Modular expansion joints are frequently used on bridges with large movements, and fatigue cracking has been noted on a number of these joints. Past studies have examined fatigue design procedures and theoretically predicted joint behavior under traffic loading. These studies have shown that the dynamic characteristics of these joints are variable, and the fatigue design loads must vary with the joint characteristics. This research report describes a field investigation of the dynamic loads and behavior of a swivel joist modular joint system. The study was performed to verify the response predicted in past theoretical studies, to establish the dynamic characteristics of the joint, and to determine appropriate fatigue design loads on the joint. The measurements are described and analyzed in detail. The report develops important conclusions regarding the dynamic loads on the joint, the distribution of load between elements of the joint, and the relative importance of different load components. The subject joint was shown to be more sensitive to vertical loads and less sensitive to horizontal loads than suggested by existing fatigue design methods. Traffic patterns were shown to have a significant impact on the behavior of this joint system. The report includes fatigue design recommendations.