| The "Suspension" Part of
the Bridge
As compared to building concrete towers 17½ feet at a time, connecting
“land to sea” with steel wire is a fairly simple proposition.
Fabrication of the new bridge suspension system began in summer 2005.
In fact, spinning cable wire marks the fourth phase of new bridge construction.
In July 2005, on the heels of completing the 510-foot tall towers, bridge
workers hauled the first cable wires from the Gig Harbor and Tacoma anchorages
to the top of the towers. Connecting this 5/8-inch “pilot line”
– connecting land to sea – is the first phase in building
a suspension system. On two different days, a workboat on the shoreline
set out to meet a tugboat on the water. The line bound to the anchorage
was ferried across the water and joined with a same-size steel wire lowered
from the tower. When the skiff came alongside the tugboat, deckhands connected
the ends of each cable wire to a steel delta plate. The whole process
– dragging the wire from the anchorage to the beach, lowering a
second wire from a 510-foot tall tower, joining the two ends in the Narrows
and raising the pilot line it into position – took slightly more
than an hour.
Between August and early October, cable crews built two one-mile long
suspended walkways that climb from the anchorages on shore to the tower
tops, draping elegantly across the waterway. Bridge crews use the walkways
as work platforms. Made of steel wire mesh and wooden slats separated
one to two feet apart, the catwalk resembles something of a rustic footbridge.
Cable spinning officially got underway in mid-October when crews pulled
the first galvanized steel wire from the Tacoma anchorage over the towers
to the Gig Harbor anchorage and back again. The continuous (and spliced)
steel wire will make 2,204 roundtrips until crews have spun 19,000 miles
of cable wire.
Unless you’re a bridge engineer or observe the process firsthand,
cable spinning is difficult to visualize. Spinning wire is a simple mechanical
process, essentially unchanged since the 19th century: A spinning wheel
pulls off individual wires – the diameter of a No. 2 pencil –
from large spools at the Tacoma anchorage, and ferries the lines across
the water and back. As wires are pulled westward four at a time, each
one is laid parallel to the other. The wheel’s guide is a tramway
haul rope; when it moves, the spinning wheel follows. (There are two spinning
wheels, by the way, controlled from inside a dispense center at the east
anchorage.)
A total of 24 tram support frames (one every 220 feet across the bridge)
help align and place the wire as it is spun. When the first 464 wires
have made the journey, they become compressed into a single strand. A
total of 19 strands, each containing 464 wires, will be compacted and
wrapped to complete one of two suspension cables.
By early March 2006, crews had finished spinning and compacting the south
suspension cable; permanent steel bands where placed every 40 feet around
the 19 strands that comprise the "steel rope." The north main
cable followed suit in mid-April. In April, workers finished installing
264 pairs of suspender cables on both cables. These suspenders –
far sturdier than the kind your grandfather wore – will connect
the future deck to the pair of 20.5-inch steel ropes. The suspenders,
together with the main cable, will support the weight of the deck
and the traffic crossing the bridge.
See glossary for more bridge engineering terms.
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At top speed, the spinning wheels go 12 miles per hour. |
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Every strand of wire has a precise position
and is laid parallel to one another. |
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When completed, the two main
cables will each measure 20½ inches in diameter, be more than
a mile long and weigh six million pounds. |
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Individual cable strands – 19 per cable –
are splayed inside the anchorage to distribute the load of 25 million
pounds exerted on each 20½-inch main cable. |
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