This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
56. It is often found that compressible soils, even alluvium and soft clay, will bear from 1 to 2 tons per square foot with but little settlement, and under a steady load, this settlement is in most cases uniform. It is very often cheaper, therefore, to spread the foundation over a large area, than it is to drive piles.
In Chicago, for example, the subsoil is of blue clay, found from 5 to 8 feet below the street grade. This clay bed, when below the level of ground or drainage water, becomes quicksand or blue mud, and has a bearing capacity of only a little over 1 ton per square foot, so the heavy weights of the high buildings are carried on spread foundations. This plan is usually adopted in the Chicago soil, because the quicksand, or muck substratum is of great depth, and piles driven through the stiff upper stratum sink 2 or 3 feet at every blow of the hammer, and are held solidly only at the top and the bottom, if that is reached.
57. Spread footings may be built either of concrete with iron tension rods, or with a base of I beams, or railroad iron, bedded in concrete; and in some cases, the footings are composed of timber and concrete. In cases where the requisite height can be obtained, concrete spread foundations, composed of Portland cement, with twisted iron bars for tension members, have many qualities to recommend them. These footings are not expensive and are very durable, as the iron or steel is so embedded in the concrete that it cannot rust, and hence there is no possibility of deterioration. Besides, the entire tensile strength of the rods is utilized, and as they are held continuously along their entire length by the concrete as a screw is held by the nut, they can neither draw nor stretch unless the concrete extends also.
58. Fig. 18 shows the section of a concrete and twisted-iron footing, where a is the concrete base, and b the twisted-iron tension bars, running both longitudinally and transversely. The base stone is shown at d, and the brick pier or superstructure at e. In building these footings, a layer of concrete, made in the proportion of 1 part cement, 3 parts sand, and 5 parts stone, should first be laid from 3 to 6 inches thick, and the first tier of longitudinal bars laid on this and tamped down. The transverse bars are then laid, and another layer of concrete, 4 inches in thickness, should be spread; as many courses as are necessary-are thus laid.
Footing courses laid in this manner, 8 feet square, 2 feet thick, with stone footings 4 feet square and 16 inches thick, the bars placed 6 inches from centers and made of 1 inch square iron, will carry 60 tons to the square foot.
This form of construction has been patented, and the rights are owned by Ransome & Smith, of New York and Chicago. When twisted bars are used, a royalty must be paid, but even then it is a cheap footing when the ultimate strength obtained is considered.
59. When buildings are on solid ground, it is claimed that steel or iron footings are cheaper than masonry. Owing, however, to the danger of rusting in steel, it is doubtful if steel footings are as durable as those composed of masonry.
In preparing the footings for laying steel beams, the bottom of the pier must first be located, and the ground carefully leveled. When the ground is of soft material and the sides of the excavation are in danger of falling in, heavy planks or timbers should be set up and fastened together at the corners, to hold the concrete in place and prevent its spreading before it is thoroughly set. Portland cement concrete, made of 1 part cement, 2 parts sand, and 4 parts broken stone, should then be laid in layers, of from 6 to 12 inches thick, according to the weight on the footings. If this concrete bed is 12 inches thick, it should be made in two layers. On the concrete, the iron or steel beams should be bedded in mortar made of 1 part Portland cement and 2 parts sand, so as to make them level and in line with each other.
All iron or steel beams should be thoroughly cleaned with wire brushes, and when absolutely dry, painted with metallic paint, or heated and coated with two coats of hot asphalt. The beams should be very carefully examined before covering them with the concrete, and if any of the paint or asphalt has been scraped off, the coating should be renewed. Every possible means should be used to keep the beams from rusting, for when unprotected, they rust very quickly.
60. The I beams may be variously spaced from 10 to 20 inches between centers, according to the height of the beams, thickness of concrete, and weight per square foot of superstructure, and should be held in place, relative to each other, by means of separators and tie-rods. They should not be spaced so far apart as to crush through the concrete, and there should not be less than 2 inches of space left between the edges of the flanges, so that the concrete filling may be placed between the beams. This concrete filling should be made in the same proportion as the concrete bedding, but the stone used for the aggregate must not be larger than will go through a 1 1/2-inch ring, and the concrete should be well rammed so that no voids will be left. It should also be carried not less than 3 inches beyond the sides and ends, and kept in place by planking or timbers.
61. If there is more than one tier or layer of beams, the top of each layer is sometimes leveled, after the cement has been rammed in place, with cement mortar, made of 1 part Portland cement and 2 parts sand, laid about 1/2 inch thick over the top of the highest beam, and on this the next layer of beams should be laid.
Some writers suggest that two thicknesses of tarred felt be laid in hot asphalt on top of the concrete before the beams are laid, and on this 1 1/2 inches of 1 to 2 cement mortar, on which the beams should be placed. The same authorities also recommend that the whole exterior of the footing be covered with two coats of hot asphalt put on over the cement.
The iron base plate or stone footing should be bedded in about 3/4 inch of cement mortar. After this is set, the whole of the beam footings, top, sides, and ends, should be covered with at least 3 inches of cement and plastered with Portland cement mortar made in the proportion of 1 to 2.
62. In some cases, iron or steel rails are used for footings, and are cheaper than I beams. The footings are built up with from three to six layers of rails placed at right angles to each other. As each layer of rails is laid, concrete is filled between and around them, and when finished resembles a concrete pier.
Fig. 19 shows a footing and pier built in this manner. At a is shown the steel or iron rails with concrete between them; b is the base stone supporting the brick pier c; while e is a bond stone in the pier, and d is the concrete base. In this case, each layer of rails diminishes in length and number, until the area of the top layer does not greatly exceed the size of the pier base, the footing thus assuming a pyramidal form.
63. An example of the use of I beams is shown in Fig. 20. The 10-inch I beams b run transversely under the walls and are supported by the layer of concrete a, 12 inches thick. At c is shown the concrete on top of the I beams; at d, the stone-footing courses of the foundation; and at e, the foundation wall.
64. When spread foundations are built under isolated piers, especially when they support iron columns, a somewhat different form of construction is usually adopted.
Fig. 21 shows the arrangement of I beams under a pier. At a and b in the plan (a) are shown the I beams, and at c the foundation. The upper tier of beams b is laid transversely on the lower tier a, and the stone footings c carry the 3-inch iron plate d under the column.
At (b), Fig. 21, is shown a section taken on line l-m on the plan (a), where the concrete base, on which the lower tier of I beams rests, is shown at e, the lower tier of 10-inch I beams at a, and the concrete around the upper tier of beams at f; b is the upper tier of 10-inch I beams shown in section, and c, the stone-footing courses of brick pier under the iron plate d.
At (c), Fig. 21, is shown a view of the pier taken on line n-o, on the plan (a), which represents the lower tier of I beams in section, and the upper tier in elevation. The 12-inch course of concrete is shown at e; the lower tier of I beams at a; the upper tier of I beams at b; the concrete covering the beams at f; the base stones of the pier at c; and the iron plate under the iron column at d.