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The size and strength needed for foundations depends upon a number of factors. One is the total weight of the structure including dead load (the house itself plus all furnishings), live load (the occupants and their activities), and weather (wind, snow) load that will be pushing down on the foundation, plus the weight of the foundation itself. Another is the height of the foundation walls or piers; the higher a foundation is the less weight it can handle, and the greater is its susceptibility to lateral forces. A third factor is lateral pressure, if any, that might be exerted by backfill shoved up against the outside of the foundation, and also the depth of the backfill itself.
There are a number of foundation design variables that can be controlled and matched to those factors to provide greater or lesser strength as necessary. The thickness of continuous walls can be varied, as can the width of the footings upon which they rest. Posts or piers can be made larger, set deeper, their footings increased in size, or their numbers in creased to support more weight. Also, interior piers and load-bearing girders can be added to spread the load, and other design devices can be used as well.
One way to figure out what you need for foundation strength is to do a bit of engineering and calculating with the proper tables, or have it done for you. Another is to look around, ask questions, and study the literature to see what foundation designs and arrangements have been successfully used, or would be appropriate, in your area. Those that have been used before can be used again. Note, though, that the standard designs used in frame houses might not be sturdy enough for a log house, or might not be suitable for the soil conditions at your building site. If your log house will be large, it will be heavy, and if you are con fronted with unstable soil conditions or some other peculiarity that might adversely affect your building program, further investigation and probably some professional help is in order.
Perhaps the most crucial factors in foundation design is ground loading. The entire weight of your house will rest upon the foundation. This weight plus that of the foundation itself will rest upon a few small strips and patches of ground (except for a slab, which is far more spread out). Different types of soils have different load-carrying capacities, and if you provide insufficient flotation at the footings so that the load is concentrated upon too small a ground area, subsidence of part or all of the structure is inevitable. The total area of the bottom of your footings—the base upon which the foundation walls or piers rest—must be large enough to prevent your house from slowly sinking down into the particular soil that you build on. This in turn means that you need to know two things: The weight of the building, and the load-carrying capacity of the soil where the house will be built.
To take the last first, soil load-carrying capacity is widely variable. Different kinds of soils can hold up different weights. To add to the fun, there is not only considerable variation from locale to locale, but often between nearby spots as well. How can you determine the capabilities? The most accurate way is to have soil tests made by a qualified engineering firm—not an awfully expensive proposition. If the ground seems soft or spongy, or you suspect a high water table or underground water pockets, such problems as well as other potential ones will show up in the report.
You can also bypass the test and , as is often done, use some general figures that are widely accepted (Table 4-1). A soft clay soil can handle a weight of about 2000 pounds per square foot. Firm clay or fine sand can hold 4000 pounds. Tightly compacted fine sand or relatively loose, coarse gravel will support 6000 pounds, and tightly compacted coarse sand or loose gravel about 8000 pounds. A compacted sand and gravel mixture will handle up to 12,000 pounds of weight per square foot. All these figures presuppose that the soil structure is native and undisturbed and contains no fill earth.
You can also use a rule of thumb, if you wish, which is applicable to all conditions except where the soil is obviously unstable. Keep the total weight resting upon the soil, house, and foundation together, to no more than 1000 pounds per square foot. While this will often result in some overbuilding—you’ll have more foundation than you really need—it also affords excellent stability and does no harm.
The next problem is to figure out the weight of the house. This weight is divided into two categories: dead weight, which, as mentioned, is the weight of the structure itself, and live weight, which includes the occupants and their belongings and also the wind and snow load on the roof. You can figure the weight of a building by consulting the proper tables and adding up all the weights of all the parts of the building. For logs, use the average diameters of the various members, determine the total cubic footage for each category, multiply by the approximate density of that species at air-dry weight, and add up all the results. There are tables for live occupancy load under various kinds of occupancies, and tables for wind and snow loads (which vary from one geographical area to another) that you can consult for those figures. Or, your local building department might have all of the locally-used figures on hand. Tracking down and tallying up all this information is tedious, but necessary.
Table 4-1. Load Capacity of Various Soils in Pounds per Square Foot.
Most residences are figured according to some averages that have proven themselves over years of construction experience, plus locally applicable load figures for wind and snow. One way to make your calculations is to allow 100 pounds per square foot of living area for the dead weight, plus 50 pounds per square foot for the live load of occupants and belongings. To this add the weight per square foot of the entire roof area, which is usually about 50 pounds (more for exceptionally heavy beaming or heavy coverings like slate or tile). Add also the wind/snow live load factor. Again, 50 pounds is usually adequate, but can run over 100 pounds in some places; this bears checking on a local basis.
To work out an example, a 25- x -40-foot, single-floor residence has a living area of 1000 square feet. The roof area is variable depending upon the roof style and pitch, but let’s assume it to be 1200 square feet. Multiply the floor area by 150 and the roof area by 100 and add the two answers together. The first figure is 150,000 pounds, the second is 120,000, and the total is 270,000 pounds for the full weight of the building. This is the load that's transmitted to the ground by way of the foundation walls, piers, or slab.
Taking the figures a bit further, it’s apparent that in order to keep the ground loading factor at 1000 pounds per square foot, you will have to have 270 square feet of footing area to bear upon the soil. A continuous-wall foundation for this house would contain 130 linear feet (both sides plus front and back). If the footing beneath the wall were 2 feet wide, the total bearing area of the footing would be 260 square feet. This would be satisfactory, especially in view of the fact that additional support would doubtless be provided, perhaps in the form of a girder and posts, down the centerline of the building to cut the floor joist span to 12½ feet. Additional support for the weight of the building would be gained there.
These figures can also be used to calculate the number of piers or posts needed for that type of foundation, along with the necessary size of the footings. For instance, by making each pier footing 4 feet square for a total of 16 square feet each, you would need 17 piers. The smaller the footings, the more piers you would need, and vice versa. The actual number of piers must also be balanced against the sizes and spans of the timbers used in the floor frame. Timber size, span length, and number and placement of piers all interrelate in terms of overall strength and stiffness of the structure. A span between piers of 8 to 10 feet is a good working maximum, with the other factors selected to suit.
Note that if you were positive that all of the soils at your building site were fully capable of carrying 2000 pounds per square foot, the bearing surfaces of the footings could be halved. The continuous-wall footing could be 12 inches wide, (but in any event should be double the wall thickness dimension) and the pier footings could be reduced to 2.8 feet square. Further variables might be introduced in the weight figures themselves. For instance, in your locale there might not be any snow load, and only a light wind load. If your plans call for building with 12-inch logs, the dead weight of 100 pounds per square foot of living area might be exceeded. Such differences would of course have to be taken into account.
Note too that for purposes of this example the weight of the foundation itself was not included. But wherever this weight is appreciable, as in poured concrete continuous-wall foundation which amounts to approximately 145 pounds per cubic foot, these figures should be included in the total weight resting on the soil. The weight of posts, on the other hand, would be negligible and could be discounted. Posts that are sunk directly into the ground, by the way, pick up ground loading area not only at the bottom but to a certain extent from the sides as well, through friction.
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