Strengthening Steel Bridge Beams with Fiber Reinforced Polymers (FRP)
and economical means to increase the load carrying capacity of short- and
medium-span steel bridges is a challenging and important task for state departments of transportation (DOTs).
According to the National Bridge Inventory (NBI), there are 96,052 off-system
bridges in the US with steel-beam super structures. Of these, 13,784 have structural
evaluation numbers of 4 or below indicating that they are either currently in
need of repair or replacement, or that the need for repair or replacement is
imminent. The average age of these off-system bridges is 50 years. As such,
these structures were not designed with current or future load demands in mind.
The increasing load demands and decreasing capacity of these structures due to
aging necessitates strengthening or replacement activities. Given that for many
communities an off-system bridge may be the only feasible means to transport
people and goods into and out of the community, prolonged closures for
replacement or repair activities is often not a viable option. Existing repair
methods, such as bolting or welding steel plates or sections to the girders
requires heavy lifting equipment which may not be easily available in remote
areas. Further, these techniques increase the self-weight of the structure
thereby limiting the actual live load increase that can be realized through the
strengthening effort. Taken collectively, these constraints highlight the
urgent need for the development of an effective and economical repair approach
for the short- to medium-span bridges that serve rural and remote communities.
Research over the
past 20 years has demonstrated that fiber reinforced polymers (FRPs) can be
used in various forms to improve the serviceability and strength of steel
bridge girders through flexural strengthening (Mertz and Gillespie, (1996); Sen
and Libby, (2001); Miller et al., (2001); Tavakolizadeh and Saadatmanesh
(2003a,b)). Recent technological advancements, such as the development of high
modulus carbon FRP (HM CFRP) (Rizkalla et al., 2007) and carbon FRP (CFRP)
strand sheets (Tabrizi et al., 2015) further improved the effectiveness of
using CFRP materials to strengthen steel bridge beams. In addition to flexural
strengthening, recent research has demonstrated the potential to use FRP
composites to enhance the stability of plate elements for shear strengthening
applications (Okeil et al., 2009, Kazem et al., 2016). While the effectiveness
of these techniques has been demonstrated, there still remain several key
barriers to the adoption of these technologies by state DOTs:
Since FRP materials are bonded to the surface of a
steel beam using polymeric adhesives, correct surface preparation of the steel
is essential to the effectiveness of the system. The predominant surface
preparation technique used in the majority of the previous research is grit
blasting to a white or near-white metal finish. Given that older structures
commonly used lead-based paints, wide-spread grit blasting of the structure
would require costly environmental controls, thereby limiting the usefulness of
the repair technique.
Effective methods for inspecting FRP-strengthened
steel beams, particularly to ensure the integrity of the bonded interface after
construction and over time, have not been developed. Bridge engineers have
cited concerns about the inability to effectively inspect the quality of
installation and the long-term bond integrity of FRP-strengthened beams as a
barrier to the adoption of this technology.
The traditional approach to selecting adhesives
for bonding FRP to steel is based on the strength characteristics (often
tensile strength) of the adhesive. However, recent findings suggest that the
adhesive toughness also plays an important role in the bond performance. The
design of the bonded joint, and the selection of suitable adhesives for bonding
(including both strength and durability considerations) is not well understood.
Various methodologies have been proposed to
facilitate the design of FRP-based systems for strengthening steel bridges.
None of the existing guidelines have been prepared in a format that is
compatible with the AASHTO LRFD Specifications, and none include a
reliability-based calibration of the reduction factors that is consistent with
the LRFD formulation.
A comprehensive assessment of the initial and
life-cycle costs of an FRP-based repair for a steel-girder bridge as compared
to the costs of a traditional repair has not been presented. Quantitatively
demonstrating the potential cost savings of this type of strengthening system
is important to stimulate the adoption of this technology.
deterioration of off-system bridges is consistent with the US vision for
transportation infrastructure as articulated in Beyond Traffic 2045 (2015). The
implications on accessibility to under-served communities and the importance to
the movement of people and goods is well defined. Developing effective and
economical techniques for strengthening off-system steel bridges is an
important aspect of this broader vision which requires additional investigation
to facilitate adoption.
i) Establish effective surface preparation techniques
for bonding FRP to steel surfaces that minimizes the disruption of lead-based
paint on older structures,
ii) Identify inspection techniques that can be used to
confirm the bond line integrity in FRP-strengthened steel beams,
iii) Provide material and design specifications to
direct the selection of adhesives and surface preparation techniques to
facilitate proper bond design,
iv) Assess the durability of FRP-strengthened steel
beams and effect of exposure to harsh environmental conditions on strength,
v) Calibrate reduction factors in the LRFD format to
facilitate adoption of the strengthening system for bridge design and repair
vi) Complete a comprehensive life-cycle cost analysis
to demonstrate in which scenarios the adoption of FRP strengthening for steel
bridges is economically feasible.
According to the NBI,
there are over 13,000 off-system steel girder bridges in the US with a
structural evaluation number of 4 or below. These structures are either
immediately or imminently in need of rehabilitation or strengthening to
maintain these vital links in the national transportation infrastructure
network. This corresponds to nearly 2% of all of the bridges in the nation.
Given that the average age of these off-system bridges is 50 years, this
problem will only continue to grow. With a total of over 96,000 off-system
steel girder bridges in the NBI (13% of bridges nationwide) the potential
impact of the proposed research is dramatic. The development and widespread
implementation of an effective and economical system to strengthen or repair
these bridges would shorten closure times of these structures for maintenance,
repair and replacement efforts thereby minimizing the disruptions to the
communities they serve. Further, an economical strengthening option could
postpone or eliminate the need for replacements thereby freeing agency budgets
to address other pressing challenges.
The first research effort conducted in the US to rehabilitate steel bridge girders using composites was sponsored by the Florida DOT, conducted by the University of South Florida and documented in 1994, (Sen et al., 2001). A total of six steel-concrete composite beams were strengthened using different configurations of CFRP composites and tested to failure. This early testing demonstrated the effectiveness of this strengthening and repair technique. Subsequently another study was conducted through NCHRP IDEA Contract NCHRP-93-ID011 (Mertz and Gillespie, 1996). In that study, four different configurations of glass and carbon FRP strengthening were evaluated through a series of small-scale beam tests. Two large-scale beam fatigue tests were also conducted to evaluate the performance of the externally-bonded strengthening under simulated traffic loading. Since then research has been conducted at various institutions around the US and the world. The international Institute on FRP in Construction (IIFC) working group on strengthening steel structures maintains a database of publications that are related to strengthening and repair of steel structures with composites. As of 2014 that database included 325 journal papers on the subject. Research in the area has continued to-date. This extensive body of research has established confidence in the potential to use FRP composites to strengthen steel bridge beams. However, to date, the barriers to adoption listed above still remain un-addressed.
It is anticipated
that state bridge and bridge maintenance engineers will be responsible for
implementing the research findings. To date there have been at least three
documented field/demonstration projects to investigate the use of externally
bonded FRP materials for strengthening steel bridge beams in the US including
in Delaware (Miller et al., 2001), Iowa (Wipf et al., 2005) and Kentucky
(Peiris and Harik, 2015). In all of these applications, the DOT personnel
interacted closely with subject matter experts at nearby Universities to
coordinate the design and installation of the repairs. In order to facilitate
the broad adoption of the research findings by state DOTs it is important that
design guidance be provided in a format that can easily and directly be
implemented by DOT engineers independently from University researchers. It is
anticipated that AASHTO Sub-Committee on Bridges and Structures, T-6 (Fiber
Reinforced Polymer Composites) would play a key role in the development and
approval of such a document.
All DOTs will be able to use the findings of this project to address the large aging inventory of steel bridges.
|Sponsoring Committee:||AFF80, Structural Fiber Reinforced Polymers
|Research Period:||24 - 36 months|
|RNS Developer:||Dr. Mina Dawood (firstname.lastname@example.org); Dr. Sami Rizkalla (email@example.com); Dr. Ayman Okeil (firstname.lastname@example.org)|
|Source Info:||Kazem, H., Guaderrama, L, Selim, H., Rizkalla, S., and Kobayashi, A. (2016). Strengthening of steel plates subjected to uniaxial compression using small-diameter CFRP strands. Construction and Building Materials, 111, 223-236.|
Mertz, D.R., and Gillespie, J.W. (1996). Rehabilitation of steel bridge girders through the application of advanced composite materials.
Miller, T.C., Chajes, M.J., Mertz, D.R., and Hastings, J.N. (2001). Strengthening of a steel bridge girder using CFRP plates. Journal of Bridge Engineering, 6(6), 514-522.
Okeil, A.M., Bingol, Y., and Ferdous, M.R. (2009). Novel technique for inhibiting buckling of thin-walled steel structures using pultruded glass FRP sections. Journal of Composites for Construction, 13(6), 547-557.
Peiris, A., and Harik, I. (2015). Steel bridge girder strengthening using postinstalled shear connectors and UHM CFRP laminates. Journal of Performance of Constructed Facilities, 29(5), 11p.
Rizkalla, S., Dawood, M. and Schnerch, D. (2007). Development of a carbon fiber reinforced polymer system for strengthening steel structures. Composites Part A: Applied Science and Manufacturing, 39(2), 388-397.
Sen, R. Liby, L., and Mullins, G. (2001). Strengthening steel bridge sections using CFRP laminates. Composites: Part B, 32, 309-322.
Tabrizi, S., Kazem, H., Rizkalla, S., and Kobayashi, A. (2015). New small-diameter CFRP material for flexural strengthening of steel bridge girders. Construction and Building Materials, 95(7), 748-756.
Tavakkolizadeh, M. and Saadatmanesh, H. (2003a). Repair of damaged steel-concrete composite girders using carbon fiber-reinforced polymer sheets. Journal of Composites for Construction, 7(4), 311-322.
Tavakkolizadeh, M., and Saadatmanesh, H. (2003b). Strengthening of steel-concrete composite girders using carbon fiber reinforced polymers sheets. Journal of Structural Engineering, 129(1), 30-40.
Wipf, T.J., Phares, B.M., Klaiber, F.W., Al-Saidy, A.H., and Lee, Y. (2005). Strengthening steel girder bridges with carbon fiber-reinforced polymer plates. Transportation Research Board – 6th International Bridge Engineering Conference: Reliability, Security and Sustainability in Bridge Engineering, 435-447.
|Index Terms:||Steel bridges, Fiber reinforced polymers, Beams, Strengthening (Maintenance), State departments of transportation, Flexural strength, Serviceability, Load and resistance factor design, |
Maintenance and Preservation
Bridges and other structures