By: Allen Zeyher
As anyone with a backyard clothes line knows, when you remove
one piece of laundry, the line springs up and reshapes itself. Cutting a
section of deck, including stiffening truss, out of a suspension bridge
produces a similar rebound, but with a suspension bridge the rebound can be
catastrophic. The weight of the bridge pulls and twists in unusual directions.
That was just one of the engineering challenges to
reconstructing the Lions' Gate Suspension Bridge in Vancouver, British
Columbia.
One of the other challenges was the fact that the bridge had
to be open to traffic every day by 6 a.m. The workers had to cut out a section
of the old bridge, lower it to a barge waiting on the water below, haul up a
new section and bolt it in place in a 10-hour window starting at 8 p.m. The
reconstruction work was done overnight and on a few weekends between Friday at
10 p.m. and Monday at 6 a.m.
Lions' Gate is a vital link between Vancouver and the
North Shore of Burrard Inlet. Built in 1938 as a two-lane bridge, Lions'
Gate was later rearranged to create a third--reversibl--lane down
the middle. The bridge now carries 70,000 cars a day. Trucks are not allowed.
The Lions' Gate reconstruction was the first time the
entire deck of a suspension bridge, plus suspenders, sidewalks and stiffening
trusses, was replaced while the bridge was kept open to traffic. The only
components of the original structure that remain are the towers, the main
suspension cables and the main cable anchorages.
The Engineers' Society of Western Pennsylvania, in
association with Roads & Bridges, awarded the Lions' Gate
reconstruction the George S. Richardson Medal, presented at the International
Bridge Conference in Pittsburgh on June 10. The Richardson Medal is given for a
single, recent, outstanding achievement in bridge engineering. The winning project
must have a significant improvement or advancement in bridge technology.
Accepting the award was Geoff Freer, regional director of
the northern region for the British Columbia Ministry of Transportation.
The original structure was built as a toll bridge by the
Guinness family, known for its beer, and it was built for economy, not
longevity. The family owned about 4,000 acres of land across from Vancouver
that they wanted to develop, and they had to build a bridge to get there.
"The bridge had to finance itself," Darryl
Matson told Roads & Bridges. Matson was the project manager for the design,
developed by Buckland & Taylor Ltd., North Vancouver, and the owner's
bridge engineer and representative during construction. "As a result of that,
everything to do with the bridge was driven by the fact that it had to be as
cheap as it could be."
Going to rust
The old structure lacked a means
for channeling water safely off the bridge: "The water just runs right
off the deck of the bridge, and it falls right on the structural elements
below," Ronald W. Crockett, contractor's representative for
American Bridge Co., Pittsburgh, told Roads & Bridges. "Over the
years, it's caused a tremendous amount of corrosion. It mainly has to do
with the drainage systems were not designed to modern standards."
Corrosion was the main reason
the bridge had to be replaced.
In 1955, the private owners had
recovered their investment, and they sold the bridge to the government of
British Columbia.
The reconstruction cost $105.6
million. The total price, with engineering studies, administration and extras,
came to $125 million.
The owner is the British
Columbia Transportation Financing Authority. The designer of the replacement
structure was Buckland & Taylor. The contractor was American Bridge/Surespan,
a joint venture of the Canadian subsidiary of American Bridge and Surespan
General Contractors Corp., West Vancouver.
Engineering for night owls
Because most of the
reconstruction had to take place during 10-hour shifts at night, the
constructors had to make sure the operation went smoothly.
Every move of the erection
procedure was planned and tested in computer engineering simulations. Because
each section of the deck of a suspension bridge is subject to different forces,
each section required its own, unique simulation.
"For each of 54 sections
that were removed, we had to do a completely different set of analysis, come up
with completely different sets of adjustments and different instructions for
the contractor to follow," Seth Condell, an engineer for Parsons, told
Roads & Bridges. "In the end, it took us well over 2,000 computer
models to complete this project."
Parsons, New York, was hired to
work with American Bridge to produce a plan for the reconstruction and to
perform an erection analysis. Parsons Brinckerhoff Inc., New York, an unrelated
company, was hired to perform an independent check of the erection analysis.
"We worked very closely
with American Bridge to develop the method," John Clenance, a senior
engineer at Parsons, told Roads & Bridges. "We worked on the
analytical end. They worked on the field procedure end."
The solution for getting one
section out and another in was to build what the constructors called a
"jacking traveler." The jacking traveler was a platform that
attached directly to the suspenders of the bridge and acted as a crane for
lowering the old section and raising the new one into place. Then it detached,
rolled along the bridge to the next section and reattached. The jacking
traveler was equipped with four strand jacks, with a capacity of 60 tons each,
to provide the lifting force.
The main span of Lions'
Gate is 472 m, with a total length of 847 m, including the north and south side
spans.
The first deck section was
replaced during the weekend of Sept. 9-10, 2000, and the last section during
the weekend of Sept. 29-30, 2001.
American Bridge/Surespan
replaced 47 sections of the bridge in order from north to south, each about 20
m long and weighing 106 tonnes (1 tonne equals 1,000 kg). The south side span
was replaced in 10-m pieces for a total of 54 sections.
Balancing act
The original deck was a very
light, steel T-grid.
To keep the weight of the new
bridge approximately the same as that of the old bridge, the engineers decided
to use a steel orthotropic deck, a steel plate with longitudinal troughs,
transverse floor beams and longitudinal stiffening trusses.
"An orthotropic deck is
typically very light, so the use of an orthotropic deck helps immensely in
terms of the weight," Matson said. "The new trusses couldn't
be what you would normally make them out of, which is built-up plates or
I-sections. To get the capacity out of them, we had to go with tubular
sections."
Also to conserve weight, the
engineers decided to move the trusses from sitting on top of the deck to
underneath. By moving the trusses, they could make the deck serve triple
duty--as the deck itself, as the top chord of the trusses and as the top
flange of the floor beams.
The weight of the new bridge
sections was almost the same as the weight of the old sections. The new
sections were actually a little lighter because they lacked the permanent
pavement wearing course, which could only be placed after the entire bridge was
reconstructed.
"If you're replacing
something on a suspension bridge with something that's heavier, the shape
of the bridge will change," explained Matson. "If I replace one
side span with something that is heavier, that will pull up the main span and
lower the side span I just replaced. As you progress into the main span
what'll happen is half of the bridge wants to go down, half of it wants
to come up, and you can't pull the two of them together without inducing
a lot of stresses in the truss."
To make the old bridge line up
with the new bridge during construction so traffic could run on it, the
engineers came up with several ideas.
One was to attach adjustable
extensions to the suspenders.
The extensions were necessary.
The suspenders attached to the top of the old stiffening trusses. But the new
stiffening trusses were underneath the deck, so extensions had to be attached
to the suspenders to reach to the new deck. Later, the suspenders and
adjustable extenders were replaced with new suspenders.
Putting the trusses under the
deck was one reason it was possible to widen the traffic lanes and sidewalks,
put the sidewalks outside the suspenders and give the bridge a more comfortable
feel.
Claustrophobia-free zone
The driving lanes on
Lions' Gate are now 3.6 m each instead of 2.9 m.
The reconstructed bridge is 35%
wider and more open than the original.
The sidewalks are now 2 m wide
instead of 1.3 m and separated from the traffic by a barrier and the
suspenders. Pedestrians have better views from the bridge. They do not have the
cars whizzing past right next to them. And they do not feel boxed in by the
trusses.
Another advantage of placing the
trusses underneath and making them integral with the deck is that the bridge is
now much more stable in a wind.
The original trusses and deck
were separate elements, and they lacked torsional stiffness. On the new bridge,
the trusses and deck--with a layer of bracing between the bottom
chords--form a closed box, a stiffer structure.
The original bridge had a
critical wind speed of 35 m/s. The new bridge's critical speed is 70 m/s.
The reconstruction is almost
complete. In July, all the work was done except for the epoxy asphalt wearing
surface.
To prevent future corrosion,
Lions' Gate now has a modern water runoff system, with channels that lead
to drain pipes that drop the water below the level of the bottom of the bridge.
"We took a lot of care to
funnel off the water and to get rid of it and not have it splash on the
structural steel," said Matson.
