|
Transportation
Williamsburg Bridge Rehab Epic Close to End
(newyork.construction.com,
October 2006 issue)
By Adrian
MacDonald
A multicontract journey for the rehabilitation
of the 103-year-old Williamsburg Bridge crossing the East
River is edging toward completion as work continues on the
eighth and final contract.
After more than two decades of reconstruction
work, the Williamsburg Bridge connecting Brooklyn and Manhattan
is finally nearing the day when it no longer will wear layers
of scaffolding.
Virtually every component of the 1903
structure, long neglected until the multiyear rehabilitation
effort begun in the late 1980s, has been replaced or rehabilitated.
"The bridge was built for the pre-traffic
era," said Hasan Ahmed, director of the city Department
of Transportation's bureau overseeing the East River bridges.
Now, the span is among the most heavily traveled in the city,
with 100,000 vehicles on eight traffic lanes and 110,000 passengers
each day on two subway lines.
While seldom spoken of in the reverent
tones reserved for the famous Brooklyn Bridge to the south,
the Williamsburg is structurally the strongest of the four
East River crossings that the city agency oversees, said Terry
Daly, senior vice president for Koch Skanska of Carteret,
N.J., the contractor on the eighth and final phase of the
rehabilitation effort. The city agency also maintains the
Manhattan, Brooklyn, and Queensboro bridges.
Despite the Williamsburg's robust structure,
it had fallen into disrepair over the years. There were even
designs for a replacement bridge before the state Department
of Transportation, which oversaw the bridge until about 1989,
let the first rehabilitation contract in 1987.
 The
state oversaw the first three contracts, before the city transportation
agency took over and issued four additional contracts in the
ensuing years that totaled $741 million in construction costs
and involved a rehabilitation of the cable suspension system;
reconstruction of roadways, piers, and columns; and replacement
of subway tracks, signals, and power systems.
Work on the $173 million final contract
began in early 2003 with Koch Skanska, a unit of Skanska USA
Civil of Whitestone, N.Y., tasked with strengthening the bridge's
main steel support structures.
While Koch Skanska achieved substantial
completion earlier this year, steel repairs, smaller seismic
upgrades, and reconfiguration of lanes and dividers to introduce
a new traffic flow system will keep the contractor busy for
another year.
The Williamsburg is unique among suspension
bridges worldwide because it has suspender cables only on
the main span between the towers. Its outer end spans independently
rely on supporting piers - an oddball design that has required
various fixes over the years, including work in the current
final contract. A more conventional design would use suspension
cables for the whole bridge.
The uniqueness lies in a miscalculation
by the original engineers, who chose to support the end spans
independently in an attempt to save money, Daly said.
But when the original bridge construction
finished, its designers realized the end spans weren't strong
enough. They added two more intermediate piers - forfeiting
the anticipated savings, Daly said.
"If you took away the rest of the
bridge, each end span would stand alone as a bridge unto itself,"
he added.
The final contract's extensive work
on those piers could have taken only eight months instead
of the planned 4.5-year schedule if the city had the luxury
of closing the bridge, Daly said. But the flow of vehicle
and train traffic has been maintained throughout.
"Anywhere outside New York City
it's cheaper to close the bridge," Ahmed said. "But
here you have the user cost. Closing the bridge just isn't
an option."
The contractors also didn't have the
luxury of time. Unlike previous contracts on the bridge that
offered bonuses for contractors to complete work ahead of
schedule when tasks involved all-day lane closures, the final
contract only involves offpeak closures - and thus no incentives.
But all contracts, including the current one, apply penalties
if the team does not hit project milestones on schedule.
"There were seven different liquidated
damage penalties in the contract and no incentive bonuses,"
Daly added.
Shoring Up with New Steel Work
The final contract has focused on the
steel support structures in the main towers, end-span piers,
and stiffening trusses. It also has miscellaneous work such
as adjusting and upgrading cables, painting trusses, and renovating
"comfort stations" on the pedestrian walkway.
Rehabilitating the main towers was a
primary chore. Over the course of a century of increasing
loads, stress on the tops of the towers - which support the
main suspension cable - had bent them inward toward the bridge
center, Daly said.
While it would have been cheaper to
replace the towers, that would have required closing the span,
Ahmed said. Instead, the engineering team devised a way to
strengthen the towers with steel plates.
The design placed the steel plates on
the river-facing side of the main towers to compensate for
the compression load. It also called for "pre-loading"
the steel by having the installers curve each plate outward
in the center by 4 to 6 in., then bolt them incrementally
from the outside toward the middle.
"That helps engage some of the
compression load earlier," Daly said.
Koch Skanska custom-cut every plate
for its location, using wood templates. To get the plates
into place, the team had a crane lift them in pairs weighing
about 3,000 lbs. to a hoist and delivery track system built
on the side of the towers. The plates had to match 21,000
existing rivet holes.
Another major feature was work on the
intermediate piers. The design team found it could completely
replace these piers without disrupting traffic by cautiously
removing only a few steel members at a time. However, the
bridge's unusual design required special attention to the
main bearings, which are located under the roadway inside
the main towers.
"Koch Skanska does a lot of bridge
bearing work, and this is definitely the most complicated
bearing we've ever worked on," Daly said.
The bearings and main river piers for
the end spans are located in the same towers that anchor the
suspension cables for the center span, Ahmed said.
To support the bridge structure during
the process of replacing the original bearings and river piers
with larger and stronger ones, ironworkers constructed a temporary
jacking tower around each pier inside the main towers. The
temporary tower allowed crews to jack up the bridge surface
just enough to "see daylight" and replace the bearing,
said Douglas Reese, resident engineer on the project with
Greenman-Pedersen of Babylon, N.Y.
"The drawings only give theoretical
load numbers for the bearings," he added. "But the
actual load differs. You measure as you go. We have gauges
on all the temporary jacks to measure pressure in the jack
and determine the actual load at the bearing location."
The new multirotational pot bearings
have sliding capability, which can counter seismic movement
in the bridge joints, Reese said.
Many Tasks Beyond Heavy Lifting
While ironwork formed the heavy part
of the final contract, the single biggest task in terms of
budget was a $50 million repainting effort on two stiffening
trusses, Daly said. The 2,800-ft. long steel support trusses
run the length of the bridge on either side with X-shaped
members.
Daily said the painting job required
sandblasting of existing lead paint and dust containment to
meet environmental regulations through a complex network of
tarps on scaffolding and a vacuum system that created negative
air pressure in the work area. The system had 11 compressors
with a combined capacity of 18,000 cu. ft. of air per minute.
The containment system also required
closing lanes on either side of the truss, cutting the bridge's
traffic capacity in half.
"The trusses are built tight to
the road," Daly said.
The city gave the contractor 176 days
to blast and repaint both trusses, with a liquidated damage
penalty of $50,000 for each day it fell behind schedule. The
team ran a 24-hour, 7-day-a-week schedule, and ultimately
finished nine days early.
Key Players
Owner: New
York City Department of Transportation
General Contractor:
Koch Skanska, Carteret, N.J.
Resident Engineer:
Greenman-Pedersen, Babylon, N.Y.
Designer:
Parsons Transportation Group, New York
Engineering
Consultant: Siefert Associates, Naugatuck, Conn.
Steel Fabricator:
Michelman-Cancelliere Iron Works, Bath, Pa.
Steel Erector:
Northeast Structural Steel, Yonkers, N.Y.
Electric Contractor:
Welsbach Electric, College Point, N.Y.
|
