Seismic risk is comprised of three components: Hazard, Vulnerability, and Exposure. Hazards relate to the “external forces” which cause damage, such as ground shaking, liquefaction, surface fault rupture, etc. Vulnerability relates to how well the structure is designed and detailed, along with weaknesses and deficiencies in the structural systems. Exposure is the value at risk, and can be expressed in terms of occupant loading (life-safety) or building value (property loss). A seismic review or building stability analysis should indicate whether the structure is likely to collapse under a specified ground motion while a seismic loss estimate should indicate the likely damage to the structure (generally as a percent of building value) under a specified ground motion(s). Together, they form a seismic risk assessment and are often referred to as Seismic Reports, Seismic Risk Assessments, or Probable Maximum Loss reports.
“Facility Surveys” are most commonly conducted to provide input to business decisions regarding acquisition, financing, or insurance for properties. A structural/seismic review is often performed as a part of the overall facility survey in order to identify earthquake risk to the property in terms of life-safety, property loss, business interruption and liability.
Seismic Reviews and Loss Estimation
In order to adequately address seismic loss, there are two primary items which should be addressed: Life-Safety and Loss of Real Property. Acceptable limits of risk for one of these items does not necessarily indicate that the other item is within acceptable limits. Other losses such as business interruption and contents damage are also often considered.
The United States Geological Survey (USGS) has developed ground shaking maps for the US which considers ground motions generated by all applicable faults as well as the return period for each fault. A commonly used return period for loss estimation is 475-yrs, which represents a 10% chance of exceedance in a 50 year period. Current building codes for new design utilize a 2% chance of exceedance in 50 years, or a 2500-yr event, which is then scaled down to be similar to the 475-yr ground motions. This change captures large but infrequent earthquakes in areas such as the mid-west, the Carolinas, and parts of New England.
Liquefaction, landslide and surface fault rupture are best determined by registered geotechnical engineers for a particular site using site specific soils information. However, for many seismic reports, regional mapping is often used to determine soil conditions. The regional maps in many cases have a high degree of uncertainty as local soil conditions may vary.
Vulnerability of a structure is best determined by qualified structural engineers experienced in the review of existing buildings, particularly in the subject building type. It is important for the engineer to review the construction documents used in the original design and to perform a site visit to observe the condition of the structural systems and see if major modifications to the building have occurred.
An analysis of the building should then be performed. Analysis procedures range from ASCE41 Tier 1 Quick Checks to in-depth computer modeling, depending on the building type, anticipated deficiencies, and clients needs. This analysis is used to determine the Building Stability requirements of ASTM E2026.
Exposure consists of the values at risk, generally life-safety and property damage. Life-Safety is determined during the building stability check, where the building or portions of the building are subject to collapse. The property damage, however, is generally defined as the damage as a percent of building replacement value. These items should both be included in a Probable Maximum Loss or PML report.
There are several sources of “data” which exist to estimate the damage to particular categories of buildings (ATC-13, Steinbrugge, Thiel & Zutty, proprietary software, etc.). For the most part, data is expressed as a probability distribution for each level of ground motion and each category of building. This distribution is in the shape of a skewed bell curve. The mean (or top of the curve) represents the best estimate of damage (half have more, half have less) given a large population of buildings. The 90th percentile is a conservative estimate in which only 10% of the buildings are anticipated to have greater damage. PML used to be defined as the PML mean or PML 90th percentile, however the ASTM standards recommend the use of the terms Scenario Expected Loss (SEL) and Scenario Upper Loss (SUL), respectively.
Experience and engineering judgment play an important role in attempting to compare a building to other “average” buildings within a classification. Loss estimates should be modified based on positive and negative features specific to each building. A qualified and experienced structural engineer should perform both the site visit and the analysis of the building to determine whether there are design features which may result in building collapse.
Seismic risk assessments are useful tools in helping define seismic risk for lenders and owners. However, as engineers we can not assess our clients “risk tolerance” and thus rely on owners and lenders to establish criteria based on their needs. The criteria should include the level of review desired, qualifications of the reviewer, risk to life-safety, and the ground shaking (return period), confidence level, and limits for potential real property damage. Engineering reports should in turn state the criteria and methodology used in the review. While many reports look similar, not all reports are created equal and vary widely based on the level of effort (i.e. cost) put into the review.
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This site is published by Cascade Crest Consulting Engineers, a national leader in seismic risk assessment reports. Our services are geared toward upholding the ASTM and ASCE standards, both in scope and qualifications of those who conduct the both site visit and perform the seismic risk assessment or Probable Maximum Loss (PML) reports.