Mothman

The General Corporation and the American Bridge Company constructed the Highway Bridge in 1928. It was designed as a two-lane eye-bar suspension type bridge, measuring 2,235 feet in total length, including the approaches. The bridge was designed under the specifications set forth by the American Society of Civil Engineers. The design criteria the society required was an H-15 load demand.(2) The load demand is the weight restrictions and guidelines that the designing engineers must factor into their design considerations.

The bridge was dubbed the 'Silver Bridge' because it was the country's first aluminum painted bridge. It was designed with a twenty-two foot roadway and one five-foot sidewalk. Some unique engineering techniques were featured on the Silver Bridge such as 'High Tension' eye-bar chains, a unique anchorage system, and 'Rocker" towers. The Silver Bridge was the first eye-bar suspension bridge of its type to be constructed in the United States. The bridge's eye-bars were linked together in pairs like a chain. A huge pin passed through the eye and linked each piece to the next. Each chain link consisted of a pair of 2" x 12" bars and was connected by an 11" pin. The length of each chain varied depending upon its location on the bridge.(3)

Some questions were raised when this design idea was brought forward. What if the two eye-bars did not share the 4 « million pound load of the bridge equally? Would the eye- bars fail under the overloaded stress? The designers thought they had an answer. (4)

The answer come in the type of material used for the eye-bars. The American Bridge Company developed a new heat-treated carbon steel to use on the construction of the Silver Bridge. This new steel would allow the individual members of the bridge to handle more stress. Along with the two eye-bars sharing the load, the steel could easily handle the 4 « million pound load. The newly treated chain steel eye-bars had an ultimate strength of 105,000 pounds per square inch (psi) with an elastic limit of 75,000 psi along with a maximum working stress of 50,000 psi. The eye-bars embedded into the unique anchorage were also heat treated for an ultimate strength of 75,000 psi, an elastic limit of 50,000 psi and a maximum unit stress of 30 psi.(5)

Because of the unique design of the structure, the anchorage design needed to be innovative. Bedrock was only found at a considerable depth, making the ordinary gravity type anchorage impractical. An unusual anchorage was designed consisting of a reinforced concrete trough 200 feet long and 34 feet wide filled with soil and reinforced concrete. The huge trough was supported on 405 sixteen inch octagonal reinforced concrete piles in which the cable pull is resisted by the weight of the anchorage and by sharing the halves of the piles.(6)