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 and continued to shake it violently several minutes afterwards. Even though this house fell from its foundation it did not suffer what we normally think of as catastrophic damage. The walls did not collapse, the roof did not cave in; even the windows did not break. In spite of all evidence to the contrary, many homeowners believe catastrophic damage like this is a real possibility.
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 rooms, as well as any interior walls such as those found in hallways, are called cross walls. These walls resist earthquake forces and the more cross walls a house has the stronger it will be.
The truth of this statement is confirmed by the photo of the white house above and the photographs below. In practical terms it means if you can keep a house on its foundation with a good seismic retrofit the amount of damage should be minor.
On the other hand there are no cross walls in the crawl space The cripple walls must therefore resist all he force and can collapse.
This Santa Cruz house is an especially interesting case where its cuboid shape prevented catastrophic damage above the first floor. Of particular interest is the intact row of windows at the front even though their wall had no lateral support. Rotational resistance certainly provided another layer of protection.
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 after the evaluation I drove by and saw an empty lot. Except for the porch 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. 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.
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 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 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.
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. 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 subsequent 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 persona judgement.
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. This 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 pounds lbs per ft. for seismic shear.” This is a minimal amount of earthquake resistance and is subject to collapse.
Behind stucco you will often find the same weak horizontal siding that can collapse once the very brittle stucco fails. This is why houses with stucco also need to be retrofitted.
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.
For a discussion of existing foundation go to this web page.