Making City Hall Safe : The Northridge earthquake lent added urgency to a warning that engineers have been sounding for years-that Los Angeles City Hall needs substantial strengthening to survive a major quake.

The Project In an expensive seismic retrofitting program that is expected to begin in 1995 and take three years, City Hall will be repaired so it can withstand an 8.1-magnitude earthquake on the San Andreas Fault and a 6.8 quake on the Elysian Park Fault. Major features of the plan include a dry, underground moat that will surround the foundation and high-tech shock absorbers that will limit shaking. Background The 65-year-old building suffered substantial damage to its upper floors in the January earthquake, even though the epicenter was miles away. After the Sylmar quake in 1971, holes were drilled in the terra-cotta facing and a ceramic bonding agent was pumped into the facing to prevent further damage. Over time it has deteriorated and is now crumbling out of the holes. The top of City Hall, left, is now shrouded in black nylon netting to prevent debris from falling during the aftershocks. Stairwells New staircases will be built to provide better emergency access. Room to Move A four-foot-wide underground moat will surround the foundation to provide room for earthquake movement in any direction. Steel sliders will be placed intermittently to act as bearings for the concrete above and below. The moat will be covered by such things as planter boxes for safety reasons. Shear Walls Reinforced concrete walls will replace hollow clay block walls, which have been loosened by more than a century of temblors. Project Cost Voters approved a $376-million bond measure in 1990 to pay for the City Hall improvements and to strengthen dozens of other city buildings and bridges. But work has been slow and engineers recently increased the cost estimate for City Hall two-thirds, from $92.3 million to $153.5 million.

How a Building Bounces City Hall will be placed on 430 high-tech shock absorbers, called base isolators, that will move up to a foot in any direction to limit shaking in the 28-story building. These will be inserted one at a time, allowing for continual support of the building during renovation. 1. New pillar foundations will be built in basement above originals. 2. The old foundation will be braced with jacks on four corners to transfer the existing load off of the column while work is being done. 3. Then a piece of the original column will be cut out and replaced with the base isolator. 4. The brace jacks will be removed so the building rests on the base isolators. 5. New basement slabs will be added, joining all the new foundations. This is one of several designs of base isolators. Vulcanized rubber layers that can move in any direction are laminated between steel sheets to form a moveable, flexible base. The steel top and bottom plates are anchored to the columns above and below. Sources: L.A. City Department of Public Works Bureau of Engineering, Architectural Division; Albert C. Martin and Associates Architects and Engineers; Lehrer McGovern Bovis, Project Management Consultants.