Code Update: ACI 318: High-Strength Reinforcing Bars
AMY M.R. TRYGESTAD, P.E., F.ACI (above)
The motivation for developing high-strength reinforcing bar (HSRB) is driven from the construction community. Design practitioners are looking for improved, efficient and sustainable design solutions, while contractors are looking to address rebar congestion and scheduling/cost issues related to reinforcing bars.
HSRB is regarded as any reinforcing bar with a yield strength greater than 60,000 psi. The adoption of HSRB started when Grade 75 bars appeared in the 2001 edition of ASTM 955. Grade 100 bars followed in the inaugural 2004 edition of ASTM 1035. The 2007 editions of these specifications first appeared in ACI 318-08, with ASTM 1035 containing requirements for Grade 100, and are based on the ACI design guide ITG-6R-10 – Design Guide for the Use of ASTM A1035/A1035M Grade 100 (690) Steel Bars for Structural Concrete.
In ACI 318-05, a yield strength of 100,000 psi was permitted for confinement reinforcement for use in non-seismic applications and then in ACI 318-08 for use in seismic applications.
The 2009 editions of ASTM A615 and ASTM A706 were the first to include requirements for Grade 80 reinforcing bars, which were adopted into ACI 318-11.
The design of any reinforced concrete member must satisfy the fundamental requirements for strength and serviceability as prescribed in ACI 318. With respect to reinforcing bars, the basic mechanical properties important in achieving safe and serviceable designs include the following:
• Yield strength, fy
• Tensile-to-yield strength (ductility) ratio, T/Y = fu ⁄fy
• Strain (elongation) at tensile strength, εu
• Length of yield plateau
The yield strength for compression reinforcement is limited to 80,000 psi for use in applications other than special seismic systems. This limit is imposed because bars with yield strengths greater than approximately 80,000 psi will not contribute to increased compression capacity: at a strain of 0.003 at the extreme concrete compression fiber of a reinforced concrete section (the strain assumed at crushing of the concrete), the maximum usable stress in the reinforcing steel would be 87,000 psi based on linear-elastic behavior (ACI 188.8.131.52). Note that Grade 100 longitudinal reinforcement may be used in columns provided the aforementioned limit of 80,000 psi is used in the calculations in accordance with ACI 318.
Shear and Torsion Reinforcement
The limit for shear and torsion reinforcement remains at 60,000 psi, intended to control the width of inclined cracks that tend to form in reinforced concrete members subjected to these types of forces. References to research that supports the use of 100,000 psi reinforcing bars for lateral support of longitudinal bars and concrete confinement, including special seismic systems, can be found in ACI R184.108.40.206.
Longitudinal Bars in Special Seismic Systems
Only Grade 60 steel reinforcement is permitted in ACI 318-14 for use in special seismic systems (special moment frames, special structural walls and all components of special structural walls, including coupling beams and wall piers) to resist the effects caused by flexure, axial force, and shrinkage and temperature.
The limitation was included because, at that time, there was insufficient data to confirm applicability of existing ACI 318 provisions for members with higher grades of reinforcement. Test results for HSRB in seismic applications were subsequently referenced in NIST GCR 14-917-30 (NIST 2014). Based primarily on these test results, it’s anticipated that the use of ASTM A706 deformed reinforcing bars with a yield of 80,000 psi will be permitted in the 2019 edition of ACI 318 for use in special seismic systems.
Provisions for the use of higher-strength reinforcing bars have been incorporated into ACI 318 as new reinforcing steel products have become available, and designers can specify and utilize HSRB based on these current provisions and limitations. Several jurisdictions in the United States also permit the use of HSRB above and beyond the limitations set forth in the current ACI 318. It’s important to note that some provisions of ACI 318 may need adjustment before HSRB can be used in applications where the yield strength is greater than the limits (for example, using longitudinal tension reinforcement with a yield strength of 100,000 psi in beams).
As new research funded by the Charles Pankow Foundation, the CRSI Foundation and the ACI Strategic Development Council becomes available related to the use of HSRB, it’s anticipated that ACI 318 will be updated accordingly.
For additional information regarding HSRB, see CRSI Technical Note ETN-M-8-16: High-Strength Reinforcing Bars at www.crsi.org and the Applied Technology Council ATC 115: Roadmap for the Use of High-Strength Reinforcement in Reinforced Concrete Design at www.atcouncil.org.
Amy M.R. Trygestad, P.E., F.ACI, is director of Codes & Standards, and Michael Mota, PhD, P.E., F.ACE, F.ASCE, F.SEI, is vice president of Engineering, both with the Concrete Reinforcing Steel Institute; e-mail: [email protected] and [email protected], respectively.