Determination of remaining fatigue life of welded stud details on overhead aluminum sign panels in Virginia

Some overhead highways signs in Virginia using a specific welded threaded stud and clip connection have failed while in service. From inspection of the signs it was determined that the failure was caused by fatigue of the weld connecting the threaded stud to the back of the sign panel. It was also observed that lower edge connections failed first and the failures progressed upwards in an unzipping pattern. A combination of natural and truck-induced wind gusts is the cause for the fatigue failure. Although signs with these connections are no longer produced by VDOT and all production was halted in early 2004, there are still approximately 4,000 signs in Virginia with this connection detail. The objective of the research project described here in was to determine priorities for an inspection and retrofitting plan for the remainder of the signs in Virginia. Specifically an S-N curve, which is a plot of stress range versus the number of cycles to failure, was to be developed to aid in predicting the remaining service life of sign panels using this connection detail. The authors opted to test single connections instead of multiple connection systems (i.e., an entire sign or portion thereof) because of material availability, the timeliness of testing, and the readily available equipment for testing. Connections simulating interior and exterior connections were tested under a pseudo-static load as well as for fatigue. Fatigue tests of interior and exterior sign connections developed failures of the aluminum panel instead of the expected weld fracture. Because the failure and fatigue threshold were not representative of failures found in the field, a proper S-N curve to help develop retrofitting priorities could not be developed. Recommendations from this program include increasing retrofit and inspection efforts, gauging and monitoring full-scale signs in service to understand loading conditions, and testing full-scale signs.