Concrete requirements reorganized

July 2014 » Feature Articles » Materials
ACI 318-14 Building Code Requirements for Structural Concrete adopts a designer’s perspective
Randall W. Poston, Ph.D., P.E., S.E.
ACI 318-14 is the first significant reorganization of the code since the 1970s — a reworking that has been more than a decade in development and included focus groups with structural engineers.

The value of a building code is demonstrated daily in its real-world application in a variety of settings. As our knowledge and experience increase, a code must change to best serve its users.

Later this year, the American Concrete Institute (ACI) will publish ACI 318-14, “Building Code Requirements for Structural Concrete and Commentary.” The code is central in our understanding of structural concrete design and the safety of concrete structures. More than 20 nations incorporate the code in whole or in part into their building code requirements.

ACI 318-14 will be the first significant reorganization of the code since the 1970s — a reworking that has been more than a decade in development. This development included focus groups with structural engineers, asking what they would like to see in a reorganized code; formal subcommittees that wrote, edited, and reviewed the work in progress; dozens of main committee ballots; and a public discussion period. In total, members of ACI committee 318 and its subcommittees have dedicated about 100,000 man hours developing a code that is completely reorganized for greater ease of use. 

Rationale for reorganization 

ACI was founded as the National Association of Cement Users at an Engineering Congress held at the St. Louis World’s Fair in October 1904. In 1919, the name was changed to the American Concrete Institute.

The first code governing the use of structural concrete was published in 1910 as “Standard Building Regulations for the Use of Reinforced Concrete.” From 1910 through 1956, the 318 structural design requirements were a mixture of limits based on working stress calculations and empirical rules. The 1963 code included both working stress and the ultimate strength approach. The 1971 code removed the working stress method, and the resulting code organization has been followed until now.

The Members chapters, the heart of reorganized ACI 314-14, include One-Way Slabs; Two-Way Slabs; Beams; Columns; Walls; Diaphragms (new); Foundations; and Plain Concrete Members.

There have been many additions and changes to ACI 318 since 1971 based on an increase in knowledge and experience, including in areas such as torsional strength, seismic design and detailing, integrity reinforcement, concrete exposure classes, strain-based strength-reduction factors, and anchoring to concrete. In all, 1971’s 750 provisions have more than tripled to the more than 2,500 provisions found in ACI 318-11.  

However, as information was added over the years, not all of it fit neatly into the existing structure. ACI 318-14, the 20th edition of ACI 318, uses an organization that follows the design process, makes wider use of graphs and tables, better incorporates valuable commentary, and makes it easier for inexperienced users to know when they have fully satisfied the design provisions.

Principles of organization

In the completely redesigned ACI 318-14, it will be easier for engineers to find the information they need quickly and be more certain that a design fully satisfies the code’s requirements. The new organization provides complete member design and detailing requirements in a single chapter, and there is a chapter for each type of member. 318-14 member chapters are organized to provide the user an explicit road map of relevant provisions.   

Under ACI 318’s current organization, when considering a common construction detail, the user has to refer to multiple chapters governing issues such as flexural strength, axial strength, shear strength, strength reduction, tie spacing and bend details, or lap splices. As a result, the existing organization relies heavily on familiarity with the code; it requires one to visit multiple chapters to design and detail members and make judgments on which provisions apply.

In contrast, with its member-based organization, ACI 318-14 “keeps” the user within a given design chapter as much as possible. This means that all design and detailing rules for each building member are provided within one chapter. It doesn’t matter if the member is cast-in-place or precast, or if mildly reinforced or prestressed — all requirements for a particular member type are contained within the member chapter. As needed, reference is made to a “universal” set of toolbox chapters that support the individual member chapters.

Overall, the 27 chapters of ACI 318-14 can be grouped into six categories: General; Systems, Members, Joints and Connections, The Toolbox, and Construction.

The General chapters include information on the scope, application, and interpretation of the code; notation and terminology; referenced standards; concrete design properties; and steel design properties. Most of this section will be familiar to current users.

The Systems chapters cover minimum requirements for structural systems; a new chapter that clarifies the overall design and detailing requirements of reinforced concrete building structures; loads and load combinations; structural analysis, with a new section on requirements for a finite element analysis; and a seismic chapter that addresses earthquake-resistant structures.

The Members chapters, the heart of reorganized ACI 314-14, include One-Way Slabs; Two-Way Slabs; Beams; Columns; Walls; Diaphragms (new); Foundations; and Plain Concrete Members. Each member chapter references all design and detailing provisions that apply to that member by way of uniform headings. 

The Joints and Connections chapters address beam-column and slab-column joints, connections between members, and anchoring to concrete.

The Toolbox chapters are an essential feature of ACI 318-14, comprising Strength Reduction Factors, Sectional Strength, Strut-and-Tie, Serviceability, and Reinforcement Details. Many requirements are common to several members, and to reduce duplication of information, these common, supporting requirements are gathered into toolbox or utility chapters. 

The Construction chapter in ACI 318-14 now consolidates construction and inspection requirements for the designing engineer. The requirements are designated either as “design information” or “compliance requirements.” In particular, this section is meant to help the engineer fulfill the responsibility to communicate field requirements to construction and inspection teams. 

Conclusion

Committee 318 has created a reorganized code for 2014 that will have greater consistency in phrasing, equations, lists, notations, and figures, with generous use of graphs and tables.

The reorganized ACI 318-14 will be published in various electronic formats plus the traditional printed copy in both U.S. Customary and S.I units; it will appear in English, Spanish, Chinese, and other languages. The 2015 International Building Code (IBC) will reference 318-14. ACI has also provided a transition key that maps ACI 318-11 provisions to ACI 318-14. The ACI Reinforced Concrete Design Manual design examples are also being updated to be fully consistent with ACI 318-14.

Development of ACI 318-14 has been a long, careful process. With something this complicated and complex, new directions became apparent as we proceeded, which have improved the document and made the code more logical in presentation. We expect ACI 318-14 to accommodate new topics, knowledge, and construction standards for many decades to come.

Randall W. Poston, Ph.D., P.E., S.E., a founding principal of WDP & Associates (www.wdpa.com) is chair of ACI Committee 318: Structural Concrete Building Code. For further information and updates, visit www.concrete.org/ACI318, as well as www.facebook.com/AmericanConcreteInstitute.


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