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DEEP FOUNDATIONS
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• Dynamic Testing (PDA)
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• Axial & Lateral Load Testing
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must be carried by the substructure. Although a shallower beam
would have been adequate for the approach spans, the team used
the same beam depth for all the exterior girders to eliminate the
discontinuity in the profile of the structure and to maximize the
visual appeal. The approach spans consist of 78-inch-deep Florida
I-beams (FIBs) and 96-inch-deep FIBs (exterior), and the main
span consists of all 96-inch-deep FIBs. Using the 96-inch-deep
FIB, the concrete beams were able to span greater lengths, which
decreased cost and the construction schedule and increased maintainability.
Construction problems and creative solutions
Delivery and installation of longest concrete beams in
the U.S.
Although more efficient and economical, the real test in using concrete
beams came in how the team would go about getting 209-foot
concrete beams fabricated, delivered and erected. The beams are
the longest beams in the state and the pre-casting company confirmed
that the beams are actually the longest single-piece beams
in the United States.
Traditionally, using concrete beams that are in the 165 to
170-foot range was the upper end of length limitations for concrete
beams. With the introduction of Florida-I Beam (FIBs), precast
beams are able to span longer and with greater efficiency than
AASHTO beams.
Delivery
Working closely with an FDOT-approved pre-stressed supplier, the
team addressed the delivery of the thirteen 265,000-pound beams
that had to be fabricated and delivered from their site in Leesburg,
Fla. The supplier performed a detailed evaluation, coming up with
the most cost-efficient route between their plant and the project
site. With two special trailer trucks, capable of supporting 300,000
pounds, they delivered two beams each night, making the delivery
a full weeklong process. The entire US 17-92/SR 436 intersection
was closed down and traffic was rerouted between off-peak
hours, 11 p.m. to 6 a.m., each day of that week to accommodate the
delivery. Due to the load the trucks were carrying, seven Florida
Highway Patrol cars accompanied each truck every night to help
ensure safety and a smooth delivery of the beams.
Installation
The construction team used two large cranes to lift the beams
from the trucks and place on the piers. The cranes had to operate
within a close distance to the relocated power lines. To maintain
safety and an effective installation, the contractor implemented
their own safety standards – which were greater than OSHA
standards. The most significant unforeseen construction challenge
was the variation between the predicted camber of the beam and
the actual camber. Several factors played into camber differences,
including how the beam was cast and additional strands added
to the beam for handling. These strands were cut at the jobsite
before concrete for the deck was placed. The effect of these strands
was more significant than expected. Since the strands were fully
stressed when the concrete was at a lower compressive strength,
and cut when the concrete was at a much higher strength, camber
growth was reduced significantly. The casting bed at the precast
facility that could accommodate the 209-foot-long beams was 500
feet in length.
To meet the schedule, two beams were cast in the same bed.
While this allowed for a faster production rate, one unintended
result was varying camber between the two beams. After all tendons
were stressed, the first beam was formed and the concrete placed.
After the concrete cured, the forms were stripped and placed for the
second beam and concrete was placed. Once the second beam’s concrete
reached the strength required for transfer, the strands were detensioned
and both beams were removed from the bed and placed
on dunnage. The difference in concrete strengths at the time of
strand release caused variations in camber values when measured at
the precast yard. Compounding the issue of camber variation was
the construction schedule. A bonus to reduce the project schedule
was offered to the contractor after award. This forced the contractor
to start erecting beams earlier than the design assumption. The
beam age at erection varied from 16 to 58 days. Beams are typically
erected at 120 days, which was assumed for design. While all beams
had achieved the specified 28-day concrete compressive strength, the
large variation in age at erection caused reduced beam cambers and
increased variation in cambers.
To get a more accurate estimate of the deflection and the camber,
in comparison to the standard design analysis, LARSA 4D, finite
element analysis software, was used. Using the software allowed the
modification of the live-load distribution factors that exceeded some
of AASHTO’s parameters. These calculations helped to determine
deflections for construction staging to field measure deflections.
Design changes
The substructure for this bridge consisted of two end bents, each
containing 13 piles, together with two piers, each containing 44
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