Structural Characteristics
FRP bridge decks are anisotropic,
meaning the mechanical properties of the laminates vary with the
volume and orientation of the fiber reinforcement (similar to
the reinforcing steel in concrete). As a general rule of thumb
for structural FRP applications, design strains are typically
kept below 20% of ultimate capacity. However, bridge deck
applications are typically stiffness-driven, many times
resulting in strains well below that level. As a result of such
low levels of strain, fatigue and creep are not an issue when
properly designed and fabricated.
Design Issues/Considerations
-
Deflection - Allowable
live load deflection in FRP (as well as timber and steel) has
been much debated. The main purpose for deflection limitations
should be to prevent local or global deformations under a
wheel load that may cause delamination or cracking of the
overlay. Initially FRP decks were held to the same stiffness
as concrete decks although there are no deflection
requirements in AASHTO for concrete decks. The thought was
that since deflection was not a problem in concrete decks the
conservative approach would be to match concrete. This
approach was ultra conservative and resulted in FRP decks that
were extremely over designed and too expensive to be viable.
The deflection criteria was later relaxed to L/800 in many
cases, but this criteria was based on the global bending of an
entire superstructure and was clearly not applicable to FRP or
any other types of decks. As the market has become familiar
with FRP materials and realize how over designed these decks
are for ultimate strength, deflection criteria has recently
settled into the L/300 to L/500 range, which is consistent
with the provisions in the current AASHTO LRFD Code provisions
for orthotropic steel and timber decks.
-
Deck-to-Girder Connections
– FRP bridge decks have been installed on steel, concrete, and
FRP girders and connection types vary with each deck type and
application. Connection options include but are not limited to
all-adhesive, mechanical fasteners, and conventional shear
studs. Composite bending action between the deck and support
girders is possible but the ability to provide this will
depend on each specific deck type and manufacturer.
-
Barrier Rail Connections
- Many types of conventional railing systems can and have been
used with FRP bridge decks. The connections will vary with the
deck type but will typically be bolted through the deck or
anchors embedded inside the deck
-
Overlay - All of the FRP
bridge deck systems require an overlay to provide adequate
skid resistance as well as abrasion protection. The type of
overlay is generally the owner’s choice and options include
but are not limited to conventional asphalt, polymer-modified
asphalt, polymer concrete, and micro-silica modified concrete.
-
Specifications - Numerous
attempts are currently under way from various private and
governmental agencies to develop material specifications for
FRP materials to be used for bridge applications. One of the
advantages of FRP materials is the ability to choose from a
wide range of constituent materials and manufacturing
processes. However, trying to develop a prescriptive-based
specification to cover all materials and applications has
proved to be a daunting task and the owners and industry
continue to struggle with how to best address this issue.
Historically a performance-based specification has been
successfully used for FRP materials and it appears for the
short term this approach will be continued for bridge
applications.
Cost/Current Market Limitations
Based on initial in-place material
cost, FRP bridge decks typically cost 2-3 times that of a
conventional deck. FRP bridge decks will probably always have a
higher initial material cost when compared to conventional
materials, but many times there are no other alternatives than
continually replacing and maintaining the existing deck types or
complete replacement of the structure. Eventually life-cycle
costs (particularly user costs more so than material cost) will
become part of the decision making process in the United States,
but until that time FRP bridge decks have to compete in niche
markets where initial costs are comparable or where there are
limited or no other alternatives. In these markets, lightweight
and rapid installation tend to be the features that can justify
the higher material cost.
Niche applications where weight is
critical include movable, truss, arch and potentially suspension
bridges. In many of these applications as well as other types,
the bridge may also be classified as a historic structure, in
which case there is a desire to preserve rather than replace.
Many of these older structures were not designed for current
traffic loads, and as a result traffic may be restricted on
these bridges. The weight savings gained by replacing an
existing concrete deck with a lightweight FRP bridge deck can
result in additional live load capacity and possibly allow
restrictions or postings of these bridges to be removed.
Off-site manufacturing and modular
construction of FRP bridge decks allows for rapid installation
in the field and is ideal for urban installations or other
bridges with high traffic counts that are sensitive to closure
times or detours. When compared to a conventional cast-in-place
concrete deck, FRP bridge deck installations can possibly cut
construction time of the deck in half. Applications for rapid
installation include all types of bridges.
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