Some people wonder why we say earthquake forces are striking a house when we are actually referring to ground movement under the house. This is done because conceptually it is much easier to understand.
Design is Everything
The design determines if the retrofit will work as intended and it determines cost. A single engineering oversight can cause catastrophic failure and an uninformed engineer can recommend far more work than is necessary, even to the point of being unaffordable. A case in point being an engineered design in Berkeley for a 1200 square foot house that was going to cost $65,000 for us to build. This happened because the engineer recommended many things, including foundation work, that were not necessary. Make sure you understand the design before proceeding with any retrofit.
A designer must consider the following: What was the building code when the house was built? How did houses like yours perform in previous earthquakes? Is the cripple wall made of redwood or Douglas Fir? How tall are they? If the house stucco, or wood? Are the floor boards straight, or are they placed at an angle? Does the entire house have a cripple wall, or just part of it? Is the foundation concrete, brick, or capped concrete? Are the walls balloon framed or platform framed? In other words, there are a lot of things you need to consider. Only someone who has taken the time to research these things will know how these things impact a design.
Someone Already Did the Engineering
In 2004 a local specialist in seismic retrofit engineering looked at built with the most common configurations of building materials and old building codes. This engineering is the basis of a regional guideline called Standard Plan A.
He took that information and created engineering calculations for the most common types of houses.
This included stucco houses, houses with wood siding, houses with old foundations, houses with new foundations, tall cripple walls, short cripple walls, one story houses, two story houses, houses with sheet rock, and houses with plaster on the walls, etc. In other words, he engineered retrofits for the most common configurations and sizes of homes found in the Bay Area. Here is a sample:
TABLE 3A – SUMMARY OF UNIT LOADS
Assume SD soil with Ca = 0.44; Na = 1.3; I = 1.00; and R = 5.5; Conversion to ASD force level: 1 / 1.4 Seismic V = 0.186 W (2001 CBC Equation 30-5)
The basic unit dead loads used to calculate the seismic loading demand for Case 3A are:
Roof/ceiling system: Light roofing and gypsum board ceiling finish. Light roofing is defined as wood shakes over spaced sheathing or wood shingles or composition shingles over solid sheathing. Vertical load adjustment for 4:12 roof slope = 1.054
Light roofing system: 5.0 psf
Rafters & ceiling framing: 2.5 psf
Gypsum wallboard: 2.2 psf
Miscellaneous: 0.8 psf
Light roof Total: 10.5 x 1.054 = 11.0 psf
Second floor/ceiling system: Gypsum wallboard is assumed to be the interior ceiling finish.
This is only a small segment of the engineering involved and tells you nothing about how to apply this engineering to a house in a practical way. The practical recommendations were a complete failure.
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.
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.