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
- The New Bridge at Christmas
- The Two Towers X: Compacting the Cable
For those who may be new to this series, I have been 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 first-hand tour taken of the construction site.
I’ve been a bit remiss on bringing you up to date on the new Narrows Bridge construction. Over the past few months, there’s been a lot of progress, but much of it in ways and places without much, if any, visible change. The cables, whose spinning progress was interrupted by problems of premature corrosion of the galvanized steel wire, mentioned in my previous post, has now been completed. The North cable has been compacted and banded, and the infrastructure supporting cable spinning has been dismantled. The catwalks remain in place, but the overhead tram system used for spinning has been removed.
After the cable is compacted, it is necessary to maintain its compressed configuration, particularly since it will be placed under enormous stress shortly by the bridge decking. Temporary metal bands are used initially, but permanent cable bands are the long-term solution.
These heavy steel clamps are designed to fit the compacted cable exactly, and their compression of the cable is precisely controlled through the use of precision bolts.
These bands will also serve the function of supporting the stringers which will suspend the deck sections. The cable bands are placed precisely 40 feet apart at the level of the deck (the deck sections are 40 feet long)–which seems simple enough, except that due to the variable arc of the cables, the calculations for precise placement are fairly tricky. Once the bands have been placed, the stringers are simply draped across the top of the cable bands in notches designed for them. The length of the stringers must also take into account the anticipated stretch of the cable which will occur when the massive weight of the deck is attached.
Several of the more interesting and events on the bridge were not exactly in the original engineering plan. The most significant of these was an accident involving a mobile crane, which was being used to remove some of the equipment from the caissons below the existing bridge. This sort of work is typically done late at night when existing bridge traffic is very light and lane closures cause relatively little disruption. The crane had completed its job of lifting the machinery from the caisson 250 feet below, and was preparing to move off the bridge. For reasons as yet not disclosed, the crane operator made a small sequential error in judgment: he lifted his outriggers before returning the crane to its neutral position, while the crane was still extended over the side of the bridge. For those of you who are crane operators, this is a decidedly bad idea, as the outriggers are there for the purpose of preventing the crane from tipping over. A tipping crane is obviously somewhat dangerous, and hugely embarrassing to the operator (who was not injured)–but it is particularly dicey when the crane is extended out over the side of the bridge.
This small judgment error resulted in the crane tipping on its side, the crane boom crashing through the existing steel railing of the bridge, and the entire rig teetering on its side 250 feet above the Narrows. This is widely considered to be a bad career move. It is also rather horrendous for morning rush hour which started several hours later. The task of righting a crane is tricky enough–but righting one on a bridge, high over the Narrows, is a job definitely not for the faint of heart. In order to right the crane, it is necessary to place another crane’s hook on the boom itself–which now hangs far out over the water. So, who you gonna call? Well, you might want to call the guys who are comfortable hanging around hundreds of feet above the water on skinny cables: the bridge iron workers. One such hero climbed out on the boom to attach the rescue cables.
If your idea of scary heights is your second floor window, this is not the job for you.
Ultimately the crane was brought upright, but the existing bridge had to be closed for about six hours–which was an absolute disaster for traffic for the rest of the day. Unfortunately, I went across the bridge before the accident, and therefore had no excuse to not make it to work. There is no justice in the world.
The next brief flurry of excitement, about a month later, was when a suicidal, mentally disturbed gentlemen decided to take a little stroll up the catwalks to the top of the West Tower — at which point he threatened to jump, while holding a knife to keep his potential rescuers at bay. Fortunately, his determination to jump waned rapidly as he glanced downward from the 550 foot high pedestal. He was safely brought under control by several of the iron workers, and escorted by police to his appointment with the mental health specialists.
Meanwhile, in a land far far away, the bridge sections were being constructed to extremely precise dimensions, required for precision fitting of the deck sections.
This construction was performed in South Korea–raising the question of how, exactly, do you get these bad boys to the Tacoma Narrows? The answer, as you might suspect, is by boat: and quite a ship it is. These specially-designed ships are partially submersible, specifically engineered for tall loads, will be used to transport the bridge sections to the Tacoma area, on three separate shipments.
The first of these has arrived, and is now in place near the West Tower. This maneuver also provided some unexpected embarrassment. Rooms full of engineers sat down with their slide rules (do they use slide rules anymore?) and carefully calculated the height of the decking, the height of the ship, in the height of the existing bridge above the water. Evidently, at least some of these engineers had been trained in the Washington public school system, and had some difficulty with basic addition (rumor has it the WASL standards were lowered to allow them to graduate–but I don’t believe it). When the ship was brought by tugboat through the Narrows, the decking was too tall to fit under the existing bridge deck–and the ship had to be towed back to Commencement Bay.
After a several remedial math classes, combined with some additional ballast in the submersible ship, a maritime Mulligan successfully hit the fairway, and the ship is now moored safely on the south side of the existing bridge, adjacent to the west tower of the new bridge. From here, the bridge sections will be lifted either directly, or placed onto barges, for placement in the new bridge deck.
This is where things begin to get very interesting, and the placement of the bridge sections is about to begin within the next one or two weeks. The next post will detail how this process is undertaken.
Many photographs above courtesy of the Tacoma Narrows Construction Company and the Tacoma News Tribune.