As the new eastern span of the Bay Bridge slowly
rises, commuters in the region are getting a close-up view of one
of the world's most ambitious engineering projects: a 2-mile elevated
roadway that will combine art and infrastructure in a showcase of
cutting-edge technology.
The goal is to create a new bridge by 2009 that will
be as aesthetically spectacular as it is functional. The span will
be graceful, even beautiful, and provide stunning, uncluttered views
of Yerba Buena Island and portions of San Francisco. At the same
time, its builders say, the structure will be seismically safe enough
to resist a catastrophic earthquake.
Caltrans, the state agency responsible for replacing
the aging and unnervingly rickety eastern span, is employing engineering
methods never attempted before in a seismic zone -- most specifically,
the construction of a self-anchored suspension span near Yerba Buena
Island. This element of the new bridge has engendered criticism
from some engineers, who believe that such designs are unsafe in
large quakes.
Caltrans insists that the structure -- visible in
the form of huge cranes involved in pier construction -- will be
far superior in every way to the existing bridge. Still, the concerns
linger, underscored by a 6.5 temblor that wreaked havoc on the Central
Coast on Dec. 22.
As the debate continues, Bay Area residents will witness
a project that harkens to the mid-20th century's construction boom
of freeways, bridges and dams.
Much has changed since that era, at least from an
engineer's perspective. Materials are stronger and construction
techniques more sophisticated, allowing engineers far more leeway
in creating a signature span designed to endure a major quake.
As for the old Bay Bridge, geologists and structural
engineers don't like to talk about it, simply because there's nothing
good to say.
"If we knew then what we know now," said
Peter Siegenthaler, Caltrans' principal construction manager for
the project, "we certainly would've built the bridge in a different
fashion. And maybe we wouldn't have built it at all."
Opened for traffic in 1936, the Bay Bridge is about
4.5 miles long, consisting of two sections separated by tunnels
on Yerba Buena Island.
The western suspension span, which begins in San Francisco,
is similar in design to the Golden Gate Bridge, using huge cables
anchored at each landfall to support the traffic lanes.
The eastern span, which extends from Yerba Buena Island
to Oakland, is a truss and cantilever span, basically a framework
of steel girders supporting a roadway.
The roadways on both bridge sections are piggybacked:
Westward traffic uses the top deck, and eastbound uses the bottom
deck.
The suspension span on the San Francisco side is considered
relatively safe from a seismic perspective, but the eastern span
is another matter.
In simplest terms, the eastern span's rigid structure
is utterly unsuitable for seismic zones, as was demonstrated during
the Loma Prieta quake of 1989, when a large section of roadway collapsed
from the upper to lower decks, resulting in one motorist's death.
But even worse, engineers say, are the span's inadequate
foundations. Experts believe that any major quake with an epicenter
in the vicinity of the eastern span would collapse the structure
like a tinker toy construction.
The new design for the eastern span consists of a
long skyway -- an elevated highway -- joined to a self-anchored
suspension span near Yerba Buena Island. Roadways are side by side,
providing clear, unobstructed views of the bay and adjoining cityscapes.
The new bridge will present an especially dramatic
configuration at its western approach.
The transition structure -- the roadway that connects
the Yerba Buena tunnels to the suspension span -- will seem to "leap
from the island," Siegenthaler said.
"During that phase of construction,'' he said,
"we're going to build a temporary double-deck span to keep
traffic moving through the tunnels."
The new bridge will feature a curve from the north
to the Yerba Buena tunnels that is less acute, and hence represents
less structural stress, than the bridge's current angle, Siegenthaler
said.
When the new span is complete, the old bridge will
be dismantled.
As most things do in construction projects, the new
span starts with the foundation -- the work under way and visible
to motorists.
The existing east span rests on 60- to 70-foot-long
wood pilings that are essentially suspended in the deep strata of
mud and silt that constitutes the bay's bottom. The pilings aren't
anchored to bedrock, which is why the current structure would be
unlikely to fare well in a huge quake.
The skyway portion of the new bridge, by contrast,
will sit on 300-foot- long steel pilings driven into the rock underlying
the sediment strata.
Right now, giant cranes are straddling recently constructed
dams -- supermarket-size structures built around the new bridge's
column sites. At high tide, the cranes hook on to the 1,600-ton
steel boxes that will secure each column's pilings, lowering them
through the water to the bottom of the bay.
Each box has holes for six pilings. A piling template
-- a tower that guides the pilings at the correct angles as they
are pounded in -- is placed over the dam. "Somebody's got to
hold the nail while you hit it with the hammer," said Dan McElhinney,
the chief deputy district director for Caltrans.
Water is then removed from the dam, and the pilings
are driven into the rock with some of the world's biggest pile hammers,
each capable of 1.2 million foot-pounds of force.
"That means the force they generate is equivalent
to dropping 1.2 million pounds 1 foot," Siegenthaler said.
The pilings are angled away from each other. The rationale
for this strategy can be better understood, Siegenthaler said, when
one contemplates a standing human being. "If you stand with
your feet together, you have very little stability," he said.
"Stand with them apart, and your stability is much greater."
After the pilings are reinforced with steel and concrete,
a column will be erected from each footing. At the top of each column,
huge cranes will place precast concrete segments of roadway. A total
of 452 segments will be used for the skyway, each weighing up to
780 tons. Cast in Stockton and barged to the site, they are actually
fairly light for chunks of concrete the size of office buildings.
The east span marks the first time such precast concrete
techniques have been used in a seismic zone, said Caltrans engineers.
"The technology is very good for controlling
the weight on precast units like this," McElhinney said.
The precast segments will be connected by cables and
a special epoxy. Special hinge pipe joints at either end are designed
to endure major seismic stress, as are additional smaller expansion
joints employed at selected points in between.
"In an active earthquake area, you don't want
a bridge (as long as the skyway) to be a single solid piece,"
Siegenthaler said. "In a quake, you want it to move -- but
move safely. These hinges allow ... movement of up to a meter. The
pipes are designed to deform, but not break. Later, we can come
back in and replace them."
As work progresses, motorists will have plenty of
opportunity to see those working-class heroes, high ironworkers,
plying their dangerous trade.
But while their labor may be visually riveting, don't
expect them to use rivets.
The traditional and picturesque method of fixing steel
to steel by pounding red-hot metal plugs through drilled holes with
a pneumatic hammer is as outmoded as pearl-buttoned spats -- at
least where seismically safe structures are concerned.
"Rivets wiggle," Siegenthaler said. "During
a quake, you want the entire bridge to move in a calculated way.
Rivets interfere with that. Now we use high-strength bolts. They
clamp -- they don't slip."
The most controversial part of the bridge is also
its signature element: the suspension segment, with its 525-foot
tower -- slightly higher than the tallest towers of the west span.
The new suspension span -- unlike the Golden Gate
Bridge and the west span of the Bay Bridge -- is self-anchored,
meaning it is not fixed to landfalls. Instead, the middle of the
main cable is threaded underneath the west end of the span with
the two ends draped across the top of the tower before descending
to the east end of the span, where they are anchored in the concrete
decks.
The aesthetic effect will be stunning -- a kind of
webbed drape that frames the bay and its environs for motorists.
The tower boasts some impressive engineering chops.
It is actually four towers, bound together by beams designed to
move and deform -- but not break -- in a quake.
"If you had a single tower, the stress load would
be mainly translated to the base, which can't be repaired,"
McElhinney said. "Here, the load is largely carried by the
beams, which we can easily repair or replace."
The tower will be the tallest self-anchored suspension
support in the world. And that has led to criticism that continues
apace with the construction.
For example, Abolhassan Astaneh, a professor of structural
engineering at UC Berkeley, said the span would be especially vulnerable
to seismic motion or terrorist attack.
"Anchored suspension bridges with an even number
of towers are the safest design in the world," Astaneh said.
"The towers brace each other and dampen motion during seismic
events." But single-tower suspension bridges, said Astaneh,
are inherently unstable.
"Basically, you're going from the best possible
design to the worst," Astaneh said. "The instability (of
single-tower bridges) is increased when you make them self-supporting.
The main cable is connected to the deck, not the ground. It's like
anchoring a ship to itself, not to the sea bottom. The whole roadway
is under such tremendous compression that any extra stress -- like
a quake or explosive charge -- could make it collapse completely."
Astaneh supported an alternative design for the bridge,
which the state rejected. Some have suggested that his opposition
to the current design is sour grapes -- a charge that makes him
bristle.
"We're not high school kids," he said. "This
isn't about envy. I'm strictly concerned about safety. At any given
time during the commute, 4,000 people could be on the suspension
span. Caltrans needs to get qualified people to look at this and
fix it while there's still time."
McElhinney, who has heard Astaneh's arguments before
and discounts them, said the technology for self-anchored suspensions
is established and sound. "This isn't the first of its kind,"
he said. "The engineering is well understood."
Self-anchored suspension designs were popular in Germany
for small bridges during World War II because they were relatively
cheap and easy to construct. However, Astaneh said, they were unstable,
sometimes collapsing from ground tremors caused by nearby bombing.
Self-anchored suspension bridges remain in wide use throughout the
world, but most are relatively small -- certainly not on a scale
with the new Bay Bridge.
As to the span's vulnerability to terrorist attack,
McElhinney said, "We took another look at it after 9/11, and
we included a peer group and military advisers in our review. We're
convinced the structure could sustain an attack (such as car bombs)."
At the bridge's level of design and engineering, McElhinney
said, there are multiple safety redundancies built into the structure
-- the tower beams, the roadway pipe hinges, the special concrete
designed to withstand high compression. "That should protect
against seismic events and terrorism," he concluded.
E-mail Glen Martin at [email protected]
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