By Jack P. Moehle, PhD, PE
It’s an acknowledged fact that construction practices are changing at a rapid pace. The American Concrete Institute’s recently released ACI 318-19: “Building Code Requirements for Structural Concrete” responds to many of these changes. With a focus on safety, economy and sustainability, it addresses many of the new technologies, products and methods that have become common in recent years.
New 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.
Based upon laboratory tests as well as observed behavior of structural walls during seismic events, other changes have been incorporated into the code. For example, to improve concrete confinement and longitudinal bar support, ACI 318-19 limits the aspect ratio of hoops in boundary elements and requires that all crossties have seismic hooks at both ends. To avoid brittle fracture of under-reinforced walls, the code also requires some walls to satisfy minimum longitudinal reinforcement requirements.
New ACI standards (the ACI 550 series) reflect changes made to the 2016 edition of ASCE 7, which incorporated new provisions that called for significant increases in seismic design forces for precast diaphragms. It established new requirements for the design and detailing of precast concrete diaphragms, particularly the connections between precast elements. The ACI 550 documents and standards are referenced in 318-19.
ACI 318-19 attempts to eliminate conflicting information in ACI 318, ASCE 7 and IBC regarding design of deep foundations for earthquake-resistant structures, with a goal of having all pertinent concrete-related seismic design and detailing provisions for deep foundations contained in ACI 318.
Among other changes, new provisions on precast concrete piles were incorporated; axial load limitations for deep foundations are now included; and strength design requirements were added that are consistent with ACI 543. This code change will allow designers of deep foundation members to use either the traditional allowable stresses that have been in the general building codes for years or the strength design method using strength reduction factors that is consistent with the rest of ACI 318-19.
Beam Column Joints
ACI 318-19 incorporates design provisions for shear strength and reinforcement detailing of beam-column joints of seismic design category (SDC) A frames, ordinary moment frames, intermediate moment frames, and frames that are not part of the seismic force-resisting system in SDC B, C, D, E, and F. Also, existing design provisions for special moment frames were expanded to include shear strength of roof joints.
Chapter 15 now requires consideration of the presence of opening and closing moments in corner joints. This includes moment transfer across a diagonal section through a corner joint, which is particularly critical where the joint connects a cantilever member for which no redistribution of moments is possible.
Design Verification Using Nonlinear Response History Analysis
The use of nonlinear dynamic analysis methods for design of earthquake-resistant buildings has been increasing recently, especially for design of high-rise buildings. To respond to this change in design practice, ACI 318-19 includes provisions for application of these methods to concrete buildings. The provisions are intended to be fully compatible with Chapter 16 of ASCE 7-16, which contains requirements on seismic hazard, selection of earthquake ground motions, load combinations and independent peer review.
High Strength Reinforcement
Current U.S. building codes limit rebar strength based on decades-old research, with most reinforcement used in concrete construction in the United States being Grade 60. Producers are now able to produce rebar, however, 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. The use of higher-grade reinforcement raised concerns about serviceability (cracking and deflections), which were 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.
Strength and ductility concerns were addressed by introducing new requirements for mechanical properties of reinforcing bars; adjusting the method for calculating the strength reduction factor for moment, combined moment and axial load; revising development length provisions; and limiting the value of fy that can be used for calculating the maximum axial compressive strength, Pn, max, of columns. Grade 100 reinforcement is likely to be used mostly for vertical bars of shear walls and columns, though it might also be used for heavily loaded floor systems.
Substantial new research funded by Pankow Foundation, ACI Foundation and others has demonstrated acceptable performance of members of special seismic systems reinforced with ASTM A706 Grade 80 reinforcement and A706-equivalent Grade 100 reinforcement. Recognizing this, ACI 318-19 now permits special moment frames with A706 Grade 80 reinforcement and special structural walls with A706 Grade 80 and A706-equivalent Grade 100 reinforcement. The provisions allow the use of the higher grades to resist moments, axial forces and shear. To accommodate these higher grades, additional restrictions on hoop spacing, beam-column joint dimensions and lap splice locations have been added that will contribute to more reliable performance of special structural systems.
Thick Slabs and Deep Beams
As more large structures are designed to include thick slabs and other large members that support upper floors, shear provisions have been updated. ACI 318-19 sections on one-way shear and two-way shear (i.e., punching shear) consolidate what were previously a wide range of equations. They also provide a method to include size effect in shear design to avoid issues wherein increasing a member’s size can reduce the unit shear strength of a section. The new shear equations also allow the design engineer to take the effect of reinforcement ratio into consideration.
Materials and Technology
Shotcrete, a method of placing concrete by projecting it at high velocity, was not explicitly discussed in previous versions of 318, but is now specifically included. The unification is expected to clarify both the design process and construction requirements for the use of shotcrete.
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 on requirements for anchorage zone reinforcement.
Post-installed concrete screw anchors are increasingly used and this anchor type is recognized in ACI 318-19. The document also introduces provisions for shear lugs comprising a steel element welded to a base plate. Shear lugs are usually used at the base of columns to transfer large shear forces through bearing to a foundation element.
Numerous changes were made to the durability of concrete sections including additional requirements for sulfate exposure classes and concrete exposed to water. Lightweight concrete provisions throughout the code received numerous changes and clarifications based on the new method for determining λ, the lightweight modification factor. The new method calculates λ based on concrete mix proportions.
Updates were also made to strut and tie methodology (STM) that included the removal of bottle-shaped struts from the code and the inclusion of minimum reinforcing requirements in STM. Other STM improvements included curved-bar nodes and knee joints.
Ease of Use
While organizational changes were minimal, some new formatting features in ACI 318-19 enhance the document’s usability. Full-color and three-dimensional illustrations are included, improving clarity. The index was expanded and interactive links were added to the online version of the document, in an effort to help users quickly find code provisions.
Chapter 26, “Construction Documents and Inspection,” has seen significant updates since 318-14. Inspection requirements are unified in this chapter, including the relocation of anchor inspection requirements from Chapter 17. 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 (such as alternative cements, crushed hydraulic-cement concrete or recycled aggregates), 318-19 Chapter 26 outlines precautions for designers who are considering their use.
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.
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 is also available to subscribers of the online ACI Collection of Concrete Codes, Specifications, and Practices. Additionally, the Institute is hosting public and in-house seminars to introduce users to ACI 318-19 – visit concrete.org for locations and to learn more.
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 has played a leading role in the development of building codes and professional engineering guidelines on subjects related to reinforced concrete and earthquake engineering. 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.