Due to high sensitivity to environmental factors, the estimated eigenfrequency of a bridge is varied
and blurred at every measurement. Thus to assess such influential factors, we conduct one year monitoring
of dominant frequency of a Gerber type composite girder bridge and weight of the vehicles passing over
the bridge at the same time. As a result, it was found that the peaks around 3Hz in the power spectrum obtained
in forced vibration depend on the vehicle weight, which may be caused by changes in vibration properties
of the vehicles, but the other properties may not strongly depend on the external factors.