After the 2000 Napa earthquake, the owner of this house explored options for retrofitting. After a seismic evaluation, his line-item retrofit proposal suggested that all beams, girders, posts, and rafters be connected with strong ties. It was also suggested that a new, steel reinforced foundation be poured. However, he decided to forego the foundation at that time and instead, performed a basic seismic retrofit tie-down with the intention of getting the foundation replaced at a later date.

At 3:20 AM on August 24, 2014, that “later date” had arrived. Fortunately for the owner of this house, the previous, basic seismic retrofitting kept the posts supporting the main structural beams from coming unattached from their piers. Those posts were all that was supporting the home by 3:21 AM. Having done a minimal, basic retrofitting after the 2000 Napa earthquake, Forbes and Sons was able to save the structure from completely collapsing in on itself during the 2014 quake.

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As you will see by clicking through the photos, the structure remained intact while the foundation crumbled. Photo 1-A shows the new foundation work being performed in January 2015. Photos 1-B, and 1-C (Click on arrows above.) show the destroyed foundation, while photo 1-D shows the seismic strong ties still in place, doing what they were meant to do—hold the building together.

 

Here is an excellent example of a foundation staying in place while the building moved two feet. The problem here is that the bolting of the house to the foundation was not upgraded to current code. These bolts are only 1/2 inch (code is 5/8 or larger), with sub sized washers, and more importantly, the bolts are placed in the foundation at least six feet apart. Code now requires a maximum 4 feet intervals for residential and 3 feet for commerical. This structure may have been saved if it were properly achored to the foundation along with other minimal seismic retrofitting. Photo 2-B (Click arrow.) shows another view.

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This example shows both a structure and an un-reinforced foundation that failed. The home moved as a result of not being retrofitted and properly bolted to the foundation. However, part of the foundation under the front porch broke away from the rest and fell in under the house. As a result of no prior seismic retrofitting, this home will need to be raised and the old foundation removed and replaced with a new, reinforced one.

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