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Fannie Mae Peak Ground Acceleration

The 2014 update of the Fannie Mae Multifamily Selling and Servicing Guide for Property Needs Assessments (PNA) has been revised to include a determination of site seismic hazard using the 475-year (10% in 50-year) peak ground acceleration. This update is a departure from the previous reference to the 1997 Uniform Building Code (UBC) Seismic Zone Map.

As of February 3, 2014 all properties located in a region with a 475-year probabilistic seismic hazard (associated with a 10% chance of exceedance in 50-years) of 0.15g or greater are subject to review for seismic risk factors that may require further seismic risk assessment reporting. Previously, all properties located in Seismic Zone 3 and 4 would require review for the seismic risk factors. The PGA determination replaces the visual determination of the Seismic Zone.

The 10% in 50-year site PGA can be obtained using the United States Geological Survey (USGS) seismic hazard database, developed under the National Seismic Hazards Mapping Project (NSHMP). The search tool illustrated below includes a number of options that are useful for engineers but can be simplified for the PGA determination according to the Fannie Mae seismic provisions.

Partner offers the following simple, step-by-step procedure to determine the 10% on 50-year site PGA:

The USGS Earthquake Hazards Website contains a number of links to hazards determination tools, but we will focus on one, the Interactive Deaggregation link: https://earthquake.usgs.gov/hazards/interactive/  

Unified Hazard Tool

Visit the updated 2011 Interactive Deaggregation map here. The 2008 map will take you to a page that will require you to accept the security certificate as shown below. The USGS site has not been updated with a current security certificate.

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Accept the insecure content based on the browser you are using. The page will refresh to the default page shown below.

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We fill focus on the search applet now. A map can be viewed by selecting the “Show Map” link in the lower left corner:

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An example property is provided below. The following items are required for the open search fields:

  • Provide a site name. It can be anything, but there needs to be something in this box
  • Type in the property address so the Google map engine can locate the property. Otherwise the Latitude and Longitude may be entered.
  • Change the Exceedance Probability percentage from 2% (default value) to 10%
  • Change the Vs30 value as follows:
  • 300 m/s for Western US sites
  • 760 m/s for all other Continental US sites
  • Click on the Compute button.
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The site will refresh the page to show the output file below the map. Click on either the TXT file or the PDF file to view the appropriate output.

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The PGA can be found in the TEXT file as shown below:

usgs pga search text output

The PGA can be found in the PDF file as shown below:usgs pga search pdf output image 8

If the site PGA is determined to be greater than .15g the property would be subject to review for the presence of one or more of the following risk factors according to Section 321.01:

  • reinforced masonry buildings and pre-cast concrete or tilt-up buildings constructed prior to 1994;
  • reinforced concrete frame or reinforced concrete shear wall buildings constructed prior to 1976;
  • wood frame buildings on unbraced cripple walls (sometimes referred to as crawl space walls – short wood perimeter walls used to elevate a Property above-ground to allow access to the substructure and utility lines or to level a building constructed on a slope);
  • wood frame buildings without anchorage to foundation;
  • any building with a weak (soft) story at the first level above grade;
  • buildings on sites with significant liquefaction potential (e.g., low-lying areas along bodies of water such a rivers, lakes, bays, the coast or waterways) unless provided with deep foundations (piles or piers); or
  • buildings with tuck-under parking constructed prior to 1988.