Significant changes in ACI 318-11

January 2012 » Features » CODES ACI 318
S. K. Ghosh

The American Concrete Institute (ACI) has published Building Code Requirements for Structural Concrete (ACI 318-11) and Commentary (ACI 318R-11) and it has been adopted by the 2012 International Building Code (IBC). Whenever the 2012 IBC becomes effective in a state or other jurisdiction, as it will be in the State of California on Jan. 1, 2014, ACI 318-11 will be law within that state or other jurisdiction.

Although the changes from ACI 318-08 to ACI 318-11 are not as extensive or as substantive as those between ACI 318-05 and ACI 318-08, some of the changes in the latest cycle are definitely significant and are the subject of this article.

Chapter 3 — Materials
1. ASTM A615 "Standard Specification for Deformed and Plain Carbon Steel Bars for Concrete Reinforcement" and ASTM A706 "Standard Specification for Low-Alloy Steel Deformed and Plain Bars for Concrete Reinforcement" have both added a Grade 80 reinforcement, having a minimum yield strength of 80,000 psi. Note that the use of this reinforcement is not permitted by Section 21.1.5 in special moment frames and special structural walls. Available data were judged to be insufficient to permit such usage.

Chapter 7 — Details of Reinforcement
1) In new Section 7.10.5.4, rigorous detailing requirements for complete circular ties around longitudinal bars located around the perimeter of a circular column are given.

2) New requirements have been added in Sections 7.12.3.2 through 7.12.3.5 concerning temperature and shrinkage reinforcement in post-tensioned slabs.

Chapter 9 — Strength and Serviceability Requirements
APPENDIX C — Alternative Load and Strength Reduction Factors

The strength design load combinations in Sections 9.2 and C.9.2 have been revised to be fully consistent with those of ASCE/SEI 7-10.

Chapter 11 — Shear and Torsion
In Section 11.7.4.2, the area of shear reinforcement parallel to the longitudinal axis of the beam is now required to be not less than 0.0025 bws2, rather than 0.0015 bws2, where bw is web width and s2 is center-to-center spacing of longitudinal shear reinforcement.

Chapter 18 — Prestressed Concrete
1) The permissible stress of 0.82 fpy (fpy = specified yield strength of prestressing steel) but not greater than 0.74 fpu (fpu = specified tensile strength of prestressing steel) in prestressing steel immediately upon prestress transfer in Section 18.5.1 has been eliminated based upon practical experience with post-tensioned concrete members.

2) The formulas for estimating friction loss in post-tensioning tendons have been eliminated from Section 18.6.2.1 as being text-book material.

Chapter 21 — Earthquake-Resistant Structures

1) In Section 21.3.3 of ACI 318-08, the required shear strength of a column of an intermediate moment frame was permitted to be calculated as the maximum shear obtained from design load combinations that include E, with E assumed to be twice that prescribed by the legally adopted general building code. In the new Section 21.3.3.2 of ACI 318-11, the multiplier of two has been increased to the overstrength factor, Ω0= 3, of the intermediate moment frame.

2) In ACI 318-08 Section 21.5.3.2, the spacing of hoops within the region of potential plastic hinging at each end of a special moment frame beam could not exceed the smallest of: (a) d/4; (b) eight times the diameter of the smallest longitudinal bars; (c) 24 times the diameter of the hoop bars; and (d) 12 inches.

In ACI 318-11 Section 21.5.3.2, Item (b) has been changed to six times the diameter of the smallest primary flexural reinforcing bars excluding longitudinal skin reinforcement required by Section 10.6.7. Item (c) has been deleted. Item (d) now is 6 inches. For deeper beams, this is a significant decrease in the spacing of confinement reinforcement in regions of potential plastic hinging.

3) New Section 21.6.3.2 requires that in columns with circular hoops, the minimum number of longitudinal bars be six for effective confinement.

4) Section 21.9.6.4(e), applicable to special shear walls with special boundary elements, has been expanded to: "Horizontal reinforcement in the wall web shall extend to within 6 in. of the end of the wall. Reinforcement shall be anchored to develop fy in tension using standard hooks or heads. Where the confined boundary element has sufficient length to develop the horizontal web reinforcement, and Avfy/s of the web reinforcement is not greater than Ashfyt/s of the boundary element transverse reinforcement parallel to the web reinforcement, it shall be permitted to terminate the web reinforcement without a standard hook or head." See Figure 1.

 
Figure 1. Development of wall horizontal reinforcement in confined boundary element

5) Door and window openings in shear walls often lead to narrow vertical wall segments, many of which have been defined as wall piers in the IBC and in the UBC before it. Wall pier provisions are now included in Section 21.9.8 of ACI 318-11. The dimensions defining wall piers are given in Section 2.2.

Shear failures of wall piers have been observed in previous earthquakes. The intent of Section 21.9.8 is to prescribe detailing that would result in flexural failure preceding shear failure in wall piers. The provisions apply to wall piers considered part of the seismic force-resisting system (SFRS). Wall piers considered not part of the SFRS need to be designed by Section 21.13.

Wall piers having (lw/bw) ≤ 2.5 behave essentially as columns. Section 21.9.8.1 requires them to be detailed like columns. Alternative requirements are provided for wall piers having (lw/bw) > 2.5.

Wall piers at the edge of a wall are addressed in Section 21.9.8.2. Under in-plane shear, inclined cracks can propagate into segments of the wall directly above and below the wall pier. Shear failure within the adjacent wall segments can occur unless sufficient reinforcement is provided in those segments (Figure R21.9.8).

An excellent new Table R21.9.1 in the Commentary effectively summarizes the new requirements.

APPENDIX D — Anchoring to Concrete
1) The onerous nature of seismic design imposed by ACI 318-08 Section D.3.3 on anchors in Seismic Design Category (SDC) C or higher is alleviated and the seismic design of anchors is made considerably more reasonable. Where the tension component of the strength-level earthquake force applied to the anchor or group of anchors is equal to or less than 20 percent of the total factored anchor tensile force, the seismic design requirements of Section D.3.3 to prevent a brittle tension failure of anchors simply do not apply any more (Section D.3.3.4.1). A similar provision concerning shear is included in Section D.3.3.5.1.

Where the seismic component of the total factored tension demand on an anchor or a group of anchors exceeds 20 percent, the following four options have been made available:

(a) Ensure failure of ductile steel anchor ahead of the brittle failure of concrete (Section D.3.3.4.3(a)). This now involves the new concept of a stretch length.

(b) Design anchor for the maximum tension force that can be transmitted by a ductile metal attachment after considering the overstrength and strain hardening of the attachment (D.3.3.4.3(b)).

(c) Design for the maximum tension force that can be transmitted by a non-yielding attachment (Section D.3.3.4.3(c)).

(d) Design for the maximum tension force obtained from design load combinations involving E, with E multiplied by Ω0 (Section D.3.3.4.3(d)).

For an anchor or a group of anchors subject to shear, three options similar to (b), (c), and (d) above have been made available. Unlike ACI 318-08, ductile anchor failure in shear is not an option anymore.

2) The maximum anchor diameter for which the provisions of Sections D.5.2 and D.6.2 can be applied to calculate the concrete breakout strength in tension and shear, respectively, has been increased from 2 inches to 4 inches (Section D.4.2.2). This expansion is based on the results of new tests using larger diameter anchors. However, a new Eq. D-34 has also been introduced for a lower-bound value of basic concrete breakout strength for a single anchor in shear, Vb, to account for the larger diameter anchors.

ACI 318-08 also imposed a 25-inch limitation on anchor embedment depth for the calculation of concrete breakout strength using the provisions of Appendix D. This limitation was effectively removed by Section 1908.1.10 of the 2009 IBC and is now gone from ACI 318-11.

3) An adhesive anchor is defined in Section D.1 as a post-installed anchor, inserted into hardened concrete with an anchor hole diameter not greater than 1.5 times the anchor diameter, that transfers loads to the concrete by bond between the anchor and the adhesive, and bond between the adhesive and the concrete. Failure modes and the corresponding nominal strengths for adhesive anchors are defined, along with requirements for testing and evaluation of adhesive anchors for use in cracked concrete or subject to sustained loads. Failure modes postulated for other anchors apply to adhesive anchors as well, except that the calculation of strength governed by anchor pullout is replaced by the evaluation of adhesive bond strength in accordance with Section D.5.5. The provisions for adhesive anchors include criteria for overhead anchors, seismic design requirements, installation and inspection requirements, and certification of adhesive anchor installers. Separately, a certification program has been established.

Although the changes from ACI 318-08 to ACI 318-11 are not as extensive or as substantive as those between ACI 318-05 and ACI 318-08, some of the changes in the latest cycle are definitely significant.

S.K. Ghosh Associates Inc. is a seismic and code consulting firm located in Palatine, Ill., and Aliso Viejo, Calif. President S. K. Ghosh, is active in the development and interpretation of national structural code provisions.He can be contacted at skghoshinc@gmail.com, or at www.skghoshassociates.com


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