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Strengthening Steel Bridge Beams with Fiber Reinforced Polymers (FRP)


Developing effective 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.

Addressing the 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 applications, and

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.

Related Research:

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:AKB10, Innovative Highway Structures and Appurtenances
Research Period:24 - 36 months
Research Priority:High
RNS Developer:Dr. Mina Dawood (mmdawood@uh.edu); Dr. Sami Rizkalla (sami_rizkalla@ncsu.edu); Dr. Ayman Okeil (aokeil@lsu.edu)
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.
Date Posted:03/18/2017
Date Modified:03/20/2017
Index Terms:Steel bridges, Fiber reinforced polymers, Beams, Strengthening (Maintenance), State departments of transportation, Flexural strength, Serviceability, Load and resistance factor design,
Cosponsoring Committees: 
Maintenance and Preservation
Bridges and other structures

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