Ambient Vibration and Forced Excitation Tests of a Composite Bridge - Environmental influences and nonlinear effects on the dynamic properties

Environmental influences, changes in boundary conditions and nonlinear structural effects play an important part when investigating the dynamic properties of civil engineering structures like bridges because all of them affect the eigenfrequencies and the damping values as well as the modeshapes. Condition or damage assessment methods which are well known in literature [1], [2], [3] and which make use of the interpretation of changes in dynamic properties due to structural defects or damage could be misinterpreted if these effects are unknown or neglected. In the following, measurement results of ambient vibration tests which have been conducted on a composite bridge within one year are presented. By means of this long-term measurement, changes in the first four eigenfrequencies and their damping values with changing environmental conditions are shown. Forced excitation tests with varying force amplitudes demonstrate the dependency of the eigenfrequencies and their damping values on the excitation force themselves. Measurements of modeshapes with same excitation forces but at different environmental conditions show small changes in the normalized shapes. The assumed main reasons for these phenomena are strongly temperature dependent material behaviour like the stiffness of asphalt layers, changes in boundary conditions like reinforced elastomeric bearings or the soil and nonlinear material behaviour itself.