Friday, August 03, 2007

Spanning the Waters

[This started out as a comment at Mockingbird's Medley that grew as I wrote. Most of what follows is there but I expanded it here to add a few more favorite bridges.]

My last project in Virginia 25 years ago evaluated highway maintenance needs. I spent a lot of time looking at bridges with engineers and learning how they are stuctured, built and maintained. Even then many were structurally deficient but politicians weren't willing to pay for maintenance. They much prefer shiny new things like new highways, bridges or even ball parks. But it's the maintenance that keeps the value and safety of public infrastructure. Effective maintenance will ensure that a structure achieves its design life and can even extend that life, thereby saving the expense of a new facility. Which, of course, denies politicians their shiny new projects.

The Minnesota failure may not be so much incompetence as a catastrophically bad decision to defer maintenance in favor of "more inspections" since maintenance money is always scarce. In the meantime, the sports palaces rise and we pour money into war and destruction.

Evaluating bridges was fun just because they are amazing structures, especially older ones that required engineers to span distances that exceeded their longest materials. Arches, trusses and suspension spans to accomplish that feat were not only engineering marvels but also works of art. These days, crews can fabricate long beams on site so designers just need to revert to the simplest form: a beam from bank to bank or pier to pier, functional but boring. I noticed that the bridge adjacent to the collapsed bridge was an arch span, probably built in the 30's or 40's. It looks like the deck was widened to carry more lanes but the main structure is much older than the one that failed.

Richmond, Virginia, where I lived before Arizona, has some beautiful bridges. The railroad bridge over the James west of Boulevard Bridge is one of the finest I've ever seen. Its broad main arches and gracefully curved deck arches are poetry in stone. The view from the Powhite Parkway crossing is spectacular. The old Lee Bridge was also an arched structure but only had the three or four main arches; the deck supports were just columns. It was far less graceful than the railroad brige upstream. I've seen the new Lee Bridge but not enough to recall it. I know the arches are purely decorative, much to the frustration The Boulevard Bridge is pretty neat too, with its complex truss under the deck, similar in concept to the collapsed Minneapolis bridge but much longer lasting, at least in part because it is weight and size restricted. Boulevard Bridge was always my favorite crossing even with the toll. The approaches in either direction were pretty steep and led to two very narrow traffic lanes. kind of like driving on to a ferry ramp. The deck was much lower to the James than other crossings. Boulevard Bridge also offered a good view of the railroad bridge upstream but, if I was driving, I had to concentrate on staying on course; there wasn't much clearance between lanes. Crossing on foot was much more fun.

Phoenix bridges are mostly pier and beam. And new. For the longest time only one bridge crossed the Salt River (which contrary to popular myth is not "normally dry", it's dammed to hell). That was at Hayden's Ferry in Tempe built in the 30's, I think. A very nice three arch span. The area had three major floods between 1978 and 81. Only the Hayden's Ferry bridge and the adjacent railroad bridge held. The rest were gone. When I came in 1982, some major thoroughfares crossing the Salt River were still unbridged. No more.

I encountered several memorable bridges on the Appalachian Trail. The many AT clubs that maintain the trail have built some wonderful bridges. The Laurel Creek crossing south of Bland Virginia is an impressive stone and wood structure. Just north is a wooden suspension bridge over Kimberling Creek. Same too at Jones Creek just south of the Salt Sulphur Turnpike. I crossed the latter in 2002 during a crashing thunderstorm. Three years later, it was washed out. The footbridge across the James River is very impressive. It's easily 100 meters long, maybe more, built on the piers or a previous railroad bridge and is the product of lots of work by the Natural Bridge AT Club. I crossed the James just after dawn on a June morning. My steps echoed on the wooden deck and the river lay under a cover of light mist here and there. The Swatara Creek crossing in central Pennsylvania is also notable. It's an old truss bridge relocated as a foot crossing. The morning I crossed was bright, sunny and the silver truss sparkled like a delicate filigree.

So now you know I like bridges.

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Anonymous Neil Fleming said...

Your first paragraphs speaks volumes as to why we need fewer politicians and more engineers, inspectors & auditors.

6:37 PM  

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