10 May Special Retrofits Methods for Hillside Homes
HOWARD COOK: Many people in the Northridge Earthquake were, in fact, killed in these homes, and you’re going to see why. You’re also going to see what you can do to make sure this kind of damage does not occur to your home or to the homes of your friends and neighbors. The information that you’re going to see here about how to retrofit a hillside home has actually been available since 1995, when the City of Los Angeles developed two building codes, one for retrofit of hillside homes, and one for new construction.
However, these codes never made it into the California building code. So as of today, you can still retrofit a hillside home with plywood. You can also build a brand new house with plywood, even though this earthquake, the Northridge earthquake, proved that plywood does not work. It is the views that attract people to hillside homes, such as this wonderful view of the Los Angeles basin. Notice the police tape that is blocking off access to the driveway. If you walk down the driveway a little bit and look down the hill, this is what you would see. This is a typical hillside home that you can see all over Oakland Hills, Berkeley, San Francisco, and all over the Bay Area.
This presentation will be discussing this type of home. This is a hillside home. But before the house was built, the ground was leveled flat and the house was built on this flat piece of ground. These houses are retrofitted in the same way as any house that’s built on level ground. The manner in which rectangular shear walls react in earthquakes is predictable. As you can see, this shear wall deflects equally along its whole length when the earthquake pushes against the top of it. This is because the stiffness of the shear wall is uniform. On step foundations and hillside homes, the shear walls have varying heights. So for example, towards the uphill side, where we see Panel A, it is quite short. And it’s also much stiffer than the Panel B. And Panel B, in turn, is stiffer than Panel C.
This can cause a problem, because earthquake forces travel to the stiffest element, such that when the earthquake force hits, it will be absorbed by Panel A. And then once Panel A fails, Panel B will come into action. And when it fails, Panel C will absorb all the force. Which means that any one time, we have a limited amount of shear wall resisting an earthquake. This is an example of what can happen to a step foundation. The segment of foundation to the left supported a very short segment of plywood. The second of foundation all way to the right, towards the downhill, supported a very tall segment of plywood. The same sort of thing happens when we have a sloped foundation.
There are a lot of these in the Oakland Hills. Here’s an example of what happened to a house that was built on a sloped foundation. Here’s another home that suffered severe earthquake damage that had a sloped foundation. This is the kind of damage that occurs to homes built on steep hillsides. People are killed in homes like this, and therefore, it is very important that all of us protect our neighbors and ourselves from this kind of damage, and implement policies to make sure homes like this are retrofitted properly. This is the next door neighbor’s house. Fortunately, this woman was not home when the earthquake occurred, and she was able to come back and salvage what few possessions she had left. Hillside homes must resist forces that are attacking it in two directions. One force, represented by the blue line, tries to push the house down the hill. The other force, represented by the red line, tries to push the house along the hill. When the earthquake tries to push the house down the hill, the upper edge of the floor can bow in the middle, which can cause the floor at the uphill side to fall off the foundation. This is a structural weakness that any hillside home retrofit must address. When the floor pulls away from the uphill foundation, it can slide off the mudsill and even break it. Once this happens, it can lose vertical support and fall.
Often the floor is attached to a ledger. A ledger is a piece of wood that is bolted to the side of the foundation, and then the floor is nailed to that. If the floor pulls away from this ledger, obviously, the floor can fall. Here is a photograph of a house that is about to pull away from the ledge and fall down the hill. Here’s a closeup of the ledger connection. If this floor had moved another inch or two, this house would have been destroyed. Here’s a photograph of a house where the ledger was torn completely away from the uphill foundation. This is what’s known as a secondary anchor. A secondary anchor is designed to keep the center of the floor from pulling away from the foundation. This is another secondary anchor that’s doing the exact same thing. Rather than using plywood on sloped foundations, large steel braces like this are used.
These are called primary anchors. Here is a primary anchor that was used on a step foundation. The principle behind their use is that rather than going into plywood, which is not stiff enough, all the force goes into this very stiff steel element. I imagine you believe me now when I say that hillside homes can kill you, and that this is a very serious public safety issue. This is something I’ve been very worried about for quite some time. And I was hoping and I did my very best to make sure something was done so that people were protected from this kind of serious damage. And so they weren’t killed. Anyway I want to relay a personal story. It sort of exemplifies why we don’t have a retrofit building code for any type of home, and especially, why we don’t have a retrofit building code here in the Bay Area for hillside homes.
In approximately 2005, in fact, it was 2005, the Mayor of Berkeley appointed me to be a member of the Berkeley Disaster Commission. One of the reasons being is he asked me to retrofit his house. I crawled under his house, and like so many houses, his retrofit was done all wrong. Completely wrong. One of the worst I’d ever seen. And when I told him about it, I said, well, this was common. This happens all the time. And he said, well, you know what. You could probably be on our Disaster Commission. I think maybe you could help the city out. I agreed. I was very eager, very happy that I could do something. And I did what I could to disseminate the information you’re getting here. Now, when it came to the hillside homes, I did go to the commission, and I talked to the commission, and I showed them the presentation that you have seen. And I told them there’s a building code that’s already been developed in Los Angeles. All we’d have to do is just adopt it. There’s nothing to it. There’s just two of them, new construction and retrofitting.
Let’s just adopt it and we can protect the community. What the Disaster Commission did, and I was actually there, we went to the building official. We explained everything to the building official. I actually personally showed her the presentation you saw right now. And so what she did was she formed a committee. Now, after three years, the committee of volunteers– who knows who they were.
But of course, it dissipated, and eventually nobody showed up and nothing happened. Now, currently I belong to the El Cerrito Disaster Preparedness Network. And at a meeting the other day when I was there, I told them the story and they said, oh yeah, we’re used to that. Death by committee. So anyway, this is what happens. The reason there’s no code, people have to have a committee, they all get together, they don’t do anything. Just like this happened with the Berkeley Disaster Commission.
So you need to protect yourself. And if you are worried about your neighbors, and you’re worried about yourself, and you’re worried about people getting ripped off in the retrofit world, please do what you can. Talk to your city councilman, talk to your mayor, talk to whoever you can. Please disseminate the information I’m giving you here, especially about hillside homes, to protect the community.