Ultrasonic instrumentation for measuring applied stress on bridges

The measurement of applied stress on bridges can provide valuable informati on on the condition of the structure. The conventional technique for measur ing applied stress is with a strain gage. However, strain gages can be time consuming to install because first the surface must usually be prepared. O n a bridge, paint removal will most likely be necessary as part of this sur face preparation. When dealing with lead-based paints, which are considered hazardous waste, many time consuming removal procedures are required. Beca use of these factors, a device that measures applied stress without requiri ng paint removal could be useful. While a "clamp-on" strain gage can also b e used to measure applied stress without requiring paint removal, this type of strain gage can not be used on some bridge details, such as webs of I-b eams and tops of box girders. An ultrasonic technique using non-contact ele ctromagnetic transducers provides a possible method for applied stress meas urement which is not limited by the same factors as those with conventional strain gages. The transducers operate through nonconductive and conductive (lead-based) paint and work on rusted, pitied surfaces. Our previous resea rch developed a technique for measuring applied stresses on bridges with EM ATs and included many laboratory tests. This paper describes field applicat ions of the technique on actual bridge structures, as well as additional sy stem testing and instrument calibration in the laboratory.