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The American Concrete Institute (ACI) recently released ACI 318-08, Building Code Requirements for Structural Concrete and Commentary. The code is revised by ACI committee 318, Structural Concrete Building Code, which includes a balance of structural engineers, contractors, academic members, government members, and industry members. Practicing structural engineers represent the largest group on the committee, so it’s fair to say that the deliberations and decision-making process about what eventually gets into many sections of the code have significant input from people who use it.

Chapter 1—General Requirements
You will notice in Section 1.1.8 that design requirements for earthquake-resistant structures are now to be determined by the Seismic Design Category (SDC) to which they are assigned. This change makes the ACI code nomenclature compatible with that used by the 2005 ASCE/SEI 7 Standard and the 2006 edition of the International Building Code. Table R1.1.8.1 gives a correlation between SDC classifications and the former terminology of low, medium, and high seismic risk.

Chapters 3, 4, and 5—Materials; Durability Requirements; and Concrete Quality, Mixing, and Placing
In Chapter 3, new requirements for headed shear-stud reinforcement, headed deformed bars, and stainless steel reinforcement are given with appropriate references to ASTM standards. Also, revisions have been made throughout the code to be consistent on how lightweight concrete is addressed. Many of the tables in Chapter 4 have been modified due to the adoption of exposure categories and classes, and the code’s coverage of durability has been reorganized to make it more parallel with the approach used in other international codes. In Chapter 5, the code has adopted the use of three 4-inch by 8-inch (100 x 200 mm) cylinders as equivalent to the use of two 6-inch by 12-inch (150 x 300 mm) cylinders for determining concrete compressive strength. Due to a concern that material properties may change with time, a limit of 12 months was set on historical data used to qualify mixture proportions. Finally, performance criteria are defined for the flexural test "Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete (ASTM C1609)" to qualify the use of steel fiber-reinforced concrete (FRC) mixtures as a replacement for minimum shear reinforcement in Chapter 11.

Chapter 7—Details of Reinforcement
To permit a more consistent application of tolerances in the code and other ACI documents, "specified cover" is used to replace "minimum cover" throughout Chapter 7. Changes were made to the anchorage and splice requirements of structural integrity reinforcement. Continuous top and bottom structural integrity reinforcement must pass through the column core. Also, the type of transverse reinforcement used to enclose structural integrity reinforcement in perimeter beams was more clearly specified.

Chapter 8—Analysis and Design: General Considerations
Section 8.4 on moment redistribution in continuous members was modified to permit moments to be redistributed away from positive moment sections as well as negative moment sections, with unchanged limits on the percentage of the moment that may be redistributed. Lateral deflections of reinforced concrete systems have become an important design parameter, so a new Section 8.7 was introduced to provide a simple modeling procedure for engineers to use when evaluating lateral displacements.

Chapter 9—Strength and Serviceability Requirements
The strength reduction factor, φ, for spirally reinforced columns has been increased from 0.70 to 0.75 to recognize the superior performance of spirally reinforced columns when subjected to extraordinary demands. Similarly, based in part on recent reliability analysis and a statistical study of concrete properties, the φ-factor for plain concrete has been increased from 0.55 to 0.60.

Chapter 10—Flexure and Axial Loads
The most significant change in this chapter is a rewriting of Section 10.10 on slenderness effects in compression members. This change updates the requirements to current practice, while retaining the style of presentation that has been used since 1971. The section was reorganized to reflect the evolution of current practice to the point where second-order effects are primarily considered through the use of computer analysis techniques. The moment magnifier method is retained as an alternate procedure.

Chapter 11—Shear and Torsion
The revisions to achieve a consistent handling of lightweight concrete impact several of the equations in this chapter. Although these equations will have a different appearance, there have not been any significant technical changes related to the shear strength of structural members that are constructed with lightweight concrete.

One of the most significant changes in this chapter is the addition of code requirements to permit the use of headed stud assemblies as shear reinforcement for slabs and footings. These provisions will better regulate the use of this form of shear reinforcement that is widely used in current practice. Both the amount of shear assigned to the concrete, Vc, and the nominal shear strength, Vn = Vc + Vs, are permitted to be larger for headed stud assemblies than for other forms of slab and footing shear reinforcement.

Significant changes were made to the list of members in Section 11.5.6.1 for which minimum shear reinforcement is not required when Vu exceeds 0.5φVc. Based on considerable experimental evidence of a reduction in Vc for deeper members (the so-called size effect), more stringent limits were placed on the depths of beams that could be exempted from the requirement for minimum shear reinforcement. Also based on experimental evidence, a new limit on the depth of hollow core units for which this requirement could be waived was established.

Finally, again based on considerable experimental evidence, the list of beams exempted from the minimum shear reinforcement requirement in Section 11.5.6.1 includes beams that are constructed with steel FRC. This addition represents the first permitted structural use of steel FRC in the ACI code. Finally, in Section 11.7.5, the upper limit for nominal shear friction strength, Vn, has been significantly increased for both monolithically placed concrete and concrete placed against intentionally hardened concrete.

Chapter 12—Development and Splices of Reinforcement
The most significant change to this chapter is the introduction of Section 12.6 for the development of headed deformed bars and mechanical anchorage of reinforcement in tension. The use of headed deformed bars should be a viable alternative to hooked-bar anchorages in regions where reinforcement is heavily congested. Another important change is Section 12.1.3, which specifically calls the designer’s attention to the structural integrity requirements in Section 7.13. There has been concern among committee members that many designers were not aware of these requirements. Section 12.15.3 was added to define the required splice length for splices of different-sized bars, and a coating factor for galvanized reinforcement is given in section 12.2.4(b).

Chapter 13—Two-way Slab Systems
Some dimension limits are given in Section 13.2.6 for the use of shear caps, which are commonly used to increase the shear strength of slabs at slab-column connections. A definition for a shear cap, which is new in this edition of the code, is given in Chapter 2 to distinguish between shear caps and drop panels, which are used to either decrease deflections, decrease the required amount of negative reinforcement in the slab, or to increase the shear strength at a slab-column connection. A simplification was made to Section 13.3.6 to permit an alternative form of corner reinforcement in two-way slabs that are supported by edge beams or walls.

Chapter 14—Walls
Based on a study by the Structural Engineers Association of Southern California, the design provisions for slender wall panels (primarily tilt-up walls) were modified to be consistent with design practice, which commonly follows the requirements of the 1997 Uniform Building Code.

Chapter 18—Prestressed Concrete
One important change in Section 18.4.1 permits an increase in the allowable concrete compression stress immediately after prestress transfer at the ends of pre-tensioned simple span members. This change was made based on research results and common practice in the precast/prestressed concrete industry. In Section 18.12, there is a modification of the requirements for structural integrity steel in two-way, unbonded post-tensioned slab systems.

Chapter 20—Strength Evaluation of Existing Structures
Since 1995, Section 20.2.3 has referenced Section 5.6.5 for determination of concrete strength from cores when evaluating the strength of an existing structure. However, Section 5.6.5 was developed for investigation of low-strength test results, not strength evaluation of existing structures. ACI Committee 214 has developed procedures for estimating an equivalent ƒ′c from core test data. The code has been changed to require an estimate of an equivalent ƒ′c, and the commentary references the ACI 214.4R-03 methods. The current test load intensity in Section 20.3.2, 0.85(1.4D + 1.7L), was not changed when the ASCE 7 load factors were brought into the main body of the code in 2002 because Committee 318 did not want to reduce the fundamental level of structural safety. However, the current format is confusing to practitioners because it appears to refer only to the former load factors, which are currently used only in Appendix C. Furthermore, test load combinations, including snow and rain loads, were not provided. To correct these deficiencies, without substantially changing the test load intensity, the required test load intensity was revised to be not less than the largest of three load combinations.

Chapter 21—Special Provisions for Seismic Design
As noted previously, the most visible change to this chapter is the use of the more widely adopted Seismic Design Category (SDC) terminology and a reorganization of the entire chapter such that requirements for low SDCs are presented first, followed by the higher categories. In the process of making this revision, many sections were rewritten to clarify which provisions apply to which SDC. New requirements are introduced for structures assigned to SDC B and using ordinary moment frames as part of the seismic-force-resisting system. Structures assigned to SDC C and using slab-column framing as part of the seismic-force-resisting system have revisions for the shear and moment transfer requirements, clarifying a part of the code that had caused confusion and excessive design conservatism in previous editions. Several important changes have been introduced for structures assigned to SDC D, E, or F. One of the most significant changes is in the design/detailing of coupling beams, for which diagonal reinforcement was required to have confinement around each of the diagonals, leading to construction difficulties. A new detailing option has been introduced that permits the confinement of the entire beam cross-section rather than the individual diagonals. The requirements defining when to use diagonally reinforced or conventionally reinforced coupling beams were also clarified.

For columns in special moment frames, the accumulation of provisions over many code cycles had resulted in a tangled series of provisions that were difficult to follow. Those requirements have been rewritten in a more logical sequence. In addition, the requirements for confinement reinforcement were modified slightly to make the design calculations easier to implement, and the design yield strength for confinement reinforcement (not shear reinforcement) was raised to 100 ksi (690 MPa) to help reduce reinforcement congestion. For structural walls and diaphragms, the requirements for special boundary element confinement were relaxed such that the longitudinal spacing of transverse steel, previously specified as one-quarter of the section width, has been changed to one-third of the section width. Requirements for design of structural diaphragms have been modernized and reorganized, with clarifications on load paths, shear strength of topping slabs, and the use of prestressing tendons to resist flexure.

Chapter 22—Structural Plain Concrete
Limitations are provided in Section 22.1.2 to clarify the scope and applicability of this chapter of the code. Also, the commentary refers to Section 1.1.4 to define which concrete elements in residential construction are within the scope of ACI 332-04, "Requirements for Residential Concrete Construction and Commentary."

Appendix D—Anchoring to Concrete
Prior editions of the code defined the beneficial effect of providing supplemental reinforcement across the potential concrete breakout cone when evaluating the strength of an anchor. To alleviate some confusion regarding this reinforcement, the code now defines two types of reinforcement that can be used across a potential breakout cone.

Supplementary reinforcement can be used to improve the deformation capacity for the breakout mode, and thus, enables the use of a higher φ-factor.

Anchor reinforcement is designed to transfer the full design load from the anchors into the structural member, and thus precludes consideration of the concrete breakout failure mode.

A modification factor for concrete breakout strength is introduced to correct the current conservative provisions for anchorages loaded in shear and located in thin concrete members. Also, seismic design provisions for anchors were modified to clarify the ductility requirements for anchors and to provide an option to permit anchor designs in seismic zones that are controlled by concrete failure modes.

James K. Wight, FACI, FASCE, is the Frank E. Richart Jr. Collegiate Professor of Civil Engineering at the University of Michigan. He is the past chair of ACI Committee 318, Structural Concrete Building Code. Wight can be reached at jwight@umich.edu.
To learn more about ACI 318-08 and for ordering information, contact ACI at 248-848-3800 or visit www.concrete.org.