Building code requirements for structural concrete

November 8, 2019

By Jack P. Moehle, PhD, PE

All images courtesy American Concrete Institute[1]
All images courtesy American Concrete Institute

The American Concrete Institute (ACI) published ACI 318-19, Building Code Requirements for Structural Concrete, in July. ACI 318 presents requirements for design and construction of structural concrete that are necessary to ensure public health and safety. It is addressed to the engineer or the building official who is responsible for the contract documents.

It has been five years since the reorganized ACI 318-14 edition was published and the latest round of changes concentrates heavily on responding to developments in materials, structural systems, and seismic design. The Canadian Standards Association (CSA) A23.3, Design of Concrete Structures, gives structural engineers discretion in using new materials and methods for building construction, and historically the profession has been proactive in doing so. It is anticipated the new provisions within ACI 318-19[2] will give engineers further background and tools to support such adoption.

Engineers in Canada must comply with CSA A23.3, as adopted by the most recent building code, in the province in which they work. It is anticipated, some, or all of the new requirements in ACI 318, will be adopted in an upcoming version of A23.3, if they have not been adopted already.

Alternative cements

The viaduct over Almonte River located in Garrovillas de Alconétar, Cáceres, Extremadura, Spain, is the 2018 American Concrete Institute (ACI) Excellence in Concrete Construction Awards Overall Winner. [3]
The viaduct over Almonte River located in Garrovillas de Alconétar, Cáceres, Extremadura, Spain, is the 2018 American Concrete Institute (ACI) Excellence in Concrete Construction Awards Overall Winner.

Portland cement, long used as a binder in concrete, has been under scrutiny because of the energy required for its production, as well as associated carbon dioxide (CO2) emissions. ACI 318-19 has added provisions allowing the use of alternative cements, defined by ACI Innovation Task Group (ITG) 10 as inorganic cements that can be used as complete replacements for Portland or blended hydraulic cements, and are not covered by applicable specifications for Portland or blended hydraulic cements. A few commonly used examples include geopolymers, activated glassy, fly-ash, and slag cements, calcium-aluminate cements, calcium-sulfoaluminate cements, magnesia cements, and CO2-cured cement.

Materials specifications in previous versions of ACI 318 have applied to hydraulic cement (cement that sets and hardens by chemical reaction with water and is capable of doing so under water). Many alternative types of cement, however, do not rely on a chemical reaction with water. Further, the materials specifications have applied to the cementitious material alone or in a mortar, without any testing done on a mixture that might be considered structural concrete.

Existing standards do not address these conditions, and until data are available industry-wide at some point in the future, materials suppliers will be responsible for conducting laboratory and field testing and distributing the findings. Design professionals and building officials will be responsible for assessing the influence a given alternative cement will have on concrete, including but not limited to:

Basic material properties, similar to those required by existing standards, must be established to ensure uniformity of supply of a given material. Concrete mixtures made with the alternative cement will also require testing to determine how production should be modified (if at all). For example, considerations should include the storage of materials, mixture proportioning, and compatibility with admixtures. Structural design and performance should be adequately tested, as should fire-resistance. In addition to warning that data must be available to demonstrate compliance with all project requirements, Section 26.4, “Concrete materials and mixture requirements,” of ACI 318-19 further warns designers the water-cementitious materials ratio (w/cm) of mixtures containing alternative cements may not have the same relationship to strength and durability as Portland cement-based concrete mixtures would.

Alternative aggregates

Another strategy for making concrete more sustainable is to incorporate more recycled materials into the mixture. As with its treatment of alternative cements, ACI 318-19 permits crushed hydraulic-cement concrete or recycled aggregate if it is approved by the licensed design professional and the building official. Here, too, the burden is on the project team to show concrete using the aggregates meet all project requirements. The material supplier is responsible for developing data to show the alternative aggregate will perform as intended, since the nature of a given recycled aggregate is likely to change from project to project. The designer must determine whether the data presented are adequate for the application.

Newly accepted materials

The American Concrete Institute (ACI) 318-19, Building Code Requirements for Structural Concrete, includes new and updated code provisions on one-way shear, two-way shear, shear wall drift capacity, seismic design, shotcrete, deep foundations, post-tensioning, precast, durability, lightweight concrete, and more.[4]
The American Concrete Institute (ACI) 318-19, Building Code Requirements for Structural Concrete, includes new and updated code provisions on one-way shear, two-way shear, shear wall drift capacity, seismic design, shotcrete, deep foundations, post-tensioning, precast, durability, lightweight concrete, and more.

Previous editions of ACI 318-19 did not address the use of shotcrete (a method of placing concrete by projecting it at high velocity). For ACI 318-19, provisions were taken from the International Building Code (IBC) and updated, with input from the American Shotcrete Association (ASA) and ACI Committee 506. In the future, ACI standards will govern the use of shotcrete. Provisions governing the use of shotcrete are located in relevant sections of ACI 318-19 rather than being covered in a dedicated section.

Familiar construction materials continue to be improved, which can cause material characteristics to evolve faster than the structural design provisions governing them. Therefore, some changes to ACI 318-19 were made to keep pace with changes to material characteristics. For example, lightweight concrete’s mechanical properties and unit density are different from other types of concrete. ACI 318-19 added a new approach for assigning l (a modification factor used in calculations to account for the reduced mechanical properties of lightweight concrete) that is based on the unit weight of the concrete. The method for determining l based on testing to assess splitting tensile strength has been deleted from the code. However, the method to determine l based on the composition of the fine and coarse aggregate has been retained in the code.

High-strength rebar is another material advancement addressed in 318-19. Progress in metallurgy has resulted in production of rebar that is almost twice as strong as it was several decades ago. This stronger rebar is able to transfer much greater stress. However, it also may lack benchmark properties of weaker steels, such as minimum strain-hardening and elongation.

Recognizing these facts, ACI 318-19 includes new requirements for material properties of higher-strength steels. Accompanying these are many changes related to strength reduction factors, minimum reinforcement, effective stiffness, and requirements for development and splice lengths of straight high-strength rebar. Design expressions for the development length of straight bars, standard hooks, and headed deformed reinforcing bars were also harmonized based on large-scale research programs that further clarified effects of steel yield stress, concrete compressive strength, bar diameter, spacing between reinforcement, and level of confining reinforcement. With these revisions, ACI 318-19 is able to increase the range of concrete compressive strengths and steel yield strengths that may be used.

University of Iowa’s Visual Arts Building in Iowa City won first place in the 2018 ACI Excellence in Concrete Construction Awards in the Low-rise Buildings category.[5]
University of Iowa’s Visual Arts Building in Iowa City won first place in the 2018 ACI Excellence in Concrete Construction Awards in the Low-rise Buildings category.

The many updates addressing high-strength rebar are expected to support adoption of these bars, which will, in turn, reduce congestion in heavily reinforced members, improve concrete placement, and save time and labour.

ACI 318-19 raises limits on the specified strength of reinforcement in shear wall and special moment frame systems. The new standard allows Grade 80 (550) reinforcement for some special seismic systems and no longer allows Grade 40 (280) rebar to be used in seismic applications. Shear walls can employ rebar in Grades 60 (420), 80 (550), or 100 (690). Special moment frames can use Grades 60 (420) or 80 (550). Hoops and stirrups in special seismic systems used to support vertical reinforcing steel have a tighter specified spacing to prevent the vertical bars from buckling.

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.

Performance-based design and seismic requirements

With several new metrics for building performance (e.g. seismic resistance) now in place, performance-based design is becoming common. The Canadian Society for Civil Engineering (CSCE) has recently introduced continuing education on performance-based seismic design of tall reinforced concrete buildings and the method is increasingly used when building on the west coast.

Performance-based requirements are not prescriptive, rather, they set measurable objectives but allow freedom in design and construction for how the objectives are met. Performance-based seismic design is commonly done using nonlinear dynamic analysis. ACI 318-19 Appendix A, “Design Verification Using Nonlinear Response History Analysis,” sets parameters for design verification of earthquake-resistant concrete structures using nonlinear response history analysis. The appendix is intended to be used in conjunction with Chapter 16 of the American Society of Civil Engineers/Structural Engineering Institute (ASCE/SEI) 7, Minimum Design Loads for Buildings and Other Structures, which includes general requirements, ground motions, and load combinations. The appendix is also compatible with “Guidelines for Performance-based Seismic Design of Tall Buildings,” a document published by Pacific Earthquake Engineering Research (PEER) in conjunction with PEER’s, partners in the Tall Buildings Initiative. With the release of ACI 318-19, ACI becomes the primary resource for nonlinear dynamic analysis as it pertains to tall concrete buildings.

The Provo City Center Temple in Utah won first place in the Repair & Restoration category in the 2018 ACI Excellence in Concrete Construction Awards.[6]
The Provo City Center Temple in Utah won first place in the Repair & Restoration category in the 2018 ACI Excellence in Concrete Construction Awards.

Nonlinear dynamic analysis has shown shear wall design forces may be amplified by inherent wall overstrength and by apparent higher-mode effects. ACI 318-19 therefore increases earthquake shear forces for design. In some cases, the design shear force will be more than double the design shear from previous codes.

After the 1994 Northridge earthquake in California’s San Fernando Valley, many research institutions focused years of study on the seismic behaviour of precast concrete diaphragms. Study findings were used to inform changes in ASCE standards and ACI Committee 550 developed the following two new ACI standards:

The new standards entail significant increases in seismic design forces and contain new requirements for the design and detailing of precast concrete diaphragms, particularly the connections between precast elements. ACI Committee 318 adopts by reference both ACI 550.5-18 and ACI 550.4-18.

The Chile earthquake of 2010 and earthquakes that took place in Christchurch, New Zealand, in 2010 and 2011 provided further examples of structural response to seismic forces. Observed behaviour of structural walls, as well as laboratory tests, showed unconfined volumetric areas of concrete can loosen and fall during seismic events, leading the structurally compromised walls to buckle and fail. This observation led to new detailing requirements for boundary elements of special structural walls. (Boundary elements, typically occurring around wall edges, corners, or openings, provide longitudinal or transverse reinforcement to confine concrete and provide longitudinal bar support.) Whereas previous standards permitted the use of crossties with 90-degree hooks at one end, ACI 318-19 specifies all crossties for special boundary elements now must have 135-degree hooks at both ends. New provisions also:

Inspection and certification

All inspection requirements are now covered in Section 26.13, “Inspection,” of ACI 318-19.

This unification includes the relocation of anchor inspection requirements from Chapter 17, “Anchoring to Concrete.” ACI 318-19 also identifies qualification training programs for inspectors/installers and lists certification requirements. All inspectors are required to be certified if an appropriate certification program is available. Whenever an ACI certification program is cited in the commentary, a reference to that program has been included. Uniform resource locators (URLs) for the programs are included in the commentary reference list to allow a code user to review the certification program in detail to see what tasks are covered. By stating certification requirements directly in the code and linking to training programs, the process becomes easier for engineers to navigate.

[7]Jack P. Moehle, PhD, PE, is the chair of the American Concrete Institute (ACI) 318 Building Code Committee and is the Ed and Diane Wilson professor of Structural Engineering in the department of Civil and Environmental Engineering at University of California, Berkeley (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 (ACI), Structural Engineering Institute (SEI) of American Society of Civil Engineers (ASCE), and the Structural Engineers Association of California (SEAOC), and is an elected member of the U.S. National Academy of Engineering (NAE). He can be reached via e-mail at moehle@berkeley.edu[8].

Endnotes:
  1. [Image]: https://www.constructioncanada.net/wp-content/uploads/2019/11/IntrepidAvenue.jpg
  2. new provisions within ACI 318-19: http://concrete.org
  3. [Image]: https://www.constructioncanada.net/wp-content/uploads/2019/11/Almonte.jpg
  4. [Image]: https://www.constructioncanada.net/wp-content/uploads/2019/11/318-19_3D-High-Res.jpg
  5. [Image]: https://www.constructioncanada.net/wp-content/uploads/2019/11/MironIowa.jpg
  6. [Image]: https://www.constructioncanada.net/wp-content/uploads/2019/11/ProvoTemple.jpg
  7. [Image]: https://www.constructioncanada.net/wp-content/uploads/2019/11/moehle.jpg
  8. moehle@berkeley.edu: mailto:moehle@berkeley.edu

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