The Earthquake Engineering Online Archive

Fatigue life evaluation of changeable message sign structures -- Volume 2: Retrofitted specimens

Chavez, Juan W.; Gilani, Amir S.; Whittaker, Andrew S.

UCB/EERC-97/13, Earthquake Engineering Research Center, University of California, Berkeley, 1997-12, 91 pages (645/G54/1997a)

This report outlines a research program undertaken to investigate the fatigue life of retrofitted changeable message sign (CMS) structure posts. This type of sign structure is an inverted "L" shaped structure, fabricated from steel pipe sections, and composed of a vertical (post) section that is connected to a horizontal (mast arm) section by a flanged connection. These steel structures are inherently flexible and have low structural damping. Following the failure of one CMS structure in Southern California, field studies were undertaken that indicated that the groove-welded post-to-base plate connections were susceptible to wind-induced fatigue cracking. Laboratory studies on CMS posts indicated that the post cross section adjacent to the electrical conduit hole was also susceptible to fatigue-induced cracking. To increase the fatigue life of CMS structures, Caltrans engineers identified the following retrofit strategies: (1) increase the section modulus of the post near the post-to-base plate connection and the conduit hole to reduce the cyclic stress ranges at the critical cross sections and (2) increase the mechanical damping of the sign structure. Two retrofit schemes corresponding to the first retrofit strategy were developed and tested: a steel-gusset retrofit (GR1) and a cast-in-place concrete-jacket retrofit (CIP1). Both retrofit schemes were designed and detailed by Caltrans. The steel-gusset retrofit consisted of welding eight gusset plates to the post-to-base plate connection. The concrete-jacket retrofit consisted of adding a reinforced concrete shell to the steel post. Specimen GR1 was tested to approximately one million cycles before significant cracking developed. Specimen CIP1 was subjected to 4,500,000 cycles of loading without failing.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-97-13.pdf (39 MB)