Modal Identification and Environmental Effects on Natural Frequencies Using openLAB Research Bridge

Abstract

Modal-based structural health monitoring only works if you can tell environmental effects apart from real structural change. This study looks at how air temperature and solar radiation shift the natural frequencies of the openLAB Research Bridge, a three-span prestressed concrete structure fitted with accelerometers and environmental sensors.

We paired triggered acceleration records from the bridge’s undamaged reference state with concurrent temperature and solar-radiation readings, then ran modal identification to get the dominant frequencies and mode shapes. These were sorted into environmental cases by high and low temperature and solar radiation.

Four frequency clusters emerged, near 4.6, 7.0, 8.5–9.0, and 10.6 Hz, and each shifted depending on the case. Temperature drove more of the variation than solar radiation did. The two also interact: solar radiation matters more when temperatures are low, and temperature matters more when solar radiation is low. These shifts can look like structural deterioration if you’re not accounting for them. Any damage indicator built on modal frequencies has to account for the environmental state at the time of measurement, or it risks flagging noise as damage.