Some conclusions from the measurements of temperatures and their gradients on eigenfrequencies of bridges

The ambient air temperature and solar radiation are affecting the soil and asphalts’ stiffness and hence the eigenfrequencies of a bridge. Very often eigenfrequencies are automatically determined by special algorithms from structural response data generated by ambient excitation and measured by permanently installed sensors that is sometimes called “output only analysis” or “operating modal analysis”. Additionally the ambient air temperature is registered and finally the eigenfrequencies are analyzed versus the air temperature. The graph is normally a scatter diagram and each point is one measurement. In general the eigenfrequencies decrease with increasing temperature, whereas often linear regression is used to determine the line of best fit. But the slope of this straight line and the width of the scatter around the regression line differ from bridge to bridge. Especially this scatter field around the mean value at a determined temperature complicates the detection of damage, which is also often based on eigenfrequencies’ reduction. Hence the difficulty among others consists in separating damage from environmental effects.