I stumbled on the information in this video by chance. I created a video that contained every house I could find on the web and in libraries that illustrated a cripple wall collapse. When I finished, I was astonished to see that only around 3% of these houses had stucco siding and 97% had wood siding made of horizontal boards on the exterior walls. One of the stucco buildings looked like an apartment building but I am not sure, so I included it anyway.
This is remarkable because all seismic retrofit guidelines recommend more retrofit components for stucco homes compared to wood-sided homes. I have been on two retrofit guideline committees and now understand that this comes out of the imagination of structural engineers.
The video speaks for itself.
What Does the Building Code Say?
Before retrofitting a home, its existing structural components should be evaluated for seismic resistance. These existing structural components are called archaic building materials. Many archaic building materials such as plaster are no longer used, but we still need to know how strong they are before we can develop a rational design. You can learn more about this science on this webpage about the earthquake resistance of existing homes.
The important this for you is that this table tells us stucco is twice as strong as wood siding when new. If you consider the fact that if just one time in the life of the building painting was neglected, the nails will be severely rusted. Add all these factors together and it is no wonder stucco houses do so much better.
Table A1-D below tells us how much earthquake force these archaic materials can resist. Most houses in the Bay Area have some combination of archaic materials. Table A1-D can be found in Appendix A Guidelines for the Seismic Retrofit of Existing Buildings and the California Historic Building Code.
Earthquake Resistance of Archaic Materials
Structural engineer Nels Roselund, who has been involved in earthquake hazard mitigation for 50 years, told me Table A1-D was the result of some testing done by structural engineer John Kariotis and several other engineers, including himself, many years ago. These tests are sometimes referred to as the North American Rockwell Tests because Mr. Kariotis negotiated with this Aero Technologies company (purchased by Boeing in 1973) to use their facility and testing equipment to conduct these tests.
The testing equipment at the Rockwell lab was very advanced. It was normally used to test the flight performance of airplanes such during take-off, wind turbulence, upward and downward drafts, and landing. This testing equipment could be calibrated to create an environment almost identical to an earthquake.
For example, the research team would build a wall made of “Plaster on wood or metal lath” and then subject it to these simulated earthquake forces. As demonstrated in table, a test proved this combination of archaic building materials can resist 600 lbs. per linear foot.
“Horizontal diaphragms” is a fancy word for floors or roofs. Except in one case, we are not looking at this part of the table because neither floors nor roofs fail in earthquakes.
Same Table, Different Values, What’s Up?
As mentioned previously Table A1-D lists how much earthquake force cross walls made of archaic materials can resist until they reach catastrophic failure. Cross walls are internal walls that create the various rooms of the house. As shown at the top of the right-hand column these force levels are referred to as STRENGTH VALUES.
Most designers multiply these tested STRENGTH VALUES by a safety factor. In other words, if a designer uses Table A1-D and determines a 10-foot-long plaster cross wall can resist 6000 lbs. of force, he will then factor in factor in a variable called a reduction to compensate for the unknown. He might say to himself “The plaster looks good, but what if the material was not mixed properly? What if the wood supporting the plaster was not installed properly? What if the nails are the wrong size? ” “It looks like this customer can afford a good lawyer” etc. Just to be safe, he decides to assume this 6000 lb. wall might only be able to resist 2000 lb.
As a rule, most designers use 1/3 of the strength value or a safety factor of 0.33. Most tables in the code have a safety factor built in. Table A1-D leaves the choice for the allowable value to the discretion of the designer.
This table can be found in the 1997 Uniform Code of Building Conservation, which was used until July of 1999. It is probably found in previous editions of the code, but I don’t know for sure. I don’t have copies of these older codes. The allowable value is built into the table and does not allow the designer any room for personal judgement.
An Important Horizontal Diaphragm Value in Table A4-A
For example, let’s say a designer is looking at a wall that is 10 feet long with plaster on one side. Looking at item 2.1 this wall can resist 2000 lbs. of earthquake force. This is all that is allowed. The same wall has 6,000 lbs. of earthquake resistance using Table A1-D., though the designer can at his discretion reduce it as much as he wants.
As mentioned previously, floors do not fail so we do not look at the values for horizontal diaphragms except in one case. A horizontal diaphragm made of “straight tongue and groove sheathing” stood up on end is the same as an exterior wall with wood siding on it. This siding can be seen on the white house above. According to item 1.3, walls built like this have an allowable value of “100 lbs. per ft. for seismic shear.” This is a minimal amount of earthquake resistance and is subject to collapse.
How Does This Information Help Me?
Imagine the cripple walls at the front and back of your house are 24 feet long. If they used horizontal siding (straight sheathing according to the Table), the allowable value in item 1.3 in Table A4-4 the cripple walls will be able to resist 2400 pounds of earthquake force before they fail. If we add 12 linear feet of plywood to this cripple wall and nail it to resist 870 pounds of force, it will now take 10,440 pounds of earthquake force before the cripple walls fail. In other words, the house is now 5 times more resistant to earthquakes than it was before.
For a discussion of existing foundations, go to this web page.