These videos illustrate how retrofits can prevent catastrophic damage by attaching the first floor of your house (the floor you step onto when you open the front door) to the foundation with a retrofit. After the earthquake you will more or less be able to continue your life as before.
History has shown when houses fall off their foundation in an earthquake, owners repair them 99% of the time. However, if your house does fall off its foundation, you’ll probably need to leave the state because other earthquake refugees occupy all the temporary housing, you may lose your job, contractors charge triple or quadruple their normal rate due to high demand, kids must leave school, and the list goes on and on. It is possible to prevent this disaster rather than cleaning it up after it happens by installing an effective retrofit. We created this website to help you make that decision.
This is what happens if you don’t retrofit
This is how you can protect your home
The 1989 Loma Prieta Earthquake Became a Natural Laboratory
The 1989 Loma Prieta Earthquake gave us a chance to see that seismic retrofits work by in an actual earthquake. An architect owned two turn-of-the-century houses in Santa Cruz that needed foundation bolts and cripple wall bracing. These houses were identical in every way: same year of construction, same builder, same materials. In 1988, the architect retrofitted one of them using the retrofit techniques described on this website. He bolted it to the foundation, strengthened the cripple walls, and installed shear transfer ties. He was preparing to retrofit the other house when: The Loma Prieta earthquake struck. The un-retrofitted house slid off its foundation and sustained over two hundred thousand dollars of damage. The retrofitted house only had plaster cracks that were relatively inexpensive to repair. This house was fully habitable. Earthquakes always cause cracks in the drywall or plaster, but seismic retrofitting can make them extremely earthquake resistant.
Why the House-to-Foundation Connection is So Important
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. Preventing earthquake damage is actually quite simple.
THIS HOUSE WOULD HAVE REMAINED FULLY HABITABLE IF A RETROFIT HAD ATTACHED IT TO ITS FOUNDATION.
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. This keeps a house more or less structurally intact above the first floor. This shape will automatically prevent earthquake damage.
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, this means if you can keep a house on its foundation with a good seismic retrofit, then the cross walls in the living area will protect the rest of the house.
THERE ARE NO CROSS WALLS IN THE CRAWL SPACE AND ONLY ONE IN THE LIVING AREA. THIS MEANS THE CRIPPLE WALL IS WEAKER THAN THE REST OF THE HOUSE.
However, the crawl space isn’t protected by cross walls, which makes it very weak. This is why all retrofitting takes place in the crawl space.
THIS SANTA CRUZ HOUSE FELL IN THE 1989 EARTHQUAKE. NOTICE THE WINDOWS DID NOT BREAK.
This Santa Cruz house is an especially interesting case where cross walls prevented catastrophic damage to the living area. On the other hand, the garage did not have cross walls to protect it. Of particular interest is the intact row of windows at the front. This shows how strong the cuboid shape is and how it can prevent earthquake damage.
Earthquake Damage on The Inside
THIS HOUSE FELL FROM ITS FOUNDATION IN THE 1992 FERNDALE EARTHQUAKE IN CALIFORNIA. THE AUTHOR OF THIS ARTICLE WAS THE FIRST FEMA INSPECTOR TO EVALUATE IT.
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 the walls as shown here. This is because plaster, which is simply a thin layer of concrete, is so brittle. This kind of damage does not occur with sheet rock.
I was the FEMA inspector who evaluated this beautiful house in Watsonville after the 1989 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.
This is a typical tract house, often found in the newer parts of the Bay Area. Contractors built almost all of these homes according to building codes that made sure the living areas were extremely earthquake resistant. Unfortunately, even though the original builders bolted these houses, the same code failed to strengthen a very vulnerable floor connection. This weak connection could result in the house sliding on top of the foundation. These houses need No Cripple Wall Retrofits.
What Does the Building Code Say?
A retrofit expert must inspect a home for weak connections before designing a retrofit. Tables such as the one below allows you to look at the old building materials, called archaic materials, your house is made of and evaluate its current earthquake resistance. The table tells us that plaster on wood or metal lath – that means plaster on wood or stucco on chicken wire – can resist 900 lbs. of earthquake force for each linear foot. Well-nailed plywood can only resist 870 lbs. per linear foot. This further explains why living areas do not collapse. 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, and this table allows us to do so.
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.
Shear Values of Archaic Materials
Structural engineer Nels Roselund was involved in studies to prevent earthquake damage for the past 50 years. He 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 rocket ship 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 performance of rocket ships such as during take-off, wind turbulence, upward and downward drafts, and landing. Researchers could calibrate this testing equipment to create an environment almost identical to an earthquake.
Why These Tests Are Important
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 the table above, 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. When placed vertically they are called shear walls. In other words, existing houses already have shear walls made of plaster, stucco, sheet rock, and wood siding. The video below looks how these old building materials were tested in a rocket ship testing center.
