| Better Bridges
Bridge Replaced in Sections — at
Night
The Lions Gate Bridge truss and deck
sections
were replaced during 10-hour night closures.
by Michael Abrahams, Joseph Tse, and John Bryson
 The bridge
stayed open to traffic during the day. Such replacement for a major
suspension bridge is believed to be unprecedented, especially where the
bridge remained in service except during the 10-hour night closures.
 The Lions
Gate Bridge in Vancouver, British Columbia, Canada is a 1,518-meter-long
bridge with a 473-meter-long suspended main span over the Burrard Inlet.
Reconstruction of the 45-year-old, two-lane bridge is a part of an
$80-million rehabilitation of the Lions Gate Crossing. The major
structural components include seismic retrofit of both the north approach
viaduct and the main span, widening of the approach span sidewalk, and
replacement of the entire suspended span deck including hangers.
Deck replacement was to be done without interruption to bridge traffic
during daytime hours; only nighttime closures lasting 10 hours were
allowed for operations that interrupt traffic.
In order to accomplish these modifications, the owner selected a rather
unique combination of the traditional design-bid-build with newer
design-build methods. The many engineering challenges combined with the
two methods of procurement make this project unique.
The seismic retrofit portion of this major project was probably the
first of its size to be carried out as a design-build undertaking. The
contractor, American Bridge-Surespan, a joint venture, retained an
engineering firm, Klohn-Crippen, to conduct a seismic analysis and
retrofit design and, as required by the contract, an independent
consultant firm, Parsons Brinckerhoff, to conduct an independent analysis
and design check.
The suspended span deck replacement was designed by N.D. Lea/Buckland
& Taylor and was included in the contract documents. However, other
than a limited erection analysis, which considered only dead and live
loads, it was left to the contractor to develop the deck replacement
methodology and erection equipment.
The erection analysis was conducted by one consultant, the Parsons
Transportation Group Major Bridge Division (PTG-Steinman), and was checked
with an independent analysis by a second consultant, Parsons Brinckerhoff.
All erection equipment required an independent check. In this case the
erection equipment was designed by American Bridge-Surespan engineering
staff with an independent design check by PB.
Seismic retrofit
The structure was retrofitted to a level of performance such that it
would remain open to traffic following a 475-year design level event.
The approaches are tall with piers up to 160 feet in height. They are
made up of steel plate girders (orthotropic deck) supported on slender
steel piers that use laced members for bracing elements. The piers are
supported by spread footing foundations.
There were three primary elements to the retrofit:
1. Build-up the existing H-bents so they can anchor the structure
longitudinally.
2. Allow piers to rock transversely.
3. Add piled foundations in areas where liquefaction potential is high.
There were two types of foundation retrofits. The first was
pile-supported reinforced concrete grade beams that tied the existing
spread footings together. This retrofit detail was used in zones of
potential liquefaction. The second type was comprised of reinforced
concrete grade beams without pile supports. The purpose of this grade beam
is mainly to tie the foundations together. These foundation designs
considered the effects of rocking, and some ductility was allowed in
foundation grade beams.
The piers were retrofitted to provide additional bracing for the
chevron bracing system to prevent buckling. Push-over analysis was used to
determine the sequence of events between yielding and failure, and to gain
a more complete understanding of the system behavior.
To capture the non-linear behavior of the structure rocking, an ADINA
model was developed to account for effects such as p-delta on the steel
piers and pounding as the rocking piers landed. The design limited the
pounding effects by incorporating a TADAS device at the base of the piers.
The non-linear behavior of this element was also included in the design
evaluation model.
The TADAS device was installed at the base of the tower legs to resist
and dampen rocking. This device uses a pinned/fixed plate to develop
resistance. The narrow end is pinned in the slotted keeper and the wide
end is bolted to the column leg. This results in predictable, ductile
behavior for the TADAS device. The device is also easy to fabricate and
easy to replace.
Seismic retrofitting was also required for the superstructure. Existing
superstructure cross-frames were not adequate to carry inertial loads into
the piers. A combination of local and global models was used to determine
the loads and load paths in the superstructure. Additional cross-frame
members were added at the pier locations in order to complete the load
paths needed to carry inertia loads from the superstructure into the
substructure.
Deck replacement
 The new
deck carries three 3.6-meter traffic lanes, and two 2.7-meter sidewalks.
The deck is of welded orthotropic steel-plate construction with stiffening
trusses utilizing welded rectangular hollow sections. The new stiffening
trusses are located below the deck and are 2.08-meters deep. The number
and location of the hangers that support the new deck remain the same as
the original deck. The new deck is 4.5% lighter than the existing deck in
the main span, and 3.1% heavier on the side spans; a significant design
consideration for the erection engineering.
The condition of the bridge prior to construction was poor for several
reasons. Traffic loads are substantially greater than originally intended.
Then, load changes and geometric changes resulted in a sag at mid span of
over 1 meter. Corrosion of the floor system had further reduced the
capacity of many of the stringers and floorbeams. The bridge required
ever-increasing load restrictions prior to reconstruction.
The challenge in this project was the very limited additional capacity
of the bridge to accommodate erection loads and deformations. The original
design of the bridge economized on every element to the maximum extent
possible, and at the time of its design, was only for two lanes of traffic
with no sidewalk. There was no consideration of seismic forces, nor were
analytical methods very accurate. Over time, the effects of section loss,
cable and suspender stretch, and cable slip further reduced the bridge’s
capacities.
The contract documents listed changes in the bridge geometry between
the original design and field survey data collected through 1998. The
contractor was required to conduct an independent survey to verify the
listed changes and provide updates as necessary. With all information
gathered, interpreted, and resolved, our computer models were adjusted to
account for these primary geometric corrections, namely, the suspension
cables had sagged around 1,120 mm at the mid-span, and the North and South
Towers had deflected 160 and 90 mm, respectively, towards mid-span.
The difference in the vertical alignment of the existing deck and the
new deck — the existing main span deck had an abrupt 8% grade change at
the center (believed to be associated with the need to reduce construction
costs). This break in grade was to be taken out with the installation of
the new deck. The computer models used by both PTG-Steinman and PB were
developed to account for this difference by systematic tracking of hanger
fabrication length and intermediate hanger adjustments for proper
alignment across the continuity link.
The existing structure had little reserve capacity for construction
loading and geometric effects. As a result, fine-tuning of the hangers,
especially around the work front, was necessary in each stage to keep
hanger loads within allowable limits. Likewise, the existing truss
diagonal members required reinforcement to avoid being overstressed. A
hydraulic spring was also necessary to prevent overstressing the chords of
the existing truss in case of high winds when replacing the main span
sections. This required 3-D computer models be developed with multi-linear
springs to simulate the hydraulic system. The flexibility introduced by
the continuity link in turn required measures to mitigate overstressing of
the chord members due to significant inclinations during construction in
the existing and new hangers.
Replacement
 After
evaluating alternatives, the contractor and PTG-Steinman settled on a
scheme that involved replacing the existing deck in sections of 19.66
meters in length for the north side span and the main span. The
19.66-meter sections were raised and lowered to, and from, the ground or
barges in the water. At the south side span, the steep slope and
environmentally sensitive nature of the park grounds below required the
deck be removed from above; sections of 9.83-meter length were
removed/delivered through the south end of the bridge.
The contractor custom designed and fabricated equipment. Essentially a
steel lifting frame, the jacking traveler advanced with the work front by
rolling along the deck on Hillman Rollers. In its operating mode, the
frame was attached to the hangers with the supporting legs retracted.
Strand jacks that were mounted on top of the frame were engaged to support
the weight of either the existing deck or the new deck during the various
stages/steps of the replacement operation. There were two versions of the
traveler — one for the north side span and main span, and a shorter
version that was used at the south side span.
When the bridge was reopened to traffic at the end of each replacement
stage, a continuity link was needed at the erection front to join the
existing and new deck sections together. The continuity link had two steel
components; each attached to one of the two bottom chords of the existing
truss. Each steel component housed an extendable/retractable key that
extended to engage its counterpart (called the link nose) attached to the
new deck. The key maintained vertical alignment across the work front
providing shear continuity and thus ensuring rideability between the new
and the existing deck sections.
The contractor provided a temporary deck section that could be deployed
rapidly to fill a gap in the deck in the event of unforeseen circumstances
such as a failure of the erection equipment or misfabrication of the new
deck section. The temporary deck was designed to support three lanes of
reduced traffic load and full design wind load.
The existing hangers were replaced by new and longer hangers attached
at the roadway level of the new deck. This replacement operation trailed
behind the deck replacement and removed itself from the critical path.
Before the new hangers could be installed, hanger extensions were required
to provide a link between the connections at the new deck and the shorter
existing hangers. The hanger extensions were detailed with adjustment
capabilities so that adjusting the hangers dispersed localized
overstresses in the hanger and truss members near the work front.
The bridge under construction was much more flexible than either the
existing structure, before reconstruction began, or the new structure,
when completed. As a result of detailed analyses, a significant number of
connections were found to require special details to accommodate the
rotations of the suspenders associated with the increased flexibility.
The bridge was vulnerable to high winds when the deck section was
discontinuous. The contractor installed fairings at the main span to
improve aerodynamic stability of the structure during reconstruction. It
was also established that the bridge could withstand a wind speed of 12
meters/second in its open condition. Therefore, night closures for deck
replacement were ordered only when the forecast wind gust speed was less
that 10 meters/second.
Typical replacement steps were:
1. Initial condition. The jacking traveler is rolled into position for
replacing a given deck section. The continuity link engages the new deck
to the existing deck. Traffic is running.
2. The jacking traveler is attached to the hangers and its weight is
transferred to the hangers as the supporting legs are retracted.
3. Traffic is stopped. The continuity link is disengaged and rolled
southward, clearing the target deck section for replacement.
4. The strand jacks take up most of the weight of the target deck
section. The existing truss hanger socket plates are removed. The existing
truss top chord, bottom chord, and diagonal are cut. The link nose is
removed from the target deck section and relocated to the end of the
remaining existing truss.
5. The section of existing truss is lowered to the ground or barge. The
continuity link nose is removed from the bridge and attached to the next
new deck section.
6. The next new deck section is hoisted into place and incorporated.
Temporary hanger extensions are installed and the strand jacks released.
7. The continuity link is moved northward as required to engage the
link nose. The continuity link traffic plates are installed. The new deck
splice traffic ramp is moved ahead. The hangers are adjusted to
predetermined forces/extensions that will accommodate all the loading
conditions expected until the next stage.
8. The bridge is reopened to traffic. The jacking traveler rail is
moved ahead and the jacking traveler legs are extended to engage the
Hillman Rollers. The jacking traveler is winched forward on the Hillman
Rollers. Various pieces of construction equipment are relocated ahead for
the next stage.
 The south
side span sections were replaced in 9.83-meter segments, which were half
the length of those for the north side and main spans. Removal of the
existing truss involved cutting the section free, lifting and rotating it
90 degrees, and lowering the section onto the back of a specially designed
transporter that was waiting on the deck at the work front. To accommodate
the heavier construction loading for these operations, both existing and
new hangers were adjusted at each stage to more favorably re-distribute
the loads near the work front. (On the north and main spans, only the new
hangers were adjusted.)
The Lions Gate Bridge is believed to be the first major suspension
bridge that was seismically retrofitted under a design/build format. It is
also believed to be the first major suspension bridge that had its deck
completely replaced while maintaining traffic.
In the case of the deck replacement, wind issues combined with the need
to maintain traffic is perhaps more complex than the construction of a new
suspension bridge. Compounding the wind issue is the fact that the
existing bridge lacks reserve capacities for construction loading and
deformations. Moreover, solutions to these engineering challenges were
provided under significant time constraints. The construction engineers
solved the engineering challenges by using sophisticated computer
software, old-fashioned hard work, and engineering judgment. The bridge
behaved close to the construction engineering teams’ prediction.
Michael Abrahams, P.E., Joseph Tse, P.E., and John
Bryson, P.E. represent Parsons
Brinckerhoff Quade & Douglas, Inc., New York, New York.
Reprinted from Better Roads Magazine
February 2003 |
