Evaluation of Long-Term Behavior of Neutral Axis and its Performance in Detecting Structural Damage

Abstract

The neutral axis represents the line in the cross section of an element that, when the element is subjected to loading, will experience no stress. In structures under uniaxial bending with no normal force, the neutral axis is a horizontal line that passes through the centroid of stiffness of the section. Shifts in the neutral axis location away from the center of stiffness can therefore be indicative of damage or abnormal behavior in a structure. Previous studies have shown the neutral axis location to be sensitive to damage in laboratory and real-world structures for the purposes of Structural Health Monitoring (SHM). However, the utility of the neutral axis as a parameter for long-term monitoring has yet to be studied. This work presents monitoring results using the neutral axis method applied on six years of strain data taken on the US202/NJ23 Overpass near Wayne, New Jersey. Monitoring results indicate sites of unusual behavior in the structure. An analysis of the neutral axis location on the bridges girders indicates a loss of composite action between the concrete and steel. Another unusual behavior, possibly indicating a loss of stiffness in one girder, was also observed. These results are consistent with observations resulting from the research carried out by other researchers on other locations on the bridge. Finally, the behavior and performance of sensors was analyzed for long-term and dynamic testing. This analysis revealed measurement drift and unusual behavior in several sensors.