This video provides photographic evidence showing that so long as you keep a house on its foundation with a seismic retrofit it should survive.
Here we look at fascinating tests done in a rocket ship testing lab that tell engineers and the building code much of what we need to know.
If You Don’t Know You Better Find Out
The 1964 Alaska earthquake, which lasted a record breaking 4 1/2 minutes, jolted this house off its foundation. Remarkably, the house is fairly intact. The walls did not collapse, the roof did not cave in; even the windows did not break. Here you will learn why houses like this, and probably yours, can do so well.
Cross Walls-Each Wall Inside a House is Called a Cross Wall
A house is a cuboid (box-shaped object). The base of the cuboid is the floor, the sides of the cuboid are the walls, and the top of the cuboid is the roof. Inside this large cuboid are lots of smaller cuboids in the form of bedrooms, hallways, bathrooms, etc. All these cuboids work together to form a very strong geometric shape that keeps a house more or less structurally intact above the first floor.
The interior walls that create these cuboids are called cross walls. The more cross walls a house has the stronger it will be.
In practical terms it means if you can keep a house on its foundation with a good seismic retrofit the cross walls will protect the rest of the house.
On the other hand the crawl space is not protected by cross walls. This is why all retrofitting takes place in the crawl space.
This Santa Cruz house is an especially interesting case where cross walls prevented catastrophic damage above the first floor. The garage did not have cross walls. Of particular interest is the intact row of windows at the front.
Same House: Earthquake Damage On The Inside
Even if the cubic shape holds the house together, if it falls from its foundation it can still sustain catastrophic damage to the interior walls, plumbing, and electrical systems. If the walls and ceiling are plaster, it is very common for the plaster to fall off as shown here.
I was the FEMA inspector who evaluated this beautiful house in Watsonville after the 1989 San Francisco Earthquake. It suffered a cripple wall collapse. Two weeks later I drove by and saw an empty lot. Except for the porch, viewed from the outside this house suffered minimal structural damage. The interior was another story. If it had remained on its foundation someone would still be living there.
The house shown below is a very typical tract house found in the newer parts of the Bay Area. Almost all of these homes were built according to building codes that made sure they were extremely earthquake resistant from the first floor up. Unfortunately, even though most of these houses are bolted, the code failed to strengthen a very vulnerable floor connection. This weak connection could result in the house sliding on top of the foundation.
What Does The Building Code Say?
Before retrofitting a home its existing structural components must be evaluated for seismic resistance. Strengthen existing the weak connections and leave the strong connections alone. These connections are made of archaic building materials. Many archaic building materials found in older homes such as plaster are no longer used, but we still need to know how strong or weak they are.
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 Gudelines for the Seismic Retrofit of Existing Buildings and the California Historic Building Code.
Shear Values of Archaic Materials
Structural engineer Nels Roselund, who has been involved in earthquake hazard mitigation for past 50 years, told me Table A1-D was the result of some testing done by the Structural Engineer’s Association of Southern California, including himself, many years ago. These tests are sometimes referred to as the Rockwell Tests because Mr Kariotis negotiated with this airplane manufacturer (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.
As a rule, most designers use 1/3 of the strength value s seem in the table by a safety factor of 0.33. Most tables in the code have a safety factor built in. The table below is identical to the table above except the safety factor is built in. For example, rather than a plaster cross wall being able to resist 600 lbs per linear foot it can only resist 200 lbs per linear foot. A safety factor accounts for the possibility that the original installation had problems and it is actually weaker than was shown in a lab.
This table can be found in the 1997 Uniform Code of Building Conservation. The safety factor, also known as an allowable value, is built into the table.
An Important Horizontal Diaphragm Value in Table 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 (10 x 200lbs).
On the other hand, here is a house with wood siding (a vertical horizontal diaphragm) made of “straight tongue and groove sheathing”stood up on end. 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 pounds lbs per ft. for seismic shear.” This is a minimal amount of earthquake resistance making this house 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 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.
“Plaster on wood or metal lath” is the same as stucco. Notice it is twice as strong as wood siding. Stucco houses do much better than houses with wood siding.
For a discussion of existing foundation go to this web page.