Intelligent Infrastructure Systems performed field testing and load rating analysis for five Northampton County, PA bridges. Each of the bridges was subjected to forced vibration testing using THMPER™ (Targeted Hits for Modal Parameter Estimation and Rating) which allows for the efficient collection of structural response measurements. Once collected, the measurements were error-screened visually and pre-processed to average and filter out noise. Finally, the measurements were post-processed using the Complex Mode Indicator Function (CMIF) algorithm to obtain the natural frequencies and associated mode shapes of each bridge. These dynamic parameters represent a structural signature that reflects the stiffness and mass properties of a structure, inclusive of longitudinal, transverse, and torsional behaviors. Load ratings for each of the bridges were developed using the Single Line Girder approach, and compared to results from refined analysis using Bridgemiser to represent a priori and calibrated behaviors.
Each bridge included in the study was found to have load ratings greater than 40 tons for the HL-93 live load model (Strength I Load Rating) employed by the AASHTO Manual for Bridge Evaluation. This indicates that each bridge has sufficient load-carrying capacity, and does not need to be posted for structural safety reasons.