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| Sunday, April 11, 1999
Copyright © Las Vegas Review-Journal If The Big One Hits Here, Will We Be Ready? Valley building officials say structures are safe, but geologists say too little is known about the threat quakes pose. By Keith Rogers
Review-Journal
When the Aladdin came crashing down last
April from the force of 233 pounds of explosives, the demolition
company's president observed, "It is not a building I'd want to be in,
in an earthquake."
The 32-year-old
landmark was so easy to drop that Controlled Demolition Inc. President
Mark Loizeaux reduced the amount of explosives by 137 pounds from what
he initially thought it would take.
If the strategically placed charges had been detonated below ground,
they would have delivered about the same amount of energy as a
magnitude-1.1 earthquake, a small blip on a seismograph probably not
strong enough to be felt by people.
But with the charges positioned above ground instead of within the
crust -- where the release of strain results in powerful earthquakes --
the Aladdin implosion didn't even register on the nearby seismograph at
the University of Nevada, Las Vegas, according to geology professor
Dave Weide.
Building supervisors
for Clark County and Las Vegas say they are comfortable with the
current seismic safety requirements for construction in Southern
Nevada, which follow the Uniform Building Code and the new
International Building Code that was developed for 2000 by engineers
and building officials from several professional organizations.
Older buildings such as the Aladdin, they say, were built according to codes of the time.
The Aladdin "was in no way ready to fall down," says county Building
Assistant Director Ron Lynn, who is also chairman of the state Seismic
Safety Council.
"Mr. Loizeaux is
the foremost at exploding structures. However, I'm not sure he is an
expert on erecting structures," Lynn said, noting that the Aladdin,
because of stress caused by settling soil, "had been reinforced, all
the way to the top floor."
Many
older buildings and parking structures were bulked up with reinforced
steel, retrofitted to withstand stronger ground motion or equipped with
braces and tie-downs to reduce the risk of falling objects through the
years.
Examples of such upgrades in
the downtown area include modifications to the Las Vegas Club parking
structure and some structures in the Fremont Street Experience. Several
transportation structures, such as the Oakey Boulevard underpass, have
been reinforced with steel bars surrounding piers that support the
weight of the roadway.
In most
cases, according to Las Vegas Building Director Paul Wilkins, buildings
that haven't been retrofitted have been torn down.
In addition, five aging casino buildings -- the Aladdin, the Sands, the
Landmark and two from the Dunes -- have been imploded since 1993.
From Weide's perspective, the chance of an earthquake collapsing
buildings in Las Vegas "is the risk you live with."
"Las Vegas exists because of probability. Gaming is probability," he said.
-- -- --
Builders gamble on the very low probability of a really big earthquake
ever hitting along the seven faults that cross the Las Vegas Valley.
Local building experts say the structures are safe enough to endure a
potentially damaging earthquake based on the types of construction
used, construction upgrades, soil conditions and on the likelihood of a
large earthquake occurring here. Nevada ranks third behind Alaska and
California in the frequency of strong earthquakes, according to the
U.S. Geological Survey.
Transportation structures such as freeway overpasses and bridges are
built to withstand 15 percent of the acceleration of gravity, according
to Nevada's chief bridge engineer, Bill Crawford.
While he said that is a "fairly low seismic design," it is based on the
probability of an earthquake strong enough to damage such structures
occurring once every 500 years.
Acceleration of gravity, sometimes referred to as g, equals 32 feet per
second, which is the speed of an object falling into the Earth's
gravitational field. Large earthquakes, at their point of origin,
generally deliver forces of up to 1g.
Earthquakes are measured by magnitude. With every increase in
magnitude, the size of the seismic waves that travel through the crust
increase by a factor of 10. In terms of energy released, that
translates to a 30-fold increase, which means a magnitude 6.7
earthquake sends out 900 times more energy than a 4.7 earthquake.
Despite the tremendous difference in energy, seismic designs are not
solely related to magnitude. The bottom line, according to Lynn, is how
a structure performs while the ground is shaking. Unless a building or
a structure is sitting directly on the place where a fault ruptures to
the surface, the real test is its ability to ride out the motion.
Freeways, roads and bridges are designed to resist seismic activity
based on criteria developed by the American Association of State
Highway and Transportation officials.
"We don't use the Richter scale," Crawford explained. "It's a measure
of energy, and that doesn't reflect how it affects a structure."
He said construction of overpasses and ramps as part of the overhaul of
the Spaghetti Bowl in Las Vegas relies on single columns to support the
weight of the roadway. The pillars are anchored into clay and rocklike
caliche.
"It's designed to resist a significant seismic load," Crawford said.
He described the difference between seismic design being used for the
Spaghetti Bowl and the one that was used for the Cypress overpass, a
double-deck structure built atop mud soil in Oakland, Calif. The
overpass collapsed during the magnitude-7.2 Loma Prieta earthquake on
Oct. 18, 1989.
Centered in the Loma
Prieta Mountains between Santa Cruz and the San Francisco Bay area, it
was the largest earthquake to occur on the San Andreas Fault since the
San Francisco earthquake in April 1906. Sixty-three people were killed,
and damage -- much of it to freeways along the bay and structures in
San Francisco's Marina District -- totaled $6 billion.
During the Loma Prieta quake, seismic waves had a dramatic effect on
the soft bay soils, up to 65 miles from the epicenter, causing
liquefaction -- where, for a matter of seconds, solid ground reacts as
if it is liquid.
"(The Cypress
overpass) was founded on soft soils that tended to amplify the
earthquake forces. The (clay) soils in the Las Vegas Valley tend to be
much better and don't amplify earthquake forces from the bedrock,"
Crawford said. "The softer soils, like mud or lake bottom, tend to
amplify it like a plate of Jello. You shake the plate and the Jello
moves more than the plate."
-- -- --
On Dec. 14, a magnitude-2.7 earthquake rattled homes and buildings in
northwest Las Vegas. It shook the pro shop at Angel Park Golf Club but
wasn't powerful enough to stir gamblers at Calico Jack's Saloon, about
a mile or two from where geologists believe the earthquake struck
without rupturing the surface. That jolt packed 450 times the amount of
energy it took to drop the Aladdin with explosives.
During the quake, Las Vegas resident Burt Slemmons, professor emeritus
of geology at the University of Nevada, Reno, happened to be at
Hualapai Way and Sahara Avenue, just a few miles from the epicenter.
"We heard it more than felt it," he said at the time. "It was more or
less a window rattler. It was really loud, almost like an explosion."
Just across from Angel Park Golf Club, Quinton Boshoff, senior vice
president of The Resort at Summerlin, said he didn't feel the
earthquake. The tremor didn't budge the seismic safety features of the
resort, which was under construction.
Mustapha Assi, Summerlin resort project manager for Martin & Peltyn
Inc. Structural Engineers, said the tremor was "very soft, very light,
but I definitely felt it." His company also designed the structures for
Bellagio, MGM Grand, Treasure Island, The Mirage and the Paris hotel.
Assi said the shaker assured him that designs used in The Resort at
Summerlin are safe for the type of earthquakes and the soil conditions
in the Las Vegas Valley.
In
January, a year after construction began on the resort, visitors to the
project could see seismic design features in the framework. Huge
tube-steel K braces, shaped like inverted V's, support the main casino.
The braces are designed to hold up during the side-to-side movement
generated by an earthquake. Frames that resist similar lateral motions
also have been put in the resort's two six-story hotels.
A two-story, reinforced concrete slab with reinforced concrete shear
walls built to withstand seismic loading was constructed to keep the
resort's parking garage from wobbling during severe shaking. The shear
wall supports the garage in the same way a large piece of plywood
positioned between the legs of a table would keep the table from
wobbling because it would move as one unit instead of four separate
legs, Assi explained.
The elevator shaft leading to the garage is "not connected to anything
else." That means the free-standing structure would react independently
of the buildings it connects.
-- -- --
Craig dePolo, a research geologist with the Nevada Bureau of Mines and
Geology who is studying faults within the Las Vegas basin, said
knowledge about the faults is scarce. Given the concentration of people
who live here -- 1.3 million -- a more in-depth understanding is
warranted, he said.
"There are
questions that have to be answered," dePolo said during a break in
February's Nevada Earthquake Safety Council meeting. "Maybe we're lucky
if we have 10 percent of that knowledge."
Just hours after dePolo's comment on Feb. 19, a magnitude-2 tremor
shook North Las Vegas. Only one monitoring station in the Nevada
Seismological Laboratory's network picked it up, an instrument in the
Sheep Range on the northern outskirts of the valley. With only that one
piece of information, geologists were unable to pinpoint the epicenter.
A month later the scientists were speculating whether the instrument
was triggered by an earthquake or noise, such as blasting or a sonic
boom.
As seismologist David von
Seggern put it, "Again, we're severely handicapped by having almost no
instruments in Southern Nevada."
Some North Las Vegas residents reported ground shaking in the early
afternoon, but they might have been confused by bombing demonstrations
at Indian Springs by aircraft from Nellis Air Force Base, von Seggern
said. The bombing exercise registered as noise on seismic equipment
around 2:20 p.m., but residents had reported feeling an earthquake
about two hours earlier, at 12:15 p.m., the same time the Sheep Range
instrument was triggered.
DePolo
agrees the Las Vegas basin needs a more extensive network of seismic
instruments. Ideally, he said, two sets of instruments would be
anchored into outcroppings at various locations. Two types of
seismographs are preferred because if only the most sensitive
instruments are installed, then strong motion waves from larger, less
frequent earthquakes could distort the recordings.
-- -- --
Wilkins said the Las Vegas city building code dates to 1927, when
information about earthquakes in the valley was virtually nonexistent.
For the most part, blueprints for those years and the next six decades
have been discarded. Since the mid-1980s there has been a requirement
to keep building plans for commercial structures indefinitely.
In Clark County, Lynn said building design records are stored on
microfilm and, more recently, on optical discs. Records date back to
1981 for most commercial buildings. A few have been kept from as early
as 1960. The same records for subdivisions date back to 1978.
After 1980, more emphasis was put on seismic resistant designs as
building codes were tightened based on lessons learned from destructive
quakes.
"Back in 1982 there were
six pages on earthquake design," Wilkins said. "By 1997 there were 14
pages and this (the 1 1/2-inch-thick International Building Code) is
not going to be the end of it."
More than just the likelihood of a strong earthquake and soil
conditions go into the formula for safety designs. The distance from
faults where large earthquakes occur is part of the equation of today's
safety codes, as are the types of structure involved.
Seismic waves from the shifting of rock masses ripple through the
crust, losing their punch at greater distances. There are two types,
the fast moving "P" waves, or primary waves and the slower,
side-to-side moving "S" waves, or secondary waves. Those are the
motions that newer buildings in Las Vegas are designed to ride out.
Critical structures, such as Hoover Dam and hospitals, need to be the
most resistant to ground motion. The dam, built in the early 1930s,
blocks the Colorado River and creates a drinking water supply for more
than 18 million in the Southwest. Disaster lurks if a breach of the
3.25 million cubic yards of concrete occurred during an earthquake.
Bureau of Reclamation officials, who oversee the dam, say the structure
is designed to sustain seismic waves sent out from the nearest threats
-- the Black Hills Fault, the Mead Slope Fault and background
earthquakes.
They point to the
dam's track record of having survived several moderate earthquakes
since it was built.
During
construction of the dam, it was revealed that two significant faults --
the Mead Slope Fault and the Black Hills Fault -- were upstream and
downstream of the dam's foundation, respectively. But it was reported
by Charles Berkey, a consulting geologist during the dam's
construction, that "the dam site could not have been located more
accurately to take advantage of the best local physical condition."
While predicting the potential effects of earthquakes is not an exact
science, Bureau of Reclamation engineers say the massive dam is
designed to withstand a significant earthquake without failure. In
fact, a magnitude-6.75 earthquake occurring on either of the two faults
near the dam likely would cause only cracking in its upper portion.
Although there weren't many structures in 1932, Las Vegas survived one
test from strong ground motion generated from outside the valley during
the magnitude-7.2 Cedar Mountain earthquake, which struck Dec. 21 that
year near Gabbs, 240 miles northwest of the valley.
The valley was tested again during the 6.2-magnitude Clover Mountain
earthquake near Caliente, 120 miles north of Las Vegas, on Sept. 22,
1966; and again during the magnitudes-7.4 and -6.5 Landers, Calif.,
earthquakes on June 28, 1992, 150 miles southwest of Las Vegas. In
California, the twin quakes killed a boy, injured 350 people, destroyed
20 homes and damaged 1,100 more. Ten businesses also were destroyed.
Still, buildings such as the Golden Gate Hotel have endured those and
smaller earthquakes. The hotel, the first in Southern Nevada
constructed of concrete, opened in 1906 as the Hotel Nevada. The
original structure -- the first two floors -- is part of today's
establishment on Fremont Street, which has endured several expansions
in its 93-year history including the addition of a third floor in the
1920s.
-- -- --
Engineers from professional organizations that developed the Uniform
Building Code and the International Building Code based requirements on
lessons learned from earthquakes such as the 1994 Northridge, Calif.,
earthquake -- a magnitude-6.7 temblor -- and the 1995 magnitude-7 Kobe,
Japan, quake.
Las Vegas is ranked
as a "2b" area for seismic safety under the Uniform Building Code. That
means structures must incorporate certain earthquake-safe features such
as reinforced walls and foundations designed to withstand the type of
ground motion expected from earthquakes in the area.
Construction in the Las Vegas Valley and most of Southern Nevada must
meet the mid-level requirements on a scale of zero through 4, where 4
is the most stringent for seismic safety design. A 4 ranking would be
required in urban, coastal areas of California, parts of Alaska and
parts of central Nevada where strong earthquakes strike more
frequently.
High-rise buildings in
Clark County, according to building chief Lynn, have been designed and
constructed to a standard much higher than the minimum requirements for
the 2b zone. Some parking structures, he said, are on par with what is
required for seismic safety of parking structures in Los Angeles,
including such features as earthquake clips -- metal rails that prevent
heavy objects from crashing down.
Geologists believe several faults around Las Vegas deliver strong
earthquakes about every 10,000 to 50,000 years. But Slemmons and others
say the intervals might be more frequent, or about every 10,000 to
20,000 years, or less in the case of the Eglington Fault.
Lynn's perspective is that civilizations, such as the Roman Empire,
which existed for about 1,000 years, will rise and fall before an
earthquake strong enough to drop buildings occurs in the Las Vegas
Valley. In fact, he puts earthquakes sixth on hazard rankings for
buildings in Clark County, preceded by soil problems, first; fires,
second; wind, third; water, fourth; and tornadoes, fifth.
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A
tube-steel K brace, shaped like an inverted V, supports the main casino
of The Resort at Summerlin during construction in January. K braces are
designed to hold up during side-to-side movement generated by an
earthquake.
Photo by John Gurzinski.
A
tube-steel K brace, shaped like an inverted V, supports the main casino
of The Resort at Summerlin during construction in January. K braces are
designed to hold up during side-to-side movement generated by an
earthquake.
Photo by John Gurzinski.
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