The Salt Lake City-County Building renovation and restoration is nearing completion. Today I would like to talk about the structure, and next week the subject will be the art in its architecture.
The building is estimated to weigh about 80 million pounds - mostly stone that is considered unstable and likely to collapse in a major earthquake. With today's emphasis on earthquake safety, it became imperative that it undergo a major structural upgrading if it were to be saved.If you asked a creative child how to keep from falling down while standing on a shaking surface, he might answer, "put me on roller skates." Several years ago a similar question was asked about the City-County Building. Those with vision said put it on roller skates - all 80 million pounds of it.
That is a simplified answer for what has been accomplished. Today the building sits on base isolators, a sophisticated term for a device that acts like a ball bearing to isolate movement between the ground and the building.
When I asked Ed Allen, the project's structural engineer, for a firsthand look, he put me under a hard hat and we toured this inventive structural phenomenon.
First he pointed out an obvious change in the building's exterior - a metal plate surrounds the base. This is the "moat" or trench around the outside of the building that can move within an open space without being impacted by the surrounding earth. The metal covering the moat is to keep dogs, children and others out of the hole.
When we crawled through an access door into the earthen basement, I was stunned by the methodology used to place the building on its "roller skates." First, special beams of concrete and post-tensioned steel were built around and within the existing foundation wall. The beams are approximately 3 feet in span and at 3-foot intervals throughout the foundation.
Once the beams were completed, the foundation wall under the beam and above the footing was removed. Within this foundation opening, a new concrete and steel platform was placed on the existing footing, to distribute that load evenly on the old footing.
Then a base isolator, a 12-inch cube of alternating layers in rubber and steel, was placed on top of each of these new distribution slabs. Between every base isolator and the new concrete and steel platform is a permanent flat jack. The flat jack is capable of about 1 inch of vertical movement within a half-inch clearance between.
Through calculations of the existing live and dead load weights of the building, the engineers determined the weight of the section of the building above the base isolator. This particular spot had 86 kips, or 86,000 pounds, written on the new beam. The flat hydraulic jack was then raised by injecting the hydraulic with resin instead of oil. When it was raised to the point that it was carrying the 86 kips, the resin injector was turned off and it was left to cure.
This method was used continually around the perimeter and under every bearing wall of the building until all of the building's 80 million pounds were resting on the new base isolators - theoretically speaking. The big test came when it was time to cut the building loose from its old foundation walls and prove that all of the brain and computer power brought to bear on this subject was fact and not theory.
The building was cut loose with large wire saws, like taking out a slice of bread one at a time. When the task was done, theory proved correct when the building settled onto its new roller skates and lost 1/10th of an inch in its majestic height. "The plaster didn't even crack," beamed Ed Allen as he finished describing this intricate process.
The building is designed for a horizontal force of two-tenths gravity in the ground. This, experts believe, could happen every 1,400 years. That magnitude of an earthquake would use up 4 inches of the moat around the building. There are 12 inches in the moat for movement, and each of the base isolators is capable of 12 inches of movement, so the building has a large safety factor.
Other structural modifications, such as new reinforced walls, floors and braces of the lofty stone pinnacles, add to the overall structural integrity of the building.
The innovations represent a remarkable solution to save a remarkable edifice - a structure of historic significance that has been given a new lease on life.