The restrictions on the strength of reinforcement in ACI 318-08 Building Code Requirements for Structural Concrete, adopted by the 2009 IBC, are not clear to many engineers. This article presents a table to clearly point out and discuss the restrictions. In the table, fy is the specified yield strength of reinforcement and fyt is the specified yield strength of transverse reinforcement.


ACI 318-08 Section
Provision
Commentary

3.5.3.2 Deformed reinforcing bars shall conform to one of the ASTM specifications listed in 3.5.3.1 except that for bars with fy exceeding 60,000 psi, the yield strength shall be taken as the stress corresponding to a strain of 0.35 percent. See 9.4. This section was introduced in ACI 318-71. In general, ACI 318 does not override ASTM specifications. But the committee felt it necessary to limit the yield strength fy to that at a strain of 0.35 percent, instead of 0.5 percent as then specified in ASTM. At that time, there was little use of Grade 75 reinforcement. There was concern that the concrete in a column would crush at levels around 0.3 percent compressive strain and thus, the steel strength based on an fy at 0.5 percent strain would not be fully additive to the concrete strength. The limiting strain of 0.35 percent was chosen as a compromise, considering that the concrete would have some long-term strain from creep and shrinkage, in addition to the short-term crushing strain of 0.3 percent.

9.4 The values of fy and fyt used in design calculations shall not exceed 80,000 psi, except for prestressing steel and for transverse reinforcement in 10.9.3 and 21.1.5.4. The reason for the 80,000 psi limit in Section 9.4, also introduced in ACI 318-71, was explained in the Commentary to ACI 318-71 as follows: “An upper limit of 80,000 psi is placed on reinforcing yield strength (other than prestressing tendons) in Section 9.4.2. Committee 318 did not choose to recommend any strength above 80,000 psi without adding other restrictions, since this steel strength is about equal to the ultimate strain in concrete multiplied by the modulus of elasticity of steel. At present the highest yield strength covered by ASTM standards is 75,000 psi, and this grade is not widely used.” It should be noted that ASTM A615-09b and ASTM A706-09b, referenced by ACI 318-11, have both added a Grade 80 steel having a yield strength of 80,000 psi. The upper limit of 80,000 psi remains unchanged.

10.9.3 Volumetric spiral reinforcement ratio, ρs, shall be not less than the value given by

ρs = 0.45 (Ag/Ach — 1) f’c/fyt(10-5)

where the value of fyt used in Eq. (10-5) shall not exceed 100,000 psi. For fyt greater than 60,000 psi, lap splices according to 7.10.4.5(a) shall not be used.

Sections 9.4 and 10.9.3 were modified for the first time in ACI 318-05 to allow the use of spiral reinforcement with specified yield strength of up to 100,000 psi. This was done to address the concern that confinement reinforcement often creates congestion in reinforced concrete members. Research has shown that reinforcement with yield strength up to 100,000 psi can be used for confinement, without any detriment to member performance. Note that Section 3.5.3.3 in effect requires such reinforcement to conform to ASTM A1035. The way that one knows that is by reading 3.5.3.1 and 3.5.3.3. Section 3.5.3.1 requires deformed reinforcing bars to conform to one of the specifications listed in that section, except as permitted by Section 3.5.3.3. The specifications listed are: (a) Plain carbon steel: ASTM A615; (b) Low alloy steel: ASTM A706; (c) Stainless steel: ASTM A955; (d) Rail steel and axle steel ASTM A996 (rail-steel is required to be Type R). None of the above standards provides for a yield strength exceeding 80,000 psi.

11.4.2 The values of fy and fyt used in design of shear reinforcement shall not exceed 60,000 psi, except the value shall not exceed 80,000 psi for welded deformed wire reinforcement. The upper limit on the yield strength of transverse reinforcement is imposed to limit the width of possible shear cracks to acceptable levels. It should be noted that this restriction does not mean that higher-strength bars may not be used as transverse reinforcement. It simply means that a higher yield strength may not be used for the calculation of shear strength.

21.1.5.1 Requirements of 21.1.5 apply to special moment frames and special structural walls and coupling beams. It is important to note that the four restrictions in 21.1.5.2 through 21.1.5.5 do not apply to anything other than special moment frames and special shear walls (including coupling beams, if any). Special detailing is required for structures assigned to Seismic Design Category (SDC) D, E, or F; it may be used in structures assigned to lower SDCs.

21.1.5.2 Deformed reinforcement resisting earthquake-induced flexural and axial forces in frame members, structural walls, and coupling beams, shall comply with ASTM A706. ASTM A615 Grades 40 and 60 reinforcement shall be permitted in these members if: (a) The actual yield strength based on mill tests does not exceed fy by more than 18,000 psi; and (b) The ratio of the actual tensile strength to the actual yield strength is not less than 1.25. Restrictions (a) and (b) are part of ASTM A706, but not ASTM A615. If actual yield strength of reinforcement is much higher than specified, that reinforcement in tension will make a structural member extra-strong in flexure. That member, then, will attract higher shear forces to itself in an actual earthquake situation. If its shear strength is not correspondingly increased, which typically is not done, brittle shear failure may result. Thus, restriction (a) is extremely important. Restriction (b) ensures that reinforcement will gain or at least retain strength over a large range of inelastic deformations; a structural member with that reinforcement in tension, then, will also not lose strength over a significant range of inelastic displacements. This is very important in seismic applications.

21.1.5.3 Prestressing steel resisting earthquake-induced flexural and axial loads in frame members and in precast structural walls shall comply with ASTM A416 or A722. The 2008 edition is the first edition of ACI 318 to permit a special moment frame beam to be prestressed, provided the four conditions set forth in Section 21.5.2.5 are satisfied. It is also allowed in special precast shear walls. The prestressing steel in such members is required to comply with ASTM A416 or A722.

21.1.5.4 The value of fyt used to compute the amount of confinement reinforcement shall not exceed 100,000 psi. Two of the functions of transverse reinforcement in a reinforced concrete member are to confine the concrete and to act as shear reinforcement. For confinement purposes, the upper limit on fyt is 100,000 psi.

21.1.5.5 The value of fy or fyt used in design of shear reinforcement shall conform to 11.4.2. For purposes of shear strength computation, the upper limit on fyt remains 60,000 psi — except for welded deformed wire reinforcement, which is 80,000 psi. It should again be noted that this restriction does not mean that higher-strength bars may not be used as transverse reinforcement.

 




S.K. Ghosh Associates Inc. is a structural seismic and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. Presidents S. K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at skghosh@aol.com and susandowty@gmail.com, respectively, or at www.skghoshassociates.com.

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