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Transportation
Suspension Span Defies Tradition
(enr.com 12/24/01)
By David B. Rosenbaum
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DISTINCTIVE New Carquinez Bridge
(top) next to older pair, has slender concrete towers
and slim box girder (above). (Photo and graphic courtesy
of Parsons Inc.)
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Commuters heading east from San Francisco
on Interstate 80 might not find the nation's first new suspension
bridge in a generation comparable to the Golden Gate Bridge in terms
of remarkable span length, tower heights or Art Deco details. But
the new Carquinez Bridge will sport its own distinctive features.
Under a $188-million contract bid nearly
two years ago–at the time the largest ever let by the California
Dept. of Transportation–the new bridge has concrete instead
of steel towers, which saved at least 50% on construction. With
both 120-m-tall towers erected, 60% of the contract dollars spent
and 42% of the schedule used up, the bridge is rising in contrast
to two nearby crossings.
It will stand out even more after cable spinning
begins next November, followed in April 2003 with delivery of 24
prefabricated segments to be lifted and assembled into an unusual
1,056-m-long four-lane superstructure–the first orthotropic
steel box girder for a major U.S. suspension bridge. Fabricating
12.7 million kilograms of steel for the $30-million box girder in
Japan saved 30% off the budget, the construction team estimates.
The contractor, a 70:30 joint venture of FCI Constructors Inc. and
Cleveland Bridge California Inc., expects to save another $1 million
by using strand jacks instead of a 1,000-ton gantry to lift the
50-m-long segments.
Scheduled to open by November 2003, the new
bridge will parallel two spans–one built in 1927 and slated
for demolition, the other built in 1958 and newly retrofitted for
earthquakes. Both older spans hang from steel trusses and three
towers; the middle towers sit in the shipping channel of the Sacramento
River 20 miles east of San Francisco. Caltrans had considered a
third tower for the new bridge as well, to support a cantilever
truss, concrete arch or cable stays. But it wanted to minimize shipping
risks and foundation costs. For economy's sake, the two-tower decision
dictated the design of a 728-m-long main span for the first major
U.S. suspension bridge since the Chesapeake Bay Bridge in 1973.
The new Carquinez Bridge incorporates advancements
in suspension bridges made since then in Europe and Asia, says Thomas
Spoth, the bridge's design manager in the New York City office of
Parsons Transportation Group Inc. For one, the design features a
shallow orthotropic steel box girder, just 3 m deep and trapezoidal
in cross section. Nosings at either side of the 29-m width allow
wind to flow over, so the girder remains flutterless at wind speeds
of less than 260 km/hr. By comparison, a wind just one-quarter as
strong collapsed the first Tacoma Narrows Bridge in 1940.
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REPLACEMENT Rendering shows new
bridge with older of adjacent two removed. (Photo courtesy
of Parsons Inc.)
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Most suspension bridges contain open trusses.
A proposal for a new Tacoma Narrows Bridge, for which Spoth also
managed the design, calls for a truss to allow for a future lower
deck. But using a truss requires heavier cables to support the extra
weight of floor framing. With a lightweight box girder, a design
team that includes OPAC Consulting Engineers, San Francisco, held
down the Carquinez Bridge's suspension cables to a diameter of 512
mm. In a first for a U.S. suspension bridge, Spoth says, the design
makes specific allowances for the loss of any one of the vertical
ropes that support the superstructure from the main cables.
According to Spoth, using large-diameter drilled
shaft foundations for the towers instead of concrete caissons is
also rarely seen in U.S. bridges. In a quake, massive caissons tend
to rock.
The 3-m-dia concrete shafts, with steel shells
about 45 m long, are socketed as much as an additional 35 m into
rock. Because rock under the south tower lacked the anticipated
strength, a $10-million change order and a 355-day extension of
the original 1,200-day schedule was needed, says Curtis Weltz, project
manager for the FCI-Cleveland Bridge joint venture in Crockett,
Calif. The team started work in February 2000 and intends to avoid
paying liquidated damages of $50,000 per day after the first 1,000
days, "so all the decisions are schedule driven," Weltz
says (ENR 1/31/00 p. 24).
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