Trends for Fire Qualification, Design of Post-Installed Reinforcing Bars & Anchors

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Fire design of structural connections is vaguely addressed and loosely understood. Structural engineers are given prescriptive code requirements to address structural capacity during fire events, leaving unanswered questions, including the applicability of these requirements to adhesives used in structural connections. In some cases, engineers and jurisdictions have steered away from such adhesives due to lack of clarity on this issue.

Chapter 6, Types of Construction, of the 2024 International Building Code (IBC) provides basic fire-resistance rating requirements for structural components of buildings. While no specific language is provided in Chapter 6 that directly addresses the fire-rating requirements of connections, it can be inferred that connections should be able to carry at least the lower of the fire-resistance rating of the two connected members, including both cast-in and post-installed solutions. Where fire ratings are not required, e.g., for nonstructural elements, it is assumed that sprinkler systems and other active fire protection measures are sufficient to address fire events.

IBC 2024 Chapter 7, Fire and Smoke Protection Features, expands on the requirements with the following statements that address connections:

Minimum cover requirements for reinforced concrete and minimum thickness of insulating materials are provided for reinforced concrete and steel members in Section 721, Prescriptive fire resistance. Reinforced concrete cover requirements align with the principles outlined in ACI 216.1-14(19), Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies, which is directly referenced by IBC Section 722, Calculated fire resistance. These prescriptive cover requirements incorporate various reinforced concrete failure modes without further discussion of connections.

Existing design of structural connections often implicitly accounts for the fire design scenario through the prescriptive requirements of IBC Chapter 7. However, some aspects of fire resistance of connections may not be fully addressed and may merit additional design checks, including the use of structural adhesives. Adhesives used in both steel-to-concrete and concrete-to-concrete fire-rated connections often include polymers that are sensitive to elevated temperatures. Even in cases where the minimum cover and protection requirements of the IBC are met, the temperatures incurred by the adhesive layer in these connections could result in reduced adhesive capacities that should be checked against the design structural loads corresponding to the fire event.

To address the issue of adhesive strength under fire exposure, multiple assessment and design procedures are underway or have recently been completed. Post-installed reinforcing bar products are evaluated in accordance with the International Code Counsel Evaluation Service (ICC-ES) Acceptance Criteria for Post-installed Adhesive Anchors and Reinforcing Bars in Concrete Elements (AC308), which serves as a basis for Evaluation Service Reports (ESRs) and a forthcoming ACI 355 document titled Qualification of Post-Installed Reinforcing Bar systems in Concrete. In AC308, criteria for evaluating the capacity of post-installed reinforcing bars at various temperatures are provided. The AC308 assessment of temperature resistance provides the necessary input parameters for design using the “Resistance Integration Method,” where the adhesive embedment is broken into contributing segments. Similarly, the resistance integration method can be applied to adhesive anchors connecting steel to concrete. Equation 1 and Figure 1 demonstrate the basic concept of design using this methodology (ignoring other applicable design factors), which can be summarized as follows:

This Resistance Integration Method will be formalized into future American design documents and guides. While other anchoring failure modes may also be affected by fire conditions, the extent of the effect and the necessity to address them explicitly remains unclear, as does the fire capacity for other shear fracture mechanisms in concrete, including sectional shear, punching shear, and interface shear, none of which are explicitly accounted for in design. In European design, highly conservative prescriptive capacities are provided for anchorage failure modes, but these requirements do not account for reduced temperatures due to typical fire protection on anchored connections. IBC Section 704.5 currently provides a prescriptive stand-in to address these failure modes. Further study should be made to verify the broad applicability of IBC Section 704.5 to anchorage and additional study should be made to determine the fire resistance of other shear failure mechanisms in reinforced concrete.

These recent and upcoming advances in qualification and design of adhesives for post-installed reinforcing bars and anchors will provide clarity for designers, paving the way for confident fire-rated design of connections in combination with the existing prescriptive requirements of the IBC. ■

About the Author

Kenton McBride, Ph.D, PE, is director of codes and standards at Hilti North America.