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Bridge construction was a fast-track process that
included removal of portions of existing concrete deck, sidewalk, railing
and substructure units, construction of new portions of a cast-in-place
abutment and wing wall, setting of the precast bridge units, precast
approach slab and precast railing, grouting of the beam seat areas and the
areas below the approach seats, grouting the shear keys, lateral and
longitudinal post-tensioning, grouting post-tensioning tendon ducts,
grinding and grooving the final riding surface, placement of the sidewalk
concrete, and epoxy-coating the sides of the fascia girders and general
clean-up.
“The Quaker City Bridge was the first of the series
six bridges that will be built in Ohio using innovative techniques, methods,
and materials to reduce the time of construction,” the researchers wrote.
“Fast-tracking hadn’t been used extensively in Ohio before, so this was a
first time effort for both the contractor and the contracting agency. The
project was a success even though it was completed three days late. In the
end, ODOT agreed that some of the delays were beyond the contractors’
control (such as rain) and the contractor received one day’s incentive.”
Some conclusions drawn were:
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Partnering is essential, and the contractor and
contracting agency formed a cordial relationship with an atmosphere
conducive to quality performance. |
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The design engineer must be aware of local
contractor capabilities. The design used was also a first in Ohio.
Post-tensioning has not been used extensively there before, and there are
just two post-tensioning companies in the state. Moreover, the two companies
specialize in different materials for post-tensioning like rods and strands.
“If a bridge is to be post-tensioned using a particular material, there will
be complete dependence on one company,” they said. “This over-dependence may
lead to complacency on the part of the supplier and any problem faced during
post-tensioning could adversely affect the schedule.” |
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Use of unusual specifications must be clearly
flagged. “There were significant issues with the selection of and the QA/QC
for the shear key grout,” they said. “Most of these issues revolved around
the fact that performance of the keyway grout for this project had to be
much better than that for a normal adjacent box-beam bridge. When there are
unusual material requirements, the design engineer needs to be sure that the
state agency and the contractor(s) are aware of these requirements. Simply
adding language to the project specifications is insufficient.”
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New horizons in steel
The dominance of precast concrete notwithstanding,
high-performance steel will grow in popularity among structural engineers
because the new steels make it possible to reduce structural dead loads and
improve toughness and weldability. This will lead to new types of steel
bridge designs, including composite designs.
In a May 1998 Better Roads article, authors Susan
Lane, Eric Munley, Bill Wright, Marcia Simon, and James D. Cooper described
five basic attributes of high-performance steel.
Strength. The FHWA is looking at the
higher-strength grades (480 and 690 MPa) in its research program, which can
be immediately used to improve the structural efficiency of today’s bridge
designs.
Corrosion resistance. Traditionally, corrosion
protection was achieved by painting the bridge. Today, the high cost and
adverse environmental impact of repainting make this approach impractical. HPS grades being developed will have corrosion resistance equal to or
greater than A588 weathering steel.
Weldability. A primary goal of high-performance
steels is to make them more weldable by reducing the carbon levels in the
steel. The payoff will be steels that are more tolerant of welding process
procedures and welding conditions, leading to improvements in fabrication
efficiency, improved weld quality, and possible reductions in the level of
quality assurance inspection that is currently required. Improving
weldability also opens the door for using field welding for construction and
repair of steel structures.
Toughness. That’s the ability of steel to ductilely deform under a load, rather than fracture like brittle glass. High
toughness allows structures to absorb and redistribute the impact of traffic
loads. This becomes even more critical at low temperatures where steel
becomes more brittle. Tougher steels provide a higher reserve capacity to
structures, and allow those structures to tolerate fabrication flaws and
withstand extreme loading events such as earthquakes.
Formability. Formability is a key factor in
high-performance steel due to the increasing use of light-gauge structural
members. Many of these new shapes may require steels that tolerate bending
and forming operations.
Missouri HPS bridges
Missouri has adopted high-performance steel in its
bridge program, report Missouri DOT engineers Bryan A. Hartnagel, P.E.,
Ph.D. and Shyam Gupta, P.E. in their TRB 2004 paper, Use of High Performance
Steel (HPS) in Missouri Bridges.
“High-performance steel is a superior version of the
older grade of high-strength steel,” they said. “HPS offers enhanced
toughness, improved weldability, superior corrosion resistance, and higher
strength when compared to typical grade 50W (345W) steel. The use of this
superior grade of steel allows the design engineer to create cost-effective
and structurally efficient bridges by using the higher strength HPS70W
(HPS485W) in the highly stressed regions.”
The Missouri DOT began using high-performance steel
in bridge girders with the bid letting of April 2001. By August 2003, seven
structures using HPS had been let for construction and at least three others
were in the design phase. “The anticipated benefits of reduced initial cost,
lower life-cycle costs, and improved performance over the life of the
structure are important considerations when selecting material for
structures designed for a life of
75 years,” the authors said. “In [these] projects,
high-performance steel allowed the Missouri DOT to: reduce initial cost,
decrease superstructure weight, and eliminate girder lines. MoDOT also
expects lower life-cycle costs on the structures built with high-performance
steel due to the improved corrosion resistance the steel offers.”
Inverted steel box design
Research also continues in conventional steel bridge
designs. For example, on behalf of the Nebraska Department of Roads and the
FHWA, the University of Nebraska-Lincoln has undertaken conceptual plans for
a new steel bridge configuration, called the inverted steel box.
In this concept, each girder consists of an inverted
box section. The web and top flange have the same plate thickness, while the
flanges are welded to the end of the web. The knee braces at the ends allow
longer overhang, if needed.
The inverted box portion of the bridge could be
fabricated in the shop or made by bending a flat plate into a U shape.
Under traffic or construction loads, there will be a
tendency for the flanges to kick out or bend in the horizontal direction.
Providing initial curvature in the horizontal plan for the flanges could
solve this problem. This initial horizontal curvature will result in initial
compressive force in the flanges, due to dead loads. This in turn will
increase the live load capacity of the girder.
The wide top flanges of the inverted box girder
could be required to be stiffened.
The knee braces will have two functions: providing
support for overhang, thereby eliminating the need for temporary knee braces
that are needed to support the overhang during the construction; and
preventing the flanges from movement in the horizontal direction.
Wide top flanges of the inverted box girder provide
a convenient platform for the construction worker.
The shape of the inverted box girder provides a very
stable configuration during construction and also allows for easy
transportation. Additionally, unlike small, conventional steel box sections
where workers cannot enter the box for inspection, the inverted steel box
girder is fully accessible and can be adapted to any span bridge.
For more information contact Nebraska DOT’s Gale
Barnhill, 402-479-3921, e-mail
gbarnhil@dot. state.ne.us, or UNL’s Dr.
Azizinamini, 402-472-5106, e-mail:
aazizi@unl.edu.
Timber for special applications
Despite strong promotional support for construction
of timber bridges from the timber industry, the U.S. Department of
Agriculture, and various state governments with strong timber industries,
timber’s share of the short and medium bridge market has declined.
“The two most common types of timber bridges are the
glued-laminated girder bridge (for both single and multiple spans) and the
longitudinal glued-laminated panel bridge,” said Jake Bigelow of the Center
for Transportation Research and Engineering, Iowa State University. “The
widths of both structures vary depending on number of traffic lanes;
however, the clear span for girder and panel bridges ranges from 20 to 80
feet and 20 to 35 feet, respectively.”
Like precast/prestressed concrete bridges, such
timber bridges reduce closure times because they are assembled in the field
with pre-manufactured components. “These bridges are constructed of
prefabricated modular components and can have a service life of 50 years or
more,” Bigelow said.
There has been no lack of support for timber bridge
construction. The 1988 Timber Bridge Initiative, passed by the U.S.
Congress, was an important pilot program, Bigelow said. In 1991, the United
States Department of Agriculture’s Forest Service/Forest Products
Laboratory, in conjunction with the FHWA, again received funding under the
1991 transportation act to continue the research and technology advancements
in timber structures.
Now called the National Wood in Transportation
program, the initiative is divided into three areas, according to Bigelow:
construction of demonstration bridges, timber research, and technology
transfer. “The main goals of these three areas are to develop a better, more
positive awareness of timber and to advance timber technology for future
needs,” said Bigelow.
Today, timber bridges are mostly used in special
applications. They are used for appearance or economy in a residential,
rustic, or recreational setting. They also remain popular for railroad use
for all types of bridges and trestles, short or long.
And new ways to reinforce wood will be refined,
including variations of glued-laminated timber and fiber-reinforced polymer
composites. In this concept the wood resists the compression load, while the
FRP composite resists tensile load.
FRP: The distant horizon
Timber and fiber-reinforced polymer (plastic)
composites will have to work together, because timber’s applications are
limited and composites are still in the development stage.
Composite or FRP decks are being studied as
alternatives to precast/prestressed or cast-in-place concrete designs. The
main characteristics that make them a viable alternative to traditional
reinforced concrete decks include better resistance to electro-chemical
corrosion, higher strength-to-weight and stiffness-to-weight ratios,
versatility of fabrication, and potential for design optimization.
The composite industry has eyed the transportation
market in the face of declining demand from the defense industry. Despite
its successful introduction into the construction market, widespread
acceptance of FRP decks by the civil engineering community has been slow,
not only by simple reluctance to change common practice, but also by
legitimate technical and economical concerns.
FRP bridge projects have shown inherent problems in
deflection, material ductility, creep, and reactivity with concrete and
steel. There have also been performance problems under long-term exposure to
ultraviolet light and other environmental factors such as moisture,
freeze-thaw, humidity, and external chemical attack, according to R.
Kannankutty, Minneapolis DPW, and Donald J. Flemming, Minnesota DOT, in
their Year 2000 TRB Special Report: Transportation in the New Millennium.
“To help resolve these issues, material testing
standards and design methodology will be developed to fit FRP material
properties,” they wrote. “A comprehensive research effort at the national
level will be undertaken to make FRP a dependable, low-maintenance bridge
material capable of delivering high performance over the life of a bridge
structure.”
The authors think the collaboration of practicing
designers, construction engineers, and bridge owners will make FRP a
feasible and competitive alternative to conventional bridge construction
materials, and they expect universities to expand their curricula to include
FRP and other composite materials in their courses.
Recently, a new prestressed FRP tubular deck system
replaced a deck on an existing steel truss and wood deck bridge in Delaware
County, Ohio, through the FHWA’s Innovative Bridge Research and Construction
Program, which supports field research in FRP bridge decks
( http://ibrc.fhwa.dot.gov/).
But practically speaking, at least for now,
full-blown FRP structures are confined to the campus or to the occasional
test structure in a controlled application.
FRP-concrete designs
Not only will FRP combine with timber materials, but
it will be combined with concrete as well. Such a “smart” composite bridge
was installed on the University of Missouri-Rolla campus in the fall of
2000. The bridge deck was designed for an AASHTO H20 load rating and
replaced a wooden bridge over a small creek.
The project is a cooperative effort among an
interdisciplinary faculty team at UMR, St. Louis-based Composite Products,
and the Navy Center of Excellence for Composites Manufacturing Technology at
the Lemay Center for Composites Technology.
The project goals were to develop a composite
materials approach for extended-lifetime, short-span highway bridges and to
demonstrate advanced composites and sensing technology. Funding sources
included a National Science Foundation grant, the Center for Infrastructure
Engineering Studies (UMR), and the Missouri Department of Transportation.
The design is based on a modular assembly of
composite elements. The technology has the advantages of performance due to
all-composite construction, of relative economy due to standard tube
elements made from a pultrusion process, and of flexibility due to the
modular use of carbon and glass tubes.
The elements are square tubes with standard 3-inch
sides. They are reinforced with longitudinal carbon or glass fibers. The
tube assembly consists of alternating layers of tubes that run
longitudinally and transversely to the span length. The strength and
deflection of the bridge assembly was tailored by the balanced use of
higher-cost, higher-stiffness carbon tubes and lower-cost lower-stiffness
glass tubes. A smart structures network of fiber optic sensors was
incorporated for long-term in-situ monitoring of strain and temperature.
Although the bridge was designed for highway loads,
it is located along a concrete walkway used by pedestrians and light
vehicles. Consequently, the wear surface and railings match this
application. The wear surface is thin polymer concrete made from Transpo
T-48 that was modified with a soybean-oil-based resin.
The composite railings consist of pultruded glass
tubes and carbon rods and of column plates made of sheet molding compound
from recycled Ranger truck parts. The aesthetic design of the railing was
determined by a contest among UMR students.
A paper describing this project is at
http://campus.umr.edu/smartengineering/bridge/Articles/SCB-SMSPaper.htm
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Hybrid FRP-concrete decks are being studied under
research funded by the New York State DOT. This work is continuing through
October 2005 at the State University of New York at Buffalo.
The primary objective of this study is to introduce
FRP-concrete hybrid deck systems through analysis and experimental
procedures, and to optimize these systems to produce durable, structurally
sound and cost-effective hybrid systems that will take full advantage of
both the FRP materials and concrete.
Two hybrid FRP-concrete bridge deck systems are
being investigated. The FRP systems can be utilized to replace concrete
decks on steel girders, or to serve as a self-supported short-span bridge
superstructure. The two concepts rely on using cellular components to form
the core of the deck system, and an outer shell to wrap around those cells
to form the integral unit of the deck.
FRP bridge deck
A recent, innovative use of an FRP reinforcement
cage in concrete bridge decking on a project in Wisconsin was investigated
by Jacobson, Conachen, Bank, Oliva, and Russell, of the Wisconsin Structures
& Materials Testing Laboratory, University of Wisconsin-Madison. Unlike many
previous installations, this one was placed on a major arterial, U.S. 151,
over De Neveu Creek near Fond du Lac.
University researchers investigated an innovative,
modular, three-dimensional, FRP-pultruded grid reinforcement system to
construct a concrete bridge deck on a major bridge structure. The two-lane
northbound bridge utilizes the FRP reinforcement system, and has a single
130-foot span with a width of roughly 45 feet. For comparison, the bridge’s
southbound counterpart has been constructed with a conventional
steel-reinforced deck.
Following lab tests, the Wisconsin DOT accepted the
FRP system for use in the Fond du Lac bridge. The University of Wisconsin
provided a quality assurance program to provide a secondary verification of
the material properties used in the design of the bridge. UW also performed
a constructibility analysis of the bridge that included site observation and
cost analysis of the project. This was done to determine if the savings in
field labor offset the extra cost of the material.
Additionally, a load test of the bridges was
performed in conjunction with the University of Missouri-Rolla. This testing
was to provide a benchmark for future performance testing of the bridge by
Marquette University.
High-performance concrete
High-performance concrete is becoming a conventional
bridge construction material as a result of Strategic Highway Research
Program research and Federal Highway Administration implementation efforts.
HPC is a set of specialized concrete mixes which
provide added durability for concrete structures. Their benefits include
ease of placement and consolidation without affecting strength, long-term
mechanical properties, early high strength, and longer life in severe
environments. They also conserve material, require less maintenance, and
deliver extended life cycles and, if designed well, enhance aesthetics.
Originally, high performance concrete was target-ed
at bridges before it was aimed at pavements. Bridges and decks built with
HPC in Georgia, Minnesota, North and South Carolina, Missouri, Iowa, Texas,
and many more states attest to the growing acceptance of HPC.
While HPC is indicated for substructural concrete
bridge elements, it may not be right for bridge decks, says one industry
expert. “Too often, too much emphasis is placed on developing higher
strength concretes,” Ernest A. Rogalla, S.E., told Better Roads. Rogalla is
a consulting engineer with Wiss, Janney, Elstner Associates based in
Northbrook, Illinois.
“These concretes have high cement contents,” said
Rogalla, “and have generally performed not as well as older, more
conventional mix designs. For example, many new bridge decks in the ‘70s and
‘80s developed excessive cracking as the industry went to higher strength
concretes.”
Higher-strength mixes tend to have denser pastes
that are more durable against freezing, said Rogalla, but most severe damage
on bridges and roadways is not related to freeze-thaw damage, but instead is
attributable to wide cracks that go through the thickness of the deck.
“Unfortunately, higher-strength concretes typically
create substantially higher shrinkage and thermal stresses, which then
create wide cracking,” said Rogalla. “The effects of this wide cracking are
much more severe than the effects of freeze-thaw damage.”
Higher-strength concretes are also stiffer than
weaker concretes. For a given temperature change of shrinkage, the stresses
will be larger in the higher-strength concrete. Usually, the added strength
of the concrete does not keep up with the added stresses, he said.
To optimize the mix for decks, Rogalla says,
determine the minimum strength required for the applied loading and do not
specify stronger concrete. “Most concrete suppliers will supply concrete
that is quite a bit stronger than specified anyway, to account for
inevitable variability during mixing and the liability of a placed mix not
meeting the specified strength,” Rogalla said. “Avoid using concretes with
compressive strengths exceeding 4,000 psi. Determine the maximum
water-cement ratio and the minimum air entrainment needed for freeze-thaw
durability. This will assure reasonable durability of the paste.”
Rogalla also recommends the maximum amount of coarse
aggregate, and using the largest aggregate possible (1.5 inches or larger,
when possible). “From a durability standpoint, there is no reason whatsoever
to not maximize the aggregate size,” Rogalla said. “I don’t understand why
so many designers specify a maximum stone size of only 0.75 inch, when the
project could easily have used 1.5-inch stone; smaller stone only adds cost
and reduces performance.”
He also recommends using the least amount of paste
(cement and water) that you can. “Specifying a minimum amount of cement for
a mix is crazy,” Rogalla said. “There is no reason for this, as the added
cement only worsens the structure by increasing shrinkage and thermal
stresses. Instead of specifying the minimum amount of cement, it would be
better to specify the maximum amount of cement.”
As much as possible, Rogalla said, avoid using Type
III (accelerated strength) concrete, or admixtures to accelerate hydration
(setting). This worsens the early heat rise and thermal stresses.
In summary, maximize the amount of aggregate and
minimize the amount of paste. To minimize the paste, specify the lowest
compressive strength possible for traffic loading, and maximize the gravel
size. For paste durability, specify the maximum allowable water-cement ratio
and minimum allowable air-entrainment. Do not allow Type III cements or
accelerators, as this can create early heat problems. Stick with aggregates
and mixes that have performed well, and do not reuse those that have not.
“Don’t forget the basics, and learn from your experiences,” Rogalla told
Better Roads.
Stay-in-place deck forms
Stay-in-place metal forms for bridge deck
construction are used commonly throughout the United States, but users in
southern states appear better satisfied with their performance, say the
authors of Survey of State DOTs on Performance of Concrete Bridge Decks
Constructed Using Stay in Place Metal Forms, presented at TRB last year.
The five researchers — Nabil F. Grace, professor and
chairman, James L. Hanson, associate professor, and Walid Farahat, research
assistant, Department of Civil Engineering, Lawrence Technological
University, Southfield, Michigan; and Roger D. Till, engineer of structural
research, Michigan DOT — developed a comprehensive survey to evaluate the
use of stay-in-place metal formwork for concrete bridge deck construction in
the USA.
“Stay-in-place metal formwork provides cost savings
and increased construction safety,” the authors write. However, they add,
after over 30 years of use, satisfaction with the performance of SIP forms
ranges from highly satisfied to unsatisfied.
The survey was distributed to all state DOTs. A
total of 39 DOTs responded to the survey. The survey results are presented
on a geographic basis, and show 67% of the DOTs that responded to the survey
allow the use of this technology.
A total of 26 DOTs allow, and 13 do not allow,
stay-in-place metal formwork in concrete bridge deck construction. Most of
the 26 DOTs that use this technology are satisfied with its performance. The
majority that does not use this system are concerned with the inability to
visually examine and access the bottom of the deck slabs.
For DOTs that use stay-in-place metal formwork, five
(all in the East) have more than 1,000 such bridges each, 15 DOTs have less
than 100, and the remaining six DOTs have between 100 and 1,000 each. Nearly
half of the DOTs that permit the use of stay-in-place metal formwork (11)
have been using them for more than 30 years.
Filling the corrugations of SIPMF with Styrofoam to
reduce the dead weight of bridge decks is not a common practice among the
majority of the DOTs that allow the use of SIPMF in bridge decks.
The use of epoxy-coated steel bars in these bridges
is a common practice in most states. Most did not observe a difference in
performance between decks with bare steel reinforcements and those with
epoxy-coated steel reinforcement.
The majority of the DOTs use conventional inspection
approaches such as visual inspection and hammer sounding for periodic
examination of their stay-in-place metal formwork bridge decks. The typical
period between each inspection ranges from one to three years.
Most of the DOTs do not believe that the
stay-in-place metal formwork increases the long-term durability of bridge
decks.
Overall, states in the southern regions of the
country were more accepting of this construction method and gave it higher
performance evaluations than those in the northern regions.
Continuous change
The technology of short- and medium-span bridges
will continue to change.
Even now, the Kansas DOT is conducting an
independent economic study comparing costs and design feasibility of
clear-span concrete or steel arches, compared to multiple opening drainage
structures and small-scale reinforced concrete bridges with cast-in-place
slab spans. KDOT will develop design criteria for possible standards,
considering the expected serviceability, the availability of material, and
site conditions. The project was to end in May.
Concrete slabs, timber, precast/prestressed
concrete, and rolled steel shapes comprise the market options for bridge
spans up to 50 feet. But even this will change. Even now, these designs are
being challenged by long-span culverts and pre-engineered, out-of-the-box,
prefabricated component bridges.
While suppliers and designers will be challenged to
keep up and stay relevant, taxpaying highway users and the economy will
benefit due to lower costs, better reliability and durability, and the
minimal interruptions in traffic flow that takes place when the long bridge
closures of the past are replaced by prefabricated bridge structures of all
types.
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