Previous posts on the new Narrows Bridge:
- History of the Tacoma Narrows Bridges
- The Two Towers I: Intro
- The Two Towers II: Concrete Thinking
- The Two Towers III: Anchor Management Classes
- The Two Towers IV: Out & Down
- The Two Towers V: The Struts
- The Two Towers VI: To the Top
- The Two Towers VII: Stairway to Heaven
- The Two Towers VIII: Spinning Beginning
- The Two Towers IX: Wheels Over Water
For those who may be new to this series, I am blogging the construction of the new Tacoma Narrows Bridge. See the above posts for more information on the Narrows Bridges, the engineering challenges, and a recent first-hand tour taken of the construction site.
It’s been a few months since my last post on the bridge construction, but as you can imagine, the project has not been standing still. The spinning process–laying down over 8800 passes of 0.5 cm steel wire, in 19 bundles, joined end to end into one continuous wire–has been proceeding along without a glitch. Well, almost without a glitch–both cables were being spun at approximately the same rate, until someone noticed that the newer spools of wire coming from the warehouse had a small problem: corrosion. Oops.
The carbon steel used for the wire–like any steel or iron product–is subject to corrosion–primarily oxidation, better known as rust. Because the oxides of iron are much weaker than the steel itself, this degrades the strength and integrity of steel wire at a rapid rate. To protect against this process, steel is galvanized–coated with metallic zinc. Zinc will also oxidize–but the oxides of zinc are much more inert and stable, and corrosion of the zinc layer takes far longer than steel. The zinc is applied to the surface of the steel by one of several processes, including heat fusion, electrolysis, immersion, and painting. These processes form a tight chemical bond between the zinc coating and the underlying steel–a metal alloy or amalgam. When the wire leaves the plant, the coating is pure metallic zinc. The metallic zinc combines with carbon dioxide and oxygen to form a very stable zinc carbonate protective layer–under ideal conditions. However, when pure metallic zinc is exposed to water (rain or excessive humidity) in an oxygen-poor environment (usually from improper storage), it will react to form zinc hydroxide, or white rust–which is a much poorer corrosion inhibitor, and further inhibits the formation of the more stable carbonate oxides. One spool affected by this process can be seen below:
Further inspection of the remaining warehoused wire showed nearly 25% of the spools were affected–although fortunately none of the wire already spun out on the bridge. The affected wire was unusable.
Now, of course, this presented a small problem: one cannot simple waltz into Home Depot and order up, oh, say, 5,000 miles of high-quality 0.5 cm galvanized steel wire (try putting that on your VISA card… “Can I see your ID, sir?”). The bridge contract mandates that the project be done by a fixed date, or significant penalties ensue–so a grand scramble began, and manufacturing sources in South Korea, China, and England were found to manufacture the wire on an emergency basis to make up the difference. Sufficient undamaged wire was on hand to complete the south cable, and about 50% of the north cable.
With the south cable completed, it was time to begin compaction. The individual wires are laid in a geometric pattern, and grouped into bundles or tendons–each comprised of 464 wires, 19 bundles to each cable. The wires are laid in bundles to facilitate their tension adjustment (the tension increasing as more wires, and therefore more weight, are added), and wrapped loosely with fabric tape. These bundles, and their component wires, must now be compressed to form a single cable 20 1/2 inches in diameter, eliminating all potential space between the wires.
To accomplish this task, the construction crew uses 4 compaction machines, each with 6 hydraulic jacks. These bad boys are capable of generating up to 1200 pounds of combined force–and you thought you were under a lot of stress!
The compactors were lifted by crane to the east and west side of each tower, and placed around the south cable–one heading toward each anchorage, and two heading toward center span. Their movement downward is passive, using gravity. Riding on wheels on the cable itself, their descent is controlled and braked by cables and pulleys from the towers.
(Does this look like your dream job, or what?? Climbing over a 14 ton hydraulic machine suspended 500 feet above the water? More exciting than Olympic curling, I’d say… )
The compactors move a few feet at a time, stopping to compress the cable, while an engineer walks along with the machine with–get this–a sledge hammer, and a pair of giant calipers:
Who said the pocket-protector geek crowd in school weren’t sexy and cool, eh? The dude “encourages” the wires to settle into place as the compactor moves along, caressing their tender flanks with a whack from the ol’ sledge–then checking with the calipers to make sure the diameter is just right. Here’s betting he doesn’t have to kick the dog when he gets home from work…
Once compacted, workers fasten temporary steel bands around the entire cable to maintain the compacted wires in position. Once the compacting is completed, permanent precision cable bands will be secured and tightened to a high level of precision, located every 40 feet along the cable. These will double as saddles for the suspension wires used to support the bridge deck.
Well, that’s all for now. I’ll post again when the saddles and suspension wires start going on.
My thanks to the Tacoma News Tribune and the Tacoma Narrows Construction Co., who provided the photographs.