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Building a Suspended Roadway
Piecing together a suspended roadway represents the represents the fifth
and final phase of the new Tacoma Narrows Bridge project. The entire deck-lifting
process will take four months to complete.
The mile-long roadway is made up of 46 deck sections. The sections,
which were built in South Korea, will arrive in The Narrows on three separate
deliveries. The first of 16 sections arrived in Puget Sound in early June
2006.
The deck sections that average 120-by-78 feet and weigh 450 tons will
be lifted one at a time with “gantry cranes” (special lifting
hoists) that straddle the cables. Two different types of gantry cranes
will be used to hoist the bridge deck sections into place. On the side
spans, the gantries’ lifting mechanisms are winches located on the
caissons. In the mid-span area, the gantry cranes lift the sections using
strand jacks located on the gantries’ main girders. Each section
will be attached to vertical suspender cables, and then connected to the
main cables.
The first deck section will be placed in the center span location. The
main suspension cables will deflect (sag) 12 feet as a result of the first
deck section being lifted into place. This change will cause a rather
dramatic change in the cables’ geometry. The weight of the first
deck section will shape the cable into a "v" at midspan. It
will appear as a “v” until crews install sidespan deck sections
to act as counterweights. The remaining 45 blocks will be lifted in an
order that, to the casual observer, may appear almost random. That is
not the case. Rather, the non-linear order of lifting sections is designed
to maintain equal stresses on the bridge’s two towers. The precise
lifting sequence ultimately will pull the towers back to a plumb, vertical
position.
During tower construction, the bridge builder used large cables to pull
the tower tops about two feet toward shore. That was done so that during
the deck installation phase, the weight of the sections would pull the
towers toward one another and back to a perfectly vertical alignment.
The weight of all 46 sections will pull and lower the main cables about
25 feet below its present and non-weighted profile. This stretching process
will place the new bridge deck at the same level as the 1950 bridge deck.
As construction gets underway in July 2006, passersby will notice crews
simultaneously building the deck out from the east and west anchorages.
In the initial assembly of the sections, the deck pieces will be joined
temporarily with shear lugs and bolting fixtures. When the majority of
deck sections are in place and the cables near their final profile, the
crews will weld the deck plates and bolt all other truss sections together.
Like the new towers, the new bridge deck is different from the 1950 span. Instead of a lightweight
concrete deck surface, the new deck will be composed of lighter steel
plates with beams running lengthwise and crosswise. (Engineers call the
surface an “orthotropic” deck.) It’s a type of deck
that enables a suspension bridge to perform better under different loads,
temperatures, winds and earthquake conditions.
The new bridge deck will have only two expansion joints - one on the
east side and one on the west side. Each will allow up to 30 inches of
bridge deck movement. That movement can be caused by thermal (temperature)
changes, seismic events, wind, and even forces generated by vehicles braking
and accelerating on the bridge. That configuration is in contrast to the
existing bridge deck, which has large expansion joints at each tower and
smaller expansion joints every 120 feet along the length of the deck.
Deck lifting and assembly of all sections is scheduled to be complete
in early 2007. Once the deck has been bolted and welded together to form
a seamless roadway, the surface will be topped by two inches of asphalt.
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The new bridge deck will be continuous (meaning individual
sections will be bolted and welded together to make one piece), with
expansion joints located only at the anchorages. |
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The new deck will be 5,400 feet in total length, 2,800
feet in the main span. The average deck section weighs 450 tons and
measures 120 feet long, 78 feet wide and 30 feet deep. |
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On the new bridge deck, there will be four lanes of
traffic, two shoulders, and a 10-foot bicycle/pedestrian path separated
by a barrier. |
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The bridge is designed for a second deck (either road
or light rail) to be added in the future. |
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