The Problem With Strengthening a Historic Building
Conventional seismic strengthening of an old building is, by definition, invasive. You are taking a structure that was built for gravity loads in the 19th or early 20th century, and you are trying to give it the lateral capacity to ride out a 21st-century design earthquake. That usually means new shear walls, new braced frames, new collectors, new diaphragm overlays, and large new foundations to anchor it all.
For a state capitol, a city hall, a parliament building, or a museum, those interventions are not just expensive. They are destructive. Historic plaster ceilings have to come down to install diaphragm slabs. Original masonry walls are cut to insert shear walls. Carved-stone foundations are dug out to enlarge. The building survives, but a lot of what made it worth saving does not.
Seismic isolation flips the problem. Instead of trying to reinforce every floor against the earthquake, isolation addresses the earthquake itself, by reducing what the building above ever has to resist. Done right, the original superstructure remains essentially untouched. The intervention happens at the basement and foundation, where the building's heritage value is usually lowest.
This is why isolation has become the preferred retrofit strategy for the most important historic buildings in the world. The DIS portfolio of historic-building retrofits is, in many ways, the strongest single case for the technology.
The First One: Salt Lake City and County Building
The Salt Lake City and County Building, completed in 1894, is a five-story unreinforced masonry building with a 256-foot central clock tower. It is the seat of city government in Salt Lake City, and it was designed in an era when "lateral system" meant "thicker walls." Unreinforced masonry of that vintage is among the most earthquake-vulnerable construction types known. The Wasatch fault runs essentially through downtown Salt Lake City, with a probable design earthquake in the magnitude 7 range.
Conventional retrofit was studied and rejected as too invasive. Instead, between 1986 and 1989, the building underwent the first seismic isolation retrofit of a historic structure anywhere in the world. The work involved:
- Cutting the building from its original spread footings
- Installing 443 lead rubber bearings underneath the load-bearing walls and column points
- Creating a continuous reinforced concrete diaphragm at the new isolation level to tie the building together above the bearings
- Leaving an open moat around the perimeter, hidden from view, that allows the building to displace freely during an earthquake
The total project cost was about $30 million, the largest capital project in Salt Lake City's history at that point. When it was completed in 1989, no historic building anywhere in the world had ever been seismically isolated. The clock tower, the masonry walls, the original interior finishes are all still there. What changed is what is under the building, not what is in it.
Salt Lake City and County Building, completed 1894, retrofitted with 443 lead rubber bearings between 1986 and 1989. First seismic isolation retrofit of a historic structure in the world.
The Largest One: San Francisco City Hall
San Francisco City Hall reopened in 1915, replacing a building that had been destroyed in the 1906 earthquake and fire. The new City Hall, designed by Bakewell and Brown, is a Beaux-Arts landmark with a dome taller than the U.S. Capitol's. It is also, structurally, a heavy stone-clad steel and concrete building sitting on a site that the 1906 history made absolutely clear was high-seismicity.
The 1989 Loma Prieta earthquake damaged the building enough to force a comprehensive assessment. Conventional retrofit options would have required gutting interior finishes, including the rotunda. Instead, between 1994 and 1999, the building was placed on a base isolation system, with approximately 530 lead rubber and elastomeric bearings installed under the columns and major wall lines. The work involved temporarily transferring the entire building load through jacks to allow the bearings to be installed at the base of each column, then transferring the load back onto the bearings.
This was the largest base isolation retrofit ever undertaken at the time, by a significant margin. The historic interior, including the famous rotunda where Marilyn Monroe and Joe DiMaggio were married, was preserved essentially intact. The building has been through multiple moderate earthquakes since the retrofit was completed, and continues to function as the city's seat of government.
The State Capitol: Utah State Capitol
The Utah State Capitol opened in 1916. It sits on a hillside in Salt Lake City, less than two miles from the Wasatch fault. A 2002 evaluation concluded that the building could collapse in a major Wasatch earthquake. Like Salt Lake City and County, it could not realistically be conventionally strengthened without destroying its historic character.
Between 2004 and 2008, the Utah State Capitol underwent a comprehensive seismic isolation retrofit. The building was placed on 265 base isolators, each designed to allow horizontal displacement of up to 24 inches in any direction, for a total possible swing of 48 inches. The work was done with the legislature continuing to operate in temporary chambers. When it reopened, the interior, including the painted dome and rotunda, looked essentially identical to the day the building opened in 1916.
The Utah State Capitol retrofit is one of the most architecturally successful seismic isolation projects ever completed. The intervention is invisible. The performance gain, in terms of design-earthquake response, is on the order of a factor of three or four reduction in base shear demand.
The International Case: New Zealand Parliament
The Parliament Buildings in Wellington include the Edwardian neoclassical Parliament House, which dates to the early 20th century. New Zealand sits on the Pacific-Australian plate boundary, with one of the highest seismic hazards of any developed nation, and Wellington in particular has been studied as a likely site for a future major earthquake.
Between 1992 and 1995, both Parliament House and the Parliamentary Library were retrofitted with a base isolation system that combined 145 lead rubber bearings, 230 high-damping rubber bearings, and 42 sliding bearings, for a total of more than 400 isolation devices. The buildings were lifted off their original foundations, the isolation level was installed, and the buildings were set back down on the new bearings. Maximum design displacement is on the order of 30 cm.
When Parliament House reopened in 1995, the chambers where the New Zealand House of Representatives meets had been preserved in their original 1922 form, sitting above an isolation system designed to keep them serviceable through a magnitude 7.5 event.
Why Isolation Works So Well for Unreinforced Masonry
The deeper engineering reason isolation is so effective on historic buildings has to do with the nature of unreinforced masonry (URM) construction.
URM walls are strong in compression and weak in tension. They are stiff, brittle, and have very little ability to absorb energy through inelastic deformation. When a URM wall is loaded in shear, it tends to fail suddenly along diagonal cracks. There is no graceful yielding the way there is in a steel moment frame.
Conventional seismic strengthening of URM tries to add a parallel ductile system, like a new shear wall or braced frame, that can take the load when the masonry cracks. That works structurally, but it adds new stiff elements that change how the building behaves and that have to be tied into the existing masonry in ways that risk damaging it.
Isolation, by contrast, just stops asking the masonry to do the job. With base shear demand reduced by a factor of three or four, the original walls can almost always stay within their elastic capacity, where they behave well. You are not strengthening the masonry. You are unloading it.
This is also why isolation tends to be the right answer for stone buildings, brick buildings, adobe buildings, rammed-earth buildings, anywhere the historic material does not tolerate inelastic deformation. The technology lets the original fabric keep working the way its original builders intended.
What Has to Happen at the Foundation
The mechanics of cutting a building off its foundation and setting it on bearings are not trivial. The typical sequence is:
1. Temporary support. Steel needling beams or hydraulic jacks are installed under load-bearing walls and column points to support the building during the work. 2. Excavation. A working level is dug below the existing foundations, with shoring as needed. 3. Bearing installation. Reinforced concrete pedestals are poured below and above each bearing location. Bearings are set on the lower pedestals. 4. Diaphragm. A new reinforced concrete diaphragm is constructed at the isolation level, immediately above the bearings, to tie the building together so it moves as a rigid block above the isolators. 5. Load transfer. The temporary supports are slowly released, transferring the building load through the bearings to the new foundation. 6. Moat completion. A continuous gap is created around the perimeter of the building, with a flexible cover for utilities and a slip detail at every door threshold and stair.
The work is done in small sections at a time, so that the building's load is never being supported entirely by temporary structure. On the largest retrofits, including San Francisco City Hall, the process takes several years.
The Test: When the Earthquake Actually Comes
Multiple isolated historic buildings have now experienced real earthquakes since their retrofits. None has reported significant damage. Salt Lake City and County, Utah State Capitol, and San Francisco City Hall have all ridden out moderate events without issue. The first proof-of-concept earthquake for an isolated historic building was actually neither of these, it was the USC University Hospital in 1994, which though a new building rather than a retrofit, established that DIS bearings perform as designed when a major event actually hits.
The case for isolation on historic buildings is now unambiguous. It is the only retrofit strategy that delivers immediate-occupancy performance without sacrificing the historic fabric that made the building worth saving in the first place. For state capitols, city halls, museums, and parliament buildings, that combination is hard to walk away from.
More from Resources
How Seismic Isolation Actually Works: The Physics, Plainly
Most earthquake-resistant buildings try to be stronger than the shaking. Isolated buildings do the opposite: they let the ground move while the structure stays roughly still. Here is the physics of why that works, with a real case from the 1994 Northridge earthquake.
Inside a Lead Rubber Bearing: The Engineering, Explained
A lead rubber bearing looks like a hockey puck and weighs as much as a small car. Inside is a precise sandwich of natural rubber, laser-cut steel shims, and a lead plug. Here is what each piece does and how DIS builds and tests them.
Hospitals That Don't Stop: Seismic Isolation in Critical Care
A hospital that survives an earthquake but cannot operate is, for the people who needed care that morning, the same as a hospital that collapsed. That is why hospital seismic design has shifted from survivability to operability, and why isolation has become the default for new critical care construction in seismic regions.