CAIT project no.: FHWA NJ 1998 010
Fiscal Year: 1996/1997
Rutgers-CAIT Author(s): Gary R. Consolazio
External Author(s): Kevin W. Johns, Lehigh University, Robert J. Dexter, University of MinnesotaNicholas Vitillo
Sponsor(s): NJDOT, FHWA-USDOT
In order to determine equivalent static pressures for fatigue loads on cantilevered highway support structures a cantilevered Variable Message Sign(VMS) located along Interstate westbound at mile marker 48.5 in northern New Jersey was continuously monitored for three months. The structure was instrumented with strain gages, pressure transducers, and a wind sentry. All the data was collected with a Campbell Scientific CR9000 digital data acquisition system. A cellular phone transceiver enabled remote communication with the data logger. The system and instrumentation was powered with solar powers and marine batteries. Short-term testing was performed on the structure to determine the dynamic characteristics such as stiffness, natural frequency, and percent of critical damping. Results of the short-term test indicated that the stiffness was 0.24 kN/mm, the first and the second modes were 0.87 cycles/s and 1.22 cycles/s respectively, and the percent of critical damping for the first and second modes were 0.57 percent and 0.25 percent respectively. Long-Term monitoring was performed to capture the structures response to natural wind gusts, galloping, and truck-induced wind gusts. This data would then be used to determine appropriate fatigue design wind loads for future sign support structures.
During the three months of monitoring the structure did not experience galloping, which is a phenomena highly dependent on location. A galloping design pressure of 1000 Pa was recommended based on previous research. The summer months, which is when the structure was monitored, were not conductive to the strongest natural winds patterns in Northern New Jersey. The highest natural wind speed that was recorded was 7.5m/s. It is believed that much stronger winds are present in winter and spring, therefore a natural wind gust design pressure of 250 Pa was recommended. Truck-induced gusts were measured and a linear gradient for the truck induced gust design pressure was determined. The truck-induced gust design pressure ranged linearly from 1760 Pa at 0 to 6m above the surface of the road to 0 Pa at 10.1m and over.