FOUNDATIONS AND EARTHQUAKE RETROFITTING
There are two basic types of foundations. First we have the raised foundation. This foundation consists of concrete (and sometimes brick) and is like a short wall that encircles the footprint of the building and acts as a base for the exterior load bearing walls. It is called a raised foundation because typically one can actually crawl beneath it. To prevent floors from sagging in the middle, there are usually one or more intermediate foundations that intersect the footprint of the building. The other kind of foundation is a slab foundation. The slab foundation is not nearly as interesting as a raised foundation. It is just a concrete pad for the whole building to sit on. Oh sure its not that simple as it has more reinforcement and thickness under load bearing walls but no attacking raccoons or deadly electrical fire hazards from field modifications. There are a few moisture vapor issues but that will be discussed at the end of this section.
The best way to describe a raised foundation is to build one verbally. In new construction concrete forms are made from wood form boards. Those are wood members that act as molds for the concrete pour and will determine the actual shape of the foundation. Once the foundation is poured, the wood form boards are pulled off. Form boards will not be shown in the illustrations. If one were to take a foundation and cut a cross section of it, it would like a thick upside down “T”. The wide part of the “T” is the footing and the vertical portion of the T is the stem wall. Without a thick footing the stem wall just cut into the soil like a knife. The footing reduces excess settlement. When looking at this cross cut one will even see evidence of reinforcement bar. These are steel bars about 20 feet in length connected end-to-end to each other to add more strength to the concrete along the stem walls and the footing.
This foundation is poured all around the perimeter of the building. To keep the middle of the floor diaphragm from sagging, we also install another middle foundation or a series piers along the middle of the building that support the floor. When piers are used, they typically consist of concrete footings, wood posts, and an “under the floor beam” that we like to call a girder.
After the foundation is poured and the form boards are removed, we place a piece of wood across the top of the foundation. This portion of the foundation is referred to at the mud sill. Embedded bolts, big square washers called sill plates, and nuts, bolt that piece of wood on to the foundation.
Once the sill plates are added, a cripple wall is built. This is a short section of wall between the foundation and the first floor that is built to assure a level floor diaphragm. Keep in mind, terrain where foundations are poured, are not necessary level. The cripple wall (sometimes referred to as a pony wall) gives one the chance to establish a level floor diaphragm despite the level of the terrain the foundation is poured upon.
From there the floors, walls, and roof framing are built and we have a house. Older homes with raised foundations often are not bolted or the bolts are small and less functional than ones in newer construction. In earthquake country foundations not only need to be bolted to the foundation, the cripple wall needs to be made stronger. Upon making the cripple wall stronger, this strengthened cripple wall must be well attached to the mudsill that is bolted and to the floor diaphragm it supports.
In the above graphic, the mudsill is bolted and have large square washers to prevent the sill from coming off by breaking round the bolt nut during an earthquake. The cripple wall that has been made stronger by nailing plywood to it is attached to the foundation with blocking. Wood blocks (typically two-by-fours) are nailed to the mudsill between the studs of the cripple wall so the plywood can later be nailed into the blocks. This creates a positive connection between the plywood strengthening and the mudsill. Above the strengthened cripple wall, the floor joists are attached to this system with connectors that are nailed into both the floor joists and the cripple wall. There are several ways to do this, in the graphic we are showing a Simpson-Tie® produce called an H-10 connector.
A slab foundation it typically a monolithic concrete pad that is steel reinforced. The concrete is thicker and deeper into the earth wherever a load bearing wall is going to rest upon that slab.
Notice the vapor retarder in the above graphic. Sometimes sand is placed over the vapor retarder so when the concrete is poured moisture from the cement can dry on both sides more evenly, which prevents the slab from curling or bowing. If you place a wet dish sponge on the kitchen counter, you’ll notice that when it dries it curls up a little. This is because the sponge is drying on one side faster than the other (which is the side resting on the counter). The problem with this method is that a reservoir of moisture is collected between the vapor retarder and the concrete slab and can result in moisture vapor passing through the floors. This could damage wood flooring placed on the slab. Better methods to prevent curling and moisture capture include using stronger rebar without the sand on top of the vapor retarder as well as better concrete water mix. If putting floors on a slab where it is unknown how it was constructed, consult a qualified flooring expert who knows about this issue and knows how to treat the slab surface before applying the floors.
