In October 1986, Sundt Corp. topped off the tallest slipformed building core in the United States, the 725 «-foot-high core for Atlanta's IBM Tower. In slipforming this concrete core, the crews used lasers to keep the form level and the core plumb.


After studying 42 combinations of floor framing and wind framing systems, the general contractor concluded that slipforming would cost somewhat more than other methods, but would shorten the total construction schedule by at least 3 months.


The slipform had three work decks: an upper deck, middle deck, and lower finishing deck. A tower crane delivered concrete and reinforcing steel to the upper deck. From there, workers installed the vertical reinforcing steel for the core walls and emptied the concrete bucket into hoppers. From the hoppers, the concrete dropped through tremies to the middle deck, where laborers distributed it with buggies to the wall forms. From the finishers' deck 17 feet below the middle deck, workers finished the concrete, applied curing compound, removed blockouts, installed steel beams for the elevators, and attached tee clips for structural beam anchorages.


Keeping the slipform level and the core plumb are two of the greatest problems during slipforming. Lasers guided the slipforming of the IBM Tower core, which had to be within 2 « inches of plumb for installation of the elevators. These lasers were set up at ground level in opposite corners of the core. They were aimed up at clear plastic targets set in holes cut in the middle work deck. By watching where the lasers hit these targets, the jacking operator could tell if the slipform was rotating or out-of-plumb.