May 10, 2013
After seeing the terrorizing evidence of the Wasatch Fault snaking its way through mountainfront Salt Lake City, our 2013 SSA field trip headed to the newly retrofit Utah State Capitol to see how the state is dealing with the looming threat of earthquakes. Reaveley Engineers’ Jerod Johnson, one of the head structural engineers on the retrofit project, led us on an in-depth tour of the facility, explaining all of the elements they had to consider and procedures they had to take to prepare this spectacular building for violent shaking.
As Jerod put it at the beginning, the building is too old, too heavy, and too brittle to withstand the kinds of ground motion expected from an earthquake on the fault that runs virtually beneath it. The building’s historic status compounded the problem by inhibiting the addition of shear walls and braces that would have obscured its architectural heritage. Thus the engineers of the retrofit decided that rather than hold up the building during strong shaking, they would limit the strength of the shaking itself. As with many other historic buildings in the earthquake-prone western U.S., the engineers opted for pricey and sophisticated base isolation.
The high price tag of this seismic resistant engineering is worth it for buildings that need to remain functional: base isolation essentially “floats” a building on its foundation, decoupling it from the violently shaking ground surrounding it and letting it slide slowly above, eliminating the relatively high-frequency shaking that could resonate through a structure and whip it to its doom.
Expressed slightly more technically, base isolation moves the frequency response spectrum of a building to frequencies much lower than its original resonant frequency, so that instead of amplifying the dangerous ground motions, the building’s protective foundation dampens them and it moves in a way that minimizes damaging stresses higher in the structure.
The way this works is well illustrated in the following video. This shows a massive shaketable test of a base isolated 5-story building. Compare the erratic input ground motion (the shaking of the concrete pad below) to the lazy oscillation of the building at the top of the rubber dampers. Also note how incredibly much the damper shears.
Base Isolation in action – San Diego earthquake test:
Because of the way base isolation reduces the horizontal forces experienced by a building it is particularly suitable to old, stiff buildings, and has the particular benefit of limiting shaking damage to the interior contents of a building as well. For these reasons it is commonly applied in retrofit or construction of government and emergency management buildings. In fact city halls throughout California have recently undergone retrofits to add base isolation systems, including L.A. City Hall, San Francisco City Hall, and the gorgeous Pasadena City Hall (which you may otherwise recognize as the painfully bemuralled office of Leslie Knope in Pawnee, Indiana). These buildings’ historic architecture preclude use of invasive seismic bracing, and their need to keep functioning in the event of a disaster emergency adds an extra benefit to applying base isolation.
Here’s a computer simulation (motions exaggerated) of L.A. City Hall’s base isolated performance if subjected to the shaking of the 1906 San Francisco earthquake. Note the difference between the building’s gentle oscillation and the erratic input shaking tracked below. There’s a video that shows a comparison of the building’s response with/without base isolation available here: http://orgs.up.edu/totle/index.php?q=node/342
In the Utah Capitol building we were led into a unique subterranean viewing room outside of the foundation’s “isolation moat,” the meter-wide gap between the building’s footprint and the ground surrounding it. From this room visitors can see beneath the building and view a handfull of the rubber dampers now holding up the whole structure. This perspective really floored me–so to speak–because of the chance to contemplate this whole massive structure gliding around atop a heaving Earth.
Each of these feet is a stack of alternating steel and elastomeric disks, sheathed in natural rubber. This structure gives them impressive vertical strength yet allows them to easily shear horizontally. Their elasticity allows up to 27 inches of motion, meaning the capitol building can move over two feet relative to the ground around it. To account for this differential motion, the building is surrounded by a ~3-foot moat, covered by cantilevered granite slabs that you’d never suspect are just a facade.
During strong ground motion, the caulk sealing this skirt to the pavers outside the building would tear, and the whole capitol would “glide” over them, probably somewhat noisily.
The retrofit included a boggling array of other procedures, including shoring up the granite block walls, literally gluing the stacked columns together, securing rosettes and cornices galore, stiffening the building’s shear strength by filling an obsolete ventilation duct system with concrete, and other impressive feats that are all summarized in this structural engineering report from the project:
Utah State Capitol retrofit project – Structural Systems Report
The tour was a great window into the impressive efforts that go into a true and robust seismic retrofit of an important structure. Fortunately, modern buildings in earthquake prone portions of the developed world are built with seismic design standards in mind. A 168-million-pound stack of stones topped with a 10-story dome wouldn’t be built these days without implementing what we’ve learned from prior earthquakes. Unfortunately, the vast majority of the world has neither the resources nor the inclination to undertake these preparatory measures, so let’s hope for increasing awareness, accountability, and decreasing costs of implementing seismic safety measures in architecture.