Seismic Strengthening of Existing Concrete Structures with FRP

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Fiber-reinforced polymer (FRP) composites have become a widely accepted solution for strengthening existing concrete structures. While established guidance exists for gravity, flexural, and column confinement applications, seismic strengthening with FRP has historically lacked clear, consensus-based design direction. In recent years, this gap has narrowed due to targeted experimental research, jurisdictional guideline development, and ongoing updates to national standards. Recent advances include large-scale testing, ongoing work in the consensus standards, and the development of retrofit guidelines like the City of West Hollywood FRP guidelines and HCAi PCS preapproval guides, whereby the community works toward a coherent framework for applying FRP to seismic retrofit projects involving columns, walls, diaphragms, collectors, and chords.

A large inventory of existing concrete buildings in seismic regions was constructed prior to the introduction of modern detailing requirements for deformation compatibility, transverse reinforcement, and complete seismic load paths. Common deficiencies include inadequate shear strength in columns and walls, insufficient confinement, thin or lightly reinforced diaphragms, and collectors designed only for gravity loads. While performance-based seismic evaluation methodologies in ASCE 41 and the corresponding concrete provisions from ACI 369 have shifted engineering practice toward displacement-based evaluation and component-level performance objectives for existing buildings, little retrofit design guidance has been provided for FRP enhancement of deficient concrete components.

FRP strengthening has grown rapidly as an attractive retrofit option due to its high strength-to-weight ratio, minimal added mass, and constructability advantages. However, much of the existing FRP guidance, particularly ACI 440.2R, was developed primarily for force-based strengthening of beams and columns under gravity or monotonic loading. Direct extension of these provisions to seismic applications, characterized by cyclic loading and the post-yield deformation demands of the retrofitted concrete component (rather than of the FRP itself, which remains essentially linear-elastic to rupture), has led to inconsistent interpretation and application.

This article summarizes recent research, code development, and field experience that collectively establish a clearer design basis for FRP seismic strengthening of existing concrete structures, with an emphasis on practical implementation for structural engineers.
Historical Context: What Existing FRP Guidance Was (and Was Not) Designed For
The ACI 440 family of documents established the foundational framework for externally bonded FRP strengthening in the United States, building on earlier evaluation criteria such as ICC-ES AC125, first issued in 1997. The first design guidance arrived with ACI 440.2R-02 in 2002 (preceded by the companion ACI 440.1R-01 in 2001), with subsequent editions in 2008 and 2017. These provisions target a minimum safety objective and applications with relatively well-defined stress states, short bond lengths, and limited cyclic demand, such as shear and flexural strengthening of beams and columns.

Seismic strengthening, by contrast, must accommodate repeated load reversals, reinforcement yielding, and force redistribution while maintaining deformation compatibility, with the governing performance objective often being deformation capacity rather than peak strength.
Absent seismic-specific guidance, engineers were forced to extrapolate from these gravity-based provisions, leaving uncertainty regarding force and ductility reduction factors, usable strain limits, anchorage and detailing requirements, and acceptance criteria. Recent research and guideline development are beginning to resolve these inconsistencies.

FRP seismic strengthening strategies can be broadly categorized by structural component type. Columns, walls, diaphragms, collectors, and chords form an integrated seismic load path and must be addressed holistically in retrofit projects. FRP strength and detailing design is often delegated to an FRP vendor-provided engineer, which leads to many disconnects between the integrated seismic system and the component action requirements that are affected by supplemental FRP. More clear guidance is needed both for consistency in design applications and for mitigating disconnects in information between the building project engineer and FRP providers. As is common practice with the design of typical seismic force-resisting systems, the building project engineer should be comfortable taking responsible charge for the entirety of the structural system, including the enhancement and interaction of FRP-retrofitted components. Indeed, certain jurisdictions do not permit delegated or deferred design of FRP strengthening at all: the Division of the State Architect (DSA), California Healthcare Agency, HCAi / OSHPD, and the City of West Hollywood do not accept deferred approval of FRP systems, and the structural engineer of record (SEOR) is therefore responsible for the full integrated design of the combined FRP and seismic system. The following design objectives are beginning to be better documented in FRP guidelines and consensus standards update proposals.

Shear Strengthening. For existing reinforced concrete columns with inadequate transverse reinforcement, the primary objective of FRP shear strengthening is to prevent brittle shear failure and ensure flexure-controlled behavior. FRP is proportioned to supplement deficient shear capacity such that the nominal shear strength exceeds the shear corresponding to development of expected flexural strength, establishing a clear hierarchy of strength and failure mode consistent with ASCE 41 objectives.

Ductility and Confinement. FRP jacketing is also widely used to enhance column ductility through confinement of the concrete core. Two complementary approaches are commonly used in practice:

Both approaches are recognized in recent retrofit guidance and allow FRP-confined columns to be integrated consistently into ASCE 41 nonlinear analysis and acceptance frameworks. NIST Guidelines for Seismic Design and Evaluation of Concrete Members Retrofitted Using Externally Applied FRP PART I: FRP Jacketed Concrete Columns provides further recommendations on modeling and acceptance criteria which is analogous to the provisions of conventional concrete columns.

For reinforced concrete shear walls, FRP retrofit objectives depend strongly on wall geometry, aspect ratio, existing reinforcement, and quality of existing concrete. It is noted that older buildings have been known to exhibit concrete strength as low as 1,500 psi or less, which can limit FRP bond.

Flexure-Controlled Wall Retrofits. Where geometry permits, FRP can be designed to increase shear strength such that flexural yielding of the vertical reinforcement governs wall response. In this case, wall behavior is governed by flexure, which is treated as deformation-controlled under ASCE 41, and FRP debonding does not control global wall performance.

Shear-Controlled Wall Retrofits With Deformation Capacity. Many existing walls are too squat to be practically converted to flexure-controlled behavior. Recent large-scale experimental programs, including testing performed by Simpson Strong-Tie in collaboration with academic and industry partners, have demonstrated that shear-controlled walls can nevertheless achieve stable, deformation-compatible behavior when robust FRP anchorage is provided (while the Simpson Strong-Tie results are not published, for example, the diaphragm test programs reported in Virginia Tech Report CE-VPI-ST-23-07 and the FRPRCS-16 work of Zhang et al. show similar behavior for diaphragm shear and collectors sustaining strength beyond delamination). In these tests, anchors maintained FRP tension capacity after initial debonding, allowing sustained lateral strength, stable hysteretic response, and distributed damage.

The City of West Hollywood FRP guidelines (Appendix F Sections 8.2.2, 8.3.2, and 10.0) explicitly recognize this deformation-controlled shear behavior and prescribe modeling parameters and acceptance criteria for FRP-strengthened shear walls whose governing action remains shear-controlled. These provisions are aligned with ASCE 41-23 wall acceptance criteria and are supported by observed experimental behavior.

Seismic retrofits frequently identify deficiencies in diaphragm shear strength, collector tension capacity, and chord continuity. FRP offers distinct advantages for these components due to its minimal added thickness (e.g., roughly a quarter inch) and ability to be installed on the top surface of slabs, thereby avoiding conflicts with architectural finishes and mechanical systems. Recent research and guideline development have clarified appropriate strain limits, anchorage requirements, and acceptance criteria for these applications.

FRP Tension Ties and Collectors. Collectors and tension ties are typically classified as force-controlled components. Research by Zhang et al. and others has demonstrated that debonding governs usable FRP strain in long and thick ties. Anchorage detailing directly influences whether conservative lower-bound or median debond strain limits are appropriate. Current jurisdictional guidance, for example West Hollywood Guideline Appendix F, Section 13.0, generally requires at least moderate anchorage, permitting median strain limits consistent with experimentally observed behavior. It is noted that minimum concrete thickness limitations are generally consistent for FRP anchors as to other epoxy anchor types.

FRP for Diaphragm Shear. Beam analogy remains the predominant framework for evaluating diaphragm shear. Experimental studies conducted at Virginia Tech have demonstrated that FRP can significantly enhance diaphragm shear strength and stiffness (concrete component yield deformations are generally constant for a given geometry, but strength is increased with FRP). Debond strain limits developed for column applications have been shown to provide reasonable predictions for diaphragm strengthening when adequate anchorage is provided.

ASCE 41 Tier 3 procedures permit both linear and nonlinear analysis methods to establish seismic demands. Beam analogy remains common for diaphragms and collectors, while finite-element shell models with section cuts are increasingly used to capture force transfer and amplification effects. Regardless of the analysis method employed, engineers must ensure that demand estimation is consistent with the assumed load path and deformation mechanisms.

FRP behaves as a linear-elastic material and, absent special detailing, is classified as force-controlled under ASCE 41. However, experimental evidence demonstrates that when appropriate anchorage is provided, FRP-strengthened components can sustain significant deformation while maintaining load-carrying capacity. As a result, identifying realistic usable strain limits, rather than ultimate fiber rupture strain, is central to seismic FRP design. It is noted that the West Hollywood Guidelines, Appendix F Sections 8.1 and 8.2, provide guidance for component action classification for both existing and FRP-retrofitted components in the context of ASCE 41.

While FRP systems have traditionally relied on adhesive bond to the concrete, progressive debonding is brittle in nature, and the combination of concrete cracking and debonding can degrade concrete more than concrete alone. Given the variability of seismic response, FRP anchoring can provide a more reliable load transfer mechanism even after delamination has occurred, allowing for more deformation capacity of the strengthened component. Recent large-scale testing, including programs conducted at University of Aukland and Virginia Tech, has demonstrated that properly detailed anchorage maintains FRP tension after debonding, limits damage localization, and significantly enhances reliability and deformation capacity.

Several anchorage approaches are used in practice, and the article distinguishes among them because their reliability can differ:

Effective anchorage design prioritizes avoidance of brittle concrete failure modes, promotion of load redistribution following debonding, and constructability. Experimental results indicate that anchored FRP provides the most robust and reliable seismic performance. Testing has also demonstrated that the type of anchorage used can result in differences in overall performance, which may be considered depending on the engineer’s desired performance considerations. The traditional embedded FRP anchors have been shown to have inconsistent results due to the required specialized installation procedures, the skill level of the installer and the inability to perform any meaningful inspection once the anchors are installed. Prefabricated composite plate anchors with conventional concrete screws or wedge anchors have been shown to provide improved performance when compared to the FRP anchors, while they are more easily installed and can be easily inspected on site. Again, minimum concrete thickness is generally aligned with other applications of the analogous anchor types.

FRP retrofit must be evaluated at the system level to ensure continuity of the seismic load path and compatibility with existing reinforcement and adjacent components. Strut-and-tie representations are often useful for understanding node behavior and force transfer. Construction quality control is critical, as FRP performance is highly sensitive to surface preparation, installation procedures, and inspection.

The City of West Hollywood has adopted comprehensive FRP retrofit guidelines for non-ductile concrete buildings, representing one of the most complete jurisdictional frameworks for seismic FRP strengthening currently in use. These guidelines integrate ASCE 41 analysis procedures with explicit FRP design requirements for columns, walls, diaphragms, collectors, and chords.
In parallel, the California Department of Health Care Access and Information (HCAi) is reviewing FRP preapproval criteria structured similarly to the West Hollywood provisions. At the national level, updates to ACI 440 and ACI 369 are underway, informed by recent research, testing, and practitioner experience.

Recent research and ordinance-level guidance have substantially advanced the state of practice for FRP seismic strengthening of existing concrete structures. Key lessons include the importance of deformation-compatible design, the central role of anchorage in governing performance, and the need to evaluate retrofit measures at the system level. As national standards continue to evolve, practicing engineers now have a clearer and more reliable framework for employing FRP in seismic retrofit projects. Effective FRP seismic design requires not only robust detailing but also clearly specified field testing for FRP fabric as well as anchorage systems. ■

About the Authors

Garrett Hagen, SE is a Principal at Degenkolb Engineers, with 15 years of experience in design, evaluation, and retrofit of buildings in high-seismic regions. Based in Southern California, his work focuses on performance-based seismic engineering, seismic risk mitigation, and the implementation of innovative structural systems.

Aniket Borwankar is the Director of Strategic Engineering at Fyfe (a Henkel Company), based in Southern California, where he works within the Civil Composites team. He brings over 17 years of experience in structural engineering, including a decade focused on the design, evaluation, and retrofit of building systems, and more than 7 years specializing in fiber-reinforced polymer (FRP) composites.

References

ACI 440.2R-23, Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures, American Concrete Institute, Farmington Hills, MI, 2023.

ASCE/SEI 41-23, Seismic Evaluation and Retrofit of Existing Buildings, American Society of Civil Engineers, Reston, VA, 2023.

ACI 369.1-22, Standard Requirements for Seismic Evaluation and Retrofit of Existing Concrete Buildings and Commentary, American Concrete Institute, Farmington Hills, MI, 2022.

Zhang, J., del Rey Castillo, E., Kanitkar, R., Borwankar, A. D., and Ramprasath, R., “Proposed Design Method for EB-FRP Ties Debond Strain Encompassing Short/Long and Thin/Thick Ties,” SP-360, FRPRCS-16, American Concrete Institute, 2024.

del Rey Castillo, E., et al., “FRP Tension Ties: State-of-the-Art Review of Existing Design Guidance for Debonding Capacity and Applicability to Concrete Diaphragm Seismic Strengthening,” Journal of Composites for Construction, V. 26, No. 2, 2022, p. 04022014.

Virginia Tech, CE-VPI-ST-23-07, FRP Retrofit of Deficient Reinforced Concrete Horizontal Lateral Force Resisting Systems, Virginia Polytechnic Institute and State University, Blacksburg, VA.

NIST GCR 17-917-45, Guidelines for Seismic Design and Evaluation of Concrete Members Retrofitted Using Externally Applied FRP, Part I: FRP Jacketed Concrete Columns, National Institute of Standards and Technology, Gaithersburg, MD.

City of West Hollywood, Mandatory Retrofit Program for Non-Ductile Concrete Buildings, Ordinance 17-1011, Appendix F: Guidelines for NDC Retrofits Using FRP, Issued October 1, 2025, Revised November 6, 2025.