Purpose of Footings
If a man stands on soft mud, marshy ground, or quicksand, he sinks into a greater or less depth, proportional to his weight. If, however, he stands on a plank or a wooden platform, or on a post or posts driven through the mud or marsh to firmer ground, his weight is distributed over a larger area in the first case and carried down to a better foundation in the second.
The same thing is true of the footings of buildings. By spreading the load, or weight of the structure, over a larger area or bearing surface, the weight of the building is more evenly distributed, and the likelihood of a settlement, due to compression of the ground, is greatly diminished. For this reason, the higher and heavier the building is to be, the wider and deeper the supports of footings for the foundation must be and if extremely soft or yielding ground is encountered, piling should be resorted to in order to carry the weight of the building to a more solid base.
Footings may be of iron, timber or large, flat building stones laid directly on the ground or on a bed of concrete, or they may be concrete alone or with reinforcement, or of concrete and stepped-up brickwork. Where piling is used, heavy capping timbers are often placed on the heads of the piles, with either stone or concrete footings resting on them; or large footing stones may be laid directly on the piles.
Timber is often used for footing courses where a large bearing surface is necessary and can be
obtained, provided, always, that the timber can be kept from rotting. In some cases, the timber is charred on the outside and, again, it is coated with asphalt. If the ground is continually wet, there is little to fear, as timber will not decay when kept constantly saturated with water but when alternately wet and dry, unprepared timber cannot be depended on.
A good method of placing planks under walls for footings is to use 3" x 12" plank cut in short lengths and laid crosswise in the trench. A layer of plank of the same size is then laid lengthwise, followed by a third layer placed transversely. In Fig 49, the stone footing b rests on the footing planks a and carries the stone foundation wall c between the sides d of the trench.
Concrete and Stone Footings
Fig 50 shows a 20 inch brick wall b erected on a concrete footing a that is 20 inches thick and 36 inches wide. Fig 51 and 52 show concrete bases a and stepped up brick footing courses b. In Fig. 51, each step of brickwork is set back 2 inches for each course, while in Fig. 52, each step is set back 4 inches for each two courses. At c is shown, a 20 inch brick foundation wall resting on the stepped-up brick footing.
F ig 5 3 ill u st r at e s a stone footing a, composed of three courses of flat stone, each course being 8 inches thick. The top course projects 6 inches on each side of the 20 inch brick foundation wall b, and the middle and bottom courses each project 3 inches making the width of the bottom stone 3 feet 8 inches.
Fig. 54 shows a stepped-stone footing a similar to that shown in Fig 53, but supporting a 24 inch stone foundation wallb. Each base course advances 3 inches beyond the one above.
Fig 55 shown a footing consisting of a single course of stone a, 8 inches thick and 28 inches wide, carrying the stone wall b, 20 inches thick.
As a rule, concrete, when of sufficient depth and width, and when properly made and laid, make the best footing course. Concrete for footings should be made of 1 part good cement, 3 parts of clean, sharp sand, and 6 parts sharp, broken stone. In very important work, such as bridge piers and the footings of very tall buildings, chimneys, etc. a mixture consisting of 1 part cement, 2 parts of sand and 4 parts of broken stone is sometimes used. The New York building laws call for 1 part cement, 3 parts sand and 5 parts broken stone.
In localities where stone cannot readily be obtained, broken brick or terra cotta may be used in the same proportion as stone, but care should always be taken to use good, hard-burned material. Well-broken foundry slag
and scoriae, clean steam-boiler ashes from anthracite coal, and clean-washed gravel, mixed in the proportions given, also make good concrete.
Quicksand, when confined, can be safely built on. Fig. 56 shows a method of confining quicksand by sheet piling and placing concrete between the piling. In this case, the sheet piling shown at a is placed 4 feet apart. The concrete, shown at b, is 2 feet thick and extends the full width of the piling. The quicksand, through which the sheet piling is driven, is shown at c, and the 20 inch brick foundation wall, at d.
Fig 57 illustrates a footing composed partly of timber. The footing from which this was taken was placed near the water-line of a marsh in New Yorkstate, to carry a factory building 50 ft X 80 ft. and 40 feet high. The soil was stiff, black muck and at a depth of about 5 feet, water soaked sand was found. After the trenches were dug, a bedding of concrete a 12 inches thick was laid. On top of this concrete, 2 inch spruce planks bwere placed crossways followed by 8" x 8" timber c, laid parallel, with the trenches filled in between with concrete. On these planks and concrete were laid the base stones d and on top of these stones was built a 20 inch foundation wall e. The trenches on each side of the wall were filled in with sand, rammed down, as shown at f.
This factory building contains an engine, shafting, boiler, and machinery, and besides, over one hundred employees are constantly at work yet no settlement has occurred, though it has been built a number of years.
Stone-footing courses should be laid with large flat stones not less than 8 inches thick. If more than one course is laid the joints should never come over each other, as that would defeat the object of bonding, which is to tie together firmly the parts of the wall.
All stone footings should lie on their natural or quarry, beds, and all joints and spaces between the stone must be well filled with mortar. The mortar acts as a bedding between the stones, and unless it is interposed, the uneven pressure of one stone on another might cause a fracture of one and produce settlement.
All footing courses, as indeed all mason work below the ground level, should be laid in cement mortar. The unusual proportion of cement and sand for cement mortar is 1 part cement and 3 parts of sand. The proportions just stated are from the building laws of New York, and have been found suitable for general mason work.
Stepped-up brick footings having concrete and stone bases, as shown in Figs. 51 and 52 are often used. The pyramidal form of stepped-up brickwork carries the load of the superstructure more evenly to the footings and reduces the risk of settlement or fracture. Nothing except good, hard well-burned bricks should be used, and these should be laid in cement mortar, and should break joints - that is, no two joints should come over each other.
Footings on Rock and Gravel. In placing foundation footings on a rock, it is sometimes found that some portions of the footings will rest on the rock, and others, owing to the diversified character of the surface, will rest on clay, sand, or gravel. The settlement of the foundation walls - and as a necessary consequence, that of the whole building - will then be uneven, as the walls resting on the rock will not settle, while those resting on the sand, gravel, or clay, by compressing the material on which they are carried will settle.
Fig. 58 illustrates the method employed to obtain equal settlement. In a are shown the rock and gravel before leveling or excavating, the clay or sand being shown at a and the rock at b. It is customary to remove the rock to a certain level, as shown in b b. The softer solid a is the removed and leveled off, as at c c, and a bed of concrete about 3 feet thick, as shown at d, is then put down. This concrete is brought to the level of the rock, as at b b, and on this base, the brick or stone foundation wall e is built.
In erecting footings on solid rock, it is not considered necessary to cut the footing bed level over the entire surface of the rock, nor even to cut a series of horizontal surfaces resembling steps, as is frequently done in softer soils, but it is necessary to roughen the surface of the rock so as to prevent the footing from the slipping on its foundation. After this is done, concrete may be put in to bring the foundation to its proper level. If the structure is to be only three of four stories in height, stone or brick may be used instead of concrete, but a concrete base is usually preferable.
Footing on Sloping Ground. Footing courses built on slopes - especially of
clay - are always liable to slide. This tendency to slide, however, may be overcome by cutting horizontal steps in the slope, as shown in Fig 59, where the slope e f is stepped off, as shown at a, in order that the footings b may have a horizontal bearing. These footings may be either of stone or of concrete, but if the former material is used, great care must be exercised to secure a perfect bond at the stepping places, and the foundations should be laid in as long sections as possible.
Inverted Arches - When a wall is composed of isolated piers, it is well to combine all their footings into one, and to step the piers down, as shown in Fig. 60. In this figure, the concrete footing course is shown at a; the stepped up foundations of the piers, at b' and the piers resting on the footings, at c.
If there is not sufficient depth to step the foundations, use is sometimes made of inverted arches. Such arches, however, are to be avoided unless the foundation wall is from necessity very shallow, as great care is required to lay them properly, and the
slightest settlement in the arches has a disastrous effect on the piers.
The end arch of the building must have pier or other support of suffucient weight or strength to resist the thrust of the arch otherwise, the weight might throw out the pier, as shown by the dotted lines at a, Fig 61.
This difficulty, however, can be overcome by using an iron rod, with iron plates and nuts, as shown in Figs. 62 and 63, thus securing the skewbacks in place.
The inverted arches turned between the piers should be at least 12 inches thick, or should extend the full width of the piers. They should also rest on a continuous bed of concrete of proper area, and at least 18 inches in thickness or, they may rest on two footing courses of large stone, the bottom course being laid as stretchers, and the top course as headers.
Fig 62 illustrates two piers, each 3 feet square, connected by a brick-and-concrete inverted arch. At a is shown the 18 inches of concrete under the 12 inches of brickwork b. At c and c' are shown the stone skewbacks from which the brick arches spring, and at d is shown the 2 inch iron rod that ties the pier e' to the second pier e, and thus prevents the thrusting out of the end pier.
F i g 6 3 shows an inverted arch built of stone, 24 inches thick. At a is shown the stone arch maintained in position by the iron tie-rod b and at c the brick foundation piers are shown on the skewbacksd.
The best form of inverted arch is the three-centered orelliptic; next, the pointed; third, the circular; and lastly, the sequential arch.
The method of getting the lines for the centering in an elliptic arch is as follows: Divide the space shown on the line from a to b, Fig 64 into three equal parts, at d, d; then draw three circles with centers c, so that the circumferences of these circles will be tangent at d. From the center of the middle circle draw the perpendicular c f' the point f where it intersects the circle is the center of the arch from g to h. From f draw lines f g and f h of indefinite lengths through points d, d. With a d as the radius, draw arc a g intersecting line f g at g. Then, with f g as the radius, draw the rcg h' and from h, arch h d with b d as the radius.
At k is shown the brick arch, which is 12 inches deep, and at l, the concrete under it. This form of arch is used frequently in the construction of sewers.