190-million-pound Salt Lake Temple transferred onto 98 seismic isolators

One of the most ambitious seismic preservation projects in the United States is approaching completion in Salt Lake City, where the historic Salt Lake Temple has been transferred onto a new base isolation system designed to protect the 19th century stone structure from a major earthquake on the nearby Wasatch fault.

Construction of the temple began in 1853 and continued for 40 years, leaving behind a massive structure formed from stacked granite blocks supported on sandstone foundations. Those construction methods helped define the building's historic character but also created a significant seismic vulnerability. Previous structural studies indicated that strong shaking could cause sliding within the stone masonry and potentially lead to partial or widespread collapse.

The temple sits within about 2 km of the Salt Lake City segment of the Wasatch fault, which is capable of producing an earthquake of approximately magnitude 7.2. Paleoseismic investigations indicate an average recurrence interval of around 1,300 years for major earthquakes on that segment, and the time elapsed since the most recent event already exceeds it. Rather than designing only for standard code-level ground motions, the retrofit adopted a more demanding criterion based on the 84th-percentile shaking from a major rupture of the nearby fault. The objective was not simply to prevent collapse, but to limit damage sufficiently to preserve the historic structure itself.

At the heart of the solution are 98 Triple Pendulum seismic isolators installed beneath a building weighing approximately 190 million lb, or 86 million kg. The difficulty was not simply inserting bearings beneath an existing structure. The temple had to remain supported throughout the entire operation while its load was gradually transferred from the original stone foundations to new footings constructed alongside them. The intention was that the building would effectively remain unaware that its support system was being changed.

One of the most unusual elements of the transfer system involved installing round horizontal pipes of 3.5 ft and 4 ft, or roughly 1 m and 1.2 m, in diameter beneath the existing foundations. These were filled with reinforcement, post-tensioning ducts and high-strength concrete to create permanent load-transfer beams. New footings were then constructed on either side of the historic foundations, together with transfer girders, flat jacks, compression rods and post-tensioning systems capable of moving the enormous structural loads onto the isolators in a controlled sequence. Only once the transfer was complete could soil beneath portions of the original foundation be removed.

Base isolation alone was ultimately not enough. Detailed dynamic modelling showed that the strong vertical component of earthquake shaking could generate significant horizontal movement and sliding within the temple's heavy stone towers. The retrofit therefore expanded to include vertical post-tensioning through the walls and towers, strengthening within the spires and a new roof diaphragm tying the major structural elements together.

The work below ground has been equally demanding. Three new underground levels were excavated adjacent to the temple, extending approximately 45 ft, or 13.7 m, below the level of the historic foundations. Extensive temporary shoring and monitoring were required to control ground movement and settlement while excavation proceeded beside a structure carrying exceptionally high foundation pressures.

The seven-year programme has also included replacement of ageing mechanical, electrical, plumbing and fire-protection systems, accessibility improvements, new entrance structures, visitor facilities and extensive redevelopment of the surrounding grounds. Completion is expected by the end of 2026, with public tours scheduled to begin in April 2027.

What makes the project unusual is the combination of scale and preservation. Rather than strengthening the temple so aggressively that its historic fabric would be fundamentally altered, the project has attempted to isolate the entire structure from earthquake motion while retaining its original stone foundations and architectural character. It is a rare example of modern seismic engineering being inserted beneath a monumental masonry building constructed more than 130 years ago.