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The total width of its deck was only 64 feet. A
unique feature of the bridge, which had significant aesthetic and
historical value, was its earth-filled concrete barrel arch approach
spans that were easily distinguished from the pile bents and rectilinear
piers of the other bridges in the area.
The replacement project, performed for the
Florida Department of Transportation, involved both the Project
Development and Environment study and design phases necessary to replace
the existing bridge. From its inception, the project was beset by
significant technical challenges, including:
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Increasing the width of the bridge to
accommodate current standards for lane and shoulder widths, providing
wider sidewalks, and adding bike lanes.
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Increasing the vertical and horizontal channel
clearance requirements of the channel span.
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Designing efficient foundations for a site
with soil properties not conducive to supporting heavy loads.
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Constructing a replacement bridge on the
existing bridge alignment while maintaining traffic flow.
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Developing an economical bridge concept that
would be acceptable to the local communities and the state historic
preservation officer.
Challenges
Environmental and social challenges were as
daunting as the technical ones. Both functionally and aesthetically, the
bridge was an important focal point for residents in the area. In
addition to providing a physical connection between the mainland and the
island roadway systems on either side of the AICW, also known as the
Lake Worth Lagoon, the bridge also carries sidewalks that connect linear
parks on both sides of the lagoon.
Royal Palm Way, the road approaching the bridge
from the east, is a scenic drive lined with massive royal palms that
immediately identify it as the main entry into the Town of Palm Beach.
The Royal Park Bridge also serves as an aesthetic and cultural conduit,
its architecture providing a visual connection between both sides. The
low profile of the bridge complements the adjacent low surroundings and
does not obstruct the views of public spaces and historic building
settings such as the Four Arts Plaza.
Given the bridge’s importance to the community
and its historic significance, a context sensitive solution was
developed with the full participation of the public, the Landmarks Commission, and SHPO.
Perhaps the most critical problem the design
team faced was the need to develop an innovative program to expedite the
project and keep traffic moving. Initially on a conventional schedule,
the project was thrust into high gear when inspection divers discovered
that the timber piles supporting the older half of the bridge had
deteriorated. The bridge was deemed unsafe to support traffic, and the
Governor of Florida declared an emergency. The bridge was closed to
traffic, and the design team was directed to accelerate the project
without deviating from its key objectives or circumventing the public’s
involvement in the process.
Innovative solutions
The Landmarks Commission’s first priority was
that the new bridge retain the appearance of the existing barrel arches.
Barrel arches are rarely constructed today because the cost is vastly
greater than using modern conventional methods. The solution was the
development of a new type of segmental concrete box girder that emulates
a barrel arch. This structural system visually resembles the
earth-filled concrete arches of the original bridge while tolerating the
poor foundation conditions.
Unlike an arch, it is a continuous girder and
has no significant thrust loads either at the abutments, bascule piers,
or intermediate piers. The arch spans vary in length proportionately to
the height of the bridge, with spans varying from about 61 to 128 feet.
Engineering and aesthetic studies demonstrated that this approach
lessened the visual impact of the increased height of the new bridge and
simplified the geometry of the structure.
The design team selected precast architectural
concrete facing to achieve a high-quality durable finish on the piers
and abutments and integrate colors and textures with the local
architecture. However, traditional methods of connecting precast panels
to structures were rejected because of their lack of durability in a
highly corrosive saltwater environment.
To circumvent the problem, the precast panels
were designed to function also as the exterior forms for the pier
concrete. Using such a technique, the core concrete is poured directly
against the panels instead of having a cold joint between the core
concrete and an applied panel. The panels were connected to the core
concrete with stainless-steel hardware to further enhance durability.
White cement and pigment were used in the precast mix, and the exposed
surfaces were pressure washed to expose the aggregate and create a color
and texture similar to the coquina stone used extensively in the local
architecture.
Although the variable depth segmental concrete
approach structure effectively eliminated thrust loads from the
foundation design, the structure — particularly the bascule piers —
still required high-capacity foundations. Since several historic
structures are adjacent to the bridge site, driven piles were ruled out
as foundations. Drilled shafts were the most desirable conventional
foundation type, but the presence of limited fragmented rock lenses
yielded low capacities.
An innovative solution was found in the use of
pressure-grouted drilled shafts. This technique had never been used on a
bridge in Florida, but it had been employed for building foundations in
Asia and elsewhere. It involves installing a jacking plate covered with
a membrane at the bottom of each drilled shaft and connecting the plate
to the surface through a group pipe.
Once the drilled shaft is installed, grout is
pressure injected to the bottom of the shaft through the grout pipe. As
the grout fills the void between the membrane and jacking plate, it
preloads the soil under the tip of the shaft and mobilizes the end
bearing capacity. Using this technique, which was tested during
demolition, shaft capacities were increased by as much as 25%, allowing
a more economical spacing of the shafts in the areas of poorest soil
conditions.
Fast track
Initially, the bridge was expected to be
completed around the middle of 2007. However, with a state of emergency
in effect and the span closed to traffic, FDOT and the design team
worked together to develop a fast-track approach. Final design was
completed 26 months ahead of schedule. To further reduce delays and
maintain traffic through the corridor, the design team developed a
construction implementation program that used an emergency design-build
temporary bridge and phased demolition projects.
At the start of demolition, excess dead load was
removed from the older half of the bridge through removal of the earth
fill. This enabled the remaining structure to provide lateral support so
that traffic could be restored on the newer half of the bridge. Using a
design-build approach, a temporary bridge was completed, restoring four
lanes of traffic. The temporary bridge was set on an alignment that
allowed the final replacement bridge to be constructed in a single major
phase, with the existing bascule leaves to be used as part of the
temporary structure.
The new wider and higher bridge, which consists
of twin parallel structures, was completed almost two years earlier than
had originally been expected. Although still a four-lane, low-level
bridge, the replacement has wider lanes, shoulders, sidewalks, and
additional bike lanes. The new bridge features a double-leaf trunnion
bascule main span that provides 21 feet of vertical clearance in the
closed position, unlimited vertical clearance in the open position, and
125 feet of horizontal clearance. The unique structure contributes to an
aesthetically pleasing bridge that meets the increased clearance
requirements while retaining a scale appropriate for its site.
James M. Phillips III, P.E. is vice president
and chief bridge engineer, E.C. Driver and Associates, a subsidiary of
URS Corporation, Tampa, Florida.
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