The isolated columns take loads to a grid of post-tensioned-concrete grade beams, set up as the yielding mechanism in the lateral system. If movement exceeds 2 ft in any direction, the isolators lock up and lateral loads go into the columns-as a jolt-and down into the grade beams, says Tipping.
Grade beams are designed to initially bend and yield but the post tensioning would re-center them, forcing the columns to go back to plumb, he adds.
Each isolator is a sandwich consisting of a 3-ft-square steel plates, each 2 in. thick, with two shallow "bowls," one inverted and set atop the other, between the plates. Each 3-ft-dia covered bowl contains three sliders or pendulums.
The bearing operates like a transmission with three speeds, says Zayas. Each slider has a different displacement period and damping level, depending on the force of the quake.
Column-mounted bearings rather than base isolators minimize the cost of the system because no site excavation is required. All building systems that go through the 25 holes in the second-floor slab, including piping, sprinklers, plumbing and domestic water, have to be flexible to accommodate the movement. Even the upper-level gangway between Tipping's current office and the new building is detailed to handle the maximum displacement, says Richard Fernau, of project architect Fernau&Hartman.
There also have to be gaps between any first-floor concrete walls and the second-floor slab. "There are 'trick' details" throughout, says Steve Cetrone, project manager for contractor Oliver & Co. "The structural issues made trade coordination difficult, especially for the dynamic slab," he adds.
After workers from Jos. J. Albanese Inc. completed the grade beams, crews from steel contractor Ogletree's Inc. installed the pipe columns-. First, workers set the 10-ft-tall section below the isolator. Next, crews placed the concrete filling, the isolator and the 4-ft-tall section above it. The second-floor slab followed.
Cetrone calls the job "unique," but not just because of the isolators. With Tipping's office next door, "our super, Tim Miller, has 20 engineers looking over his shoulder," he says. But on the positive side, Cetrone says, "Tim can whistle and an engineer comes out and immediately solves any conflict."
