In September 1924, the north and south Hamma Hamma bridges are built on the Olympic Loop Highway (now Highway 101) over branches of the Hamma Hamma River, which drain into the Hood Canal near Eldon, Washington, in Mason County on the Olympic Peninsula. Both bridges are on the National Register of Historic Places.
The bridges are identical three-hinged, reinforced-concrete through-ribbed arch bridges. ("Through-ribbed" indicates that a bridge is braced both above traffic and below the roadbed.) These bridges represent a common type built in the United States during the 1920s through the 1930s. The bridges consist of 150-foot arch ribs with the decks being supported with hangers inside the arch and with spandrel columns outside of it. Hinges at the crown and at the skewbacks allow the arch to adjust itself as it bears loads. (Arches are set on skewbacks -- supporting elements such as masonry or stone. A skewback combined with the masonry that in turn supports it is called the abutment -- the complete structure that supports the arch.) Two expansion joints separate the bridge deck from its approaches, and permit movement of the two halves of the arch ribs under load stress or during expansion or contraction.
Engineers finished the bridge design by the spring of 1923, and State Highway Engineer James Allen approved the design and applied for federal funding in March (under the Federal Aid Road Act of 1916). “Federal Project No. 108” was granted approval in May.
The Highway Department called for bids, and the Colonial Construction Company of Spokane won the contract for $77,838. The firm began construction at the end of July. Convict labor, or “state day labor,” as the incarcerated men were called in the records, built the bridge approaches and the gravel roads leading up to them. By the end of September, the bridges were about 80 percent complete.
Steel guardrails were added to prevent vehicles from accidentally crashing into and damaging the bridges at the approaches, but the only major renovation to either bridge was done in 1977. Overhead lateral braces near the ends of both sides of both bridges were removed, increasing the clearance for vehicles from 13 feet, 6 inches, to 16 feet.