BOOM FLOWN TO STAGGER TWO-TOWER CONDO POURS 2001

A separate placing boom flown between adjacent towers is keeping a condominium project on schedule in downtown Oakland, Cal. The flying boom took over at the towers’ second levels after a rotating fleet of truck-mounted boom pumps poured a thick foundation mat, 3 1/2 levels of below-grade suspended slabs and two upper decks.

Swinerton & Walberg Co., San Francisco, is general contractor for the $47 million Essex on Lake Merritt project that features a 22-story highrise and a l0-story tower. The two structures are 30 ft apart, and are connected by a pedestrian bridge at the second level. The Essex Group, Palo Alto, is project owner; Lakeshore Partners, Oakland, is the development manager.

Concrete work was started in May 2000, with mat pours made into a 30-ft-deep excavation, shored with tie-backs and sheetpiling. The 7- to 9-ft-thick, 8,200-cu-yd mat extends below both towers and was completed over a two-week period in four separate pours of l,200, 5,000, l,200 and 800 yds, tied with construction joints. On the largest pour of 5,000 yds the contractor used a 52-meter, two 42-meter and a 36-meter Schwing boom pumps. These units were rotated, one at a time, to complete the remaining pours. Concrete for the foundation slab, poured by Conco Concrete Pumping, Concord, Cal., accounted for nearly one-third of the project’s total volume of 27,500 yds of mix.

"We started pumping the parking decks below-grade before the foundation slab was done," says Mike Alameda, superintendent for Conco. "Usually we ran just one pump to complete individual pours of 300 yds to cover about l5,000 square feet of deck, and we use no more than 50 to 60 feet of added system. This limits the amount of hose that we move over the post-tensioning strands."

Jim Clark, superintendent for Swinerton & Walberg, says site restrictions limit the use of an additional number of large pumps like the 52-meter Schwing. "We weren’t allowed to close the streets," says Clark, "so we used the smaller pumps on pours, as needed. The parking deck slabs are 8-in-thick and they run for l50 ft below both towers. The upper deck contains a game room, swimming pool and a spa under the plaza area between the towers. We averaged 80 yards an hour on these decks."

Booming the towers After the two suspended decks above grade were poured by the rotating series of Schwing pumps, a 32-meter Schwing separate placing boom, flown between mounting masts on each tower, simultaneously began concrete placement on the upper decks of the fast-track project. The towers face east on Lakeside Ave., a street that arcs around the front of the structures.

Following the curvature of the road, the 22-story tower is designed with a curving front facade that provides a view of Lake Merritt from three sides. Each level of the high tower requires 300 cu yds of deck, wall, column and beam mix for the all-concrete structure. A typical suspended deck on the high tower requires 13,000 sq ft of concrete coverage.The lower tower coverage is l0,000 sq ft, requiring 250 yds of mix.

Conco positioned a Schwing 32XL with detachable boom and Generation III pump at l7th St., near its intersection with the curving Lakeside Ave. to pump the mix horizontally for 50 ft to the standpipe of the lower tower. As pours were staggered between the two structures, the pump was easily relocated to within 75 ft of the higher tower.Vertical line is run through the 3-ft-sq mast cutouts of the clamp-style floor frames in the decks of each tower. The standpipe is secured with a collar and bolt system..

An elaborate forming system is in place for the cores of the structures. Each requires an elevator and two stairway cores, with wall thicknesses starting a 2 ft thick and tapering to 1 ft at the upper levels. The Efco systems were assembled and erected on site by Conco forming crews.

Staggering the pours Operating on a l0-day cycle to complete a 7 _-in-thick deck and fly the Schwing separate placing boom to the lower tower, Conco’s pumping crew first places the l3,000-sq-ft slab concrete. Two days later vertical forms for columns and walls were pumped. After forms are stripped the deck slab is post-tensioned, the flying table forms are crane-picked to the next level. All 27,500 yds of ready-mix is delivered by RMC Pacific, Oakland.

"We’re working at a steady 50 yds an hour in place, including wall and column concrete," says Conco’s Alameda. "The job requires 230 yds of lightweight aggregate mix and 70 yds of hardrock mix at each level, and this slows things down a little at the ready-mix plant. It requires close coordination with the plant, but we’re still on schedule for completion."

California regulations for seismic resistance require that the 6,000-psi hardrock mix be placed at moment-frame points, where walls converge at the corners and at all column locations.

From its 45-ft-long, heavy-duty 24-in. mounting mast at each tower, the Schwing boom reaches over the decks to cover all surfaces except one distant corner at each level. "We chose not to add more hoseline because of the exposed post-tensioning strand," Alameda says. "We bucketed the corner. Overall, though, the crane and bucket will place less than l percent of all concrete on the entire job"

Swinerton & Walberg superintendant Jim Clark says: "Pumping the mix is absolutely the only way to go. Bucketing would put us behind our deadline. The pumping has been flawless. It’s giving us a steady and predictable flow of concrete."

By mid-March construction had reached Level 7. The low-rise tower, Clark says, will be completed a month or two before the high tower. The on-schedule project will be wrapped up by its completion date of April 2002.