How long should a bridge last?

Give us more money --  "the sooner the better".

The state Department of Transportation (DOT) maintains that the 520 bridge, which opened in 1963 and underwent a major seismic retrofit in 1999, is near the end of its useful life, and must be replaced for the safety of people in the 115,000 vehicles that cross it each day.

The bridge is now 42 years old. Doesn't it seem to you that a bridge of that age should have many years of useful life left in it? It may well be true that the bridge needs replacement; on the other hand it might be a make-work project by the transportation engineering folks.

And no one comments on how odd it is that (at least in Seattle) we accept such rapid deterioration as to be expected.

Comments

[JRoth]

We probably need an engineer's opinion, but I'm not sure I'd be so dismissive of a 50 year lifespan for a highway bridge. The question that leaps to my mind is what the structure is: it seems to me that while steel bridges with concrete decks can be "renewed" indefinitely, concrete beams are subject to the same corrosive effects as the decks themselves, and an all-concrete bridge just might not last.

Of course, with the mild climate out there, everything should just last forever, shouldn't it?

[Matthias Ripp]

We do have similar problems in Bamberg. For those of you who can read german or want to view a small movie of the demolition of a bridge, take a look at this website:
http://www.brueckenprojekt2010.de/

[Kent]

The floating bridges in Seattle are also rather unique. Essentially they are a series of concrete pontoons that are attached together with a roadway laid on top. I don't know anything specific about these particular bridges, but it seems plausible that a bridge made of concrete that is floating in water would be subject to more rapid decay than a traditional concrete/steel structure that is suspended in air. In addition to the corrosive effects of the water on the concrete, the bridges are also subject to serious wave action during every storm as anyone driving across one of those bridges during a windstorm can attest.

Again, I don't know if these are plausible reasons for the shorter life span. I'm just pointing out that these really aren't the regular highway bridge.

Another problem with the 520 bridge is that you can't bike across it. That means the only way to bike into/out of the northern half of Seattle is to ride all the way south to the I-90 bridge, or ride around the entire northern perimeter of the lake on the Burke Gillman trail. Some of the buses going across the bridge have bike racks on the front but this is less than ideal.

I used to live in the Montlake neighborhood right at the west end of that bridge and it is just a horrendous daily bottleneck with traffic backing up to all the surrounding surface streets. I expect that a rather significant motiviation of DOT is to add extra lanes to the bridge also.

[Brian Miller]

Are there not seismic issues, as well? Modern engineering for earthquake resistance is much superior to the 1953 standard. (I'm talking about the terrestrial access points to the floating bridge. I assume the floating pontoons could ride out a quake? )

[David Sucher]

You folks are so much more trusting than am I.

You may well be right but after looking at the manner in which the authorities have proceeded with regard to the Viaduct/Seawall in downtown Seattle I am very cautious about believing, anything.

I have blogged on that subject before:
Alaskan Way Seawall
Location of Seawall Test Pits
"Viaduct?" --- Do the math
As well there is the whole issue of the lack -- until very recently of anything you could call a an emergency contingency plan.

So when it comes to such matters let's just say I like what Ronald Reagan said: "Trust but verify."

[Kent]

David:

It is rather convenient that this 520 rebuild hits the news ONE DAY after the legislature passes a gas tax increase and suddenly funds might be available.

I suspect the DOT has a LONG LONG wish list of projects that have been planned and analyzed.

Perhaps there is an agenda at work here that is somewhat at odds with the priorities of other regional agencies. The city of Seattle has been pushing the viaduct. Bellevue has been pushing 405 expansion. Perhaps the DOT bureaucrats are now starting to weigh in with their own priorities.

[David Sucher]

Kent: We all want job security and to be needed.