Structural engineers at Simpson Gumpertz & Heger have taken performance-based seismic design to new levels by placing compliant isolators, typically used under structures, on the 825 x 100-ft roof of an existing three-story building in San Francisco. The location a first in the U.S. according to SGH allowed the addition of 150,000 sq ft in two levels above the roof, instead of 50,000 sq ft planned when the concrete building was constructed.

Simpson Gumpertz & Heger Crews had to thread in addition’s isolation system on cluttered roof of old building.

The strategy allowed the $44-million project to proceed with minimal disruption to the mid-1980s building’s 400 occupants. Like a mother hen nesting, the 770 x 100-ft addition has become a giant untuned mass damper, reducing earthquake forces and displacement demand on the old building.

The system not only allowed for the new space without a seismic upgrade to the old building, “it also improved the seismic performance capability” of the old building, says John Sumnicht, senior principal for the China Basin building addition in SGH’s local office.

But there were prices to pay. At about $195 per sq ft, the 225,000-sq-ft addition, counting the roof-turned-interstitial floor, was more costly than a shear-wall building. “It was not the cheapest way to go but it was the least disruptive,” says Jim O’Callaghan, senior project manager for the local general contractor, Hathaway Dinwiddie Construction Co.

The isolators will allow the addition to displace laterally 4 ft in each direction relative to the old building, and that had ramifications for the architect. “A big difficulty was designing stairs and elevators from the old to the new and addressing the movement,” says Mark Borchardt, associate in the local office of HOK.

For example, there is 3-ft-wide dry moat around the building’s four cores. A tunnel made from flexible materials connects the elevator towers to the rest of the building.

Construction, which started in July 2006, was complicated. Working on the roof, which already contained in-use mechanical equipment, was “technically challenging and a logistical nightmare,” says O’Callaghan.

For starters, three miles of existing piping or conduit had to be raised 38 in. above the as-built top of the roof to allow the 27-in.-deep steel beams, each weighing 75 tons, to be threaded under and around the equipment, says O’Callaghan.

Simpson Gumpertz & Heger Crews erect steel grids below and above drum-shaped isolators (above, at right).

Negotiated contracts with key subcontractors steel erector Herrick Corp., Stockton, Calif.; ACCO Engineered Systems, San Leandro, Calif., for mechanical and plumbing systems and the local Paganini Electrical Corp. allowed early coordination on the design side. That helped, says the contractor.

Challenges aside, O’Callaghan reports the Hathaway Dinwiddie reached substantial completion on Dec. 24, on schedule and on budget. The interiors will be complete by next fall.

To satisfy the Dept. of Building Inspection, SGH had to demonstrate to a three-person peer review panel that the existing structure would perform as well as a new code-conforming building of similar occupancy. Peer reviews typically take six months. This one took 21 months. SGH responded to the panel’s 355 questions...