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
- The Two Towers XI: Cable Banding
- The Two Towers XII: The Cranes
- The Two Towers XIII: Life on the Bridge
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.
The towers are completed, the cables strung, the gantry cranes are in place–now it’s time for the big show: building the bridge deck.
Two shipments of bridge deck sections (of three total) have arrived from South Korea, and are waiting on the deck of specialized transport ships. One of these is moored just under the west end of the new bridge; the other waits in the Port of Tacoma until the first has been relieved of its load.
Several weeks passed after the gantry cranes were constructed on the cables. They moved around a bit–and were seen with odd-looking orange bags hanging from their cables:
Just before the first section lift, their purpose became apparent: they were for testing the cranes under load. The bags–12 on each crane, holding 9000 gallons of water–cumulatively weight over 450 tons.
Man, I haven’t seen water bags that big since my plastic surgery rotation…
Since the heaviest deck section weighs about 600 tons–and is lifted by two gantries–this test far exceeded the weight loads required to lift the deck sections.
Having passed the weight test, it was time for the first section lift.
The gantry cranes can move along the cables–albeit very slowly–but cannot move under the load of a deck section. So the cranes can be used to lift the sections vertically, but cannot position them horizontally after they are lifted. The transport ship is far too large and cumbersome to move around under the cables, so the sections must be moved to more mobile barges. Special barges are used which have lateral thrusters to stabilize and fine-tune their position under the cables in the rapid, ever-changing currents of the Narrows. But there’s one more problem: how are you going to get the sections from the transport ship to the barge, since the cranes can’t move the sections horizontally?
The answer is rather ingenious and surprising: move the transport ship–laterally.
The transport ship is equipped with a dozen large winches along its sides and ends, just above the water line, which are attached to fixed anchors. The cranes position themselves over the transport, lift a section vertically–then the winches pull the transport ship to the side, allowing tugs to move the barge under the bridge section. This is quite a treat to watch–take a gander at the time-lapse video below, which recorded the lift of the first section off the ship and onto the barge:
Tacoma Narrows Bridge Deck Build Time Lapse from CurrentCom on Vimeo.
The first sections were then lifted to center span, where their uncompensated weight caused the cables to drop nearly 12 feet–resulting in a shape more like a V than a gentle parabola between the two towers.
The degree of this dip can be appreciated by following the curve formed by the free suspension cables hanging from the main cables. Prior to lifting the center deck sections, they formed a gentle parabola upwards toward mid span; after the deck sections are in place, they gradually slope downwards from the towers to mid-span.
The sections are held in place by the gantry cranes as workers prepare them to be connected to the suspension cables and to one another.
One might think that the deck sections would simply be placed end-to-end, starting at the anchors or mid-span. But instead, they are placed in seemingly random fashion–as is this section between the Gig Harbor anchor and the West tower.
This is done to balance the weight load on the cables, preventing excessive stretch or unequal tension which could cause alignment problems later. The engineering math involved in calculating these loads is exceedingly complex. While I know my readers are well-aware of the formulas used to calculate these stresses and the resulting length of the suspension stringers (and have already worked most of these formulas out in their heads), I have provided them here for the less enlightened–and for graduates of the Washington State public school system:
Well, that’s all for now–next time we’ll take a little swing on a trapeze…
