High Speed Provider Gets High Speed Construction May 2006
Tallest Building in Philadelphia Uses Special Concrete Mix, Special Equipment
STURTEVANT, Wisconsin (May 1, 2006) – In January 2005, Comcast Corporation signed a 15 ½-year lease with Liberty Property Trust of Malvern, Pennsylvania to locate its headquarters within a planned new office tower in center city Philadelphia. The new skyscraper will rise 975 ft., surpassing the city’s existing signature tower by 30 feet, and providing the most significant addition to the Philadelphia skyline in over a decade.
At an estimated $468 million, the project is said to be the largest private commercial development undertaken in Pennsylvania. Named the “Comcast Center”, it provides a new head office for the country’s largest cable-TV provider. The company had outgrown its current Philadelphia headquarters when purchasing AT&T’s cable division in 2002.
To establish a new benchmark in skyscraper design, Liberty Property Trust secured the services of New York architect Robert A.M. Stern to design it and construction manager L.F. Driscoll of Bala Cynwyd, Pennsylvania to build it. The resulting tower will feature an unembellished geometric form sheathed in a glass curtain wall, along with a dramatic lighting scheme that changes colors at night. Created to be environmentally friendly, the structure will also be the tallest Leadership in Energy and Environmental Design (LEED) certified green building in the United States.
However, just as various facets of skyscraper design are being challenged for the 58-story building, so are several concrete placement methods. Presently, Madison Concrete Construction of Malvern, Pennsylvania is meeting these challenges head on.
As one of the largest concrete contractors in the Philadelphia area, Madison has over 30 years of specialized experience in large commercial concrete construction jobs such as stadiums, prisons and casinos. To its list of major projects, the company is now adding what is soon to be the eleventh tallest building in the United States.
“Although definitely the tallest high-rise ever undertaken by our company,” says Jim Dolente, Jr. Madison’s co-owner, “it ties with the Eagles stadium as the largest contractual job we’ve ever done in the area.” To accomplish the project, over 80,000 cubic yards of concrete is needed, which includes about 45,000 yards for the foundation mat and core walls and 35,000 yards for the metal deck slabs.
Concrete construction started in early October 2005 with a large concrete mat pumped for the building’s 76 ft. by 156 ft. foundation. Madison employed four of their fleet-based Putzmeister BSF 36-Meter truck-mounted boom pumps to handle the ten foot thick mat, which contained 700 tons of reinforcing steel.
The four boom pumps were strategically arranged around three sides of the perimeter, with each unit responsible for pumping one quarter of the surface. The mat pour was uncommonly large for the Philly area, so the flawless performance of the pumps in placing 4,400 cubic yards of concrete in under eleven hours was a notable achievement.
As the structure advanced upward from its foundation, concrete was pumped up to 80 ft. heights with a truck-mounted boom pump. Then a BSF 36-Meter boom was detached from its truck-mounted base to assume the role of a placing boom. The unit’s hopper received concrete at ground level so it could be pumped up to the eighth floor. After that point, the world’s largest trailer-mounted concrete pump, the Putzmeister BSA 14000 HP was called into duty. Madison is renting the trailer pump from the Alexander Wagner Company in Paterson, New Jersey. Madison has a unit of its own on order with Putzmeister – one that will be equipped with an even more powerful 630 hp engine.
“We’re doing sizeable 710 cubic yard pours for the thick 54-inch walls needed up to the 21st floor, so naturally it takes time to pour,” says Paul Ricciardi, Madison’s Project Manager. “However, we’ve been averaging 70 to 80 yards an hour since using the trailer pump. It’s made a big difference in output and performance, especially with the unique concrete mix involved.”
The special concrete specified is a high-strength mix with a minimum 20-inch flow and 10,000-psi compressive strength at 56 days. To meet the spec, Madison teamed up with concrete producer Action Supply near Philadelphia. After nine months of creating and testing several different cementitious material and mix designs, the result was a highly specialized self-consolidating mix incorporating polycarboxylate superplasticizer.
“We researched self-consolidating mixes in high-rises throughout the western states and became convinced it would work; and it has worked, thanks in great part to Action taking the ball and running with it,” says Dolente Jr. “Initial strengths have been outstanding, as we’re achieving compressive strengths between 9,000- and 10,000-psi in seven days instead of 56 days.”
Although strengths are proving advantageous, the mix itself is posing challenges. “It is a ‘love-hate’ mix,” says Dolente Jr. “It’s highly flowable and self-leveling to produce an extremely professional finish; which in turn, reduces labor and finishing costs.”
“The trade-off is that it creates a great deal of friction and doesn’t pump as easily as conventional mixes, so we’ve had to learn some tricks along the way,” adds Dolente Jr. “Fortunately, the high pressure 14000 trailer pump has plenty of power and is doing a good job pumping the high slump concrete.”
To place concrete with the combination trailer pump and placing boom, the mounting configuration includes a square PM tower utilized with an adjustable Putzmeister wall bracket. The wall bracket is secured to an elevator core and raises the PERI automatic climbing system formwork simultaneously as it climbs to each level. This results in a very time efficient approach.
PERI Formwork Systems claims that this is currently their biggest hydraulic self-climbing formwork rental because of the massive 40 ft. by 150 ft. size of the elevator core. The huge core accommodates a bank of 30 elevators on each 150 ft. by 200 ft. floor of the tall, slender building. As of early May, the project is on the 20th floor with Madison pumping a level every fourth day in a single pour. According to Ricciardi, “To maintain a four day cycle, the concrete needs to reach 1500-psi the morning following an afternoon pour, which is about 18 hours. That means we can strip the forms, raise the formwork and keep the project moving at a more efficient pace.”
The 10,000-psi self-consolidating mix was utilized up to the 20th floor and then switched to an 8,000-psi mix for the next 20 floors. About this same point in the job, a 36-inch wall thickness and 580 yards of concrete per level are required along with the expectation of achieving a shorter three day cycle. For the remaining floors, a 6,000-psi mix is specified.
Having broken ground in January 2005, the structure is scheduled for a fall 2007 completion. At that time, Comcast will initially occupy 873,000 square feet of the building's 1.25 million sq. ft. of rentable space.
SPECS Owner/Developer: Liberty Property Trust – Malvern, PA Architect: Robert A.M. Stern Architects – New York, NY Construction Manager: L.F. Driscoll Company – Bala Cynwyd, PA Ready mix supplier: Action Supply Company – Sharon Hill, PA Concrete and pumping contractor: Madison Concrete Construction – Malvern, PA Equipment: Putzmeister BSF 36-Meter truck-mounted concrete boom pumps (4), a detachable BSF 36-Meter placing boom, a Putzmeister tower and a BSA 14000 trailer-mounted concrete pump
RANK BUILDING CITY, STATE HEIGHT YEAR 1 Sears Tower Chicago, IL 1451 ft 1974 2 Empire State Building New York, NY 1250 ft 1931 3 Aon Center Chicago, IL 1136 ft 1973 4 John Hancock Center Chicago, IL 1127 ft 1969 5 Chrysler Building New York, NY 1046 ft 1930 6 Bank of America Plaza Atlanta, GA 1023 ft 1992 7 U.S. Bank Tower Los Angeles, CA 1018 ft 1990 8 AT&T Corporate Center Chicago, IL 1007 ft 1989 9 JP Morgan Chase Tower Houston, TX 1002 ft 1982 10 Two Prudential Plaza Chicago, IL 995 ft 1990 11 Comcast Center Philadelphia, PA 975 ft 2007