58. For buildings of moderate height timber may be used for giving the necessary spread to the footings, provided water is always present. The footings should be built by covering the bottom of the trenches, which should be perfectly level, with 2-inch plank laid close together and longitudinally of the wall. Across these heavy timbers should be laid, spaced about 12 inches from centres, the size of the timbers being proportioned to the transverse strain. On top of these timbers again should be spiked a floor of 3 inch plank of the same width as the masonry footings which are laid upon it. A section of such a footing is shown in Fig. 20.
All of the timber work must be kept below low water mark, and the space between the transverse timbers should be filled with sand, broken stone or concrete. The best woods for such foundations are oak, Georgia pine and Norway pine. Many of the old buildings in Chicago rest on timber footings.
The size of the transverse timbers should be computed by the following formula:
Breadth in inches = 2 X w X p2 X s / D2 X A ...............(3) w representing the bearing power in pounds per square foot; p, the projection of the beam beyond the 3-inch plank in feet; s, the distance between centres of beams in feet, and D, the assumed depth of the beam in inches. A is the constant for strength, and should be taken at 90 for Georgia pine, 65 for oak, 60 for Norway pine and
55 for common white pine or spruce.
Example I. - The side walls of a given building impose on the foundation a pressure of 20,000 pounds per lineal foot; the soil will only support, without excessive settlement, 2,000 pounds to the square foot. It is decided for economy to build the footings as shown in Fig. 20, using Georgia pine timber. What should be the size of the transverse timbers ?
Answer. - Dividing the total pressure per lineal foot by 2,000 pounds, we have 10 feet for the width of the footings. The masonry footing we will make of granite or other hard stone, 4 feet wide, and solidly bedded on the plank in Portland cement mortar. The projection p of the transverse beams would then be 3 feet. We will space the beams 12 inches from centres, so that s = 1, and will assume 10 inches for the depth of the beams. Then by formula 3, breadth in inches = 2 X 2000 X 9 X 1 / 100 X 90 = 4, or we should use 4"X10" timbers, 12 inches from centres. If common pine timber were used we should substitute 55 for 90, and the result would be 6½.
60. When building on quicksand it is often advantageous to lay a floor of 1-inch boards in two or more layers at right angles to each other on which to start the concrete footings.
When temporary buildings are to be built over a compressible soil, the foundations may, as a rule, be constructed more cheaply of timber than of any other material, and in such cases the durability of the timber need not be considered, as good sound lumber will last two or three years in almost any place if thorough ventilation is provided.
The World's Fair buildings at Chicago (1893) were, as a rule, supported on timber platforms, proportioned so that the maximum load on the soil would not exceed 1¼ tons per square foot. Only in a few places over "mud holes" were pile foundations used.
The platform foundations consisted of" 3-inch pine or hemlock planks, with blocking (transverse beams) on top, to distribute the pressure from the loads uniformly over all the planks and to furnish support for the posts which carry the caps supporting the floor joists and posts of the building. The blocking was well spiked to platform planks and posts, and caps and sills drift bolted."
Fig. 21 shows the general arrangement of the blocking under the posts.