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Shock Proof
Utah State Capitol Gets a Seismic Facelift
A $212-million restoration at the
Capitol is highlighted by life-safety improvements and a base-isolation
system that should diminish earthquake damage
By Brad Fullmer
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The Utah State Capitol
in Salt Lake City is undergoing a complex restoration
highlighted by a base-isolation system.
Photo courtesy of Harrison Co. |
Designers and contractors working on the $212-million restoration
of the Utah State Capitol in Salt Lake City have teamed up
to bridge a 90-year gap between the past and future, with
a complex base-isolation system at the core of the project.
"Initially, when we were looking at the building, the
number-one concern was life safety," says David Hart,
executive director of the Capitol Preservation Board and the
Capitol architect. "And as we looked at life-safety issues,
we realized that to save the building, it would basically
need to be gutted."
That's because the 320,000-sq-ft historic structure is located
along the Wasatch Fault, a 170-mi.-long segmented normal-fault
system that geologists and other seismic experts predict will
eventually produce an earthquake with potentially cataclysmic
results.
Built in 1915, the concrete Capitol is being stripped down
to its bare essentials while leaving most of the historic
pieces intact.
tructural improvements include new concrete load-bearing
walls in addition to the base-isolation system of 265 base
isolators and 15 sliders being installed under 280 of the
380 concrete columns that support the building.
"It's a monumental task," says Kevin Brown, project
manager for Jacobsen-Hunt, the joint venture general contractor
of Jacobsen Construction Co. Inc., Salt Lake City; and Hunt
Construction Group, Scottsdale. "It's extremely challenging
installing that many isolators in a very confined working
space."
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Construction includes
new exterior and interior shear walls as part of the structural
upgrade. All columns and exterior decorations will also
be better tied to the structure to prevent tipping or
falling during an earthquake.
Photo courtesy of Harrison Co. |
Hunt's Dennis Sexhauer adds, "Just the fact that it's
an existing building makes it a more difficult process. The
historic nature and all the irreplaceable historic fabric
require so much more time and effort. One-third of construction
time is purely in the protection of historic elements."
A De-Coupling Strategy
The design strategy of the base-isolation system is basically
"to de-couple the building from the ground," says
Jerod Johnson, project engineer for Reaveley Engineers and
Associates Inc., Salt Lake City, which teamed with Forell/Elsesser
of San Francisco on the structural design. "The objective
is not to move the building at all. If it does move, we want
to minimize that to 1/16-in. tolerances. So far, we've experienced
nothing that big-more like one-thousandth of an inch."
In June, crews began installing the first base isolators-cylindrical
devices consisting of laminated layers of rubber and steel
plates that surround an energy-absorbing lead core. Manufactured
by DBI, Reno, the isolators range from 34 to 44 in. across
and are about 20 in. tall. Harris Rebar, Salt Lake City, built
the rebar-intensive reinforcing systems under each of the
four massive rotunda piers.
The 15 sliders-located where the columns are not carrying
the extreme loads supported by base isolators-are larger because
the entire building must be able to handle 2 ft of earthquake
displacement in any direction. A slider is a Teflon pad-bearing
mechanism resting on an 18-in.-diameter stainless steel plate.
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Before installing the base isolators, the soil at the
Utah State Capitol site had to be reinforced with a
unique micropile suspension system designed by Jerry
Bishop of Geotechnical Design, Salt Lake City. The system
must be able to handle the immense loads of a building
weighing more than 130 million lbs, with another 30-plus
million expected to be added in new structural concrete
and steel.
Bishop utilized a Titan Hollow Bar system, which originated
in Europe more than 20 years ago, and was initially
introduced to the Intermountain region in the mid-'90s.
Approximately 3,000 individual micropiles were drilled
up to 30 ft into the ground. The micropiles, which are
hollow and include a sacrificial drill bit, are filled
with a water-cement grout mixture (45% water-to-cement)
discharged out the bottom of the hole after it has been
drilled. The water-cement mixture combines with sand
and gravel to form a 3,000 to 4,000 psi concrete shell
around each micropile.
"We kind of pioneered the use of this system in
Utah," says Bishop. "Doing it on this type
of scale has never been done as far as I know, at least
not in Utah. Once they transfer the load outward, it
could not have movement of more than three-eighths of
an inch."
David Hart, the Capitol architect, adds: "Soil
reinforcement on this project-to be able to hold the
loads of the transfer-has been a key aspect. Jerry had
to make sure that whatever he was putting into the ground
was going to be able to support those loads."
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One of 265 base isolators
being installed under columns.
Photo courtesy Michael Dunn, Dunn Communications |
The isolators and sliders are designed to dramatically reduce
the seismic impact of a major earthquake. If they work as
intended, a quake that measures 8.0 on the Richter scale would
have an impact on the Capitol equal to a 5.5 earthquake. When
the lateral force of the earthquake pushes against the isolator,
it stretches horizontally, so that, rather than shaking apart,
the Capitol will slide back and forth on the isolator bearings,
reducing the structural impact of the seismic forces.
But installing the isolators is tricky. New 5-ft-wide concrete
load-transfer beams are cast around existing columns and utilize
temporary loading, pancake jacks for support. When the jack
is opened, the column loads travel through the load-transfer
beams and into the jack, temporarily taking the load off the
column, which is then cut off below the load-transfer beams.
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| The project also includes
restoration of the plaza to the design of original Capitol
architect Richard Kletting, with new walkways and access
points. |
Crews slide the base isolators under the beams and lift them
into place, essentially hanging them from the load-transfer
beams. An area below the isolator is excavated so that a new
footing and pedestal fills the gap between the footing and
the isolator. The load path from the column is transferred
into the isolator and the new footing.
Any remaining space is filled using a water-inflated flat
jack to transfer the column load onto the new footing. Eventually,
the water in the flat jack is replaced with epoxy under full
load for permanent structural support.
"One of the ways we evaluated this structure was with
a finite element analysis model," says Mason Walters,
principal for Forell/Elsesser. "The computational method
allows us to mathematically characterize the properties of
all the components of the load-transfer system in great detail.
It allows us to mathematically simulate the effects of the
loadings, as well as the reinforcement in the post-tensioning.
In so doing, it allows us to calculate the stresses and the
deformations [the building] will go through in a lifetime,
including earthquake loading."
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Eighteen months before ground was broken on the Utah
State Capitol restoration in September 2004, design
and construction team members went through a series
of 20 workshops to ensure that the project stayed within
stringent budget and schedule requirements.
The workshops covered numerous issues, ranging from
designing complicated structural, electrical and mechanical
system elements, to items as basic as what kind of light
fixtures to install. There was one goal in mind-to preserve
the historical integrity of the Capitol while incorporating
into the building all the modern conveniences of the
21st Century.
"A fairly unique part of this project is that
the contractor and the designers were selected at the
same time," says Lonnie Bullard, president and
CEO of Jacobsen Construction Co. Inc., Salt Lake City.
"So we started working jointly on this project.
In fact, the team moved on site together-that rarely
ever happens.
ow you have the preconstruction team, the architectural
team and designers in the same complex working together
and solving problems."
The workshops allowed the team to carefully analyze
every aspect of the building, then put it back together
conceptually. "It was a very intense process, but
a very worthwhile process on a project like this,"
Bullard says.
Dennis Sexhauer, a principal with Hunt Construction
Group, Scottsdale, says: "It was an extensive process-the
owner mandated it. It was phenomenal. We brought in
the entire team early on in the process. As each session
went on, they would bring in whatever players were necessary
and tear away ideas and thoughts to determine the best
way to go forward."
The best part is that the budget has remained intact,
despite major hikes in construction material costs and
a shortage of skilled labor, says David Hart, the Capitol
architect. "From a budget standpoint, [Jacobsen-Hunt]
has done a great job managing this project," Hart
says. "One of the things I feel best about on this
project is the collaboration that has happened. The
architects and the contractors are working so well with
us. We have been able to find things and solve things
before they become problems."
Change orders to date have totaled between $500,000
and $600,000-less than one-half of 1% of the $200-million-plus
budget, Hart says.
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The entire building
was gutted down to the original support walls and columns,
then reinforced with new concrete walls.
Photo courtesy Michael Dunn, Dunn Communications |
Hart adds: "Base isolation is driving the entire ship.
The reason for base isolation is to allow the building to
experience an earthquake at a much lower force. When base
isolation is complete, the building will be subjected to only
1 G of force as opposed to 7 Gs of force."
The Capitol-scheduled for completion by November 2007-is
the second historical restoration project in Utah to use base
isolation. The first was the City County Building in Salt
Lake City, done by Jacobsen Construction in 1987, and the
first historic building in the world to utilize a base-isolation
process.
"It's certainly a once-in-a-lifetime project,"
says Lonnie Bullard, president/CEO of Jacobsen Construction.
"We're fortunate to be associated with a project that
is a historic building, that has been here for decades and
will be here for another hundred years."
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