Image:089srUSGSoffFoundation.jpg

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Public domain USGS image from [USGS site]

89. Houses not bolted down securely were easily dislodged from their foundations in downtown Watsonville. [J.K. Nakata, U.S. Geological Survey]

Image prepared for the seismic retrofit article.

As the raw ground may be seen exposed under the house, the construction is upon a continuous perimeter foundation (rather than upon a concrete slab). Construction of the perimeter foundation type will typically have interior pylons supporting a grid of beams upon which the floor joists are laid. If such were present, they tumbled or the building was displaced relative to them. Such protrusions or other objects such as floor mounted heaters can cause great damage to the structure internally - even though it appears superficially that the building need be only jacked up and over onto its foundations.

Also note that in this case the "mud sill", the wood directly in contact with the foundation, remained in place. It was the connections between the joists, their end blocking, and the perimeter members that detached. The design and construction of retrofits requires a great attention to detail, as a strengthening of one element may only transfer stress to the next weaker element in the structure.

This image is a work of a United States Geological Survey employee, taken or made during the course of the person's official duties. As a work of the United States Government, the image is in the public domain. For more information, see the USGS copyright policy.

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(del) (cur) 05:20, 10 September 2004 . . Leonard G. (Talk | contribs) . . 752×512 (51,317 bytes) (Public domain USGS image from http://wrgis.wr.usgs.gov/dds/dds-29/web_pages/watsonville.html USGS site)