Bundled Reinforcement in Reinforced Concrete Structures: Provisions, Requirements, and Design Considerations

General Requirements for Bundled Reinforcement (ACI 318-19, Clause 25.6.1.1 – 25.6.1.3)

According to ACI 318-19, a bundle consists of two to four parallel bars in contact with each other, arranged to act as a single reinforcement unit. The number of bars in a bundle shall not exceed four (Clause 25.6.1.1). Bars within a bundle must be enclosed by transverse reinforcement such as ties or stirrups to provide adequate confinement and maintain their relative positions during construction (Clause 25.6.1.2).

ACI permits various configurations—triangular, square, or rectangular—depending on the number of bars. Nonetheless, no more than two bars shall lie in the same plane to prevent voids and ensure proper concrete consolidation around the group. These restrictions maintain effective bond between the surrounding concrete and each bar within the bundle.

Confinement and Transverse Reinforcement

For members in compression, bundled bars must be completely enclosed by transverse reinforcement that provides confinement to the concrete core and prevents local buckling of the bundled bars. As specified in Clause 25.6.1.2, the minimum size of the enclosing tie shall not be smaller than No. 4 (12 mm). This requirement ensures that all bars within the bundle remain confined and act as a unified element, particularly in columns where concentrated compressive stresses are critical.

Termination and Lap Splicing (ACI 318-19, Clause 25.6.1.4 – 25.6.1.5)

When bundled reinforcement is terminated or spliced, ACI 318-19 imposes specific spacing and staggering requirements to prevent stress concentration.

According to Clause 25.6.1.4, when bundled bars are terminated in tension zones, the ends of individual bars within the bundle shall not terminate at the same point. Instead, termination points must be staggered by at least 40 bar diameters (40 d_b) between adjacent bars. This staggering prevents the formation of weak planes or abrupt stress changes that may initiate cracking or slip.

For lap splices, Clause 25.6.1.5 specifies that the development length for each bar in a bundle must be increased relative to that of a single bar.

• For two-bar bundles, no increase is required.
• For three-bar bundles, the development length must be increased by 20 %.
• For four-bar bundles, the increase must be 33 %.

These factors account for the reduced effectiveness of bond in the inner bars of the bundle and the limited surface area available for stress transfer.

Equivalent Bar Diameter Concept (ACI 318-19, Clause 25.6.1.6)

To simplify the application of spacing, cover, and development-length provisions, ACI 318-19, Clause 25.6.1.6 introduces the concept of an equivalent bar diameter (d_eq).
A bundled set of n identical bars is treated as a single equivalent bar whose cross-sectional area equals the sum of the individual bar areas. The equivalent diameter is calculated using the following relationship:

where is the nominal diameter of an individual bar.

Example:
For a three-bar bundle of No. 6 bars (20 mm each),

This equivalent diameter is then used to determine minimum concrete cover, clear spacing, and development length. In essence, the bundle is considered to behave as a single larger bar located at the centroid of the group.

Development Length in Bundled Bars

The development length of bundled reinforcement must be based on the equivalent diameter described above. ACI specifies that the anchorage length should ensure complete stress transfer between steel and concrete while accounting for the reduced bond efficiency of inner bars.

For tension members, the increased development lengths specified in Clause 25.6.1.5 (20 % or 33 %) must be applied directly to the development length calculated for a single bar using the equations in Section 25.4.2.
This approach maintains structural reliability while minimizing the risk of premature bond failure, particularly in heavily reinforced regions such as beam-column joints or anchorage zones.

Cover Thickness and Durability Considerations

Although ACI does not provide separate cover requirements specifically for bundled bars, Clause 20.6.1.3 requires that minimum clear cover be measured from the outermost surface of the outer bar in the bundle.
Therefore, the cover and spacing provisions are applied as though the bundle were a single bar of diameter .

For members exposed to harsh environments—such as soil contact or marine exposure—the design cover must satisfy the durability provisions in Table 20.6.1.3.1(a) of ACI 318-19, ensuring adequate protection against corrosion.
Because inner bars in a bundle are more difficult to inspect and repair, providing slightly larger cover or using corrosion-resistant coatings is often recommended as a best practice.

Practical Recommendations and Summary

Bundled reinforcement can be an effective design solution where space is limited, but its application demands careful attention to code requirements. The following key recommendations are derived from ACI 318-19:

• Do not exceed four bars per bundle (Clause 25.6.1.1).
• Provide complete confinement using ties or stirrups not smaller than No. 4 (12 mm) (Clause 25.6.1.2).
• Stagger bar terminations within the bundle by at least 40 d_b (Clause 25.6.1.4).
• Increase development length by 20 % for three-bar bundles and 33 % for four-bar bundles (Clause 25.6.1.5).
• Compute spacing, cover, and development based on the equivalent bar diameter (Clause 25.6.1.6).
• Ensure concrete placement allows for full consolidation around bundles to prevent honeycombing or voids.

Proper detailing and execution of bundled reinforcement contribute to enhanced safety, durability, and constructability of reinforced concrete structures. When applied correctly, bundles act as efficient reinforcement units without compromising bond integrity or structural performance.

References

• American Concrete Institute (ACI). Building Code Requirements for Structural Concrete (ACI 318-19) and Commentary (ACI 318R-19). Farmington Hills, MI, USA: ACI, 2019.
• ACI Committee 315. Details and Detailing of Concrete Reinforcement (ACI 315R-18). American Concrete Institute, 2018.
• StructurePoint. Bundled Reinforcement in Concrete Columns and Bridge Piers—ACI 318, CSA A23.3, AASHTO. Technical Note 2020.