Fatigue-life evaluation of steel post structures. I: Background and analysis

Changeable message sign (CMS) structures are cantilever, interverted L-shap ed structural components that are used to transmit information on road cond itions to the driving public. The high-cycle fatigue failure of one CMS str ucture in southern California led to a detailed analytical and experimental study of CMS structures. In this paper key data from held studies are pres ented together with results of finite-element studies of CMS structural com ponents. A companion paper documents the results of the experimental progra m and presents recommendations for future CMS construction. Analysis of fie ld data shows mechanical damping in CMS structures to be approximately 0.5% of critical in the first two modes and that galloping instability was prob ably the cause of failure of the CMS structure. Finite-element models were developed for one as-built post, one gusset-retrofitted post, and different conduit hole geometries. The finite-element analysis showed that there wer e high stress concentrations around the conduit holes in all cases, that co nduit hole geometry appears to play only a small role in the value of maxim um stress, and that the gussets served ro reduce the longitudinal and von M ises stresses at the post-baseplate weldment bur nor around the conduit hol e where the gussets created stress risers at the gusset tip.