Previous posts in the New Narrows Bridge Series:
- The Two Towers I: Introduction
- The Two Towers II: History of the Narrows Bridges
- The Two Towers III: The Caissons
- The Two Towers IV: Anchor Management Class
- The Two Towers V: The Tour Begins
- The Two Towers VI: Struttin’ My Stuff
- The Two Towers VII: To the Top
- The Two Towers VIII: Stairway to Heaven
- The Two Towers IX: Spinning Beginning
- The Two Towers X: Wheels Over Water
- The Two Towers XII: Compacting the Cables
- The Two Towers XIII: Banding the Cables
- The Two Towers XIV: The Bridge Deck Cranes
- The Two Towers XV: Life on the Bridge
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 bags that big since my facelift…
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 by itself, 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, and follows with subsequent sections to (nearly) complete the deck build. Notice how they “walk” each section with the gantry cranes to move each section into its position:
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 catenary curve 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 the time lapse video above demonstrates.
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 our Washington State public school system:
Well, that’s all for now — next time we’ll take a little swing on a trapeze…
