Why retrofit and how?
What are the advantages of retrofitting?
The safety and survival of your family is one of the biggest reasons to retrofit your home. If a collapse can be avoided, the extent of injuries or even death would be greatly lessened. Your home would also be potentially safe and usable in the aftermath of a major quake.
The cost to retrofit your home will be FAR less than the cost to repair damages to your home after an earthquake. And that is if your home is reparable. Houses shaken off their foundations in an earthquake are often not worth fixing and usually get demolished. Example: If you can fix your home, costs could easily exceed $100,000.00 to lift your house, rebuild the lower supports, re-level the frame, reconnect gas, water and plumbing lines, repair interior and exterior walls that have been twisted - the list could go on. So you can see, roughly calculated, 3-5 thousand dollars for a retrofit could easily be a minimum of 20 times cheaper then repair damages.
Having earthquake insurance is great but this does not protect your family while an earthquake is happening and often doesn't include temporary housing while your home is being repaired. A lot of damage will occur in a large quake and you may be out of your home for a while. Deductibles will also be expensive - many times more than the expense of a retrofit.
Having an emergency plan and emergency preparedness kit will enable your family to safely deal with the aftermath of an earthquake, resulting tsunami or other major environmental disaster.
How a Retrofit Works
To keep a house from falling off its foundation in an earthquake, seismic retrofit strengthens three different areas of the house. These areas are all located in the crawl space as defined in the terminology illustration above. If any one of these three areas is not adequately retrofitted, the house will be susceptible to damage in an earthquake.
These three areas are:
1) The bracing of the cripple walls with plywood or metal fasteners
2) The fastening of the braced cripple walls to the foundation.
3) The attachment of the floor of the house to the braced cripple walls.
In addition any free standing masonry structures need to be assessed to ensure that they do not pose a hazard as a result of collapsing during an earthquake.
The following are simple illustrations to clarify these areas of retrofit.
Bracing the Cripple Walls with Plywood
Why you need shear walls.
Figure 1: Damage to a house due to lack of cripple wall bracing
Figure 1 shows what can happen to a house if it is not properly retrofitted with plywood on the cripple walls. Most people are concerned about bolting their house to the foundation but history has shown that unbraced cripple walls are the first area to fail in earthquakes, often leaving the residents homeless. If a house is bolted to the foundation, but the cripple walls are not braced, the cripple walls can still easily collapse. Bolting a house to the foundation without bracing the cripple walls offers very little protection.
Bolting the Braced Cripple Walls to the Foundation
Why you need bolts.
Figure 2: Damage to a house due to lack of foundation bolts.
The base of the house at the mudsill should be bolted to the foundation for obvious reasons. Without foundation bolts, a house can slide off of the foundation even if its cripple walls are braced with plywood shear panels. Notice in Fig. 2 that plywood is only on the corners of the cripple wall. It is not necessary to put plywood on the entire cripple wall.
Attaching the Floor of the House to the Braced Cripple Walls
Why you need shear transfer ties.
Figure 3: Damage to a house due to no connection of floor to cripple wall.
As figure 3 shows, the floor of a house can slide off the top of the cripple wall. In Fig. 3 the cripple wall is braced with plywood to prevent collapse and it is also bolted to the foundation. However, since the floor of the house is not attached to the top of the cripple wall, the house can slide off the cripple wall during an earthquake.