Some people wonder why we say earthquake force is striking a house when we are actually referring to ground movement under the house. This is done as a practical matter to make designing retrofits easier.
Some people find it much easier to read about earthquake engineering rather than watching this video. If you are one of those people please refer to the Homeowner’s Guide to Seismic Retrofitting.
Seismic retrofit engineering for houses is much simpler than you might imagine. All you need to know is the weight of the house and the anticipated ground acceleration measure as a G force. This is found in the building code. Plug these numbers into a simple formula, called the base shear formula, and it will tell you how many foundation bolts, linear feet of plywood, and shear transfer ties your retrofit will need to resist the force geologists tell us to expect.
If you don’t use enough components the retrofit will fail. If you use too many, you are wasting your money. The most exhaustive attempt to standardize the retrofit of homes come from these engineering calculations. The author was a member of the committee that wrote this guideline which is why he has a copy of the calculations.
This formula appears quite simple but it application represents a merging of geology, physics, and empirical evidence to tell us how much earthquake force is going to attack the base of a building where it is connected to the foundation. That is why it is called the Base Shear formula. Watch this video and find out how the base shear formula is used to design a residential retrofit. This information prevents overdoing it and spending more money than you need to, but makes sure you get the protection you need. The video will only make sense if you know what cripple wall retrofit components do and look like.