The Problem
We live in earthquake country. Washington is the third most seismically active state, behind Alaska and California and the Puget Sound is crisscrossed with a large number of serious fault lines. Plus, there are the Pacific and San Juan de Fuca plates pushing against each other with tremendous force off our coast. Finally, there are the other miscellaneous type quakes, like those of 2001. Experts are in agreement that one or more of these hazards could go off at any time - it is really "when" not "if".
Scenario for a Magnitude 6.7 Earthquake on the Seattle Fault. Projected: 1,660 dead, 24,200 injured, 9,700 buildings destroyed, 29,000 buildings severely damaged and unsafe to occupy, 154,500 buildings moderately damaged with use restricted, 130 fires burning, All six major highways experience partial closures lasting months due to substantial damage, collapsed bridges. Utilities cut in areas with poor soils. Port facilities badly damaged, use restricted. Operations of businesses relying on "just-in-time" deliveries disrupted by collapsed supply warehouses, transportation closures, communication outages.
And What About Our Homes?
Most of our older houses were built with old growth fir and were constructed to last a long time. Plus, they have been through more than one earthquake and most are still standing. That is the good news, but there is another side to this situation. Most of these same homes are held in place largely by gravity. And, during a serious earthquake, gravity can get very iffy. As experts study the earthquakes in California and Japan, a serious trend showed up in the failures of houses. Over and over, the points of failure were between the foundation and the first floor. Where the wood structure contacted the foundation itself many houses jumped off the still solid foundation due to a lack of proper connections. Next, where there was a short pony or cripple wall, wall sections would domino over to cause the houses to crash into the side or front yard because those sections were not adequately braced. Finally, a number of houses actually broke loose at the floor level and just slid out of position. Obviously, any of these situations rendered the houses unusable without extensive and expensive repairs.
That is our problem.
Our homes built before roughly 1975, were not constructed to withstand that type of catastrophic quake. The City of Seattle estimates that almost a quarter of a million houses in greater Seattle are in need of seismic retrofitting. If we can get that done, we can cut those loss figures greatly. That is what Secure Retrofitting Company is all about.
The Solution
As we mentioned in The Problem section, our older homes have 3 areas below the first floor that are the weak links in the structural chain for the house to stay together during a catastrophic earthquake. Seismic retrofitting is designed to address each of these 3 weaknesses and bring the house up to a level that we can count on them staying put during an earthquake. They have been doing retrofits in California for over 40 years, with a great deal of success if it is done properly. We in the Northwest seem to have missed the memo that we are also in Earthquake country and so retrofitting didn't take off here until 1998-9. At that time the City of Seattle got a federal grant to assist with earthquake preparedness and created Project Impact. After much study and bringing in experts from WA and the rest of the country, they published a prescriptive plan to allow homeowners to have their house retrofitted without the need and expense of having a structural engineer custom design the work. The only problem was this was a basically one size fits all approach - great if your house matched the criteria, but no help if, like most, it did not.
Still, they were out raising public awareness and started a good work. In its simplest form, a seismic retrofit has 3 parts. The first is bolting down the wood structure to the foundation per a formula based on the size and structure of the house. Second is the bracing of any pony or cripple walls with plywood to minimize lateral movement during the quake. Finally, there is the attachment of the flooring system above to the braced pony wall or sill. It sounds simple, but as with most remodeling projects on older homes it is usually an adventure - a noisy, dusty, physically demanding, intense adventure. See below for illustrations.
Parts of the solution
- Crawlspace/Basement - The appropriately named space between the ground under the house and the floor you walk on. How much space you have to crawl in depends on how tall your cripple wall is. (see #5) The retrofit contractor does almost all of his work in this crawl space area.
- 1st Floor - This is the area you walk on.
- Foundation - This is the cement structure that supports the house and gives it stability. The primary goal of seismic retrofit is to attach the house to the foundation so it doesn't fall off the foundation during an earthquake.
- Mudsill - This is a piece of wood that rests on top of the concrete foundation. The entire house rests on it. When an earthquake hits, the mudsill can be jolted off the foundation causing the house to collapse.
- Cripple or pony wall - Not all houses have a cripple wall. If you go up a set of stairs to enter your home, you probably have one. Un-reinforced cripple walls are the weakest part of a house because they readily domino over and collapse in an earthquake.
- Floor framing - also called floor joists. In an earthquake, the floor framing can be jerked off the cripple wall, causing the floor of the house to collapse.
- Top plate - This is made of 2x4 lumber lying on top of the cripple wall. The floor framing is nailed into this.
The same structure after retrofitting.
- Bolts - These attach the mudsill to the foundation. There are several types, but basically they are long threaded steel rods that pass through the mudsill and deep into the foundation. Bolt strength is greatly affected by the type of washer that is used.
- Bearing Plates/Mudsill Plates - These are specially designed large square washers that increase the strength of a bolt by 60%. Newer homes can be significantly strengthened by merely changing the traditional washers to mudsill plates.
- Shear wall - This protects and strengthens the cripple wall. Basically, it is a sheet of plywood nailed onto the wood framing of the cripple wall to prevent it from collapsing in an earthquake. Specifics like the type of nail used, nail spacing, type of plywood used, the spacing of the framing and many other factors effect the strength of a shear wall.
- Shear Ties or A35s - These are specially designed pieces of steel hardware that securely attach the floor framing to the cripple wall. Retrofitting is technical work. Done correctly, it increases the chances of your home surviving an earthquake. However, an improperly or incompletely retrofitted home might actually suffer more damage during a quake than if nothing had been done at all. Almost any contractor can perform a seismic retrofit, but why would you trust your home to anybody but a specialist?
