Load-bearing structural elements such as rock bolts and reinforcing bars (rebar) are ubiquitously present in both mining industries and in infrastructure projects, for securing tunnel walls and ceilings. Being able to detect and localize defects in previously installed rock bolts, or even quantitatively estimate the load they are taking, would be very valuable when planning maintenance and service. Unfortunately, there are no such techniques available today. Previous work shows that ultrasound, being a mechanical wave, is sensitive to mechanical changes in rock bolts. The ultrasound signature is, however, rather complex, and traditional modeling of the wave propagation rapidly becomes challenging. In this paper, we show that the ultrasound signature measured from a 3.2 m long dynamic rock bolt can be used to accurately determine the load applied to the bolt and the resulting elongation of the bolt. The method is based on training a Partial Least Squares Regression (PLSR) model to estimate the force an elongation from power spectra of backscattered ultrasound signatures.