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November 18, 2004

What owners need to know about seismic design

  • Consider building beyond code to ensure business continuation after a big quake.
  • By MICHAEL W. LaNIER
    Berger/ABAM Engineers

     Sea-Tac Airport's main terminal
    Photo courtesy Berger/ABAM Engineers
    A seismic upgrade was performed at Sea-Tac Airport's main terminal in phases over seven years. The result was minimal disruption.

    Over the last several weeks, as Mount St. Helens churns with nearly constant low-level earthquakes, people in this region are reminded of the dark side that comes with the beautiful vistas provided by our Cascade Mountains.

    The dark side is that the Cascades and their embedded string of volcanoes are the product of the Cascadia subduction zone, which has the potential for significant earthquakes. This potential develops as the San Juan tectonic plate, moving in a generally easterly direction, deflects downward and moves beneath the North American plate — upon which sit many cities of the Pacific Northwest.

    The two principal earthquake belts are the Circum-Pacific belt, which encircles the Pacific Ocean, and the Alpine-Mediterranean-Trans-Asiatic belt. The West Coast of America is located in the Circum-Pacific belt, where 90 percent of all earthquakes occur.

    In general, the largest and most intense areas of surface damage are produced by earthquakes whose focal points fall in the intermediate depth range of 44 to 188 miles. Nearly all the deep shocks, which ordinarily produce less surface damage, have occurred in the Circum-Pacific belt. Eighty percent of the shallow shocks, which typically produce the most damage, have occurred in this belt.

    The heightened awareness from current volcanic activity provides an opportunity for design consultants to talk with their building owner clients about plans for operating in our earthquake-prone areas.

    While earthquake design is an actively developing science, as reflected by continuing changes in building code requirements and an ever-improving understanding of local seismicity, the awareness of seismic risks among most people in this region tends to rapidly diminish after an earthquake.

    Because seismic risk to business operations is perceived as a lower risk than more visible issues, such as theft or fire, property managers often find they lack the technical and communication tools to justify significant investment to enhance earthquake resistance.

    Architects and engineers have a special obligation to communicate the realities of seismic risks and viable mitigation strategies to building owners and mangers. An understanding of effective strategies to communicate seismic issues to our facility owners in laymen's terms is critical.

    Berger/ABAM used that process when it collaborated with Port of Seattle staff on a major seismic upgrade of Seattle-Tacoma International Airport's main terminal. The upgrade was accomplished as a series of phased programs over a 7-year period, resulting in minimal disruption to daily operations.

    Strategic decision making

    The strategic decision-making approach for dealing with seismic risk considers the total allocation of resources by a business. This approach recognizes that while a full response to a situation may not be immediately possible, there is an advantage to applying resources toward a solution over the long run.

    In general, businesses want their available resources allocated to provide a balance between short-term profitability and long-term continuation of operations. Assuming that goal, decisions can be made to promote the company strategy in a consistent and continuing manner.

    An advantage of a strategic approach is that it allows all business concerns to be considered, independent of current resource allocations, as the operational strategy is developed. The developed strategy is then used to allocate scarce resources to prioritized action items.

    A seismic risk plan

    The following six steps lead toward an effective strategic response to earthquake risk in a facility. While the emphasis is different for an educational or research facility versus a healthcare or industrial facility, the overall process of developing a strategic response is similar.

    Develop awareness — Facility managers must be aware of the existence and nature of the seismic hazard and know the relative risk in relation to other business risks the operation faces.

    Analysis -- The likely effects of different levels of seismic events on the business need to be investigated. Critical aspects of industrial processes or other operations need to be identified.

    Quantification -- The effects of a likely seismic event for a specific facility need to be quantified to determine the impact on the various elements of the operations. At this stage, specialist support in geotechnical and structural engineering may be helpful.

    Assessment -- The consequences of earthquakes need to be considered in the context of their potential impacts on the business, and the cost and effectiveness of mitigating measures.

    Insight -- Facility managers can make predictions regarding resource allocation based on potential seismic-related effects, costs to the business and mitigating measures.

    Strategic response -- With the background developed from the above steps, the facility manager can produce a specific seismic response plan. The plan may include a variety of options: seismic upgrades to the existing facility, site characteristic considerations for new facilities, design performance level objectives for new facilities, redundant critical operations and insurance strategies.

    Building code philosophy

    While building codes and design methodologies used in the United States are some of the most advanced in the world, reliance on local building codes to determine the adequacy of seismic resistance may be misleading.

    Seismic provisions in building codes generally prescribe minimum lateral forces, often applied statically to the structure, combined with the gravity forces of dead and live loads.

    The general code philosophy of seismic design can be summarized as:

    • Resist minor earthquakes without damage.

    • Resist moderate earthquakes without significant structural damage, but with some non-structural damage such as cracked plaster and broken windows. Damage to building contents is often not addressed by the code.

    • Resist the strongest expected earthquakes in the region without collapse, but with some structural damage. In most buildings, structural damage will be repairable. This depends on a number of factors, including the type of construction and the magnitude of the earthquake.

    While architects and engineers understand that the building code philosophy for non-critical buildings is focused on assuring public safety, this distinction is often lost on the public.

    With the exception for critical facilities, there is little consideration in the building code for the operational requirements of building contents, and no differentiation of equipment that may be critical to an industrial or other business operation.

    In the interest of protecting the continuation of the business, some critical aspects of the operation may justify special seismic provisions above those required by building codes.


    Michael W. LaNier is the executive vice president of Berger/ABAM Engineers, a structural and civil engineering, and project management firm headquartered in Federal Way.



     


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