Newly accepted materials
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.
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.