SMALL PUMP HELPS SHORE UP RENOVATED CARNEGIE HALL 2002

n a Carnegie Hall performance they hoped would not bring down the house, foundation renovators at the venerable New York City music hall cautiously excavated tons of rock, and shored and underpinned structural columns, beams and walls for tricky load transfers. Their work in the dismal basement deep below the 8-story building in mid-town Manhattan is nearing completion.

The $19 million foundation contract clears the way for construction of a new concert facility by creating more vertical space below street grade. The new $50 million Zankel Hall is designed by Polshek Partnership Architects, New York City. Construction manager Tishman Construction Co., New York, retained Schiavone Construction Co., a Secaucus, N.J., firm with heavy experience in demolition and foundation support work. Several other contractors declined bidding the job, where they would spend an estimated three years removing rock from the musty, cavelike workplace three floors below grade.

With no existing drawings of utility or hidden beam locations – some supporting loads of over 1 million lbs – Schiavone started the job in October 1999, with demolition and shoring required before rock could be removed to lower the basement floor by 15 to 30ft. The painstaking process took six months, according to Anthony Del Vescovo, project manager for Schiavone.

"We took out cast-iron columns to create the required open space for the auditorium, and transferred the load to temporary pipe columns and steel transfer beams. The cast-iron columns have been there since the hall was built in 1891," Del Vescovo said in mid-December.

"The basement," he says, "is a footprint of the building that has dimensions of 200 by 100 ft. We demolished old structural elements and underpinned the entire structure. Then we removed 8,500 yds of rock, and we’ve pumped most of the 2,000 yds of concrete."

Opening the cellar door An early logistical problem was obtaining access for equipment to the site on 7th Ave. between 56th and 57th Sts. The existing basement door is only 9 ft high by 10 ft wide. An excavation was made through the sidewalk at the base of the door, large enough to position a 20-ton, 10x16-ft hydraulic lift that lowers all equipment and materials to the subterranean site. Larger equipment must be disassembled before entering the pit.

Underpinning the existing structure required temporary support, while removing two rows of 27-ft-long cast-iron columns supporting a girder and truss system that carries the load from the hall’s main floor system. The temporary shoring consisted of pipe columns up to 45 ft long, drilled 16 ft deep and grouted into rock. Other cast-iron columns will remain in place.

The small access opening at the street limited equipment to mostly hand-operated come-alongs, chain-falls, sinking hammers and hydraulic splitters, says Del Vescovo. Rock excavation that started in early 2000 was completed by January 2001. The mica schist was fractured mostly by tightly controlled blasting, although chemical agents (expansion mortar), hydraulic hammers and an excavator with hydraulic ram were used.

The 3-level basement of the 110-year-old building had typical 13-ft ceilings that were removed to accommodate the new 100x50-ft auditorium, whose sloping walls of poured concrete are rising to the mezzanine. The walls, together with footings, perimeter columns, slabs, underpinning panels and load-transfer blocks are built with ready-mixed concrete delivered by a small concrete pump.

Positioned on the sidewalk, 20 ft from the 7th Ave. basement opening, a Schwing WP 1000X trailer-mounted pump started making small individual pours in January 2001. The pump is provided by A&B Preferred , Inc., Long Island, whose crew is placing volumes that range from 20 to 90 cu yds several times a week, as needed, according to Al Spitzer, partner in A&B.

"We’re there when they need us, sometimes only a couple times a week," says Spitzer. "Much of the work is pumping walls, where we’ve got to go slow to avoid form blowout. The pumping has to be coordinated with the shoring work and the removal and replacement of old wiring and piping – and a million other things."

Ready-mix is delivered from Anthony Concrete and Jenna Concrete, related companies with plants in The Bronx, on hauls that vary from 20 min to an hour, depending on traffic conditions. Certain restrictions remain on city streets and tunnels following the World Trade Center tragedy. Spitzer says a 12-yd truckload of mix is pumped in about 40 minutes for the project that consists mostly of small spot pours and multiple-lift wall pours.

The 4,000- and 5,000-psi mix is specified as 900 yds for perimeter walls up to 35 ft high; 800 yds for three slab levels, including the mat or "pit level"; 150 yds for footings; and 70 yds for elliptical walls. Smaller, additional quantities are pumped into new support areas. The elliptical walls, rising 35 ft from slab to mezzanine and battered at 7 degrees, shape the Aaron Auditorium with a 100x50-ft footprint. They slope inward at the top, "like a giant egg," according to Del Vescovo.

Typical pours from the Schwing 1000 line pump start with perimeter footings up to 2x2 ft for the full length of the outer walls. Walls are poured up to the first level, where the deck is started, resume to the mezzanine level deck, and are then completed up to street level. Deck levels are spaced at 15 ft. Generally working north to south, the concrete placement required a maximum horizontal line extension of 200 ft from pump to pour target.

Specialized pours Shoring requirements where existing cast-iron columns had to be extended to meet steel transfer beams posed a chemistry problem: Because steel can’t be welded to cast-iron, a pick-up point had to be devised. Accordingly, studs were drilled into the cast-iron and the Schwing 1000 was called on to cast a concrete transfer block around the studs on each column. Then, says Del Vescovo:

"We used come-alongs and chain falls to position the transfer beams under the concrete block, and jacked the columns off their footings. Then we excavated to about l5 ft below the columns and pumped a footing extension before transferring the loads back."

Another series of support pours was made below a 3 _-ft-thick, 50-ft-long rear wall and a pair of wing walls. Rock below the walls was removed to a depth of 25 ft and underpinned with a concrete replacement panel to excavated depth. The concrete was pumped in 13 sections, each 3 ft wide, in staggered pours. No more than three sections of leapfrogged wall sections were left unsupported before they were underpinned by the pump.

The Carnegie Hall job is one where pumping is clearly the most practical and efficient method of concrete placement, the renovation contractors agree. A conveyor system or buggying, A&B’s Spitzer says," would never work in a confined tangle of replacement framing. And with all these high wall pours, you would risk form blowout with anything but a pump, where the hose shortens the fall distance of your mix. We’re running the pipeline downward from the pump with no thrust blocks or elbows. The line bends easily wherever we need to make a pour."

Adds Schiavone’s Del Vescovo: "The pump has been reliable ever since we started placing mix last January. It’s helped keep us on schedule, and we expect to wrap up our phase of the job in February."