Hardy Bridge is a Warren through truss, three-span, two-lane bridge in the western United States. It crosses the Missouri River and is located at milepost 6 on Old U.S. Route 91, about southwest of Cascade, Montana, which is southwest of Great Falls.
Constructed in 1931, it was one of many similar bridges built during a great expansion of bridges and roadways in the state of Montana. It was added to the National Register of Historic Places on January 4, 2010.
Hardy Bridge is named for the unincorporated small community of Hardy, which is about downstream from the bridge below the Pine Island Rapids. The bridge is located on a northwest-southeast axis in the Adel Mountains Volcanic Field, a spectacularly eroded 75-million-year-old volcanic field; the approximate elevation of the river is above sea level.
Description
The toll-free bridge is long and is wide. Two lanes of traffic (with a curb but no shoulder) are carried by the bridge. The two longest of its three spans are 198 feet (60.4 m) in length. The trusses are riveted steel, and the deck is cast-in-place concrete (with no membranes or weathering protection). The bridge has an overhead clearance of . The bridge also spans the BNSF Railway tracks on its northwestern approaches, where the clearance is .
The trusses are the defining feature of the bridge. Two through-trusses with eight panels on each side and one through-truss with six panels on each side form the body of the bridge. The arc of the truss consists of steel beams connected to one another by "batten plates" (single-piece, flat, cast steel plates). The chords (the vertical and sloping beams) fit within a flange (a U-shaped, single-piece, cast steel plate) which is battened to the arc of the truss. The portal braces (the criss-crossing small beams that form a box hanging from the "roof" of the truss) are fastened to the truss with gusset plates. The horizontal struts at the top of each portal brace is made of "laced" steel (beams with holes in it), since the struts are in tension (being pulled apart) rather than compression (crushed from the ends). This allows them to be lighter and save weight. The sway braces (criss-crossing beams perpendicular to the portal braces) are attached to the truss with gusset plates, and bolted to one another at midpoint. In the deck, the 19 long floor I-beams are held together by five "stringer" I-beams. The deck itself is a single concrete slab, cast in place.
Three reinforced concrete piers support the bridge. The piers at either end are on land, while the central pier is in the river. The central pier has a base that is solid concrete. The approach span in the southeast is supported by a reinforced concrete T-beam, and the deck is supported by concrete girders.
The bridge carried about 100 vehicles per day in 2002, of which about 3 percent were heavy trucks. No increase in traffic is expected between 2010 and 2026.
The last inspection was in September 2010. At that time, state bridge inspectors found the deck to be in fair condition. The superstructure and substructure were both in satisfactory condition, with only minor deterioration. With no appreciable river traffic in this area, there were no pier protections. There was only minimal scouring of the riverbed around the piers, and minor bank erosion around the approaches.
History
Early highways
In the summer of 1860, U.S. Army Lieutenant John Mullan and his party of scouts, surveyors, and trail blazers finished construction on the Mullan Road, the first route to connect Fort Benton, Montana, with Fort Walla Walla, Washington. North of Helena, the Mullan Road passed through Prickly Pear Canyon in the northwestern tip of the Big Belt Mountains. After it exited the canyon, the road went north across the prairie rather than follow the Missouri River through the Adel Mountains Volcanic Field. In 1865, the Montana Territorial Legislature gave the Little Prickly Pear Wagon Road Company the right to build a toll road along the path of the Mullan Road through Prickly Pear Canyon, and from there along the Missouri River through the river's canyon in the Adel Mountains Volcanic Field to Great Falls and then Fort Benton. The following year, Helena businessmen James King and Warren Gillette purchase the road from the Little Prickly Pear Wagon Road Company. They made $40,000 in improvements (a huge sum at the time), and traffic on the new "Benton Road" boomed.