FIELD NOTE · SUPPLEMENTAL SET
Room to move: what base isolation costs a site
ISSUEDJULY 11, 2026DRAWN BY THE NIGHTLY SWEEP
A base-isolated building is not held still. It is allowed to move, and somebody has to buy it the room.
That room is the requirement, not the hardware. ASCE 7 sets the minimum separation between an isolated structure and any surrounding retaining wall or fixed obstruction at not less than the total maximum displacement. So the hole is wider than the building. The bearings are a catalogue item. The gap is a site cost, and it is permanent.
Los Angeles City Hall is the local proof, and its engineers published the numbers.
The seismic rehabilitation finished in 2001. The plane of isolation sits below the basement and above the foundation.
On that plane: 416 high damping rubber bearings, from 29.5 inches to 51.2 inches in diameter and 9.6 to 16.6 inches tall, plus 90 flat sliding bearings. The sliders are a teflon pad on a natural rubber bearing, running against a stainless steel plate. They carry less than 10 percent of the building's weight.
The bearings were bolted loose on purpose. The engineering paper is plain about why. A loose-bolt connection allows uplift to occur without loading the bearing in tension. Rubber is strong in compression and weak in tension. So the detail lets the building lift off the bearing instead of pulling on it.
Tested stability at 21 inches of lateral displacement.
WHAT IS UNDER LOS ANGELES CITY HALL, BY COUNT HIGH DAMPING RUBBER BEARINGS 416 FLAT SLIDING BEARINGS 90 VISCOUS DAMPERS AT THE ISOLATION PLANE 52 VISCOUS DAMPERS AT THE 26TH FLOOR 12
Nothing may occupy it. Not a footing, not a planter, not a utility vault somebody adds in year forty. A retaining wall has to be built to hold the surrounding earth back off the moving structure, and that wall is a permanent structure with no building on top of it, which is a cost with nothing to show for it.
That is the part an owner argues about. It is also the part that cannot be value-engineered. Reduce the gap and the building hits the wall. A building that hits its own retaining wall at speed is worse off than one that was never isolated.
City Hall has 52 viscous dampers at the isolation plane, 26 in each direction. One end connects to the foundation. The other connects to the underside of the basement slab. They span the joint deliberately, and they are dimensioned for it: a mid-stroke length of 143 inches, a rated capacity of 300 kips at 50 inches per second, and a stroke of plus or minus 21 inches. Another 12 dampers, 6 each way, sit between the 26th and 27th floors at 225 kips and a stroke of plus or minus 4 inches.
Now apply that to everything else crossing the same plane. Water, waste, gas, conduit, fire main, elevator rails, stairs, the ramp at the entry. Every one of them is connected to the ground on one side and to a building that moves 21 inches on the other.
None of that is exotic work. Flexible connections, loops, slip joints, expansion assemblies. Ordinary trades install all of it, and they have to understand that the joint is real and is going to open.
The blind spot worth naming is that this is where the retrofit gets lost. The isolators arrive tested and certified. The pipe crossing the plane gets installed by whoever is on site that week, and a rigid connection across an isolation joint quietly deletes the thing the owner paid for.
Isolation hardware has to be tested to confirm the parameters used in the design. A system whose adequacy is not proved by testing may not be used. City Hall followed that. The bearings were tested against the properties assumed in analysis, and full-scale prototype 225 kip dampers were cyclic load and drop tested before installation.
Then the building was wired to check the theory against reality. It carries 27 accelerometers and 3 displacement sensors. Every one of the displacement sensors sits at the plane of isolation. The instrumentation is there so the next earthquake produces a measurement instead of an opinion.
Which is the difference between this and most seismic work. A shear wall gets built and nobody ever learns what it did. An isolated building is a standing experiment with sensors on the joint.
The structure above it is the easy half. What decides an isolation retrofit is what the ground under it turns out to be. The gap, the retaining wall and the foundation all get designed off that. Filings scored nightly for structural contractors across the Los Angeles record show the seismic upgrades while the approach is still open.
A fixed-base building resists the ground. An isolated one is separated from it by a gap somebody had to pay for, maintain and keep empty for a hundred years. The bearings will still be under there long after everyone who argued about the moat has stopped arguing.
That room is the requirement, not the hardware. ASCE 7 sets the minimum separation between an isolated structure and any surrounding retaining wall or fixed obstruction at not less than the total maximum displacement. So the hole is wider than the building. The bearings are a catalogue item. The gap is a site cost, and it is permanent.
Los Angeles City Hall is the local proof, and its engineers published the numbers.
What is under City Hall
The building went up in 1926. It was the first in Los Angeles allowed past the 150-foot height limit on private construction. It stands 32 stories and 460 feet. And it was designed before explicit seismic requirements existed, so it was never designed to resist an earthquake at all.The seismic rehabilitation finished in 2001. The plane of isolation sits below the basement and above the foundation.
On that plane: 416 high damping rubber bearings, from 29.5 inches to 51.2 inches in diameter and 9.6 to 16.6 inches tall, plus 90 flat sliding bearings. The sliders are a teflon pad on a natural rubber bearing, running against a stainless steel plate. They carry less than 10 percent of the building's weight.
The bearings were bolted loose on purpose. The engineering paper is plain about why. A loose-bolt connection allows uplift to occur without loading the bearing in tension. Rubber is strong in compression and weak in tension. So the detail lets the building lift off the bearing instead of pulling on it.
Tested stability at 21 inches of lateral displacement.
WHAT IS UNDER LOS ANGELES CITY HALL, BY COUNT HIGH DAMPING RUBBER BEARINGS 416 FLAT SLIDING BEARINGS 90 VISCOUS DAMPERS AT THE ISOLATION PLANE 52 VISCOUS DAMPERS AT THE 26TH FLOOR 12
The gap is the requirement
Twenty-one inches does not sound like much until it has to exist on all four sides, below grade, for the life of the building.Nothing may occupy it. Not a footing, not a planter, not a utility vault somebody adds in year forty. A retaining wall has to be built to hold the surrounding earth back off the moving structure, and that wall is a permanent structure with no building on top of it, which is a cost with nothing to show for it.
That is the part an owner argues about. It is also the part that cannot be value-engineered. Reduce the gap and the building hits the wall. A building that hits its own retaining wall at speed is worse off than one that was never isolated.
Everything that crosses it
The dampers make the problem visible because they are built to straddle it.City Hall has 52 viscous dampers at the isolation plane, 26 in each direction. One end connects to the foundation. The other connects to the underside of the basement slab. They span the joint deliberately, and they are dimensioned for it: a mid-stroke length of 143 inches, a rated capacity of 300 kips at 50 inches per second, and a stroke of plus or minus 21 inches. Another 12 dampers, 6 each way, sit between the 26th and 27th floors at 225 kips and a stroke of plus or minus 4 inches.
Now apply that to everything else crossing the same plane. Water, waste, gas, conduit, fire main, elevator rails, stairs, the ramp at the entry. Every one of them is connected to the ground on one side and to a building that moves 21 inches on the other.
None of that is exotic work. Flexible connections, loops, slip joints, expansion assemblies. Ordinary trades install all of it, and they have to understand that the joint is real and is going to open.
The blind spot worth naming is that this is where the retrofit gets lost. The isolators arrive tested and certified. The pipe crossing the plane gets installed by whoever is on site that week, and a rigid connection across an isolation joint quietly deletes the thing the owner paid for.
Tested, not calculated
The last thing about isolation is that the code does not take an engineer's word for it.Isolation hardware has to be tested to confirm the parameters used in the design. A system whose adequacy is not proved by testing may not be used. City Hall followed that. The bearings were tested against the properties assumed in analysis, and full-scale prototype 225 kip dampers were cyclic load and drop tested before installation.
Then the building was wired to check the theory against reality. It carries 27 accelerometers and 3 displacement sensors. Every one of the displacement sensors sits at the plane of isolation. The instrumentation is there so the next earthquake produces a measurement instead of an opinion.
Which is the difference between this and most seismic work. A shear wall gets built and nobody ever learns what it did. An isolated building is a standing experiment with sensors on the joint.
The structure above it is the easy half. What decides an isolation retrofit is what the ground under it turns out to be. The gap, the retaining wall and the foundation all get designed off that. Filings scored nightly for structural contractors across the Los Angeles record show the seismic upgrades while the approach is still open.
A fixed-base building resists the ground. An isolated one is separated from it by a gap somebody had to pay for, maintain and keep empty for a hundred years. The bearings will still be under there long after everyone who argued about the moat has stopped arguing.