The subject of foundation construction is such an extensive one that it is impossible to consider it exhaustively within the limits assigned to this book. It will be evident to all, however, that the design of foundations for the great majority of shop buildings is not a difficult problem, since the site selected for them will usually be in a location free from water and treacherous soils.
For the outside lines of columns either a continuous foundation wall, if the columns are close together, or, individual piers, if the columns are widely separated, may be employed. In either case the foundations must have ample area to distribute the loads over a sufficient area of foundation bed to ensure safety from settlement. The bearing power of different soils is given in Table V.
If the building is large and any doubt exists as to the nature and quality of the foundation soil, soundings should be made and the bearing power tested by placing weights on a small known area. The bottom of the walls should always be carried to a sufficient depth to make certain that the original bed soil is reached. A few layers of wet sand or gravel placed in the bottom of the excavation, filling it from side to side, and thoroughly rammed will help to distribute the pressure evenly. The wall or piers should have two good footing courses and the projection of each course beyond the one immediately above should be so small that the lower footing will not be cracked by the bending strain from the load above. Each column should rest on a cut stone cap except where the load is so small that the foot of the column may rest directly on the regular masonry without danger of crushing. The usual safe load for stone is 250 lbs. per sq. in. and for brick is 125 lbs. per sq. in. In the opinion of the writer hard brick or concrete are superior to stone for small foundations on account of their better bond.
Table V. Showing Supporting Power of Various Foundation Soils in Tons per Square Foot.
3 to 30
Dry clay in thick beds.....
Gravel and sand well cemented.......
Cland and dry sand......
Quicks and soft soils....
For very light loads a wooden box may be set in the ground and filled with concrete, the column base resting directly on the concrete or on a thin layer of cement mortar covering the top of the concrete. In special cases of heavy loads on soft soil a grillage of concrete and I-beams or of concrete and railway rails will enable the load to be distributed over the requisite area with a saving over masonry.
Where there is a tendency toward overturning, the column bases should be anchor-bolted to the foundation masonry. Generally the anchor bolts should extend through the masonry and be fastened on the underside. These bolts are set in position by means of wooden templates and the masonry is built up around them. In some cases a small plug anchor set in the capstone, with sulphur or lead, will provide sufficient anchorage.
It is the practice of the writer in designing wall columns for buildings to consider the same rigidly fixed at the base, provided there is sufficient load on the column to hold it down. In some cases even though the load may be considerable, if the post is small there is still a liability to pin ended action.
In the construction of floors as in other parts of the building the requirements of each case will determine the design and construction to be adapted. A very solid floor is made as follows: The soil is excavated to a death of about 18 ins. and leveled up. Upon the bottom of this excavation is placed a 6-in. layer of broken stone which is thoroughly rammed and then covered with a layer of concrete 8 ins. thick. After the concrete has set it is covered with a wearing surface of cement 4 ins. thick. A combination of asphalt, Portland cement and sand makes a good wearing surface. Fig 11 shows a section of this floor.
Fig. 11. Concrete Ground Floor Construction.
Asphalt floors are becoming very popular where small cost is not the chief consideration. Rock asphalt is limestone impregnated with from 8% to 17% of bitumen. It is found in many localities, but the principal workable deposits are at Limmer in Germany, Neuchatel in Switzerland, and at Seyssel in France. Less well-known deposits exist at Ragusa in Sicily, near Santa Barbara in California, and in Kentucky, Colorado, Utah and New Mexico. For shipping the rock is usually made into asphalt mastic in the following manner: The rock is ground into powder and heated in kettles with 8% of Trinidad asphaltum added to prevent burning. The mixture is heated to a temperature of 350° and kept at that temperature for about five hours, being constantly stirred the whole time. The next step of the process is to mold the mixture into blocks weighing from 50 lbs. to 60 lbs. each. These blocks as purchased in the market always have the name of the mine from which they come plainly stamped on them. When marketed the mastic should contain 14% of bitumen and 86% of carDon-ate of lime.
To prepare the mastic for flooring it is mixed with Trinidad as-phalt and sand in the following proportions: Mastic blocks, broken,
60 lbs.; Trinidad asphalt, 4 lbs.; fine gravel and sand, 36 lbs. This mixture is heated for about five hours at 4000 F., and is constantly stirred during the heating. At the termination of this heating the material is taken out of the kettles and spread.
Fig. 12. Asphalt Floor with Concrete Foundation.
For a mill floor the asphalt should be spread I in. thick on a foundation of concrete or on boards. The concrete foundation should be 3 ins. or 4 ins. thick, and if boards are used they should be covered with a layer of sheathing paper before the asphalt is placed. Fig. 12 is a section of asphalt floor having a concrete foundation and Fig. 13 is a similar section with a foundation of wood.
Any composition of coal tar becomes useless in a short time on account of the evaporation of the tar which causes the material to disintegrate and crumble away. Felt saturated with coal tar becomes brittle and finally useless. The oils of asphalt, however, are not volatile at any natural temperature, and hence property prepared asphalt flooring composition remains absolutely unchanged during years of exposure to the air and sunlight. Other important advantages of asphalt for flooring are that it is impervious to water and is so elastic that cracks do not develop. An asphalt floor has no joints to accumulate dirt and can be easily and thoroughly cleaned. It is pleasant to walk on, not tiring the feet as do stone blocks or flagging. It is not worn away by traffic as are stone blocks, but is simply compressed. Asphalt flooring costs 16 cts. per square foot when laid 1-in. thick, the cost running higher or lower according to the location and size of the floor.
Fig. 13. Asphalt Floor with Wood Foundation.
There are many imitations of asphalt made of coal tar and crushed limestone which it is almost impossible to distinguish from the genuine article, but none of these imitations has the properties of asphalt. These imitation asphalts will all crack and crumble after a few years' service.
Asphalt is softened and finally destroyed by oil and it cannot, therefore, be recommended for floors subjected to oil drippings from machinery and materials.