Luxury Condos in Bay Area Go Up With Something Unique to Area - A Placing Boom
Luxury Condos in Bay Area Go Up With Something Unique to Area - A Placing BoomOnly second time a placing boom used in Bay Area
To the casual observer, there's nothing new about the construction of a 27-story condominium building next to the Bay Bridge in San Francisco, but those in the industry will quickly notice the use of a separate placing boom.
"This is only the second time a placing boom has been used in the Bay Area, and we were the ones who used the first one a few months ago. That one was for a high-rise apartment building in San Francisco," said Andy Paulazzo, vice president of Interstate Concrete Pumping in Hayward, Calif. "For some unexplained reason, placing booms just aren't commonly used in this part of the country," he said.
Chris Lotti-Chun, assistant project manager with Webcor Builders, the general contractor, agreed. "A placing boom is something that's rarely used here," he said. "We've had people walk by and ask, 'What the heck is that thing?' However, these placing booms will probably be used more as word gets around about how efficient they are for placing concrete, especially on high rise projects."
The reasons placing booms aren't considered for certain projects are due in part to the type of construction specified by the builder. One example is steel with slab on metal deck structures, which cannot utilize a placing boom system. Another explanation is that contractors may be accustomed to typically pouring with crane and bucket. However, as the cost of crane rental has skyrocketed, other, more economical methods such as the placing boom are now getting more attention to handle high rise jobs such as the one at hand.
13,000 cubic yards of concrete to be poured in tight area
The building, called Bridge View Towers, will take more than 13,000 cubic yards (9929m3) of concrete to complete, or about 400 cubic yards (306m3) per floor, including the basement and two sub-basements. All the concrete - even that for the basements - was delivered via the placing boom, as planned by Webcor.
A Putzmeister 32-Meter truck-mounted concrete boom pump was selected to do double duty. Its boom featured a special detach kit, which allowed convenient removal from the truck and placement atop the towers to access 105-ft horizontal reaches. To get the high-strength, 7,500 psi concrete up 27 floors, Interstate kept the same Putzmeister 32-Meter unit (now without boom) on-site. It could pump up to a high volume of 210 cubic yards an hour (160 m3/hr) and attain a high pressure of 1233 psi (85 bar) on the rod side with its .16H pump.
"We used Putzmeister's free-standing anchor system to put two towers down in the basement so we could use the placing boom to pour all decks, including the three floors below grade. Then we ran a pipe 35 feet (11m) down from the 32-Meter truck pump," said Paulazzo.
The placing boom alternated between towers for each floor. Fortunately, the Putzmeister design offers the easiest placing boom to raise and move. With its four-pin connection, it was easily disconnected, flown to the next tower and reconnected - all in under a half-hour. Upon completion, an estimated 730 feet (223m) of pipe will have been used in conjunction with this setup.
With the narrow streets and commuter traffic, the city allowed use of only 20 feet (6m) along the east-side property line and a small area on an adjoining road. Nothing more was available without involving substantial up-front fees or expensive ticket violations. However, the most limiting factor was not the size of the truck pump, but the ability to back two concrete trucks simultaneously into the unit's hopper while staying off the street. The crew tucked the truck pump close to the tower crane and then crowded two mixers at a time into the small area, affording them the ability to avoid incurring any additional costs.
Three different mixes used
Three different concrete mixes were used - one for the elevator shaft walls, another for each of the 42 columns per floor and a third for the deck pour, which was a high early-strength, 7-sack type A mix that set up at 3,000 psi in 72 hours. It had a 4-inch (100mm) slump, easily handled by the 32-Meter pump and placing boom.
"This mix was used because it allowed us to stress the post-tensioning cables in the decks three days after pouring," said Lotti-Chun. "This method saves time and money as it minimizes the time spent supporting the recently poured deck, allowing us to continue building vertically.
By maximizing the versatility and efficiency of the combination 32 Meter Putzmeister pump and placing boom, the 245 condominiums in Bay View Towers will be ready for their scheduled occupancy in May 2002.
Owner: Bridge View Towers, LLC
General contractor: Webcor Builders, San Mateo, Calif.
Pumping contractor: Interstate Concrete Pumping, Hayward, Calif.
Ready-mix supplier: Bode Sand & Gravel, San Francisco
Equipment: Putzmeister 32-Meter concrete boom pump with detachable placing boom