Making Better Concrete Connections in Shear-Wall Cores
Northwest teams improve steel-fiber-concrete link beams to reduce rebar congestion in seismic-zone shear walls
Structural engineer Cary Kopczynski once penned a prediction: “There may come a day in the not-too-distant future when concrete building structures will commonly be reinforced with a combination of steel fibers and steel reinforcing bar. Rebar requirements could be reduced, perhaps significantly.” He wrote “Beyond Rebar: A Revolution in Concrete” for the Seattle Daily Journal of Commerce in April 1994.
But the not-too-distant future came and went without a concrete revolution. And the 40-year veteran of earthquake engineering, who practices mostly on the West Coast, still is making the same prediction about reinforced concrete dosed with steel fibers as rebar decongestants. “Looking downstream, there are applications for steel fiber throughout the shear wall,” says Kopczynski, CEO of the 30-year-old Bellevue, Wash., firm that bears his name. “That’s still a way off.”
In quake-prone areas, rebar congestion has long bedeviled contractors. Kopczynski’s solution—adding high-strength wire reinforcement—enhances ductility and shear strength, allowing for less rebar. That, in turn, increases constructibility, productivity and material performance.
Kopczynski’s problem: The buildings sector, in addition to having a general aversion to risk and innovation, “is set up to work with rebar,” says Kopczynski. And in seismic zones, rebar fabricators and installers typically don’t like wire in any shape or form.
That hasn’t deterred Kopczynski. About 13 years ago, tired of hitting his head against a “shear wall” on the topic of steel-fiber-reinforced concrete (SFRC) in seismic cores, he narrowed his sights to focus on the most shear-stressed and rebar-congested seismic element—the link, or coupling, beam.
Like doorway headers, the typically 2.5-ft- to 3.5-ft-wide link beams span openings in walls. At four or five per floor, there can be hundreds in a tall building.
Kopczynski views SFRC link beams as stepping-stones toward structural SFRC in walls, column-beam joints and other structural elements. “You’ve got to dip your toe in the water before you jump in,” he says.
To meet the high shear demand, seismic engineers specify link beams with heavy X-shaped rebar, in addition to transverse rebar with ties and stirrups. The diagonals, which Kopczynski has long called the “Gordian knot,” often collide with transverse bars. In that case, frustrated rebar installers have been known to cut the transverse bars.
Batch-plant-mixed SFRC, which slashes link-beam rebar by about 40%, not only unties the Gordian knot by eliminating diagonal bars but also reduces the size of the flexural and shear bars that remain, says Kopczynski. Less-dense rebar allows beam penetrations for electrical conduit, pipes and sprinkler lines, he adds.
Initially, Kopczynski’s innovation met with resistance. “It is now clear that SFRC beams add value, but this was not the case a few years ago,” he says. “The first several owners we approached were understandably concerned about being a guinea pig for a new idea, outside the code, whose cost benefit had not been demonstrated.”
Thanks to Kopczynski, SFRC link beams, which come with their own challenges, slowly are gaining traction. To date, the 45-person Cary Kopczynski & Co. has gained approval for four CKC link-beam towers—one is finished and three are underway. With each project, teams improve the new routines and their rhythms.
CKC’s beams would have stayed a pipe dream were it not for laboratory tests. In 2009, University of Michigan researchers subjected four-story coupled-wall specimens to quake-type lateral displacement reversals, comparing them to conventional beam behavior. The link beams contained 200 lb per cu yd of Bekaert Corp.’s 445-ksi steel wire—1.18 in. long and 0.015 in. in diameter. The tests showed the superior performance of SFRC beams (ENR 4/19/10 p. 12).
Kopczynski was at the ready, expecting to move forward with two 40-story Seattle towers, but they became victims of the recession. Undaunted, the trailblazer ultimately succeeded in a proof of concept by pioneering CKC link beams—on a limited basis—in the 12 upper stories of The Martin, a 24-story building in Seattle (ENR 2/25/13 p. 10).
The $1.2-billion, 76%-complete Lincoln Square Expansion, a 2.6-million-sq-ft multiuse project in nearby Bellevue, followed on a grander scale. Of 392 link beams in its two 450-ft-tall towers, the 341 that are above grade have fiber.
LSE’s Kemper Development Co. went with CKC’s fiber system only after it saw a mock-up of a conventional link beam so jammed with rebar that there was little room for concrete, says Fritz Walter, Kemper’s senior vice president of construction.
Next in line is Seattle’s 40-story 970 Denny, a concrete residential tower with 386 fibrous beams in the core. Workers currently are building the subgrade levels.
Kemper and its LSE team support future use of CKC’s beams, with caveats. “There are a few drawbacks,” says Johnny Luttrull, an associate with HKS, LSE’s architect. “SFRC requires building-official approval and bucketing [due to its density], and the steel-fiber cost is high due to a single-source supply.”
Eyes wide open, Kopczynski is working to eliminate the drawbacks. More research—now underway—will support a code proposal and allow other brands and grades of wire. And CKC aims to reduce the dense fiber dosage to a pumpable 150 lb per cu yd of concrete.
LSE’s general contractor, which is self-performing concrete and vertical-element formwork under a $600-million contract, says SFRC link beams speeded up the job. “With diagonal bars in the link beams, we would not have been able to maintain the schedule,” says Alan Kniffin, project executive for GLY Construction.
The SFRC system, with 3-cu-yd link beams, allowed for a five-day floor cycle, instead of six, which sliced two months off the schedule of the 42-story North Tower, a hotel-residential high-rise with a cast-in-place reinforced-concrete structure and post-tensioned slabs.
The tower, whose lower three floors are set to open in March, with the rest opening in August, has five 3-ft-wide by 2-ft-deep link beams per floor—four in the core and one in a separate shear wall. Compressive strength is 8,500 psi.
LSE’s South Tower, a 31-story office building, has a steel frame with composite metal decking and a concrete core. There are five link beams per floor until the 20th floor and two per floor in a smaller core above that. Beams are typically 2.5 ft wide by 3 ft deep.
GLY built the core to level 20 before starting the steel because steel goes up faster. If not for CKC’s beams, GLY would have had to delay the steel another day per floor. The tower is set to open in January.
GLY embraces CKC’s beams. But its list of disruptions caused by them is longer than Luttrull’s. Until further notice, all fiber jobs are limited to the wire and dosage tested in 2009. The lack of competition for wire did increase the mix cost—$735 per cu yd compared to $135 for self-consolidating concrete (SCC) alone, says GLY. But the expenses were offset by eliminating 180 tons of rebar and associated labor.
The job contains 168,300 lb of wire in 841 cu yd of concrete. GLY says it kept a “continual watch” on wire shipments from Europe to maintain the job’s pace.
Beyond that, extra early effort at the batch plant is required to create a mix with the proper fiber distribution and slump, says Keith Smyth, senior sales representative for concrete supplier Cadman Inc., which had learned to use SCC when it supplied The Martin.
“It’s a sticky mix, like a hair ball,” says Kniffin. Among other tasks, concrete mixers have to be cleaned.
Initially, even getting the mix out of the bucket was an issue until GLY figured out it needed to use a low-slump concrete bucket, which has steeper sides.
Quality control is different, too. GLY casting crews had to get up to speed. The dense mix needed to be spread with shovels and then vibrated. “There were early learning curves,” says Kniffin.
GLY also had to coordinate two concurrent concrete operations: pumping for the shear walls and crane-bucketing for the link beams.
Kniffin says construction of the North Tower was more complicated than the South Tower because of the need to coordinate the core work with the rest of the concrete structure. But on both towers, the basic sequence for the cores was the same (see p. 34).
Ironworkers led with link beam and wall rebar. Tony Gerde, LSE project superintendent for Harris Rebar-Central Steel, calls CKC’s approach “more straightforward” than any Harris has seen. It made rebar installation “faster and easier and helped free up crane time,” typically needed to pick diagonal bars, says Gerde.
Mike Dolder, senior principal of Mayes Testing Engineers Inc., which inspected rebar placement and tested samples of the concrete, is another fan. “Less rebar congestion makes our job easier,” he says. “The strength tests were fine, and the mix performed well.”
After the rebar, workers installed steel-mesh screens at each end of the beam to prevent the fibrous mix from flowing into the wall area. Formwork followed.
Then, crews moved the fibrous mix, trucked about 15 minutes from Cadman’s batch plant, into the low-slump bucket, which was lifted by crane.
Next, crews maneuvered the bucket into position so workers could coax the mix into place, using shovels and vibration. It took seven to 10 minutes to cast each 3-cu-yd beam.
Then, other crews pumped the wall concrete to either side of the SFRC beam. A finished beam looks no different from any other.
On the 970 Denny job, all 386 link beams will contain SFRC. Instead of a mock-up, the team decided to use the below-grade link beams, which typically do not need diagonals, as trial runs for the above-grade SFRC link beams. “By the time we get up to the street, we will certainly have the cycle down and a good [placeing] rate,” says Mike O’Leary, general superintendent for the concrete subcontractor, Conco Northwest, which built The Martin.
The trial strategy has paid off. “We learned just a few weeks ago that the fiber creates enough internal cohesion that the mix can be cast without” the steel mesh at each end, says Kopczynski.
“This is a big deal,” adds Kopczynski. The installed mesh would have cost about $80,000, mostly because the installation around rebar is labor-intensive.
For CKC, fibrous beams take extra time and effort, so far unremunerated, especially during plan review. Until SFRC—now used most often for crack control in the U.S.—is in the seismic code for structural applications, CKC has to apply for approval—job by job—under Section 104.11 of the International Building Code.
Familiar to innovators, the clause allows for use of alternate materials, design and methods. But the engineer has to demonstrate, through performance-based design (PBD), equivalent or better performance compared to a prescriptive-code-complying structure.
“As a building department, it is important to be open to considering new designs but to be cautious not to get too far out ahead of the design community,” says Gregg Schrader, the building official in the Bellevue Development Services Dept.
PBDs and shear-wall core buildings taller than 240 ft on the quake-prone West Coast trigger peer reviews for plan approval. In the case of LSE, the green light was based on the reviewer’s assessment that the design correlated with the 2009 SFRC link-beam tests. The structural engineer’s experience also counted.
The design had to match the test specimens’ fiber dosage, span-to-depth (aspect) ratio and wire tests. The peer reviewer listed every parameter for each condition in the building—sizes, configurations and rebar amounts—and then listed the corresponding shear-stress parameters and aspect ratios for the 2009 test beams.
“We created a table and did a side-by-side comparison to make sure [the design] was similar to test specimens in shear-stress demands, dimensions and details,” says peer reviewer Joe Maffei, principal of Maffei Engineering. “It generally tracked.”
The link-beam review accounted for 10% of the cost of LSE’s $100,000 peer review. Maffei, who also reviewed The Martin, says the LSE link-beam review amounted to about a third of The Martin effort.
Bellevue granted LSE approval in March 2013, based on Maffei’s recommendation. “As far as this project is concerned, we embrace this technology,” says Schrader. “We think it might be a better system for seismic resistance, resulting in less, more evenly distributed damage, and we would support having it introduced into the code.”
But until SFRC link beams are in the code, CKC will have to spend extra time on plan review. That’s one reason why Kopczynski and other proponents of SFRC link beams are involved with research aimed at providing technical material for a code provision.
Recent tests, supported by a $214,240 grant from the Charles Pankow Foundation, evaluated different types of fibers, from two suppliers, “to determine if we can use fibers that are not as good as the Bekaert fibers [already] tested or if can we use fewer fibers if we use the same [high-quality] fibers tested,” says Gustavo J. Parra-Montesinos, principal investigator and an engineering professor at the University of Wisconsin who also led the 2009 Michigan tests. Parra-Montesinos expects Pankow to release the report this month.
For the code change proposal, the aim is to create a chart with different classes of SFRC, based on bending performance and linked to a given drift capacity in link beams. The chart will be wire-brand blind, to keep it generic for the code, says Parra-Montesinos.
Andrew W. Taylor, an associate with KPFF Consulting Engineers who is involved with the Pankow research, chairs the seismic subcommittee of the “ACI 318 Building Code: Requirements for Structural Concrete,” published by the American Concrete Institute. “We would like to formulate provisions for SFRC link beams for the 2019 edition,” but this will depend on deliberations of both the seismic subcommittee and the main ACI 318 committee, says Taylor.
The recent research, “as well as implementations of SFRC link beams by CKC, have been instrumental in laying the technical groundwork and demonstrating proof of concept,” he adds.
Though, so far, CKC has focused on tall buildings in seismic zones, rebar congestion can be nearly as bad in short buildings in seismic zones. “There is definitely a use for SFRC in shorter buildings,” says Kopczynski.
The engineer also says he isn’t committed to steel wire as the fiber. “We’re using steel because that’s where the research is, but other materials have potential, including carbon and graphite, which have the same strength and modulus of elasticity as steel,” he says. “They might be more expensive but they are more workable,” he adds.
Kopczynski says he hasn’t forgotten his decades-old dream of expanding the use of fibrous concrete to shear walls, column-beam joints and other elements.
“Seismic design is largely about maximizing shear strength and ductility,” he says, adding, “Fibrous concrete does both.”