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calendar   Wednesday - April 24, 2013

bridge post or bedpost?

Here’s one you don’t see very often. I didn’t visit it myself, I only read about it.



image



What you are looking at is a Bedstead Warren Pony bridge with alternating verticals. This one was built in 1914 by the Oklahoma Iron Works, of Tulsa, OK, and it still stands, although it is now abandoned and falling apart.

A Warren truss is the kind made from equilateral triangles. This kind of truss really didn’t come into favor until the advent of inexpensive good steel. While cast iron excels at compression loading, and wrought iron excels at tension loading, both are poor when used for the opposite purpose. Steel is very good for both compression and tension loading, and in a Warren truss, as the dynamic load shifts (ie vehicles drive across the bridge), the diagonals shift from one loading form to another.

Alternating vertical uprights on a Warren truss operate in tension (ie being stretched) and shift some of the dynamic and static load to the top chord sections. In other words, they make the bridge stronger. In a Warren truss the top chords are always in compression, so the alternating verticals pull those beams into tension, which actually neutralizes most of the stress on the top chords. Which is pretty neat.

A Pony truss, as I’ve pointed out before, is a truss bridge that is open across the top. It isn’t a big box of girders like a Through truss is. Pony truss bridges are generally smaller than Through truss ones, although with modern H beams and construction techniques they can be up to a couple hundred feet long.

A Bedstead truss is what makes this kind of bridge unusual. Instead of the typical diagonal end chord that most Warrens have, the Bedstead truss has heavy weight vertical end sections added. Instead of the bridge sitting on rocker or roller feet as most bridges do, the Bedstead extends those vertical ends well past the bottom of the bridge and sets them directly in the stone abutments. So the naked bridge really does look like an old iron four post bed frame. And this works just fine ... unless the abutments shift or breakdown. Rocker feet or roller feet allow a bridge and it’s base to expand and contract more without damaging anything; Bedstead posts set directly in concrete don’t allow for any movement at all, so any expansion joints have to be in the bridge itself, which weakens the design.

The above picture was taken in 1993; information on this bridge that spans the Snake Creek south of Bixby can be found here. A photo essay showing the current dilapidated condition of the bridge once the abutments shifted can be found here. I gather it is the last bridge standing built by the Oklahoma Iron Works with a clean pedigree. Shame it will be little more than rust in another 10 years.

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Posted by Drew458   United States  on 04/24/2013 at 08:38 AM   
Filed Under: • Bridges •  
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Oh, and here's some kind of visitor flag counter thingy. Hey, all the cool blogs have one, so I should too. The Visitors Online thingy up at the top doesn't count anything, but it looks neat. It had better, since I paid actual money for it.
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