Conditions of Site and Building Instability

Our second blog post in the series on seismic risk assessments (SRAs) highlights site and building stability assessments, and the conditions of instability.

These two important aspects of SRAs can reveal issues not outright expressed by loss estimates. Additionally, an understanding of these aspects better positions those with CRE interests to make sound investments and/or protect their assets.

An ASTM Requirement

When the American Society for Testing and Materials (ASTM) updated its seismic standards in 2016, it required that site and building stability assessments be included in all seismic reports. This important distinction signifies the need for greater scrutiny and varied approaches in determining seismic risks, and identifies site and building stability as necessary measures in the process.

Passing, but Vulnerable?

While site and building stability are factors used in determining loss estimates, a passing loss estimate (typically under 20% for SEL) does not necessarily convey confidence in the site or building stability. It’s not unheard of for a building to have an SEL in the 18-19% passing range, not raising flags, but remain vulnerable. In addition to an updated SEL that conforms to the most recent ASTM standards, the details of a building or site stability assessment could identify conditions of instability that might otherwise be overlooked if relying solely on a loss estimate.

Conditions of Site Instability

Site stability focuses on external earthquake-induced threats to a site’s stability, the most common being landsliding, soil liquefaction, or faulting.

Landslide

Landsliding, in response to an earthquake happens when soil and rock material rapidly move downslope, which is particularly concerning for properties built on or backing to steep slopes or hillsides.

Soil Liquefaction

Soil liquefaction occurs in areas with loose, saturated, sandy soil, which transforms into a fluid-like state. It is especially a concern in areas where there is a high water table and the structure is not provided with a deep foundation system (caissons or piles) or other form of ground mitigation.

Earthquake

Earthquake ground fault rupture concerns the risk of active fault movements breaking through to the ground surface, thereby creating site instability. Typically, shallow crevasses will form   as the ground is displaced, so structures built across fault lines can be subject to this tearing.

Site Stability Assessments

A site stability assessment consists of the review of published maps and databases from federal, state or local government agencies identifying landsliding, liquefaction, or active fault zones. Where available, the assessment also includes the review of site-specific geotechnical investigation reports addressing the potential for seismic-induced hazards. They are typically the most accurate sources of information, provided they are relatively up to date. Geotechnical investigation reports also increase the level of confidence of the site stability assessment.

Conditions of Building Instability

Unlike site stability, which is concerned with external hazards, building stability is concerned with the construction type, configuration, and condition of the structural elements. Specifically, it determines whether a building will remain stable through an earthquake by assessing a building’s structural integrity and load carrying-capacity during a seismic event. Certain conditions may determine if there is instability in whole, or in part, for the building, creating the potential of total collapse or localized falling hazards during a code level earthquake event.

Instability 1    Instability 2

Some examples of instability in whole are the soft or weak story at a building’s first level, discontinuous shear walls or other types of vertical irregularities, lack of anchorage of wood sill plates to foundations, or lack of substantial wall to roof anchorage ties (for example, in masonry or concrete tilt-up buildings).

Instability 3   Instability 4   Instability 5

Some examples of instability in part are unbraced canopies or parapets, brick veneer lacking sufficient anchorage to the primary structural or curtain wall framing, or architectural pre-cast panels lacking sufficient anchorage or bracing. These conditions have the potential to create falling hazards during a code level earthquake event, but not a global instability in whole condition such as those mentioned above.

Building Stability Assessments

Similar to a building code review of a structure, a building stability assessment involves reviewing available construction documents (e.g. structural drawings), as well as onsite visual observations of the structural elements to assess the lateral load-resisting systems (the elements of the structural system that provide support and stability to the building under seismic and wind forces). Construction document review, especially structural drawing review, is the best approach for revealing the presence of structural framing conditions that have the potential to create an instability in whole condition during a code level seismic event. Structural drawing review also increases the level of confidence of the building stability assessment.

The Bottom Line

Both building stability and site stability contribute to the damageability and corresponding loss estimate of the building(s) during a code level earthquake event. Consequently, it is important to have a clear understanding of any issues that may contribute to instability of either the building or site. A property with a stability concern is also expected to be more damageable than a similar property with no stability concerns. The availability of information such as structural drawings (either original or those pertaining to a seismic retrofit) and site-specific geotechnical reports helps to increase confidence of the building and site stability assessments. EBI continues to provide clients with seismic risk assessments that meet the latest ASTM standards, thereby addressing both building and site stability concerns.

If you have any questions or concerns regarding seismic risk assessment reports, the current ASTM seismic standards, site or building stability issues, please contact us or read more about our seismic risk assessment services.

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