Previous posts on the new Narrows Bridge:
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.
Continue reading “The Two Towers II: