ANCHORAGES: You can’t see much because they’re mostly buried at each end of the bridge. The huge blocks of concrete hold down the ends of the main cables. Each is as big as a large building and weighs about 81 million pounds. Inside, the strands of the main cables spread out like fans to distribute the 50 million pounds of tension exerted by the weight of the bridge.
CAISSONS: You can’t see much of them either, but from bottom to top they’re as tall as 20-story buildings. The giant underwater columns of concrete – visible above the water only as little platforms – establish bases for the towers. They go 233 feet to the bottom of the Narrows on the Tacoma side and 207 feet on the Gig Harbor side. The Tacoma caisson extends 62 feet into the sea floor and the Gig Harbor caisson 57 feet to make them good and sturdy. Each contains more than 30,000 cubic yards of concrete. Some workers say they were the hardest part of the bridge to build because the water moves through the Narrows so fast – up to 10 miles an hour.
TOWERS: Their job is to hold up the cables. The towers must be strong because most of the weight of the bridge (and all the cars and trucks) rests on them. They have to bend without breaking when the wind blows or if an earthquake hits. In a major earthquake, the tops of the towers could sway as much as 6 feet without breaking. The legs get closer together as they go up. This makes them more stable, like a person standing with his legs spread apart. The towers are 510 feet tall, only 95 feet shorter than the Space Needle.
MAIN CABLES: They stretch from one anchorage over the tops of the towers and attach to the anchorage on the opposite shore. They look like one fat cable, but actually are tightly packed bundles of little wires, each about as big around as a drinking straw. To make them, workers looped the wires back and forth across the water thousands of times, a process called “cable spinning.” Because the cables needed to be so big and heavy, there was no way to lift them if they were built on the ground. Altogether, the cables have about 19,000 miles of wire inside them. That’s enough to wrap around the United States twice.
SUSPENDER CABLES: These up-and-down wires attach the bridge deck to the main cables. They work in pairs, each of which is pinned onto the deck and looped over hangers on the main cables. The suspender cables are evenly spaced 40 feet apart. In all there are 264 pairs, 132 on each side of the bridge.
DECK: It’s made of steel and designed to be lightweight, but rigid enough so it won’t dip when traffic moves across it or sway too much in the wind. The deck was built in 46 pieces in South Korea and sent to Tacoma on two large ships. Each piece was lifted into place from the ships by powerful cranes that moved along the tops of the main cables. When all 46 pieces were hanging in place, workers fastened them together by welding their tops and bolting the braces underneath.
Rob Carson, The News Tribune
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