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Planning for earthquakes in a seismically active region
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Sophisticated steel supports help counteract earthquake tensions, snow loads in the hundreds of inches, and micro-bursts of wind in excess of 90 miles per hour, efficiently transferring the stress to the ground. While building codes of the past were designed only to spare lives, current regulations may save buildings as well.
Peering through the windows of a plane as it descends into Jackson Hole, every last passenger can identify the land’s dominant trait: dramatic topography. The vast sage-covered plains are violently interrupted by mountains that cause rivers to carve serpentine routes along the valley floor. Momentous grinding of the earth’s crust formed the peaks millions of years ago during the Miocene Epoch, in which modern grasslands and rodents also made their debut.
This symphony of jagged peaks, expansive views, and moving water has attracted tourists, as well as homeowners, for decades. The ability to recreate in the majestic Teton Range and watch wildlife congregating around geysers and warm springs makes this area unique. These geologic features, which draw people from all over the planet to bathe in their beauty or shred incredible powder, aren’t just photogenic perks for the tourism industry. Rather, the Teton region hosts an intense collection of past geologic phenomena, as well as a continually evolving drama.
The landscape is a
dynamic thing
The earth in this area constantly shifts, slides, and burps. The Tetons are one of the youngest mountain ranges on the continent. In Windows into the Earth: The Geologic Story of Yellowstone and Grand Teton National Parks, authors Robert B. Smith and Lee J. Siegel write that “the modern Teton Fault, which was born as long as 13 million years ago, has existed for a mere trifle in the earth’s history.” Due to their relative immaturity, the Tetons have not yet locked themselves into place, as is the case with some older fault-line ranges. Therefore, periodic tremors or earthquakes continue to ripple through both the Wyoming and Idaho valleys.
Code evolution
A hundred years ago, when this portion of the country was being settled, building codes didn’t exist. The value of building codes was proven only
after years of building where snowfall accumulations range in the hundreds of inches, winds can be expected in micro-bursts in excess of 90 miles per hour, and powerful earthquakes occur. In 1925, for example, an earthquake likely caused the Gros Ventre Slide, which moved cubic miles of rock in
seconds and eventually caused a flood that obliterated the town of Kelly, killing six people. Likewise, in 1959 an earthquake measuring 7.3 on the Richter scale redefined Hebgen Lake just north of Yellowstone National Park. Twenty-eight people died in the rock slides, and area residents felt aftershocks
for months.
The basic rationale for these “4,000-plus pages” of building codes, according to Joe McGrath, a [former] building official with the Teton County Planning and Development Department in Teton County, Wyoming, is to “protect the health, safety and welfare of the general public.”
These codes are published by the International Conference of Building Officials, now known as the International Code Council, which has been working since 1927 to keep codes current with the latest information gathered from the ever-changing science within the construction industry. They set minimum guidelines and standards for what should be considered good construction practices in this contemporary world.
Current codes and
their reasoning
Both valleys are located close to the Teton fault line, which runs north and south at the eastern base of the Teton range. Because of this close horizontal proximity to the fault line, any structures within a certain distance have to be constructed to standards adopted by the municipality exercising control over such construction. Currently, both Teton counties subscribe to the 2003 version of the International Building Code (IBC).
According to Richard Scheerer and Dean Tracy of G&S Structural Engineers in Idaho Falls, Idaho, emphasis on earthquake design only started to become apparent in the 1994 version of the UBC. Prior to the 1994 earthquake portion of the code, the UBC prioritized “life safety” considerations, while the building’s structural integrity was secondary. “If a building such as a school was occupied during an earthquake," Scheerer explained, "and either during or immediately after the event everyone were to get out of the building with no loss of life, the building, according to code, would have been considered a success, even though some people suffered bumps and bruises from falling books or cuts from shattering glass. It would have still been considered a success even if the building had to be torn down the next day because structural damage from the earthquake had rendered the building condemnable.”
The general trend of code development as it pertains to seismic design since the 1994 UBC has been, obviously, toward better performing structures. This progression is somewhat political, partially determined by insurance companies lobbying the code council to increase minimum construction requirements so insurers are likely to repair structures rather than replace them.
Techniques for new construction
Construction practices have had to change with the times, much to the dismay of contractors accustomed to outdated building methods. Since both valleys now host homes with more square footage than those prior, more care needs to be exercised when building structures to resist the various snow, wind and seismic loads that were inadequately accounted for in the past.
Architects have seen a trend toward bigger, more spacious rooms along with larger glazed openings or even window walls to take advantage of the area’s views. Designing such rooms can become a structural nightmare with regard to resisting lateral earthquake and wind loads. To be able to give clients what they want, contractors use more full-penetration, welded steel frames. A few well-placed steel frames and/or reinforced concrete walls or piers can absorb tremendous amounts of load and distribute them to the ground efficiently. Rigid horizontal diaphragms also distribute forces in an efficient manner—much more so than standard wood construction. This all sounds daunting, but the majority of the specialized systems utilized by architects and structural engineers have been around for quite some time and have been used successfully and cost-effectively in commercial construction. The trick is providing a thoughtful, creative solution to clients’ needs both functionally as well as aesthetically, all while implementing only the necessary amounts of these specialized systems.
