Monday, July 13, 2009

Industry Along the Piscataqua River - The Memorial Bridge

The Memorial Bridge in Portsmouth, NH has been in the news recently due to concerns about its safety and because of the high cost of some long overdue repairs.    The plan for its refurbishment was all set to go, but bids came in $15 million higher than expected.  That turned out to be a 35% cost increase, and it put the project on hold.

Well, it turns out that the bridge has a history of construction bids coming in higher than expected.

While I was googling around for information about industry along the Piscataqua, I stumbled on an interesting article about the Memorial Bridge's construction in a book by the American Society of Civil Engineers that dates back to 1922.   Thanks to the magic of Google Books and expiring copyrights, I was able to find and read this interesting little bit of history.

On September 14th, 1920, proposals were asked for the construction of the foundations and superstructure, in two separate bids. The two bids exceeded the amount available, $1,500,000, and the contracts were not awarded.

The original plans called for piers about 100 ft. long, with large sloping ice-breakers on the up-stream end, with a double Strauss lift bridge of 300 ft span. As the authorities were anxious to complete the bridge, further studies were made to reduce the cost. A Waddell lift span was substituted for the Strauss lift, and the piers were re-designed and made symmetrical by omitting the ice-breaker. The result of these studies was that contracts were let, the substructure being awarded to the Holbrook, Cabot and Rollins Corporation, and the superstructure to the American Bridge Company. The work was awarded in November, 1920, and the speaker's firm began immediately to get plant and material on the site.

I'm no structural engineer, so I had to do some more googling to learn what a double Strauss lift bridge and a Waddell Lift span were.  My task was slightly complicated because the double Strauss lift is a type of Bascule bridge and the Waddell lift is a type of vertical lift bridge.   BTW, you've got to check out those links if for no other reason than to see the cool animated gifs that show the bridges in action.  I'd sure be lost without Google and Wikipedia!

Anyhow, the book I mentioned is embedded below in Google's book reader.  If you click on "more about this book" on the bottom right of the reader,  you can read and even download the entire book (all 1900 pages) for free if you like. 

The article is titled "Difficult Foundation Problems for Piscataqua Bridge at Portsmouth, NH" and that's the real story of the article.  It goes into great detail about the engineering difficulty of setting the piers for the bridge in the fast-moving Piscataqua River.  The Piscataqua has some of the fastest currents of any river in North America and the challenges forced construction crews to build the two piers in fits and starts, improvising as they went along.  

After setting the foundation for the northern pier basically where they wanted, the crews were unable to find the expected bedrock in the river bottom to set the southern pier as planned.  Instead the foundation for the pier was set 12-19 feet below the river bottom in hardpan, which was determined to be good enough.

Despite the challenges and trips back to the drawing board, they did still manage to complete the bridge on time in 1922.  I suppose the fact that it's survived so well for over 95 years is a testament to their good workmanship and their ability to make good tradeoffs.

Below is a You tube video with some rough footage of the construction from the 1920s.

Finally, a few parting links on the bridge and some community efforts to save it:
> Save Our Bridges community group
Hartford Courant Elevates Memorial Bridge - (7/09)
> Why Save Endangered Memorial Bridge? - (12/08)
> Joined, Divided by a Bridge - (12/08)

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