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Introducing ACI 318-19: Building code requirements for structural concrete
New versions of the American Concrete Institute (ACI) 318 — Building Code Requirements for Structural Concrete — are typically released every three years. Due to changes in the International Code Council’s schedule for reviewing and publishing the International Building Code (IBC), this year’s July release of the document occurred five years after the previous version. It is anticipated that ACI 318–19 will be referenced in the 2021 IBC.
ACI 318 includes the requirements for design and construction of structural concrete that are necessary to ensure public health and safety. The latest round of changes concentrates heavily on responding to developments in materials, structural systems and seismic design. Not only have structural design provisions changed but also new requirements address materials advancements and placement techniques, which will result in procedural changes for manufacturers and contractors.
Seismic design
Many new metrics for building performance, including seismic resistance, are being implemented worldwide. Therefore, performance-based design is becoming more common. Performance-based requirements set measurable objectives but allow freedom in design and construction for how the objectives are met.
Performance-based seismic design verification is commonly done using nonlinear dynamic analysis. Appendix A in ACI 318–19 sets parameters for design verification of earthquake-resistant concrete structures using nonlinear response history analysis. Appendix A is intended to be used in conjunction with Chapter 16, “Nonlinear Response History Analysis,” 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. Appendix A is also compatible with Guidelines for Performance-based Seismic Design of Tall Buildings, a document published by Pacific Earthquake Engineering Research in conjunction with their 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.
For seismic design of structural walls, ACI 318–19 introduces several new design requirements. Whereas previous designs permitted the use of crossties with 90-degree hooks at one end, all crossties for special boundary elements must now have 135-degree hooks at both ends. New provisions also restrict the locations of vertical reinforcement lap splices near intended plastic hinge zones. Another new design provision provides a check that detailing is adequate for the calculated earthquake displacement demands. Perhaps most significantly, new provisions will amplify wall design shears based on considerations of wall flexural overstrength and the effects of higher dynamic response modes, which may result in substantial increases in design shears for some walls.
ACI 318–19 adopts the precast concrete diaphragm design procedure of ACI 550.5, Code Requirements for the Design of Precast Concrete Diaphragms for Earthquake Motions. The design method in ACI 550.5 gives designers connection options for selecting the target performance of a precast concrete diaphragm when subject to seismic forces. ACI 550.5 requires connections to be qualified in accordance with ACI 550.4, Qualification of Precast Concrete Diaphragm Connections and Reinforcement at Joints for Earthquake Loading.
The code now clarifies the application and effect of the vertical ground component on earthquake load. There are numerous clarifications and additions to the requirements for column tie spacing in special moment frames — this includes clarifications of tie spacings for columns not considered part of the earthquake-resisting system. Intermediate moment frame requirements for tie spacing are also reduced.
ACI 318–19 includes revisions and additions aimed at eliminating conflicting provisions in ACI 318, ASCE 7 and the IBC regarding design of deep foundations for earthquake-resistant structures. These differences have been a source of confusion for both engineers and code officials. The purpose of the code change is to have all the pertinent concrete-related design and detailing provisions for the seismic design of deep foundations contained in ACI 318–19.
The document also contains numerous miscellaneous clarifications and simplifications. For example, the column-to-beam flexural strength ratio is adjusted for roof-level connections where the column axial load is low. The shear area of concrete walls, Acv, is clarified so it is clear it does not include the area of wall openings.
Code integration and ACI 318–19 reorganization
In addition to its compatibility with the ASCE 7 requirements for design verification using nonlinear dynamic analysis, ACI 318–19 achieves code integration by incorporating information from IBC and modernizing those provisions. ACI 318–19 also identifies areas where personnel are needed to be certified and references appropriate requirements. By providing certification requirements directly in the code and commentary, the information becomes more easily accessible to engineers.
Two chapters of ACI 318–19 have been reorganized. Chapter 17, “Anchoring to Concrete,” which covers anchor design, is reformatted to match other chapters initially adopted for ACI 318–14. Post-installed concrete screw anchors are increasingly used and this anchor type is now recognized, as are shear lugs, which comprise 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. ACI 318.2–14, Building Code Requirements for Concrete Thin Shells and Commentary, which replaced ACI 318–11 Chapter 19, has also been reorganized to be consistent with the rest of ACI 318–19.
Chapter 26, “Construction Documents and Inspection,” has seen significant updates since ACI 318–14. Inspection requirements are unified in this chapter, including the relocation of anchor inspection requirements from Chapter 17. The chapter now recognizes projects may have roles for multiple design engineers and provides a framework for their coordination of work.
New materials
Structural concrete materials, quality control measures, and construction methods are continually evolving, requiring changes to ACI 318 provisions. Therefore, some changes to ACI 318–19 have been 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 adds a new approach for assigning λ, a modification factor used in calculations to account for the reduced mechanical properties of lightweight concrete and is based on the unit weight of the material. This allows it to be defined as early as the project design stage. The method for determining λ based on testing to measure splitting tensile strength has been deleted from the code. However, the method to determine λ based on the composition of the fine and coarse aggregate has been retained. Lightweight concrete provisions throughout the code have seen numerous changes and clarifications based on the new method for determining λ.
As higher strength concretes have been developed over time, using the standard definition of modulus of elasticity may be inadequate for certain projects (such as tall buildings). Therefore, the definition for modulus of elasticity has been updated using data from external documents and best practices.
With the release of 318–19, the use of shotcrete is explicitly addressed in the document for the first time. The new content was developed by updating relevant provisions from IBC, with input from the American Shotcrete Association and ACI Committee 506. In the future, the IBC will reference ACI standards to govern the use of shotcrete.
High strength rebar is another material advancement addressed in 318–19. 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. Progress in metallurgy, however, 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 greater stresses. However, it also may lack benchmark properties of weaker steels, such as required strain-hardening and elongation. Recognizing this, ACI 318–19 includes new provisions for material properties of higher-strength steels. Accompanying these are myriad changes related to strength reduction factors, minimum reinforcement, effective stiffness, and requirements for development and splice lengths of straight high-strength rebar as well as hooks and headed bars. 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 labor.
ACI 318–19 raises limits on the specified strength of reinforcement in earthquake-resistant shear wall and moment frame systems. The new standard allows Grade 80 reinforcement for some seismic systems and no longer allows Grade 40 rebar to be used in seismic applications. Shear walls can employ rebar in Grades 60, 80 or 100. Special moment frames can use Grades 60 or 80. 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.
See next post
Introducing ACI 318-19: Building code requirements for structural concrete
New versions of the American Concrete Institute (ACI) 318 — Building Code Requirements for Structural Concrete — are typically released every three years. Due to changes in the International Code Council’s schedule for reviewing and publishing the International Building Code (IBC), this year’s July release of the document occurred five years after the previous version. It is anticipated that ACI 318–19 will be referenced in the 2021 IBC.
ACI 318 includes the requirements for design and construction of structural concrete that are necessary to ensure public health and safety. The latest round of changes concentrates heavily on responding to developments in materials, structural systems and seismic design. Not only have structural design provisions changed but also new requirements address materials advancements and placement techniques, which will result in procedural changes for manufacturers and contractors.
Seismic design
Many new metrics for building performance, including seismic resistance, are being implemented worldwide. Therefore, performance-based design is becoming more common. Performance-based requirements set measurable objectives but allow freedom in design and construction for how the objectives are met.
Performance-based seismic design verification is commonly done using nonlinear dynamic analysis. Appendix A in ACI 318–19 sets parameters for design verification of earthquake-resistant concrete structures using nonlinear response history analysis. Appendix A is intended to be used in conjunction with Chapter 16, “Nonlinear Response History Analysis,” 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. Appendix A is also compatible with Guidelines for Performance-based Seismic Design of Tall Buildings, a document published by Pacific Earthquake Engineering Research in conjunction with their 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.
For seismic design of structural walls, ACI 318–19 introduces several new design requirements. Whereas previous designs permitted the use of crossties with 90-degree hooks at one end, all crossties for special boundary elements must now have 135-degree hooks at both ends. New provisions also restrict the locations of vertical reinforcement lap splices near intended plastic hinge zones. Another new design provision provides a check that detailing is adequate for the calculated earthquake displacement demands. Perhaps most significantly, new provisions will amplify wall design shears based on considerations of wall flexural overstrength and the effects of higher dynamic response modes, which may result in substantial increases in design shears for some walls.
ACI 318–19 adopts the precast concrete diaphragm design procedure of ACI 550.5, Code Requirements for the Design of Precast Concrete Diaphragms for Earthquake Motions. The design method in ACI 550.5 gives designers connection options for selecting the target performance of a precast concrete diaphragm when subject to seismic forces. ACI 550.5 requires connections to be qualified in accordance with ACI 550.4, Qualification of Precast Concrete Diaphragm Connections and Reinforcement at Joints for Earthquake Loading.
The code now clarifies the application and effect of the vertical ground component on earthquake load. There are numerous clarifications and additions to the requirements for column tie spacing in special moment frames — this includes clarifications of tie spacings for columns not considered part of the earthquake-resisting system. Intermediate moment frame requirements for tie spacing are also reduced.
ACI 318–19 includes revisions and additions aimed at eliminating conflicting provisions in ACI 318, ASCE 7 and the IBC regarding design of deep foundations for earthquake-resistant structures. These differences have been a source of confusion for both engineers and code officials. The purpose of the code change is to have all the pertinent concrete-related design and detailing provisions for the seismic design of deep foundations contained in ACI 318–19.
The document also contains numerous miscellaneous clarifications and simplifications. For example, the column-to-beam flexural strength ratio is adjusted for roof-level connections where the column axial load is low. The shear area of concrete walls, Acv, is clarified so it is clear it does not include the area of wall openings.
Code integration and ACI 318–19 reorganization
In addition to its compatibility with the ASCE 7 requirements for design verification using nonlinear dynamic analysis, ACI 318–19 achieves code integration by incorporating information from IBC and modernizing those provisions. ACI 318–19 also identifies areas where personnel are needed to be certified and references appropriate requirements. By providing certification requirements directly in the code and commentary, the information becomes more easily accessible to engineers.
Two chapters of ACI 318–19 have been reorganized. Chapter 17, “Anchoring to Concrete,” which covers anchor design, is reformatted to match other chapters initially adopted for ACI 318–14. Post-installed concrete screw anchors are increasingly used and this anchor type is now recognized, as are shear lugs, which comprise 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. ACI 318.2–14, Building Code Requirements for Concrete Thin Shells and Commentary, which replaced ACI 318–11 Chapter 19, has also been reorganized to be consistent with the rest of ACI 318–19.
Chapter 26, “Construction Documents and Inspection,” has seen significant updates since ACI 318–14. Inspection requirements are unified in this chapter, including the relocation of anchor inspection requirements from Chapter 17. The chapter now recognizes projects may have roles for multiple design engineers and provides a framework for their coordination of work.
New materials
Structural concrete materials, quality control measures, and construction methods are continually evolving, requiring changes to ACI 318 provisions. Therefore, some changes to ACI 318–19 have been 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 adds a new approach for assigning λ, a modification factor used in calculations to account for the reduced mechanical properties of lightweight concrete and is based on the unit weight of the material. This allows it to be defined as early as the project design stage. The method for determining λ based on testing to measure splitting tensile strength has been deleted from the code. However, the method to determine λ based on the composition of the fine and coarse aggregate has been retained. Lightweight concrete provisions throughout the code have seen numerous changes and clarifications based on the new method for determining λ.
As higher strength concretes have been developed over time, using the standard definition of modulus of elasticity may be inadequate for certain projects (such as tall buildings). Therefore, the definition for modulus of elasticity has been updated using data from external documents and best practices.
With the release of 318–19, the use of shotcrete is explicitly addressed in the document for the first time. The new content was developed by updating relevant provisions from IBC, with input from the American Shotcrete Association and ACI Committee 506. In the future, the IBC will reference ACI standards to govern the use of shotcrete.
High strength rebar is another material advancement addressed in 318–19. 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. Progress in metallurgy, however, 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 greater stresses. However, it also may lack benchmark properties of weaker steels, such as required strain-hardening and elongation. Recognizing this, ACI 318–19 includes new provisions for material properties of higher-strength steels. Accompanying these are myriad changes related to strength reduction factors, minimum reinforcement, effective stiffness, and requirements for development and splice lengths of straight high-strength rebar as well as hooks and headed bars. 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 labor.
ACI 318–19 raises limits on the specified strength of reinforcement in earthquake-resistant shear wall and moment frame systems. The new standard allows Grade 80 reinforcement for some seismic systems and no longer allows Grade 40 rebar to be used in seismic applications. Shear walls can employ rebar in Grades 60, 80 or 100. Special moment frames can use Grades 60 or 80. 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.
See next post