The de Young Museum Revisited
BASE ISOLATION AND CONSTRUCTION AT GROUND LEVEL
Visitors approaching the museum are not informed that they will cross a moat with a minimum width of three and one half feet before they reach the low passage into the entry court. Formerly associated with ancient fortifications, the term, moat, is now commonly used to refer to the trench that is part of a system called base-isolation increasingly used to protect important buildings from damage in areas prone to earthquakes.
Although the contents of the system’s subterranean space under the main floor would interest visitors, theyare not visible. Photographs reveal the seventy-six elastomeric and seventy-six slider bearings that support the columns of the building’s steel lateral brace-frame; they are located at the intersections of an orthagonal grid of concrete beams.The photo below of part of the underground crawl space shows one of the rubber bearings before the mechanical, electrical and plumbing services were installed. Sun shining through the vent holes in the metal deck above produced this dappled-light effect, which disappeared when the concrete fill was poured.
The following three photographs are copyright: 2004 by Rutherford and Chekene
A sliding bearing after fireproofing and mechanical, electrical and plumbing services were installed.
One of twenty-four fluid viscous dampers (the tubelike element shown above) which transmit kinetic energy, shaking, entering the system to a fluid inside the damper. The movement of the fluid within the damper absorbs this kinetic energy by converting it into heat
Should there be an earthquake of sufficient magnitude to cause the building to move its permissable three feet in any direction, the system would be tested. If any video cameras are operating then, their recording should rival the “reality effects” now artificially created for films or television.
The path that typically defines the moat around a base-isolated building has been hidden here by a layers of hardscape, earth and vegetation. The concrete moat cover and styrofoam panels designed to pop up in a seismic event and allow the building to move independently of the earth around it were set well below these surface layers.
The architects wished to tie the building to the ground by having the paving material aligned with the interior floor surface. Sills around windows and doors where water tends to collect would have disrupted the horizontal plane. To avoid this disruption gutters were set in trenches below grade. A trench drain sits on top of the concrete moat cover, but does not protrude above the surface landscaping. Water from the trench drain goes from a downspout in the moat cover down into the basement drainage system and cannot infiltrate the building.
