The award-winning Fifteen Fifty project is an iconic building and its design, via the use of Grade 80 vertical reinforcement (80 KSI) rebar, was an important early application of a proposed code change.
The Fifteen Fifty project includes a 39-story luxury residential apartment tower and 49 SVN, a 16-story office building on Mission Street in San Francisco. The project was an early application of a change in the American Concrete Institute Building Code ACI 318-19 for reinforced concrete buildings in seismic-activity-prone areas that will be adopted nationwide Jan. 1, 2023.Prior to the change, the code limit for high-strength vertical reinforcement in shear walls was 60 KSI.
DCI Engineers designed the building system, which has both towers containing performance-based design-lateral concrete shear wall systems with Grade 80 vertical reinforcement. It was a first for the city and the U.S., according to the engineers.
Using the high-strength reinforcement resulted in a reduction of more than $1 million in rebar material costs and lowered labor costs.
DCI’s team was led by Scott D. Erickson, PE, SE; Roger L. Heeringa, PE, SE, LEED AP; Gaelyn Krauser, PE, SE; and Nigel Lyons, PE. They coordinated low-cycle fatigue testing of the 80-KSI rebar in a laboratory at the University of Texas at San Antonio to confirm the reinforcement met strict ACI guidelines, specifically ACI 318-19.
"Using higher-strength reinforcing reduces the area of reinforcing required to provide the same level of strength,” DCI said in a statement released on its website. “Using Grade 80 reduced shear wall congestion and made for easier construction in the lower half of the buildings. Spacing and size rules sometimes govern minimum reinforcing area, usually higher up in the building where demands are lower, and Grade 60 is more appropriate.”
Alamillo Rebar fabricated the rebar and installed it, and Pacific Structures served as the concrete contractor. Build Group Inc. and Charles Pankow Builders Ltd. were the main contracting companies on the project.
“It’s all about using higher-strength rebar to use less material, which reduces congestion, creates a more economical structure, and reduces the carbon footprint at the same time,” said Erickson. “When the codes were originally written, it wasn’t something to consider. A long time ago, rebar was 30 KSI and eventually it became 40 KSI, then 60 KSI.
“Some testing for 80 KSI had been done, but not enough,” he said. “For this project, we decided to push it over the edge and use it, because this type of rebar was being manufactured and it was time to progress to higher strengths.
“Limited low-cycle fatigue testing meant there was insufficient data on its compression capacity during a high seismic cycling situation in a shear wall,” Erickson added, noting that the testing in Texas confirmed the ability of the rebar to withstand the compression and tension strains imposed during an earthquake given proper spacing of transverse reinforcement used to support the vertical rebar. “We gave the lab the strain parameters and the number of cycles that they needed to achieve for the tests to pass.”
John Wallace, of UCLA's Samueli School of Engineering and chair of the ACI 318 concrete building code seismic subcommittee, served as one of the peer reviewers for the project and helped develop the testing requirements.
“The low-cycle fatigue testing changed the spacing requirements for the transverse reinforcement for higher-strength rebar (the spacing of the ties that go around the verticals in the concrete shear walls)," said Erickson. "The spacing for Grade 80 is a bit tighter.”
DCI was confident that Grade 80 would meet the test.
“We had come up with the different rules during the performance-based design process and we had reasonable confidence that it would work,” said Erickson, who pointed out that the design was based on very limited test results. “We needed to get 30 cycles and we ended up getting several hundred. It was more than adequate."
Various elements of the rebar system were shipped to the Texas laboratory.
“The university lab was the best one to do the testing properly because they had developed a procedure to properly grip the bar without damaging it,” said Erickson. "That particular process, which was accepted, was only done in San Antonio. They conducted many tests on all grades of rebar, including 8, 9, 10 and 11 sizes that went into the building.”
Erickson appreciated the cooperation and efforts of Alamillo Rebar and the general contractors.
“They did a great job and were willing to go through the process,” he said. “We had all the right people and players involved, including Wallace, who was key to it.”
Since this project, DCI has built at least six buildings using high-strength rebar, the engineer said.
The Fifteen Fifty project has won several awards, including an honorable mention for the Precast/Prestressed Concrete Institute’s 2021 Design Awards for the multifamily category; the Concrete Reinforcing Steel Institute’s 2022 Honors Award in the commercial and mixed-use category; the 2021 AIA California Leading Edge Award; and the Merit Award at the 2021 AIA San Francisco Design Awards.
Preparing for ‘the big one’
While no one can predict when the next big earthquake will hit the Bay Area or the greater Los Angeles area, experts know the danger is real.
"It’s very likely that a damaging earthquake will impact Southern California or the Bay Area in the next 25 to 30 years,” said Wallace. "In some areas, the research suggests that we are overdue for a large earthquake. So, it could happen next week or next year; when it does happen, we will learn a lot about how real buildings perform.
“In the meantime,” he added, “we continue to implement new ideas to enable more economical and sustainable building designs that we expect will meet building code and societal performance objectives."
Prior to the code change, a large research program was ongoing for the use of high-strength reinforcing, implemented by the Charles Pankow Foundation and the ACI Foundation to develop the new ACI 318-19 code provisions.
"The testing extended far beyond that done at San Antonio for the Fifteen Fifty project," said Wallace. "There were tests on columns, shear walls and beams. The proposed changes were well-developed and mostly approved by the time the project was undertaken. Because we knew these updates were coming, we were able to justify the use of Grade 80 on this project. Early adoption of new approaches is critical to thoughtful updates to building codes. It's a case of merging interests."
Approval was given, Wallace said, "because the project was using an alternative design approach for the city with independent peer review. Projects can adopt new ideas as long as there is sufficient research to back up the use of these concepts and materials."
Asked how the code change will affect construction in the future, Wallace replied: "We'll see. It’s my understanding that many projects are using Grade 80 for vertical reinforcement within shear walls to reduce congestion, which can simplify construction, reduce costs and rebar fabrication time, and address sustainability issues."
Wallace added that Japan conducted similar research on high-strength reinforcement in the 1990s and has been using it for a long time. He said countries that experience heightened seismic activity, such as Italy, Taiwan, Chile and New Zealand, can benefit from this research, which helps with future building code developments.