic loads on the identified modal parameters will be investigated. For this purpose, finite element simulations are used, in which a vehicle moves over a bridge model. Different vehicle-models are considered. Using the simulated bridge response, the eigenfrequencies and modeshapes of the bridge vehicle system are determined by the stochastic subspace identification method and afterwards compared with those of the original, unloaded structure.