Raising of historic rotunda will be delicate task
When work kicks off next summer to convert San Francisco's historic Emporium department store into retail, entertainment and office space, raising of the store's landmark, 102-ft.-wide rotunda will be the center of attention. Partially covered by a 1916 addition, the glass and steel dome will be jacked up 54 feet to extend above the roofline, permitting daylight to flood into the public space.
"In 1916, a third floor was added to the structure because the department store expanded up, and the clerestory band of the rotunda was covered." says Norman Garden, vice president in charge with design architect RTKL Associates, Baltimore. "By raising the dome, the clerestory band will become part of the exterior again and daylight will pour through into the public space."
Heading up this delicate portion of the project is structural engineer Nabih Youssef and Associates, Los Angeles, along with specialty contractor Sheedy Crane and Rigging, San Francisco. The firms have been working closely with RTKL and historical architect Carey & Co., San Francisco, to ensure the fragile rotunda makes the trip without compromising its historic appearance.
According to Michel Kalin, project manager with Youssef and Associates, the team will employ a series of hydraulic jacks that will raise the entire structure continuously at a rate of approximately 1 inch per minute.
"A series of beams will first be needled underneath the dome's existing ring beam," says Kalin. "The jacks will then attach to the needle beams and raise the dome in a continuous lift on what are essentially very large shoring towers."
Initial preparation, adds Kalin, will involve stripping the dome of most of its finishes, including plaster and glass, and then strengthening the ring beam. "We have to make it as light and flexible as conceivable, while also ensuring the ring beam — the base structure of the dome — stays intact. It would be a catastrophe if we lost the ring beam."
Accommodating the hotel
While challenging, Kalin admits that the raising of the rotunda is not the most difficult structural aspect of this 1.58 million-sq.-ft. mixed-use project. For instance, a 23-story hotel will be placed on top of the seven-story building, directly over a two-story cinema.
"Instead of having a 30-ft. column grid for a base, we have to accommodate the high-bay, long-span structure required for the movie theaters," says Kalin. "To support the hotel, we used massive plate girders to design a structural system that will span more than 120 feet. The system is not all that different from a highway bridge."
Another challenge with the hotel, says Kalin, is the fact that a tenant deal has yet to be finalized. This means that the basic structural design cannot be fully completed, because it will need to be customized to fit the hotelier's needs.
"There's been four different hotel operators interested in the project so far, and we've had to design various structures for each," he says. "Once hotels get their operations people involved, they start looking at, for instance, how the kitchen and circulation is laid out. I just know there's basic parts of the structure that will come under fire.