Foundation Contractors Branson MO
Functions of the Foundation
Considering the importance of the foundation, the dearth of attention it gets is amazing. An excavator digs a big hole in the ground; a bunch of plywood panels are assembled into a form, and the form is filled with a slurry of gray mud. For many homebuilders, the excitement of construction begins when the first piece of wood is bolted to concrete. Then, after the house is complete, we bury the concrete foundation as completely as possible and plant shrubs against it, as if ashamed to have passersby see it. In fact, the only time most of us think of our foundations is when something goes wrong. And go wrong it does. The National Association of Homebuilders reports the item most responsible for contractor callbacks is not the roof, windows or doors, but the foundation. The reason our foundations perform so poorly is that little thought is given beforehand to the functions we expect them to perform. How can we be said to have designed a foundation unless we have first specified the requirements by which we can judge its success? So here is the, no doubt surprising, list of things we expect a foundation to do: Expected Functions Fundamental:
Bear the load or weight of the house
Anchor against the forces of wind and earthquakes
Prevent vertical motion due to frost
Isolate from ground moisture Less universal, but still important:
Protect against termites
Prevent winter heat loss
Store heat in the winter
Absorb heat in the summer
Provide storage, working or living space
Contain mechanicals Bearing the Load Soils of different types exhibit a wide range of weight-bearing abilities. I'm always amazed by the properties of beach sand. Crossing the upper beach is like walking on a pile of ball bearings - your feet sink several inches into the uncompacted sand. But below the high-tide line, where it has been gently agitated and compacted by wave motion, the same sand is nearly as hard as concrete. A dirt road may be hard as the turnpike in summer, but come mud season, the same road has the consistency of pancake batter! I'm reminded of a story. A farmer was sitting on his porch waiting for the mail. He sees the mailman slogging up the road waist-high in mud. "Hard goin'?" he yells. "Not too bad," is the reply. "But pretty hard on my horse." These examples illustrate two of the situations that must be avoided if your foundation is to bear its weight (load) for a lifetime: uncompacted earth and wet soil. A third situation to avoid is soil that contains vegetation. Undisturbed soil beneath the frost line has been settling and compacting for thousands of years. Soil disturbed during excavation should be carefully removed before placing the foundation. If that proves impossible, then the disturbed soil should be compacted either by vibration or by flooding with water. Soil above the water table (the level where the soil's interstices are saturated with water) can generally be dealt with through proper drainage. Unless your site is worth hundreds of thousands of dollars (like the Back Bay in Boston), reject filled areas and sites with saturated soils. The problem with organic material in a soil is that it is likely to continue decomposing, ultimately leaving you with just 1 percent solid matter and 99 percent void. If the organic material can't be removed, the foundation has to reach through it to more solid ground. Building codes require you to either test the soil-bearing capacity of your site or to assume the bearing capacities listed in the table in Illustration 1. Actually, as the following example will show, it's very easy to keep building loads smaller than the numbers in the table. In addition, soils loaded to these values are expected to compress a small amount. However, since even the smallest amount of non-uniform settling can lead to cracks in rigid concrete foundations and plasterboard walls, distribute the loads evenly and limit them to a conservative 1,000 pounds per square foot. Illustration 1 shows the design (maximum expected) loads on a 1,000-square-foot, single-story house in much of the northern United States.
1) The snow load is the maximum weight of snow expected once in 50 years. Of course, it might be higher in the mountains and lower farther south. Every architect in town knows what it should be for the area. Note that the total weight is computed over the floor area rather than the larger area of the sloped roof, since no more snow falls on the roof than on the projected area of ground beneath.
2) The roof dead load is that of its materials - altogether about five pounds per square foot unless covered with an unusually heavy roofing material, such as slate or clay tile.
3) The ceiling dead load is the weight of the ceiling joists and finish, and doesn't include any junk stored in the attic. If you plan on finishing the attic for bedrooms or storage, add a live load of 30 pounds per square foot.
4) The live load is the weight per square foot imposed by people living in the house, their furnishings and, occasionally, their rowdy, party-loving friends. Residential building codes specify 40 pounds per square foot for the first floor. That's equivalent to a waterbed or one partying friend per four square feet.
5) Wood walls actually weigh only two to three pounds per square foot, but the total of exterior and interior wall surface areas tends to be about twice that of the floor area, so assume five pounds per square foot spread over the exterior walls. Of course, it would be much greater for masonry walls.
6) A carpeted or wood-finish floor weighs around five pounds per square foot, but round up the floor dead load to 10 pounds per square foot to allow for areas of heavier tile.
7) The basement wall, if masonry, can weigh as much as the rest of the entire house, including a maximum snow load and maximum number of friends. By itself, an 8-foot-high poured concrete wall exceeds the recommended 1,000 pounds per square foot. The solution is to spread the load over wider footings at the base of the wall.
8) Finally, there may be a masonry chimney - in this example, a 24-foot-high, two-flue brick chimney. The weight of this chimney per square foot is even greater than that of the basement wall, thus violating the second rule: uniform distribution of loads. The solution for excessive and/or uneven loads is load-spreading footings: Like snowshoes, they provide greater area to prevent sinking into the earth. First add up all the weights to be carried by the foundation footing (205,000 pounds in our example). Then divide by 1,000 pounds per square foot to get the required area of footing (205 square feet). Next divide this footing area by the perimeter length of the footing (25 feet + 25 feet + 40 feet + 40 feet = 130 feet) to get the required width of the footing (about 1.5 feet, or 18 inches). Turning next to the chimney, divide its total weight (6,000 pounds) by 1,000 pounds per square foot to get the required area of its own separate footing (six square feet). So an equivalent footing for the chimney would be 2 by 3 feet. Anchoring Against Wind Now that we've taken care of holding the building up, let's consider holding the building down. Unless you have been in a category 4 or 5 hurricane or a tornado, it may be hard to visualize a flying building. First of all, we're not talking here about a breeze; we're talking about big wind, the kind of wind the old-timers talk about from 50 years ago, the kind of wind that knocks you off your feet and uproots ancient oak trees. We're talking about air moving at 100 miles per hour or more. Second, we're talking about an empty house (take away the live loads) with no snow on the roof (deduct the snow load) that is possibly disconnected from its foundation (deduct the weight of the basement walls). What we are left with are the dead loads (weights of construction materials), in the above example, a weight of just 25,000 pounds. The maximum force of the wind against building surfaces in an exposed location is usually taken to be 20 pounds per square foot. Our example house has an exposed height of about 16 feet, from the top of the foundation to the top of the roof, and a width of 40 feet. The area of the largest vertical projection is thus 640 square feet. At a wind pressure of 20 pounds per square foot, we find that the total force of the wind against the house could be as much as 640 square feet times 20 pounds per square foot, or 12,800 pounds - comparable to the weight of the building. The building could be pushed off its foundation in such a wind. Much to my chagrin, this happened to a summer camp I once owned. The Maine natives were vastly amused. I had been replacing the camp's wood-post foundation and had returned to Massachusetts with the job unfinished. One of the locals called me at my office and said, enigmatically, "You might want to look at your cottage. Had a bit of a blow the other night." The building had been blown 12 feet off its foundation. Not a window was broken, but you should have seen what the chimney did in falling through the roof and two floors! If the wind gets under the house it might lift and flip the house over. This way, the falling chimney would spare the floors but take out a wall or two. Also, it is very common for a roof to lift off if it's poorly connected to the wall and its other half. It is thus clearly advantageous to include the weight of the foundation in the total weight of a house.
This we can do, of course, if we take care to bolt the two together securely. You'll notice that some of the foundations we'll be considering in the next issue don't weigh very much, but neither does a fence post, and have you ever tried to pull a fence post out of the ground? The ground exerts tremendous lateral pressures on things in its embrace, so embedded lightweight foundations work just as well as the more massive versions we are accustomed to. Accepted practice is to bolt the building's sill to masonry foundations with one half-inch anchor bolt every six feet and two bolts within one foot of each corner. Pier foundations require at least one half-inch bolt per pier. Wood foundations are secured by nailing the sill of the building to the top of the wood foundation wall. Preventing Frost Heaves Water expands upon freezing. That's all we need to know to anticipate trouble when the ground freezes under a building. Since the expanding ground has nowhere to go but up, up goes the building, too. Sometimes the results are merely annoying, like discovering the front step a bit higher than where you last remembered it. Others can be more serious, like shearing off the incoming water pipe that you safely buried beneath the frost to prevent freezing. Mobile homes without below-frost foundations are connected to water and sewer through flexible pipes for this reason.
There are three ways to prevent frost from lifting your house:
1) Make sure there is no water in the soil to freeze in the first place. Provide drainage around the building by surrounding the foundation with gravel and by lowering the water table to below the footing with drainage pipes (more on that in the next issue).
2) Make sure the foundation rests on footings that are beneath the depth of maximum frost - see Illustration 2. If you have any questions about the depth of frost in your area, ask the building inspector or a local foundation contractor.
3) Prevent the cold from penetrating the ground beneath the building. A layer of waterproof insulation (extruded polystyrene) placed horizontally one foot below the surface - see Illustration 3 - will retard the loss of heat from the earth and block the frost under a slab on grade. Note that termites have been known to sneak into houses between the foam and the foundation. Fortunately, areas where this technique is most useful (those with more than 6,000 heating-degree days) are too cold for termites. Isolating the Building from Ground Moisture Water seeks its own level (Illustration
4), which is to say that if the ground outside a basement is saturated with water, the water will attempt to flow into the empty space of the basement until it reaches the same level inside and out. How hard will water try to get into the basement? With a pressure equal to the weight of the column of water above the basement floor. For example, assume it is spring and the ground around a building is saturated to the surface six feet above the basement floor. By Archimedes' principle, the upward pressure of water under the basement floor is the weight of a 6-foot column of water - 6 feet x 62.4 pounds per cubic foot, or about 375 pounds per cubic foot. The solution is not caulking and painting the basement as if it were a boat. If you, by some miracle, succeeded, you might also succeed in launching your house like a boat, since the total uplifting force of 375,000 pounds on the 1,000-square-foot basement floor exceeds the total weight of the house and its foundation. More likely you'd simply buckle the basement floor. Instead, the solution is to purposely lower the water level around the house, either by building into a hillside (the walkout basement) or with perforated drainage pipes. These pipes are placed around the outside of the basement wall footing, below the level of the basement floor. They slope and drain away to an even lower point on the site, to the city sewer or to a sump pump in the basement. Porous, sandy or gravelly backfill around the wall ensures that the water table immediately around the basement is at the level of the drain - in other words, lower than the basement floor. Other Functions We'll cover the issues of termites, heating, cooling, livability, and utilities in the next installment when we consider the full foundation and alternative solutions.