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

Changeable message sign (CMS) structures are cantilever, interverted L-shaped structural components that are used to transmit information on road conditions to the driving public. The high-cycle fatigue failure of one CMS structure in southern California led to a detailed analytical and experimental study of CMS structures. In this paper key data from field studies are presented together with results of finite-element studies of CMS structural components. A companion paper documents the results of the experimental program and presents recommendations for future CMS construction. Analysis of field data shows mechanical damping in CMS structures to be approximately 0.5% of critical in the first two modes and that galloping instability was probably 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 were high stress concentrations around the conduit holes in all cases, that conduit hole geometry appears to play only a small role in the value of maximum stress, and that the gussets served to reduce the longitudinal and von Mises stresses at the post-baseplate weldment but not around the conduit hole where the gussets created stress risers at the gusset tip.