The Two Towers II:
Concrete Thinking

Previous posts on the new Narrows Bridge:

  1. History of the Tacoma Narrows Bridges
  2. The Two Towers I: Intro

 
Concrete Plant

Our tour starts at the concrete plant. To minimize transport costs, a new concrete plant was assembled on the west side of the bridge. It is a mobile plant, with multiple components trucked in and assembled on site. The components were assembled into a complete plant within a few days, and shortly thereafter it began producing concrete at a rate of 150 to 200 yards a day (the average concrete truck carries 9-10 yards, so 15-20 truckloads a day). Bridge construction requires some serious quantities of concrete: each of the towers alone requires 8000 yards (1000 truckloads, as trucks were filled to 8 yards to match lift capacity), and the caissons and shore anchors required substantially more.

One question which naturally arises: since the existing bridge is a steel structure, why not build the new bridge from steel girder to compliment the appearance of the old? The answer is simple: cost. Steel is extremely expensive. The reasons for its high cost are many: huge increases in Chinese consumption (China’s structural steel utilization went from 100 million tons in 1997 to 260 million tons in 2003, and continues to rise rapidly); the scarcity and expense of raw materials, scrap iron and iron ore; and the high cost of refined coal (coke) to fuel blast furnaces due to environmental restrictions. So concrete wins hands down on a cost basis. It is also far lower in costs of long-term maintenance, not requiring regular painting and rust prevention–a big problem in the salty air of Puget Sound.

Concrete trucks make lousy amphibious vehicles, so how do you get the concrete out the caissons (bridge piers) and the towers? Barges are too slow, and the steep, high banks of the Narrows prevents nearby water access. This problem is solved by pumping it out, through surprisingly small pipes.
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Concrete Thinking”

The Two Towers I:
Introduction

Two TowersOpportunities like this don’t come along very often in life.

He was the last patient of the day. I asked him his occupation, and he replied, “construction.” “Contracting? Heavy equipment?” “No, I supervise the concrete work on the new Narrows Bridge.”

Whoa. Cool!

Thirty minutes later–with the staff itching to go home–I finished picking his brain on what is a most amazing engineering feat of the last 30 years: the construction of a new bridge over the Tacoma Narrows. As he started to leave past the checkout desk, he turned and asked: “I’m giving a tour of the new bridge on Sunday. Interested in coming along?”

I considered the pros and cons, weighed the alternatives, and finished the complex decision-making process–nanoseconds later: “What time?” “9 A.M.” “See you there.”

What ensued was an experience you get once in a lifetime. And thanks to the wonder of weblogs, you get to share in it.

There are no public tours of the construction site of the new bridge–the environment is dangerous, and the engineers and work crew are obsessive about safety. Employees face immediate termination if they are on site without proper safety gear, or in riskier areas (such as outside of barriers or railings) without harnesses. As a testament to the rigorousness of the safety precautions, in over 2 years of construction, there have been only 2 injuries (a fractured wrist and a fractured ankle), and no mortalities. No one is allowed on site without hard hat, goggles, and safety vest.

My patient–we’ll call him Mike, although that’s not his real name–was giving a tour for the owner of the site concrete plant and several of his employees–five people initially, but two who were decidedly not interested in going up on the towers–and me, the odd man out. The tour, scheduled to take about 1 1/2 hours, ended up taking three. It flew by like just a few minutes. The weather was typical early May for the Pacific Northwest–mild, overcast, about 50 degrees, but fortunately with almost no wind.

There’s a lot to cover here, so I’m going to break this up into multiple posts, with lots of pictures. For those interested in the photography specifics, the photos were taken using a Panasonic DMC-FZ20, with a remarkable 12x optical zoom Leica Lumix lens and optical image stabilization. Wonderfully light, sturdy camera with one of the easiest user interfaces I’ve used. Images were tweaked a bit in Photoshop, mostly levels and contrast adjustment.

I would recommend my earlier post on the history of the first two bridges over the Narrows (“Galloping Gertie”, which collapsed in 1940, and the current span), which also covers some of the engineering challenges of bridging the Tacoma Narrows.

So put on your hard hat, observe all safety rules, and try to keep you acrophobia in check–we’re off to see the towers of the new Narrows bridge.

Bridge Blogging

I am fortunate to live near an engineering marvel in progress: the new Tacoma Narrows bridge. Most folks have heard of the Tacoma Narrows bridge – at least the first one, “Galloping Gertie”, which catastrophically failed during a windstorm in November 1940.

Built at the cost of $6.6 million dollars, designed by world-famous bridge architect Leon S. Moisseiff (who also designed the Golden Gate bridge), it embraced the light, elegant design principles in vogue at the time – and was designed with complete ignorance of the aerodynamic effects of high winds on bridges. Moisseiff had inadvertently created a mile-wide airplane wing, with its light-weight narrow deck and plate-girder sides. It survived only 4 months after completion. In a strong-but-typical November windstorm, the wave-like undulations were severe enough to unseat a cable from its saddle on the West tower, creating a corkscrew torsional motion which ripped the bridge to shreds. The only casualty, surprisingly, was Tubby the three-legged dog.

May he rest in peace.

The fallen span of the bridge remains at the bottom of the Tacoma Narrows, and has been designated a National Register of Historic Places to prevent salvage. It is one of the world’s largest artificial reefs, and home to a plethora of marine life, as well as the world’s largest octopuses. The remainder was disassembled and sold for scrap during WWII. The caissons and anchors (for the cables, on either bank) were used, largely unmodified, to support the towers and cables of the second Narrows Bridge.
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