Seismic sustainability of building enclosures

February 2012 » Columns » SEISMIC ZONE
James Parker, S.E., P.E.

Extreme events should be part of the life cycle cost (LCC) decision-making for sustainable design of the building envelope and other nonstructural components of buildings. Recently, concerns over population growth, depletion of the world's natural resources, and global warming have led to a renaissance in designing buildings and cities to be more sustainable. For seismic regions, a major problem with the current procedures is that they neglect the impact of seismic events on a building's sustainability. For an enclosure of a building to be considered sustainable, it must have durable performance. Most enclosures are designed considering durable performance to withstand water, air and vapor intrusion and aging due to environmental loadings including wind, rain, temperature variation, exposure to sun, etc. Most do not consider seismic sustainability.

Figure 1 - Annual probability of damage state being equaled or exceeded
Simpson Gumpertz & Heger Inc.

In seismic regions like Los Angeles, you can expect a moderate earthquake during the lifetime of a building, if not an even larger event. Moderate earthquakes can require repairs or reconstruction that utilize additional materials and resources and generate waste of replaced portions. Downtime results in loss of building utilization and efficiency. With replacement costs from seismic damage potentially in the billions of dollars, the level of resources that have to be replaced calls into question how sustainable our building designs really are. Current procedures of evaluating building performance through LEED metrics neglect the impact of seismic events on a building's sustainability.

Currently, most designers do not take the evaluation of seismic performance into any more depth than that required by the governing building code. The intent of building codes is to design for a minimum objective of life safety in a design-level event (a major earthquake). The additional cost and resources to make all buildings survive this level of earthquake undamaged would counter the intent of sustainability by utilizing too much energy and resources for a low probability event. For many years now, the minimum objective of life safety has been clear in the codes for the design of the base building structure itself. However, only recently have codes been explicitly providing seismic design requirements to mitigate failure of glazing systems and other nonstructural façade components that can pose a hazard as they fall from the building.

Designers currently have little direction on procedures and lack the data necessary to have a better understanding of seismic performance of enclosures. Understanding probable damage and repairs is necessary for performance-based designs, as well as for informed sustainable design. Performance-based design provides choices for the enclosure based on knowledge of the performance for various levels of earthquake ground motion.

The annual probability of a given damage state being equaled or exceeded is found by integrating the probability of the seismic hazard h(a) with the fragility curve F(a) over all possible values for a, a site-specific parameter. The curve representing the product of h(a) times F(a) is shown in Figure 1 and the area under this curve is the probability of the damage state being equaled or exceeded. The probability of the given damage times the cost of the damage is the seismic risk and this risk should be included in the LCC of the enclosure along with the risks of other potential damage states.

With this approach, one would make design choices that appropriately reduce the potential for seismic loss (risk) and account for seismic loss estimates in LCC studies. The lower the LCC from seismic risk the greater the seismic sustainability. The losses could be defined in economic terms, environmental impact, or both.

James Parker, S.E., P.E., is a senior principal at Simpson Gumpertz & Heger Inc. (SGH). He can be reached at jcparker@sgh.com. Anders Carlson, Ph.D., S.E., is a staff consultant at SGH as well as an assistant professor, University of Southern California School of Architecture. He can be reached at aecarlson@sgh.com.

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