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The structure was subjected to 12 months of
continuous traffic loading, during which time the sensor array and remote
data acquisition system measured and recorded strain, temperature, and
deflection data. Weigh-in-motion sensors recorded traffic loads by measuring
weight, axle number, and axle spacing to identify vehicle types.
The purpose of the test was to provide data to
evaluate the performance of the composite bridge structure. The goal was to
learn whether it could carry the required loads, withstand weather and other
environmental extremes, and match or exceed the aging characteristics of
more conventional materials. The project demonstrated the structure’s
successful performance, a conclusion that was validated by a successful
instrumentation program.
Bridge Street Bridge
Bridge Street Bridge in Southfield, Michigan, was
the first major concrete vehicular bridge that incorporated
carbon-fiber-reinforced polymer as its principal structural reinforcement.
The project has won several awards for innovation. The project consisted of
two parallel bridges — Structures A and B — over the Rouge River. Structure
A was constructed of conventional concrete, with a new substructure.
Structure B was constructed of precast, prestressed double-tee girders with
FRP prestressing strand, FRP reinforcement and external post-tensioned
carbon fiber composite cable tendons. Structure B was built on and is
supported by an existing substructure.
The project is expected eventually to demonstrate
that the use of composite material as structural reinforcement can increase
a highway bridge’s service life, enhancing public safety and reducing
maintenance costs. The project’s extensive program of testing,
instrumentation, and monitoring serves multiple purposes.
The preconstruction phase included testing to
failure of a full-scale double-tee girder to confirm design assumptions and
refine fabrication details. A single, 68-foot-long test girder was shipped
to CTL for testing. During its fabrication, instruments were installed to
measure forces in pre- and post-tensioned tendons, concrete strains, and
deflections. A 3-inch-thick concrete topping was cast on the top surface of
the girder, and the girder assembly was subjected to a static flexural load.
The testing of the prototype girder addressed design, fabrication and
transportation concerns.
Once in production, girders had sensors installed
during fabrication that measure strain, temperature, and camber. These
sensors allowed constructors to monitor the girders’ condition during
transport by crane, truck, barge, truck, and crane from the precast plant in
Windsor, Ontario, to the jobsite. After erection of the girders, additional
sensors were installed to measure deck strain and temperature as well as
beam deflection. Load cells were installed to measure prestressing forces
and both longitudinal and transverse post-tensioning forces.
The 400-sensor array is connected to an on-site
data-logging system, and threshold response values are assigned to each
sensor. The system is remotely accessible through modem communication and
provides automated dial-out response should any of the sensors exceed their
assigned response value. The system is being used to monitor the long-term
performance of the innovative bridge structure and to provide performance
data during scheduled load tests in its first five years of service.
Schuyler Heim Bridge
Open-grated decks have historically been used on
movable bridges, because they are lightweight, but they have drawbacks. They
require more effort to maintain acceptable skid resistance and their ride
quality is less than optimal. The Schuyler Heim Bridge is a 55-year-old
vertical-lift bridge in the Port of Los Angeles with an open-grate steel
deck that is subject to heavy truck traffic. Because of the volume of
traffic, the deck has been in continuous distress despite a complete
replacement in 1997.
The California Transportation Department selected
the Schuyler Heim Bridge for a trial project that replaces part of its
deteriorated steel deck with polymer composite material. The replacement
deck panels have been instrumented to monitor their performance in service.
The design of the composite deck panels will serve as an example in the
development of specifications for future deck replacement projects.
Design constraints include limitations on weight
(nominally 22 pounds per square foot maximum) and thickness (height above
stringers 5.1875 inches maximum), and the need to mount the new deck on
existing stringers and curbs. The replacement deck also is required to
conform to AASHTO’s more stringent HS-25 performance standards rather than
the HS-20 standards that governed the previous deck.
Both numerical analysis and laboratory tests were
used to validate the replacement deck design. Four fully-featured prototype
decks were tested to destruction under simulated wheel load. The ultimate
load capacity of the test specimens exceeded the minimum 104 kips required
for an ultimate factor of safety greater than four.
The design verification program included a
demonstration of damage repair. A damaged test deck was successfully
repaired, then tested to higher failure loads than in its original state.
For the actual deck replacement, an instrumentation
and monitoring program has been set up to measure strain, deflection, and
environmental conditions including temperature, humidity, and exposure to
solar radiation. The instrumentation system is set up to provide a manual
mode for monitoring seasonal load tests; an automated mode for monitoring
the long-term effects resulting from environmental changes; and an
alarm-condition mode that can alert appropriate staff when response values
in selected sensors reach predetermined levels.
Manufacturers, bridge designers, research engineers,
and forward-looking government agencies are advancing bridge technology by
developing and testing composite materials in a range of applications. For
now, the ability to instrument these innovative structures and monitor their
performance is crucial, because we lack existing standards and methods with
which to evaluate them objectively. Monitoring specific properties over time
allows us to note and investigate changes. Eventually, the data gathered and
experience gained through these projects will help promote innovation and
improve the safety and durability of bridges.
Thomas L. Weinmann is principal engineer and manager
and Andrew E. Lewis is a senior engineer and technical sales specialist for
the Sensors and Structural Diagnostics group at
Construction Technology
Laboratories, Inc., in Skokie, Illinois.
Reprinted from Better Roads Magazine
November 2004 |
