Constructing a high-rise building is quite a challenge no matter how you look at it. But two projects currently under way in Los Angeles and Chicago are providing their respective concrete contractors an opportunity to demonstrate both their ingenuity and their skills.
When Morley Builders' veteran superintendent Larry Sevilla began considering how to go about constructing the 24-story Carlyle, he knew he needed a forming system that would give him flexibility. The luxury residence tower on Wilshire Blvd., just east of I-405, would be built on a very tight site. The close proximity of adjacent buildings eliminated the use of flying forms. Also, because tieback anchors could not be installed under the existing structures, it meant the foundation walls would require raked bracing anchored in the excavation that would remain until the four, 36,000-square-foot underground parking decks had been placed.
Sevilla decided to try the new A-Deck system offered by Atlas Construction Supply Inc., San Diego. The handset post, joist, and drop-head shoring system is designed to use a 6x8-foot grid pattern but easily accommodates angles and areas around columns that need to be filled in.
Forming the parking deck for the Carlyle's subterranean parking levels was fairly straightforward except for the rakers on three sides of the foundation, around which the formwork had to be built. Morley provided Atlas Construction's Kelan Quinn with shop drawings showing the rakers' exact locations, including the double row on the 200-foot-long south wall. From that information, Quinn produced CAD drawings and material lists for erection of the shoring for each level, taking advantage of the five different girder lengths up to 8 feet to accommodate the interruption of the rakers.
The tower portion of the Carlyle is based on a moment-frame design, which means deep beams and dense column spacing. But it also has a broad v-shaped footprint, so half the columns are rotated with respect to the other half. The sheer number of columns—45 in the space of a 16,000-square-foot floor—posed two challenges. The first, filling in the decking around the rotated columns requiring angle cuts, was simplified because the deck shoring system was so adaptable. The bays are “very easy to downsize,” Sevilla says.
The second challenge was the concrete placement. The slabs are a 6000-psi mix but require a higher strength mix at the beam/column intersections—10,000-psi through level nine and 8000 psi to level 21—above that all the concrete is 6000 psi. Rather than pouring a puddle of the higher strength mix followed by the main slab, Morley crews worked in reverse. They began by placing slab concrete and leaving a void around each column, later filling the voids with the column mix. They poured half of each floor, or 8000 square feet, in a day, and worked on a seven-day cycle.
Further complicating the placement sequencing was the fact the higher strength mix required a longer mixing time. Sevilla addressed this by estimating the point at which they would first need the column mix. For example, he would have the producer send 10 loads of the slab mix seven minutes apart. Load number 11 would then be the first of four loads of the column mix delivered 20 minutes apart. That required booming back and forth between the column locations and the slab area, and was complicated by having to work around 22 or 23 tall columns.
On the ground, the concrete delivery posed a challenge because of the tight site conditions. To make the best use of the staging area—a 16-foot-wide 200-foot-long space—Sevilla placed the pump at the center and provided access from each end.
The Carlyle will have a precast concrete curtain wall, which contributed to the need to place 10,000 embedded plates in the concrete. The 24th floor, placed in August, will be the base for two additional levels of steel framing. Morley will place a concrete pad for a heliport on the 26th level.
No two are the same
Halfway across the country, McHugh Construction is making good headway on another unique project: the Aqua building on Chicago's lakefront. The wavy exterior that gives this mixed-use high-rise its distinctive appearance also presented Chicago-based contractor James McHugh Construction with a major challenge: constructing the curved, cantilevered balconies that extend from 8 inches to 12 feet beyond the exterior columns, each with a distinct outline.
McHugh's concrete superintendent Paul Treacy, who is on a three-day cycle and ahead of schedule, is quick to point out that the efforts of many people are what is making this project a success. The bulkhead used to shape the floor slabs was made onsite by the “miscellaneous metals” crew. Project engineer Tom Kruszewski came up with a utility program from Carlson Software that makes the layout quick and efficient. As he receives the drawing for each floor, Kruszewski does a digital takeoff of points along the perimeter. AutoCAD uses the data to generate a list of point coordinates, which then are loaded into a robotic total station. With that, tech engineers lay out the points as the deck is being formed. Treacy says coming up with that process was key to being able to stay on schedule.
Because the balcony shapes are so variable, a nontraditional approach was adopted for placing the epoxy coated rebar. Treacy's head iron worker suggested using three standard lengths of rebar rather than cutting every one to size. Although that means many of the lap splices are longer than necessary, it greatly simplifies material handling and speeds installation.
Another valuable suggestion came from the head cement mason on the job. Crews are striking and moving the flexible bulkhead while the concrete is still workable, just hours after placement, permitting the same broom finish to be put on the balcony edge that's on the surface. That sameness will be particularly appreciated as the waterproofing is applied, which can exaggerate differences in surface texture and finish.
McHugh is using two sets of flying forms from Aluma Systems for the main portion of each floor. The girders attach to the core and columns, transferring all loads directly into the building frame, so reshoring is unnecessary. It also helps keep construction loads off the cantilevered balconies.
For the decking, Treacy is using a composite overlay from Maxam Industries. The hard, smooth surface is slip resistant, which has been useful as workers move about the surface in wet weather. Designed for a very high reuse factor, the panel surface also provides a very smooth concrete surface. With no additional touch-up beyond removing occasional fins, the as-cast surfaces will receive only a skim coat of plaster before becoming the finished ceilings.
Project executive David Alexander says that although the tower is its most visible element, the project is very complex. This becomes apparent when you look at the 20 turnover dates for various portions of the facility and the logistics of working at all its different use areas spread over a 4-acre site. As the project's general contractor and design/build contractor for roughly half of the $310 million construction project, McHugh's design/build responsibilities include the mechanical, electrical, plumbing, fire protection (MEP), and concrete, but its largest crew is devoted to concrete. “Doing the concrete work ourselves give us control over the project schedule,” Alexander says.
While concrete work proceeds—the 50th floor was placed in August, with topping out expected next February—the first ballroom was scheduled to be turned over in August. Knowing that led Treacy to try something new in the pumping setup. Because so much concrete had to be placed after the early turnover—requiring pumping through finished building areas—Treacy opted to embed the pump pipe in the building core. “The structural engineer's only requirement is that we fill it when we're finished,” Treacy says.
Although concrete placement is a high-profile activity, such a project always has other challenges along the way. With a 172,000-square-foot footprint, the Aqua is being built over several old, abandoned freight tunnels, which had to be filled with 500-psi lean grout. The foundation work included 305 drilled bell and rock caissons from 4 to 12 feet in diameter reaching as deep as 112 feet belowgrade.
One early aspect of project strategy had to be revised due to scheduled turnover dates. Because the ballroom was to be completed so early in the schedule, work had to begin on that area very early, making it unavailable for staging a large crane that would be used to place 65-ton girders spanning an electrical substation on the site.
As an alternative, McHugh increased the concrete strength for the garage columns from 5000 to 8000 psi, put grillage on top of the garage, and located the crane there. Even though the lifting load was 1.5 million pounds, reshores were only needed in the garage as the crane was brought in.
Treacy gives a lot of credit to the McHugh executives for committing the people and resources to the job to do what needs to be done. That kind of commitment and the attitudes and experience its people bring to the job are what enable McHugh to take on unique and challenging projects such as this, he says.
“The whole team at the Aqua is working together,” Treacy says. “We've had excellent support from our suppliers, and everyone has thrown themselves into achieving the three-day cycle.” And that's what makes it work.