Thursday, March 16, 2017

Pittsburgh's Bridges March 16, 2017

Copyright © 2017       John F. Oyler

March 16, 2017

Pittsburgh’s Bridges

The Bridgeville Area Historical Society’s February program meeting was an extremely interesting talk by local Civil Engineer Todd Wilson, entitled “The History of Pittsburgh’s Bridges”, which coincidentally is the topic of an Arcadia “Images of America”  book he recently authored.

The speaker began with a drawing of Fort Pitt which clearly showed footbridges across the moat around the fort, confirming that the topic went back at least two hundred and sixty years. By 1818 the first bridge across the Monongahela River was constructed, at the Smithfield Street location which currently is occupied by the historic bridge with that name,

The first Smithfield Street Bridge was a wooden covered bridge built by a well-known engineer, Louis Wernwag and opened in 1818. It was destroyed by the Great Fire of 1845 which burned a third of the city to the ground. John Roebling replaced it with an eight span suspension bridge with short towers. When a dramatic increase in traffic made it obsolete, it was replaced by a lenticular truss bridge designed by Gustave Lindenthal in 1883, the landmark bridge that is at that site today.

Connecting Allegheny City with Pittsburgh produced a similar story on the Allegheny River side. In 1819 a wooden covered bridge was built connecting the public square on the North Side to Sixth Street downtown. Ten years later Roebling built a wooden aqueduct across the Allegheny, bringing the Pennsylvania Main Line Canal into Pittsburgh. He replaced it with a suspension bridge in 1845.

The Pennsylvania Railroad built a wooden bridge across the Allegheny in 1857 as part of their link to Chicago via the Fort Wayne Division. In 1867 it was replaced by a wrought iron lattice work truss, based on the Ithiel Town design pioneered in covered bridges, and in 1904 by the rugged double deck steel truss bridge still in use at that site.

A particularly interesting story was the role played by the Pittsburgh Municipal Art Commission from 1911 to 1939. The Commission’s function is to foster “excellence in design to City property”. It prospered during the City Beautiful era when city fathers advocated bridges with open designs to permit people using them to enjoy the “beautiful vistas that can be seen from structures like the “Three Sisters bridges” over the Allegheny River.

In the1920s it became necessary to replace the existing Allegheny River bridges at Sixth, Seventh, and Ninth Streets, to provide navigational clearance beneath them.

Through truss and cantilever bridges were proposed for their replacements. The Commission however, led by its chief architect Stanley Roush, insisted upon suspension bridges even though there was no suitable anchorage for their heavily loaded cables at either end. The designers, the Allegheny County Engineering Department, responded by utilizing a unique, self-anchored support system which transferred the cables’ loads into massive compressive forces in the bridge deck.

To erect such a bridge would require closing down the river channels and installing falsework to support the deck until the cables were fully strung. The designers countered by utilizing steel eye-bars instead of cable, so the bridge could be erected following the same procedure as a cantilever bridge. The recently completed Oakland Bay Bridge is the latest example of a self-anchored suspension bridge.

In retrospect one must be grateful to the Commission for their stubbornness which eventually produced the iconic Three Sisters Bridges (now renamed for Andy Warhol, Roberto Clemente, and Rachel Carson. Their influence probably peaked with the design of the Sixteenth Street (now David McCullough) Bridge with its grotesque non-functional pillars at each end. Our opinion is that any bridge well designed to perform its required function is inherently artistic.

The Fort Pitt, Fort Duquesne, and Birmingham Bridges are examples of the first extensive use of computer analytical capability to design a multiply redundant bridge. Each is a suspended deck, tied arch bridge in which the very stiff deck support trusses accept the large tension forces from the arch members.

The Tenth Street Bridge, connecting Pittsburgh’s South Side with the Lower Hill District via the Armstrong Tunnel is a lovely conventional suspension bridge designed by George Richardson and opened for traffic in 1933. In many ways its design and erection methods were prototypes for the much longer Golden Gate Bridge whose construction followed four years later.

Richardson is also credited for design of the Liberty Bridge in 1928 and the West End Bridge in 1932. The bridge design output of the Allegheny County Engineering Department in the years immediately after World War I is remarkable. In that era Vernon R. Covell was Chief Engineer and A.D. Nutter, Design Engineer.

Any discussion of Pittsburgh’s bridges would be derelict if it neglected the magnificent cantilever bridge which brought the Wabash Railroad into downtown Pittsburgh. With a clear span of 812 feet, it was the connection between a tunnel through Mt. Washington and the gaudy Beaux Art Terminal in the heart of the Golden Triangle. The bridge was erected in 1904 and demolished in 1948.

Mr. Wilson’s knowledge of his subject was especially impressive. Being a fellow Civil Engineer I was thrilled with the topic; the audience was quite complimentary about his presentation.

In March the Historical Society returns to its “last Tuesday” evening schedule. At 7:30 pm on Tuesday March 28, 2017, Edd Hale will speak on “The Great Castle Shannon Bank Robbery” in the Chartiers Room of the Bridgeville Volunteer Fire Department, on Commercial Street. The public is cordially invited.

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