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Code Update: ACI 318-19: Building Code Requirements for Structural Concrete

Jack Moehle on September 6, 2019 - in Articles, Column

The American Concrete Institute (ACI) recently published “ACI 318-19 Building Code for Structural Concrete. ACI 318 includes the requirements for design and construction of structural concrete that are necessary to ensure public health and safety.

This edition of ACI 318 is the first to be published since the format of ACI 318 was reorganized in 2014, and it’s anticipated that it will be referenced in the 2021 International Building Code (IBC).

Important Updates

Now in full color with 3D illustrations to improve clarity, ACI 318-19 includes major changes in higher reinforcing steel yield strengths; the addition of shotcrete and deep foundation provisions; seismic requirements for deep foundations and other applications; vertical seismic motions; nonlinear analysis for seismic design; modification to development length equations; and updated shear design provisions and equations.

A major purpose of the 2019 code cycle was to expand permissible applications of high-strength reinforcement. Current U.S. building codes limit rebar strength based on old research, with most reinforcement used in concrete construction in the United States being Grade 60. Producers now can create rebar that is almost twice as strong as it was several decades ago. ACI 318-19 permits the use of Grade 100 reinforcement to resist moments and axial forces from gravity and wind load combinations and, with some restrictions, for gravity and earthquake load combinations. Serviceability concerns are addressed through a series of changes for slab and beam minimum reinforcement, effective moment of inertia, and requirements for deflection calculations for two-way slabs.

A variety of other industry needs now are addressed in ACI 318-19. Updates were made to provisions on post-tensioning, precast concrete, concrete durability and lightweight concrete. Post-tensioning updates included clarifications of the construction requirements regarding loss of prestress, use of a new reference document for determining prestress losses, deformed and bonded reinforcement spacing limitations, and several clarifications to requirements for anchorage zone reinforcement. Precast concrete received several clarifications with specific attention on bearing connections. Numerous changes were made to the durability of concrete sections, including additional requirements for sulfate exposure classes and concrete exposed to water.

Further Improvements

ACI 318-19 features clear language and style consistency, and it’s organized so engineers have increased confidence they have satisfied all necessary code requirements. The index also was completely redone and expanded as an aid to help code users locate provisions.

ACI 318-19 also identifies qualification training programs for inspectors/installers and lists certification requirements. By stating certification requirements directly in the code, the information becomes more easily accessible to engineers.

Chapter 26, “Construction Documents and Inspection,” has been significantly updated since 318-14. Inspection requirements, including the relocation of anchor inspection requirements from Chapter 17, are unified in this chapter. The chapter now recognizes that projects may have roles for multiple design engineers and provides a framework for their coordination of work. As higher-strength concretes have been developed over time, using the standard definition of modulus of elasticity may not be adequate for certain projects. Therefore, the definition for modulus of elasticity was updated using data from external documents and best practices. For certain materials that are becoming commonplace in the industry (e.g., alternative cements, crushed hydraulic-cement concrete or recycled aggregates), 318-19 Chapter 26 outlines precautions for designers who are considering their use.

New seismic design provisions amplify code-prescribed wall design shears based on considerations of wall flexural overstrength and the effects of higher dynamic response modes. The result, in many instances, is a substantial increase in design shears for some walls. The amplified shear forces will better reflect the higher shear forces that have been seen in buildings designed by nonlinear dynamic analysis methods.

Printed and digital formats of ACI 318-19 are available at concrete.org. Versions are available in inch-pound units and SI units. ACI 318-19 also is available to subscribers of the online ACI Collection of Concrete Codes, Specifications and Practices. In addition, the Institute is hosting public and inhouse seminars to introduce users to ACI 318-19; visit concrete.org for locations and to learn more.


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About Jack Moehle

Dr. Jack P. Moehle is the Chair of ACI 318 Building Code Committee and is the Ed and Diane Wilson Professor of Structural in the Department of Civil and Environmental Engineering at UC Berkeley. He is a Fellow of the American Concrete Institute, Structural Engineering Institute of ASCE, and the Structural Engineers Association of California, and is an elected member of the U.S. National Academy of Engineering; www.concrete.org.

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