Subscribe / Renew
|► Subscribe to our Free Weekly Newsletter|
|print email to a friend reprints add to mydjc|
February 27, 2002
Photo by Michael Dickter, Skilling Ward Magnusson Barkshire
How will this building fare during the next quake? Engineers will try to answer that question using building evaluation and design approaches that seek to quantify the likelihood of damage. The Palmer Building, just north of Safeco Field, was damaged during the Nisqually earthquake.
The one-year anniversary of the Nisqually earthquake reminds the Pacific Northwest of the potential devastation that can occur at any time. Even though the magnitude 6.8 earthquake, located over 50 kilometers below the ground surface, was considered a moderate event, it caused in excess of $1 billion in damage, but, fortunately, no loss of life.
In fact, the three most recent earthquakes in the United States, the 1989 Loma Prieta, the 1994 Northridge and the 2001 Nisqually, resulted in losses that reached $7 billion, $30 billion and $1 billion, respectively. These losses, beginning with the Loma Prieta, have caused the earthquake engineering community to re-evaluate, refine and refocus efforts on the techniques used to predict building performance.
Scope is broadened
Over the past decade, the major emphasis and research has concentrated on the performance of existing buildings. The latest research, currently being led by the Army Corps of Engineers, Federal Emergency Management Agency (FEMA), and the Pacific Earthquake Engineering Research Center, has broadened the perspective to include the design of new buildings as well.
Included in this research, which began in 2001 and is likely to take more than 10 years to complete, is the possible redefinition of current building performance levels. "Life safety" is the target performance for new buildings and the one that is generally selected when evaluating and upgrading existing buildings. Life safety performance is associated with a "design earthquake," which is an earthquake ground shaking that is likely to occur at the building site once every 500 years.
However, life safety is not the only performance level that society, building owners and project design teams need to consider. Although saving lives remains the fundamental objective, there are numerous building uses where disruption of facility usage can be detrimental to society at large. Or, it may be critical for a particular business to remain operational following an earthquake.
These types of occupancies, designated as "essential facilities" in the building code, include hospitals, fire and police stations, hazardous storage facilities, and businesses whose interruption can affect livelihood. For these situations, a performance level above life safety known as "immediate occupancy" is specified. With an immediate occupancy design performance level, buildings are capable of being reoccupied quickly in the event of a 500-year earthquake ground shaking.
Very big earthquakes
Although inferred in the building codes for several decades, a performance level of "collapse prevention" was officially added in the mid-1990s as part of the guidelines for existing buildings. Generally, collapse prevention performance is associated with a "maximum considered earthquake."
For most of the country, a maximum considered earthquake is defined as an earthquake ground shaking that is likely to occur at the building site once every 2,500 years -- a very big earthquake indeed! The collapse prevention performance level was added for several reasons, one of which was to minimize the likelihood that structures designed to meet a life safety performance level in a 500-year design earthquake event would not collapse if subjected to the unlikely -- but still considered -- 2,500-year ground-shaking maximum considered earthquake.
Building safety levels
Immediate occupancy: The building receives a green tag (safe-to-occupy) rating from building officials immediately following an earthquake. Any repairs would be minor, and the facility can be reoccupied quickly.
Life safety: The building remains stable, with reserve capacity, and the threat of hazardous debris is minimized. Overall risk of life-threatening injury is low. While it may be possible to repair the building, for economic reasons it may not be practical.
Collapse prevention: The building barely remains standing and would be a total economic loss, although occupants would still be able to exit the building with some difficulty.
Although a collapse prevention performance level is now defined in the latest guidelines for seismic design, current seismic design approaches for new buildings have not changed to explicitly consider this performance level. Rather, collapse prevention performance is inferred, mainly from actual U.S. earthquake experience and research in the seismic performance of buildings. However, for existing buildings, this performance level is not only defined but specified, with guidance regarding evaluation and rehabilitation provided in the most recent FEMA standards.
Currently, the major shortcoming in the way that seismic performance levels are defined is their black-and-white nature. The performance level definitions infer that a building will either meet the designated performance level or it won’t. It’s all or nothing. This level of certainty, unfortunately, does not exist in the earthquake engineering community and is something that, at least until now, was difficult to quantify.
The earthquake engineering community has, for years, been able to communicate the issues that reflect our ability to predict seismic performance. Only in the recently completed research into the performance of welded steel moment-frame buildings has there been a scientific attempt to quantify our predictive capabilities.
The technique developed in this research allows the engineer to determine the confidence of achieving a particular performance level. For instance, the result of an analysis could be "There is a 90 percent confidence level that the building will perform at an immediate occupancy level, given a 500-year earthquake ground shaking." No longer will it be a black-and-white issue of whether or not the performance level is met, but rather, how likely it is that the building will meet that performance.
Other research initiatives are following the same approach. Although these efforts will take time to develop, perhaps more than 10 years, the hope is that the earthquake engineering community will be able describe the likely seismic performance of both existing and new buildings in terms of how likely the building will be to achieve a selected performance objective. The hope is that the next time a Nisqually-type earthquake affects the Pacific Northwest, or anywhere in the United States, we will not only be better prepared, but we will also be less surprised by the resulting performance of the built environment.