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).Although the changes from
ACI 318-08 to ACI 318-11 are not as those between ACI 318-05 and ACI 318-08,
some of the changes in the latest cycle are definitely significant.
Chapter 3 � Materials
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 (550 MPa).
However, the use of this reinforcement is not permitted by Section 21.1.5 in
special moment frames and special structural walls.
Chapter 7 � Details of Reinforcement
- In new Section 188.8.131.52, rigorous detailing requirements for complete
circular ties around longitudinal bars located around the perimeter of a
circular column are given.
- New requirements have been added in Sections 184.108.40.206 through 220.127.116.11
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 18.104.22.168, the area of shear reinforcement parallel to the
longitudinal axis of the beam is now required to be not less than 0.0025bws2,
rather than 0.0015bws2, wherebw is web width ands2 is center-to-center spacing
of longitudinal shear reinforcement.
Chapter 18 � Prestressed Concrete
- The permissible stress of 0.82fpy (fpy = specified yield strength of
prestressing steel) but not greater than 0.74fpu (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.
- The formulas for estimating friction loss in post-tensioning tendons have
been eliminated from Section 22.214.171.124 as being text-book material.
Chapter 21 � Earthquake-Resistant Structures
- 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 includeE, withE assumed to
be twice that prescribed by the general building code. In the new Section
126.96.36.199 of ACI 318-11, the multiplier of two has been increased to the
overstrength factor, ?0= 3, of the intermediate moment frame.
- In ACI 318-08 Section 188.8.131.52, 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: 1. d/4;
- eight times the diameter of the smallest longitudinal bars;
- 24 times the diameter of the hoop bars; and
- 12 inches. In ACI 318-11 Section 184.108.40.206,
Item (2) 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 (3) has been deleted. Item (4) 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 220.127.116.11
requires that in columns with circular hoops, the minimum number of longitudinal
bars be six for effective confinement.4. Section 18.104.22.168(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 (150mm) 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.
Development of wall horizontal reinforcement in confined boundary element5.
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 22.214.171.124 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 126.96.36.199. 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
An excellent new Table R21.9.1 in the Commentary effectively summarizes the new
APPENDIX D � Anchoring to Concrete
- 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.188.8.131.52). A similar provision concerning shear is
included in Section D.184.108.40.206.
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
(a) Ensure failure of ductile steel anchor ahead of the brittle failure of
concrete (Section D.220.127.116.11(a)). This now involves the new concept of a stretch
(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.18.104.22.168(b)).
(c) Design for the maximum tension force that can be transmitted by a
non-yielding attachment (Section D.22.214.171.124(c)).
(d) Design for the maximum tension force obtained from design load combinations
involvingE, with *E *multiplied by ?0 (Section D.126.96.36.199(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.
- 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.
- 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.