In 1922, the Great Northern Railway builds the Harpole Bridge to span the Palouse River. The bridge is located in the Colfax vicinity, in Whitman County, about one mile down Dry Creek from the intersection of County Route 4400 and County Route 4377. This bridge is a wooden Howe truss bridge. In 1928 the railroad will "house" or box in the truss, making the bridge technically a covered bridge. The Great Northern builds the Harpole Bridge upon acquiring the Spokane & Inland Empire Railway.
The Harpole Bridge, also called the Manning Bridge and the Curtis Lowe Bridge, is the only extant structure of its kind in the state and possibly in the nation. It was built according to a general plan (Great Northern Railway Plan No. 115-1562), which specified a 150-foot span. The Great Northern built the bridge in order to upgrade the interurban electric line to steam use and this required a heavy-timbered structure.
A bridge bears up under the weight of itself (called the dead load) and its traffic (called the live load) due to the opposing forces of tension and compression. Tension "pulls or elongates a bridge component" and compression pushes the components together (Holstine and Hobbs, 6). The Harpole Bridge has a Howe truss, a type of truss patented by William Howe in 1840. A truss consists of triangular members some of which are vertical and some of which lie at a diagonal. In the Howe truss, the vertical members are in tension and the diagonal members are in compression. The bridge has timber piles and wooden approaches. The truss is covered but the bridge has no roof. It is cross-braced above the roadway with timbers and iron rods.The railroad abandoned the bridge in 1967. It was purchased by Ruth Lowe, who had planks laid across the roadway so that the old railroad bridge could provide vehicular access to her rural home.
