The Importance in Load Positioning During Experimental Load Testing of Highway Bridges

Experimental load rating and testing of highway bridges is the most common approach for determining the load capacity a bridge can withstand. In many respects, it is a method of determining the suitability of a bridge to be in-service. Strain transducers are positioned at various critical locations of the structure to capture parameters such as neutral axis of bending, transverse load distribution factors, impact factors and end fixity. The bridge is loaded at predetermined locations using a calibrated test vehicle. One such problem that can exist however, particularly in heavily deteriorated bridges, is the load may not adequately shed to all structural members. As a result, the strain measurements may be fairly low and be significantly affected by any noise within the sensor system. This paper discusses the field testing of a 3 span simply supported bridge superstructure at the end of its service life. Substantial deterioration from corrosion of the steel girders and heavy spalling of the bridge deck were noted at the time of testing. A load test was completed with a custom developed wireless sensing system monitoring strain at the midspan of each girder. Neutral axis of bending, transverse load distribution factors and impact factor response were each determined using methods outlined in department of transportation protocols. Upon processing of the data it was noted that the system produced +/- 0.1 microstrain of peak-to-peak noise. Select load cases illustrated the inability of some load to adequately shed to an exterior girder. This, in effect, reduced the signal strength of the actual strain measurement by highlighting the measurement noise. A sensitivity analysis is discussed to show what effect +/- 0.1 microstrain has in determining the accuracy of the neutral axis, distribution factor and load rating of the bridge. This paper demonstrates the importance of vehicle positioning when determining the appropriate load testing values used in load rating.