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Creating a New Motor City Gateway: Complex
Geometry
Detroit's arched bridges were erected
using alternative methods that eliminated
the need for expensive shoring
By Sheila Bacon
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Streamlined engineering
made the erection of these landmark Detroit-area bridges
easier and less expensive than originally anticipated.
Photo by Tim Hunter |
The interstate 94 gateway Bridges near Detroit-with their
soaring blue arches and football-shaped braces-are a study
in complexity. Their design and erection, although simplified
through a number of creative approaches to reduce construction
costs, had to minimize disruptions to traffic and meet a deadline
that could not be extended.
The bridges are located in Taylor, at the east of the Detroit
Metropolitan Wayne County Airport and west of downtown Detroit.
The $20-million project is part of a $75-million reconstruction
job that is creating a new alignment along 4.2 miles of I-94
through Taylor.
The twin tied-arch bridges-each supporting a 240-ft-long
deck span designed to carry four lanes of traffic-were designed
by Lansing-based Alfred Benesch & Co. for the Michigan
Dept. of Transportation. AGC of Detroit member Ruby+Associates,
Farmington Hills, Mich., was brought on board by subcontractor
C.A. Hull Co., Walled Lake, Mich., to do construction engineering.
Ruby proposed an alternate construction method that eliminated
the need for massive and expensive shoring. They also designed
a set of lifting devices that would satisfy strict design
criteria by lifting each of the 12 arch sections into place
without any bolts or welds.
As originally designed, temporary shoring towers were to
support not only the bridge deck steel, but also the formwork,
rebar, screed machines and deck concrete. The towers, as specified,
would have had to support 120,000 lb while maintaining an
installation tolerance of plus-or-minus 1?8 in. for the deck
elevation.
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The Gateway Bridges
serve as a welcoming icon for visitors coming to Detroit
from the Detroit Metropolitan Wayne County Airport.
Photo by C.A. Hull |
The available window below the deck steel for the spandrel
system over heavily traveled Telegraph Road was approximately
30 in., says Brian Volpe, Ruby + Associates' project engineer.
Creating a spandrel system that was stiff enough to resist
240,000 lb of vertical load over the road while limiting the
deflection to 1?8 in. would not have been cost effective.
The Ruby team came up with an alternate plan that reduced
the weight on each shore to just 32,000 lb and minimized tight
tolerances during installation. Instead, only the bridge steel
would be supported by the shoring towers while the remaining
bulk of the span's weight would be shifted to the bridge's
arches through the tensioning of the cable strands.
By using the arch to support the dead load during construction,
crews were able to use a less expensive, rented shoring system
instead of larger, specially fabricated shoring towers.
Had the deck been installed on heavy shoring as specified,
the foundations would have likely been driven with pile caps,
which would have been required to reduce foundation settling.
These foundations would then have to be removed to accommodate
final Telegraph Road paving.
The complicated nature of the heavy shoring system also would
have made the maintenance of continuous traffic along Telegraph
Road more difficult. The smaller shores required only timber
mats at the base, and since they were easily removed once
the load was transferred, they could be immediately reused
for construction of the second bridge.
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Erecting the bridge
arches was complicated, requiring specially designed hitches
to lift and tilt the steel 25° inward.
Photo by C.A. Hull |
Additional efficiencies were realized during the pick, lift
and placement of the arch system. Each of the four arches
(two on each bridge structure) has three members for a total
of 12 sections. Each section was built on its side, lifted
into place, then rotated 25° into its final orientation.
The complex geometry of the arch sections required specially
designed hitch fixtures to securely grasp the sections and
also tilt the pieces inward once in place. This exercise was
complicated by strict design requirements that did not allow
any penetrations or welds to the arches.
"The geometrics are this job's largest challenge,"
Volpe says. "Everything's at a 25-degree slant. There's
not a straight line on the bridge."
The three hitches-two outside devices designed to lift the
sections and a middle "lever arm" to rotate the
arch-never directly touched the steel. Half-inch strips of
polyurethane material were placed between the hitches and
the arch sections. The polyurethane provided padding and friction
resistance so that no bolts or welds were required to be attached
to the arch segments.
The friction connection to the arch segments was made with
a series of high-strength rods that clamped the fixtures to
the arch. Specially designed tower jacking heads were also
implemented to support the arches during construction.
In an attempt to minimize the amount of fabrication, the
system was designed and located so the same tower heads and
rental towers could be reused for both bridge installations.
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The project team simplified
the shoring beneath the bridge decks, eliminating the
need for a complex support system. Only the bridge steel
was supported by the shoring towers, while the remaining
bulk of the span's weight was shifted to the arches through
cable tensioning.
Photo by C.A. Hull |
The two-way jacking heads accommodate adjustability of plus-or-minus
3 in. so they could be accurately located to ensure a precise
fit once the arch segments were moved into place.
The intricacies of the Gateway Bridges' construction kept
steel erectors on their toes. "It was complex enough
that it was enjoyable," says Jim Whaley, vice president
of Whaley Steel Corp., Mio, Mich., the project's steel erection
subcontractor. "We do a lot of stuff that's square and
mundane. This was fun."
Construction of the second bridge took just half as long
to build as the first once the kinks were ironed out and processes
improved, says C.A. Hull project manager Mike Malloure. "This
was the most complex bridge project I've ever managed, and
probably the most complex the company has ever been a part
of," he says.
The tied-arch design cost $2 million more than a conventional
plate-girder bridge and caused controversy locally. But planners
wanted Detroit to have a signature crossing for visitors and
the bridges had to be finished before last January's Super
Bowl. Design money was raised by local economic development
groups. The building team won the race and the Gateway Bridges
opened to traffic last fall, with plenty of time to welcome
fans to Super Bowl XL.
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