This project was aimed at developing and demonstrating the application of a nonlinear acoustics-based technique for identifying the stress state of critical highway bridge components through laboratory-scale and field testing. Work in Stage 1 involved a laboratory-scale demonstration of the proposed approach. Theoretical and numerical models were developed to identify the most sensitive ultrasonic waves to the level of stress on structural steel. The selected ultrasonic waves were tested on an L profile loaded uniaxially and a gusset plate loaded uniaxially and bi-axially. The load value was increased incrementally, and ultrasonic measurement was taken at each loading step to develop a sensitivity curve of the stress–ultrasonic velocity relationship. Work in Stage 2 involved a field demonstration of the proposed approach and understanding the uncertainty in the measurements. A fracture-critical steel truss bridge provided by the Illinois Department of Transportation was modeled using a finite element program. Truss elements were modeled using discrete elements; the gusset plate was modeled as a two-dimensional plate element. The stress levels of the selected points on the gusset plate were identified using numerical simulation. Field measurements, using the hand-held ultrasonic device, the ultrasonic setting, and the correlation curve developed in Stage 1, were taken for comparison with the finite element results. The second bridge testing was performed at Norris Bridge in Virginia, which is a multi-span truss bridge. The measurements from multiple points in gusset plates were taken in two orthogonal directions. The difference between the numerical and experimental results was about 13%, considering the complete elastic stress range. Higher error comes from uncertainties in the boundary conditions of the gusset plate model, which requires further study. The bridge was modeled with a sub-structural concept using Abaqus: the gusset plate modeled with shell elements; and the connected truss members modeled as simple beam elements. The approach eliminates the assumption of any fixed/free boundary conditions of the gusset plate, which do not represent the actual gusset plate boundary conditions. The algorithm that includes the steps as coupling error correction, combining vertical, horizontal, and angled measurements and simultaneous solution of equations is planned to be discussed with instrument manufacturers for automated stress measurements.