The Main Street Bridge in Half Moon Bay, California was constructed in an era when horse-drawn vehicles were the norm and has been in continuous service to date, carrying motor vehicles today that it was never designed for. The Main St. Bridge is a single-span, unreinforced concrete arch bridge originally built in 1900 over Pilarcitos Creek. The vault is approximately 2 ft. (0.6 m) thick at midspan and tapers to nearly 6 ft. (1.8 m) thick at the spring line. The bridge spans approximately 60 ft. (18 m) and carries two lanes of traffic in and out of downtown Half Moon Bay. In 2015, the City of Half Moon Bay requested an independent evaluation of the bridge to help resolve the question of whether to preserve the historic bridge or replace it. The evaluation team, consisting of engineers from both Atkinson-Noland & Associates (ANA) and Bridge Diagnostics, Inc. (BDI), proposed a complete survey of the bridge and in-depth investigations using invasive and nondestructive evaluation (NDE) techniques. Concrete thickness, void location, and location of reinforcing was done using microwave radar. The concrete was evaluated visually, by use of a Schmidt rebound hammer, and by compression tests and petrographic examination of cored samples. A diagnostic load test was performed using a truck of known weight moving across the bridge along prescribed wheel paths. Instruments installed on the arch included 21 strain gages to measure strains in the concrete under load and 14 LVDTs to measure vertical deflections of the arch bridge. Data from the load test was then utilized to calibrate a finite element model of the bridge. The finite element model was used to evaluate the demand on the structure when rating vehicles were applied. This demand was then compared with the capacity of the bridge, assisted by the NDE and load test data, to develop a bridge load rating. The comprehensive evaluation, using both diagnostic load testing in conjunction with NDE, ultimately provided a load rating for the bridge in excess of 2.2. The implications of the higher than expected load rating is that the bridge is structurally adequate for all legal loads and that remedial repairs to the bridge should be substantially less than the cost of a new replacement bridge.

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