ACI released the print edition of ACI 318-25 on July 9, and it is also available as a continuously updated digital version known as ACI 318-25 PLUS.

New to the guide is a supplemental guide for sustainability, which, rather than requiring use of sustainable materials and practices, offers frameworks and templates for meeting locally enforced requirements, explains Greg Ziesler, senior engineer and secretary for the ACI 318 committee. “Typically a code puts a limit on whatever you’re doing—thou shalt have this much steel and this is the spacing,” he says. “This is more that the local jurisdiction says you must [meet] this GWP [Global Warming Potential], so how do you meet that? We now have a framework to meet those requirements.” 

The sustainability section offers guidance on meeting carbon neutrality and low-carbon goals in design and material selection. It also includes resiliency against various natural disasters such as tornados and tsunamis, which the core ACI standard does not call out directly, but does offer guidance on the types of structures that would meet those challenges. The appendix offers recommendations on how to achieve resiliency goals working from what is already covered in the existing code. “These are different types of loads, not in the body of the code. So now if someone says you have to do this, you have a body of knowledge to do it,” says Ziesler.

And low-carbon concrete is covered by its own standard, the ACI-323, explains Mike Tholen, managing director for technical operations at ACI. But the appendix in 318-25 offers a new way to address sustainability requirements without setting a new standard. “We still lean on local jurisdictions to set the limits, but there are some default options in there, based on the size of the project and what sort of targets you have to meet for GWP,” he says. “One reason it’s in a separate standard is that it’s not the law of the land until a local jurisdiction adopts it.”

Needed Clarifications and Updates

As with many standard updates, the ACI 318-25 includes multiple revisions to clarify information in the previous ACI 318-19 edition. “Particularly, the changes in seismic design are driven by new information brought in with 318-19,” says Ziesler. “Very early in the cycle, we got a lot of questions in from California, Washington and Oregon, areas heavily affected by seismic design conditions.” The changes in seismic design for 318-25 include a great deal of detailed clarifications on what was in 318-19. “ [A lot of what] we changed in 318-25 will help seismic engineers in those high seismic regions get some designs that are a little more efficient,” notes Ziesler.

Other clarifications include an expanded section on deep foundations, which Zielser says is building on work started in 318-19, which was the first revision to include deep foundations. While 318-19 saw the first attempt to bring in the deep foundations provisions from the International Building Code into the ACI standard, the 318-25 revision has consolidated all of the IBC deep foundation information in one place.

A further update from 318-19 can be found in the section regarding shear friction, particularly in certain types of basements and retaining walls. A government agency noted that the provisions in 318-19 were causing some problems for their existing designs, and Ziesler says the committee sought in 318-25 to clarify the standard so certain calculations were not being applied too broadly. “It just created a bit of a stir … sometimes when you jump from one part of the industry to another, different folks have done more testing in certifying performance, and they could show it works with a different equation than in 318-19.” The updated version in 318-25 seeks to address some of these discrepancies, but Ziesler says further clarifications may be due in the next update.

Performance-based Wind Design

Another major supplemental addition to the ACI 318-25 is the inclusion of performance-based wind design. As with the sustainability section, this is not a part of the core code standard but rather a guide for properly deploying performance-based wind design when it is called for by a client or local jurisdiction.

“Performance-based wind design—that [inclusion] is a response to the industry from committee members. We have a lot of committee members who do tall and supertall buildings,” Ziesler says. While the American Society of Civil Engineers already issued a pre-standard on the wind performance-based standard in 2019, ACI is only offering an advisory section on the method for modeling loads. “It’s not an appendix to the code; it's not in main body of code; it only applies when you do it,” says Ziesler. 

Implementing performance-based wind design, either through modeling in a real wind tunnel or with advanced computer simulations, does come with an additional cost, and ACI is not requiring it for any particular structure. Rather, “the instant you take a design to a wind tunnel to nail down the design better—with a tall or supertall structure in a city—you’re probably going to have to talk about doing a performance-based wind design,” he says. 

The key in offering a optional standard for wind was getting to the point where everyone involved feels they have reached a good final design, says Ziesler.  “[The committee] spent a ton of effort on that … there’s a lot of detailing requirements that are not going to affect a lot of structures, and [those designers] won’t think about doing this, but when you get into the height of a city or a supertall you will think about doing this, and this sets a standard for that.”

The ACI 318-25 is available for purchase from ACI, in both print and digital formats.