ACI Releases New Building Code Requirements for Structural Concrete

ACI 318-19 was published in response to new engineering practices and industry changes.

318 19 Cover Highres2

The American Concrete Institute (ACI) published ACI 318-19: “Building Code Requirements for Structural Concrete in July 2019, in response to new engineering practices and industry changes.

ACI 318 presents requirements for design and construction of structural concrete that are necessary to ensure public health and safety. The document is intended for engineers and building officials, but because it addresses materials advancements and applications, it is expected to have an impact on jobsite procedures. It is anticipated the final code requirements of 318-19 will be referenced in the 2021 International Building Code (IBC).

New Engineering Practices Translate to Changes in the Field

Changes to the way architects and engineers work—notably, their use of computers to perform extensive design and analysisprompted some changes within the code. Other industry changes, such as the increasing construction of tall buildings and new requirements for seismic resistance, led to further code updates. These will translate to new detailing and construction methods commonly seen on the jobsite.

To prevent punching failure in two-way slabs, there is a new requirement for minimum reinforcement on interior column-to-slab connections. Similarly, there are new requirements for bar extensions. Previous versions of ACI 318 established minimum lengths and extensions of bars in relatively thin, two-way slabs supporting typical roof or floor loads. Thicker two-way slabs, such as transfer slabs, podium slabs and mat foundations, were determined to need greater bar lengths and extensions to ensure that bars extend past projections of potential shear cracks.

Additional transverse reinforcement, commonly referred to as hanger reinforcement, is now suggested for some reinforced concrete beams. This change is in response to the determination that beams cast monolithically with a supporting beam, and intersecting one or both side faces of the supporting beam, could potentially cause the supporting-beam soffit to fail under some conditions. Hanger reinforcement helps to transfer high shears from the end of the supported beam into the supporting beam.

There are also new code provisions for precast concrete bearing connections. Prior editions of ACI 318 contained specific provisions for restraint forces only for corbels and brackets; 318-19 includes consideration of restraint forces at all bearing connections.

An ACI 318-19 update that was eagerly anticipated across the industry was the expansion of permissible applications of high-strength reinforcement. Higher-grade reinforcement is now allowed, with bars up to Grade 100 permitted for resisting moments and axial forces from gravity, wind and some earthquake load combinations. Grade 100 reinforcement is most likely to be used for vertical bars of shear walls and columns, though it might also be used for heavily loaded floor systems. Due to concerns related to the use of higher-strength barsnamely, that they may lack benchmark properties of weaker steels, such as minimum strain-hardening and elongation318-19 includes new requirements for material properties of high-strength steels.

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. The new standard no longer allows Grade 40 rebar to be used in seismic applications.

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.

Several changes were made related to the strut-and-tie method (STM) for design of discontinuity regions. The requirements for distributed reinforcement in deep beams were expanded to include most other discontinuity regions, although distributed reinforcement is not required where it is impractical and unnecessary, such as in pile caps. The terminology “bottle-shaped strut” was removed from the code based on recent research showing inclined struts are weakened by diagonal tension rather than bottle-shape behavior. There are new requirements for ties extending from bend regions of reinforcing bars (curved-bar nodes) and for knee joints.

ACI 318-19 has extended provisions for development length for deformed bars, standard hooks and headed deformed bars in tension based on higher concrete compressive strengths and higher-strength reinforcement. The provisions for development of deformed bars are similar to those in past codes but have a requirement for transverse reinforcement when higher grades of reinforcement are used. The provisions for standard hooks and headed deformed bars are substantially different from those of past codes and better represent the effects of bar diameter, concrete compressive strength, spacing between reinforcement and level of confining reinforcement on required lengths.

Anchorage-zone reinforcement is clarified in ACI 318-19 for unbonded monostrand tendon anchorages in slab edges, including slabs of varying thickness. Test data have shown that for slab edge anchors to perform reliably, horizontal bars (known as “back-up bars”) must be provided, positioned parallel to the slab edge and in proximity to the anchorage devices.

New Materials Addressed

ACI 318-19 addresses the use of some materials that were not covered in previous editions. For example, provisions for the use of shotcrete have historically been in the IBC with no explicit mention of shotcrete in previous editions of ACI 318. Many IBC shotcrete provisions were incorporated into 318-19 and, working with the American Shotcrete Association and ACI Committee 506, were updated to reflect current practice. The unification is expected to clarify both the design process and construction requirements for the use of shotcrete.

Post-installed concrete screw anchors have also seen increased use and are now 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.

Alternative Cements and Aggregates

ACI 318-19 chapters covering concrete materials and mixtures now include provisions for alternative cements as well as alternative aggregates. This reflects an ongoing effort in the industry to improve the sustainability of concrete.

The code does not, however, cover specifications for design criteria and performance for these alternative materials because not enough industry testing has been done. The solution, for now, is to rely on the design team, materials suppliers and concrete producers to identify criteria, perform testing and provide data on the expected performance of products. It is hoped that once data for a given product or process have been generated, they should have applicability for many projects.

Clarifying Certification Requirements

ACI 318-19 now identifies qualification training programs for inspectors and installers and lists certification requirements. All inspectors are required to be certified if there is an appropriate certification program available.

Dr. Jack P. Moehle is chair of the ACI 318 Building Code Committee and 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.