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.

Pump

A pumping station was set up at each end of the bridge, underneath the existing span near the anchors. Each pump, driven by two large pistons, can move about 1 1/2 yards of concrete a minute, generating about 140 dB of noise while running (that’s front-row, heavy-metal Def Leppard concert levels, folks). The pumps are powered by electricity; at peak use, while pumping for the caissons, electric bills ran about $10,000 a day.

Pump

Mike tells me the pumps–called “Putzmeisters”–are manufactured in Germany, and that this term means “pump master” (but some of us are not convinced, preferring to speculate about the real meaning of the name).

Pipes

The pipes–about 6-8 inches in diameter–run on a walkway constructed under the existing bridge, which also allows walking access to the towers. At the tower, they descend about 150 feet, to the caisson of the current span, then across a catwalk to the new caisson. Here the concrete is loaded into a bucket, then lifted by construction crane to the top of the towers. Total distance from pump to bucket is about 1/3 mile.

At the end of each pour, the pipes must be cleaned. The solution is ingenious: Nerf balls. A section of pipe is removed near the pump, and several large Nerf balls are placed inside. Water pressure is used to push these through the pipes, forcing the residual concrete ahead. When they arrive at the caisson, they are then returned through the now-empty pipe with high-pressure air. One non-official pastime of the crew is shooting the returning Nerf balls from the hose near the pump; they sometimes travel 3-400 yards after exiting. One Nerf ball thus launched ended up on the grill of a Mack truck driving over the bridge; it’s location is unknown, presumably in southern California.

Caisson

The caissons themselves are a rather amazing engineering feat (photo source: Morning News Tribune). An initial steel frame was constructed in a shipyard in the Port of Tacoma, then towed by tugboats several miles up the Narrows to the site. The frame has a sharp cutting edge at the bottom, like a cookie cutter, and a false bottom made up of a series of steel drums welded together which act as flotation tanks. The frame was stabilized in position initially by tugs, then by a system of 8 anchors, radiating from each caisson, to control the horizontal position–no small task with alternating tidal currents of 4-6 knots twice a day. Concrete walls were then poured on top of the frame, with a honeycomb internal cross-brace support–essentially building a 25-story structure from the water’s surface to the bottom of the Narrows 250 feet below.

Rate of descent was controlled by compressed air pumped to the flotation tanks. When the bottom was reached, in order to allow the cutting edge to sink through the sediment to bedrock, the false-bottom flotation tanks had to be removed; this required divers, in some of the most difficult diving conditions around: murky waters, strong currents, very little light, and water temperatures below 50 degrees. Not to mention the Giant Pacific Octopus…

Diver & Octopus

Some of the world’s largest octopus (octopi?) live in the waters of Puget Sound (photo source: Northwest Diver.com). Mike related a story of one diver sent down to cut away the flotation tanks. As the crew at the surface monitored the diver on surveillance camera, he began using his cutting torch. Suddenly, a long tentacle reached out and grabbed the torch from the diver. The diver grabbed it back, only to have the octopus grab it once again. After several such exchanges, the diver ascended to the surface–complaining that there wasn’t room for both of them down there. Apparently, the octopus wasn’t in the Divers Union…

Once the flotation tanks were cut away, the cutting edge settled to bedrock, 75 feet below the bottom of the Sound. No further settling has occurred, despite the additional massive weight of the towers. The caissons were positioned with 6 inches of their planned location.

That’s all for this portion of the saga, folks. Coming next: a look at the cable anchors, and a walk to the towers.

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7 thoughts on “The Two Towers II:
Concrete Thinking

  1. How fascinating! I especially enjoyed the details about the nerf balls and the octopus. Looking forward to the rest of the series.

  2. engineering is so amazing… a tangible testimony to the brilliance of the human mind. glad that an octopus can still undermine all that!

  3. Fascinating stuff, as usual on this blog.

    I have a professional interest in concrete (as a lawyer — I’ve tried cases about it).

    It’s a fascinating technology. So I have questions — What admixtures, for example, are used to resist the chemicals in the water? If pumping is used, presumably the aggregate size is small. Does this affect the concrete design? if the pour is almost continuous, hence “monolithic,” there are other issues. Given the size of the concrete components, the process must generate a great deal of heat — how much? Is flyash or other admixtures used?

    Americans used to be fascinated by technology and built all kinds of stuff in their garages. Now we tend to be quite ignorant of what we drive, what we drive across, and how the gizmos we work on do what they do. Too bad.

    I could rant on. But this is a comment. So, ’nuff said.

  4. Hearing two concrete guys talking is like listening to two doctors talk shop: a lot goes by that’s hard to grasp. The concrete does use a silica admixture, which apparently makes it harder. Different strengths of concrete (presumably related to cement/aggregate ratio) were used on the towers and the anchors)–the specifics escaped me (my brother-in-law, a civil engineer, asked about their test strengths, to which I answered, wisely, “I dunno”). The piped mixture is called a “slick pour”, which I would assume has smaller aggregate. There are other tricks to prevent clogging, especially where the pipe drops 150 feet vertically, which, by Venturi effect, the aggregate tends to fall faster than the cement, and clogs at the bottom. They vent it at the top to prevent this.

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  6. Totally facinating stuff. Seeing how people meet the challenges before them to accomplish such amazing and life altering things is always a pleasure to learn (and much better than the evening news). Nerf balls and giant cookie cutters?? You don’t have to be a mechanical whiz to appreciate stuff like that. BTW “Putzmeister” means, ‘cleaning master’. It might even be a last name, they tend to be very straight forward in German as in other languages. The German word for ‘to pump’ is ‘pumpen’. Not to complicated. ;-)

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