Centenarian Bridges

The requirements for the bridge being designed by the Senior Design team I am mentoring include a design life of one hundred years. My initial reaction was that it is unrealistic to design a bridge today to last one hundred years. After all, how many bridges built before 1920 are still in existence?

With a little reflection, I answered my own question — quite a few! One website, “Bridgehunter.com”, lists at least eighty-seven in Allegheny County, including three reported to be even older than the Smithfield Street Bridge, which was built in 1883.   

Let’s begin our search for centenarians by focusing on the Bridgeville area. The bridges associated with today’s Wheeling and Lake Erie Railroad were constructed as part of the Wabash Pittsburgh Terminal Railway before 1904. I presume this includes the bridge over Washington Avenue, as well as the trestle over Chartiers Creek.

If that assumption is true, there are Wabash centenarians all through southern Allegheny County, including the trestle and two through truss bridges at Morgan.

Heading east we have the deck girder bridge over Presto-Sygan Road, the deck girder bridge over Millers Run, and the through truss bridge over Chartiers Creek before we get to Bridgeville.

Beyond Bridgeville the railroad is characterized by the familiar high trestles at Vanadium Road, over Hope Hollow Road, over the Parkway West east of the Carnegie interchange, at the south ends of Fort Pitt and Liberty Tunnels, and at Castle Shannon, plus numerous minor bridges. A worthy collection of centenarians!

In Bridgeville proper, the eponymous bridges at both ends of Washington Avenue are each in their fifth or sixth generation. The current four bridges over McLaughlin Run were built in the 1930s, so they do not qualify.

My candidate for the oldest bridge in the area is the railroad bridge over the old Chartiers Creek channel north of town, a rugged Pratt through truss typical of several on what originally was the Chartiers Branch of the Pennsylvania Railroad.

This particular bridge is a magnificent example of the ingenuity of nineteenth century bridge builder, full of clever details. Spanning about one hundred feet, the bridge consists of seven bays. The portal diagonals and upper chords are box sections built up of plates and angles, riveted together.

The four interior posts are built-up laced channel sections; the ones closest to the portals are laced eyebars. The inner diagonals are flat bars, ranging from one to four per bay, depending upon location. The lower chords are pairs of eyebars, connected by large pins to each other, and to the posts and diagonals.

At the bottom of each post a transverse built-up deep plate girder is hung from a pin. Longitudinal plate girders frame into the transverse ones, supporting the timbers on which the tracks are mounted. This bridge is an ideal candidate to be surveyed for the Historic American engineering Record. I hope at least one example of this design is preserved somewhere.

There is a similar bridge on this railroad in Carnegie and one at Morganza. I suspect they all were constructed prior to 1900. The one in Carnegie and one on the Pittsburgh Chartiers and Youghiogheny Railroad, are clearly shown on Thaddeus Fowler’s bird’s eye view depiction of Carnegie in 1897.

Much to my surprise, the oldest surviving bridge reported in Allegheny County is on Beaver Road in Edgeworth, not far from my daughter’s house. It is a closed-spandrel stone arch bridge spanning about twenty-seven feet over Little Sewickley Creek, three blocks from Quaker Valley High School. According to the website it was originally built in 1841, and widened in 1918. It carries about 3,500 vehicles per day.

Not far from this bridge is the Ferry Street Bridge, an underpass under the Ohio River Boulevard. Built around 1875, it too is a closed-spandrel stone arch bridge which spans about twenty-six feet over Ferry Street. When the Ohio River Boulevard was built, in the 1930s, it followed the previous right-of-way of the Pennsylvania Railroad’s Fort Wayne Division through Sewickley. The railroad was relocated closer to the Ohio River at that time. Daily traffic on this bridge is about 20,000 vehicles — not bad for a centenarian!

Farther up Little Sewickley Creek from the Beaver Road Bridge is another lovely stone arch bridge carrying Woodland Road Extension over the creek. Built in 1889, it spans about thirty-one feet with the very light duty of about three hundred vehicles per day. We bridge buffs are fortunate that some skilled mason (or masons) created these three structures that have survived the decades gracefully.

Also older than Smithfield Street is the Iron Bridge Lane Bridge in Natrona Heights. It is a wrought iron pinned Pratt pony truss, fifty-four feet long, spanning Bull Creek. According to the website, it was built in 1878 by the Morse Bridge Company of Youngstown, Ohio. Thirteen and a half feet wide, it has a wooden plank deck with steel plates aligned with the wheel tracks, and stone abutments.

The Smithfield Street Bridge remains the Queen of our centenarians. Designed by Gustav Lindenthal, it was constructed in 1883. It featured two spans of lenticular trusses supporting a two-lane wide deck between them. In 1891a third truss and two additional lanes were added. In 1911 the advent of larger streetcars was accommodated by moving the third truss several feet upstream.

The next renovation was in 1933 when the steel deck frame was replaced by aluminum, reducing the bridge’s dead load by seven hundred and fifty tons. In 1994 a major restoration replaced many of the truss members, managing to increase its load carrying capability from three tons to twenty-three tons. A new paint job and the addition of architectural lighting produced the lovely bridge we now enjoy.

Our original intention was to extend this discussion beyond Allegheny County. That is a worthwhile subject that will be reserved for a future column.      

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