Bridge Load Testing Versus Bridge Load Rating

The current method of load rating bridges according to AASHTO specifications can underestimate the capacity and behavior of bridges. Analytical equations do not account for the degree of rigidity of the supports, unintended composite action due to friction between girders and the slab, and other factors. Load testing of individual bridges can produce a load rating that much more accurately reflects the capacity of a non-composite bridge. However, current methods of load testing require significant time commitments to instrument a bridge profile to record data, rendering it impractical in many cases. But there are new commercially available strain gauges that greatly reduce the time required to instrument a location. This study evaluated the accuracy and efficiency of set up of the magnetic Sensormate QE-1010 strain gauge and reusable BDI ST350 strain gauge. Both gauge types were interfaced with wireless data transmission capabilities, tested in laboratories alongside the more traditional foil-type strain gauges, and deployed on two Kentucky bridges to test their overall efficacy. Although magnetic strain gauges performed well in the field and reduced installation time, due to the rugged requirements of field testing, they will not be considered for future deployments given the current status of the technology. Reusable strain gauges coupled with wireless transmitters balance rugged performance with short installation times. The field-tested bridges were load posted due to the load rating factor for several truck types being less than one. Field load tests revealed the load rating factor for strength was adequate for the KY 220 Bridge in Hardin County while the load rating for the KY 1068 Bridge in Lewis County could be increased by 68%.