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calendar   Thursday - April 25, 2013

Open Your Spandrels!

“The Most Photographed Spot In Oregon”

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The Benson Bridge is the footbridge that traverses Multnomah Creek between the two cascades of Multnomah Falls. It has served as a distinguishing characteristic of the falls since it was built in 1914 by Simon Benson, one of the builders of the old Columbia highway. The bridge is probably the most photographed piece of architecture in Oregon and is a favorite of tourists from around the world. To be sure, the bridge is an obvious and inviting destination for visitors to the area to enjoy. On a summer weekend the short footpath to the bridge is as busy as a bus depot as people make their way up the feel of the spray from the falls, click photos, and get a great top-down view of the lower tier.

Simply gorge-us. And it isn’t too often that you get to see open spandrel arch bridges this small; the span is only 45 feet and the whole bridge is just 52 feet long. But it’s art, in concrete. Done as a public works project.

The Benson Bridge is a reinforced concrete deck arch bridge over Multnomah Creek between the two tiers of Multnomah Falls on the Multnomah Fall – Larch Mountain Trail. The 52-foot long structure consists of a single 45-foot parabolic open spandrel barrel arch span. The design of the main span is unique for bridges designed in Oregon. The main arch is reinforced with a built-up steel lattice frame similar to a built-up truss member, rather than traditional tied steel reinforcing bar. The spandrel columns and the rest of the concrete in the structure utilized the traditional reinforcing bar for concrete reinforcement.

Topping each of the spandrel columns on the Benson Bridge is a half elliptical spandrel curtain wall. The spandrel curtain walls complement the main arch span and create a seamless transition between the superstructure and the bridge’s deck and railing.
The 3.5-foot open balustrade railing that was utilized on many of the early structures on the Historic Columbia River Highway is another key feature on the Benson Bridge. The top of the balusters have quarter elliptical arches coming from each side of the baluster. When the balusters are placed side by side they create a series of small arches just under the rail cap that also accent the parabolic main span.

So technically this is still a truss bridge, although the truss is inside the concrete. Concrete is superb at bearing compression loading. It’s like, rock or something. And it’s a deck bridge (vs a pony or a through style) because the deck is on top and the support parts (arch, truss) are underneath. A spandrel is the area above the arch and below the deck; it’s the triangular-ish zone on either end of the bridge. This bridge has several pairs of columns in each spandrel, carrying the deck load directly to the arch. The “half eliptical spandrel curtain walls” can be seen here. These are just little arches between the tops of the spandrel columns. They don’t contribute much of anything to the strength of the bridge, which is why they’re called curtain walls, but they provide architectural continuity - big arch with smaller arches on top with smaller arches (the railings) on top of that - which gives the bridge an organic, slightly fractal, appearance. And makes it art. Nice job Mr. Benson.


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h/t to Soylent Siberia, who has it as one of those animated .gifs.


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Posted by Drew458   United States  on 04/25/2013 at 08:26 AM   
Filed Under: • Bridges •  
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