West Seattle Bridge. Courtesy of the Seattle Municipal Archives. Identifier number 158585, April 26, 2005
It’s Time Move Onto a New Option to Move Across the Duwamish
I was in high school and remember when, at the end of my sophomore year, the freighter Antonio Chavez collided into one of the old 1924 West Seattle draw bridge spans. The damage to the bridge forever left it in the up position until it was later replaced by a new swing bridge for local traffic and a hi-rise bridge for commuter traffic to cross over the Duwamish River separating Seattle from its west neighborhoods. The old West Seattle draw bridge had been a regular fixture in my life. I frequently crossed it to visit many extended family who lived in West Seattle, and to cheer on my high school’s home football games and compete in track at West Seattle Stadium. When I was young, my older brother, cousins and I would walk from my cousins’ house to the old bridge just to watch it go up and down. Something many young boys would do.
I left Seattle right out of high school in 1980, enlisting in the Marine Corps and spending most of my time away from Seattle. It was during my time away that Seattle had built the current hi-rise bridge that enabled West Seattle residents to again commute into the city without a wide detour upriver, and the hi-rise removed the inconvenience of frequent traffic-stopping openings from the old draw bridge.
Bridge Design Risk
The current hi-rise bridge is something called a cantilevered segmental bridge. Rather than be a series of beams laid on columns (piers), the bridge’s main span is a broad cast concrete deck resting on only two very tall piers. The span was cast up in the air by devices that both contained the forms for the concrete and mechanisms that allowed the span to extend outward as the concrete set. It’s rather neat technology, actually. The bridge literally grows outward from its pier, crossing space below, and eventually connects to a second span that simultaneously grew toward it.
When I first saw the then new-to-me West Seattle bridge, I was unfamiliar with the technology in its design and construction. My experience and limited knowledge about engineering made me suspect about such long span being made of concrete rather than steel box girders, which I was familiar with. Over time, however, as I watched more and more cast concrete bridges built, I became quite comfortable and confident in the design and technology. I actually find the design quite neat since such bridges have much cleaner and aesthetically pleasing lines compared to riveted steel cross braces comprising many older bridges.
But concrete is a challenging material to work with. While it’s known for compressive strength, it’s not very good when stretched and bent. To make up for this weakness, concrete structures are reinforced with steel bars. You’ve likely seen these steel cages assembled at construction sites prior to being covered with concrete.
Also, long spans such as the West Seattle hi-rise bridge need more than steel cages to hold themselves together. More so when considering that dynamic traffic loads crossing the road deck make the bridge flex and twist, creating even more stress on the concrete forming the bridge span. To increase strength and resistance to these dynamic forces, long steel cables run through the bridge deck. These cables are tightened and for the most part act as elastic springs that support the bridge deck.
The problem is that concrete is concrete, and behaves like concrete. When stresses flex it, it will eventually crack (there is a very new flexible concrete introduced not far from the West Seattle Bridge). After 36 years, the West Seattle Bridge reached a critical point where cracks below the road deck exceeded safety and the bridge was closed to traffic. Since the closure the cracks have expanded and some worry about the bridge collapsing.
Induced Demand
While there are many reasons why the bridge failed so early in its life, one speculation is that an additional traffic lane was created on the bridge. As goes all things with traffic, you build more lanes, and more cars will appear to fill the extra lanes. This is called induced demand. The Katy Freeway in Houston, Texas, is a perfect example of induced demand. Even at 26-lanes wide it remains filled with traffic. Some might have heard of the Carmageddon in the L.A. Area, when a ten-mile stretch of the normally filled Interstate 405 was closed so that another lane could be built and relieve its congestion. Carmageddon was coined because it was feared that without the freeway available, traffic around the closed area would be the end of the L.A. region as locals knew it. People who normally clogged the interstate would instead clog surface streets and traffic would be deadlocked.
It turns out that visions of Carmageddon were completely wrong. Rather than filling surface streets with their cars, people simply stayed home. I do think I read something about people hearing birds singing near the freeway, something not heard since the freeway’s creation decades earlier, but I could be wrong about that specific. The area near The Four-O-Five was quiet during the closure. The problem is that the peace was short-lived. Very soon after the freeway reopened with its new lane, it quickly filled and traffic returned to its stop-and-go norm.
Induced demand can be understood as “build-it-and-they-will-come”. The more lanes that are built for traffic, the more traffic will appear to use those lanes. This happened with the West Seattle Bridge. An extra lane was created for high-occupancy vehicles, which gave reason to add more busses onto the bridge. However the bridge was designed for this type of load. Back in the early 80’s when the bridge was built, West Seattle was less populated than it is today. The old draw bridge kept population low since it wasn’t convenient to commute between West and downtown Seattle. But with the new bridge, that inconvenience was removed and West Seattle boomed. As with all systems, the interaction between West Seattle’s growth and its population’s need to get to downtown Seattle for work and play increased the burden on the West Seattle Bridge to sustain the West Seattleites lifestyles.
In light of the expanding cracks despite closure, it’s now thought that the bridge will remain closed until 2022. For now the coronavirus lockdown is easing the need for the bridge, but demand will return after the pandemic passes, however once life returns to normal and folks start getting out again, the loss of the bridge will be felt more than was the loss of the old 1924 bridge since the region is far more populated.
This is a good time to think about induced demand and use it to our advantage. Save for some homebodies, people generally want to move. Several millennia ago, we’ve learned to tame and breed animals in order to expand our ability to move farther and faster than we could by foot alone. We learned to harness wind to push boats along and across seas. We discovered and improved the wheel to make the task of movement easier for both us and the animals we bred to move us and our stuff. We invented machines – trains, planes and automobiles that made it possible to move us and our heavier stuff farther and faster than our beasts of burden could. Machines that required less care and had longer lives than our horses (and visibly and odoriferously less polluting on the streets). All of this past and continuous development exists because we want and often need to move from points A to B for many, many reasons. To visit family and friends. To workplaces for income, to schools for education, to venues for entertainment. And we want and need to move our goods from places able to make them to people who want to buy them. As we increase in population, productivity and consumers demand more stuff, our transportation machines and related infrastructure will work harder and often compete for more space. As was the case with the current West Seattle Bridge. More people wanted to move between the West Seattle peninsula and downtown Seattle (and points beyond). This eventually filled up all of the original lanes on the bridge and the fastest solution was to create a new lane. As is with all new lanes, induced demand filled up, adding more weight and dynamic stress on the bridge, leading to its current failure. Seattle needs a better way than a future airborne Katy Freeway to help people move to and from West Seattle.
Opportunity Knocks
Right now Sound Transit, the regional transit system serving Seattle, its home county and neighboring counties is exploring options for extending its light rail system to West Seattle. As with all things publicly funded and built on available public land, it’s taking a while to complete. According to the Sound Transit website, planning the West Seattle extension is scheduled to be completed in 2022. This is only planning, design for the new line won’t start until 2022 and is scheduled to occur over four years, with construction beginning in 2025 and ending in 2030. But this timeline was created when the bridge was still available. How will its closure affect planning for the rail extension.
This morning I read that some are thinking that the bridge is damaged beyond repair. Especially since its cracks are expanding even after traffic was stopped. I am not engineer, but based on what little undergrad physics I know, I wonder if the bridge is failing under its own weight. As I wrote earlier, its segmented cantilevered design and construction was new to me at the time it was built. I wonder how new the technology was almost four decades ago when it was built, and how reliable 1980’s technology is compared to today. If the bridge is failing and needs to come down, how will that impact Sound Transit’s plan for the West Seattle Link extension?
In my perfect world, Sound Transit will jump at the opportunity to fast-track the West Seattle Link. As I am not a bridge engineer, I am not a transit planner, so it’s important to emphasize “my perfect world.” I admit idealism. However, this is still an opportunity for Sound Transit. Without a bridge for cars and busses, there is a need for something that will move people between West and downtown Seattle. Something with a large carrying capacity for people. Something that will move quickly and require less effort to build – at least less effort to build across the Duwamish River, which the current and past bridge spans.
Of course fast-tracking the West Seattle Link is idealism on my part. Again, politics over land for both the right-of-way and station placement. But the opportunity is there, and I think it’s worth the effort to see how eager West Seattleites are for anything to get them across the Duwamish.
Another Opinion
As I was composing this article, I read an op-ed proposing a tunnel under the Duwamish. If this suggestion was a snake, it would’ve bit me. While I immediately questioned a potential weakness with the suggestion, my apprehension came only from my past familiarity with the viaducts on both ends of the current bridge system. But reading the detailed engineering suggestion in the op-ed, my I am jumping on board with the idea. But my current stance remains, this is a great time for Sound Transit to move forward with the most efficient and cost-effective way to replace the failing hi-rise bridge, build the West Seattle Link light rail line.
