|HAER PA,2-OAK.V,1--10 from pa3714|
Oblique view of cantilever truss over main channel, looking NNE from south abutment. - Bessemer & Lake Erie Railroad, Allegheny River Bridge, Spanning Allegheny River, East of Pennsylvania Turnpike (I-76), Oakmont, Allegheny County, PA
The bridge was designed by two U.S. Steel subsidiaries - the railroad and the American Bridge Company - as a showpiece for their parent company. This structure not only introduced new silicon steel to American bridge construction, but also helped revive continuous truss design. [pa3714]I included the label "metalIron" because this was evidently the first bridge built with silicon steel, which was stronger than existing steel. It is also a pioneering bridge because it used continuous spans rather than simple spans that were cantilevered and generated an upward force on some of the piers, especially during construction. This double track bridge replaced a single track bridge. The old bridge had a steel trestle for the north approach. That trestle was buried in an embankment when this bridge was built. Furthermore a "felling trestle" was built parallel to the existing trestle so that dump trains would not interfere with the normal ore trains. "Engineering News remarked, 'It is not often in any part ofthe country that a perfectly serviceable steel viaduct is treated in this way;... [Here] two such viaducts, side by side, are being buried.' The embankment, however, allowed U.S. Steel to dispose of 1.2 million cubic yards of slag from its mills, which may have outweighed the scrap value of the steel trestles." "With the trusses continuous across two piers, train loads are distributed among the three spans, requiring less material than an equivalent series of simple spans....The bridge was clearly intended as a showpiece for U.S. Steel" [pa3714data]
Note that the piers of the old bridge were extended using cut-stone even though reinforced concrete was becoming a more common construction material by 1918.