May 12, 2012

Earthquake Retrofitting

The new addition has been built in accordance to modern California building code.  This includes the use of hold-down brackets which are designed to connect the posts and cripple wall framing directly to the foundation to increase uplift resistance during Earthquakes.  Also used were galvanized foundation bolts which are secured 12" deep into the foundation by SET-XP epoxy (ICC 2508).  The installation of the epoxy and bolts must be overseen through physical observation by a licensed deputy inspector (about $100).  Also used are 3" square galvanized plate washers to increase the hold down strength.  Finally, the walls are braced with 5/8" structural grade plywood.
We've learned a lot during this process.
Given that California has more than a 99% chance of having a magnitude 6.7 or larger earthquake within the next 30 years, building experts recommend that older homes be retrofitted because of the effects of earthquakes (house slides off foundation, or cripple walls collapse).  Houses built in the 1950s and 1960s were bolted to the foundation using simple washers and nuts.  Overtime these bolts and nuts have rusted resulting in a weakened connection to the foundation.  The purpose of earthquake retrofitting is to fasten the house to the slab and concrete foundation so that its kept from being displaced when an earthquake happens. 
We decided as long as the walls were open that we would earthquake retrofit the existing portion of the home.  All materials were purchased at our local California Home Depot.  Materials include:  hold-down brackets (about $20/each), galvanized 5/8" x 12" long bolts (about $9 each),  3" galvanized plate washers (used extras from the addition), and set-XP epoxy (ICC 2508) (about $44 each double tube).
I'm certain we now have the safest house in the neighborhood.  :-)
Retrofit-ed hold-down bracket.  You can see the SET-XP epoxy used to secure the 5/8" galvanized bolt into the existing foundation (12" deep).

Retrofit-ed 3" square plate washer with new retrofitted 5/8" galvanized bolt secured by SET-XP epoxy 12" deep into existing foundation.

New addition section.  Same hold-down structure as retrofit.  Completed during new construction.  Work was observed live by deputy inspector.

New addition section.  Same hold-down and 3" square plate washers as the retrofit.  Completed during new construction.  Work was observed live by deputy inspector.

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