|Home | Fire
Safety | Skyscrapers
Home Emergencies | Glossary
It’s an old perhaps trite, but true saying that: “A house is only as good as its foundation.”
A foundation consists normally of two components: the footing(s) and the foundation wall(s). Footings are generally of poured concrete. The foundation walls are usually of poured concrete or concrete block; stone is found occasionally in the walls of older houses; brick is also occasionally used. There are even foundations built of specially treated wood, but these are more experimental and rare, and as such will not be further discussed herein.
Footings are the structural elements which receive loads from all of the other portions of the building (walls, floors, partitions, roofs, etc.), and transmit or spread those loads uniformly to the soil. Different types of soils have different load carrying capabilities, technically called load bearing capacity. Solid bed rock has the highest load bearing capacity, varying from a low of 2 tons per square foot, increasing to very significant figures depending on the type of rock. By contrast, clay or silty clay may have a load bearing capacity as low as 1/2 ton, or less, per square foot. For major structures with significant loads, such as multi story buildings, an extensive sub-surface investigation and testing procedure is required to investigate, analyze and determine the soil characteristics and its load bearing capacities. These extensive investigations are not usually undertaken for very light buildings such as the average residence; however, some investigation must be conducted to categorize the type(s) of soil to be encountered, and to ascertain if any serious problem areas exist, such as high water tables, very poor quality soils, extreme differences in kinds of soil, presence of fill, rubbish or land-fill dump areas (yes, it does occur). A common method of exploration for residential construction is to dig open pits at least to the depth of the deepest expected footing levels and have a soil expert visually examine the soils encountered. Oftentimes, samples will be taken for laboratory analysis to further assist in the determination of soil suitability. In any event, the builder must know by some exploratory basis, what he is expecting to encounter for sub-surface conditions. His responses to some simple inquiries on your part should confirm his basis, and dispel any doubts. Building codes will assign specific (and usually very conservative) load bearing capacities to be used for various soils if testing is not undertaken.
(CAUTION) Footings must be deep enough into the earth to be below the levels of any possible frost penetration or frost action As explained earlier, footings which are subject to frost action are subject to movement and differential settlement. For examples of footings and foundation details in regions which are subject to ground freezing, see ( Figs 1 and 2). For examples of those items in the warm south and southwestern areas of the country, see ( Figs 4 and 11).
ill. 11: A Typical Footing and Foundation Detail In Warn Climates
Footings are generally of two basic types: Continuous wall footings which are long continuous lengths having one constant cross-sectional profile ( Figs 11 and 12); or individual column, post or pier footings which are usually square or rectangular in plan, by several inches in thickness ( ill. 13).
ill. 12: A Typical Footing and Foundation Detail In Cold Climates
ill. 13: Column Footing; Pier Footing
Continuous wall footings in residential structures are usually a minimum of 12 inches to 16 inches in width, but always at least 6 to 8 inches wider than the wall resting on it, and a minimum of 8 inches thick. If the foundation wall is thicker than 8 inches, the footing should be also. The actual width and thickness are subject to determination by calculation based on the imposed loads vs. the determined soil bearing capacity. However, except for unusual soil conditions, the above rule-of-thumb sizes will usually suffice for normal single story residences.
The top surface of continuous wall footings should have a depressed slot, called a keyway, cast into them, for the purpose of providing improved connection between the foundation wall and the footing. This adds resistance to movement of the foundation wall during backfill operations, as well as from lateral or sideways forces which take place from the earth itself. See ( ill. 14).
ill. 14: A: Trench Formed Footing; B: Wood Plank Formed Footing
Column or pier footings’ sizes should at all times be determined by calculations. Their plan dimensions, thickness and reinforcing requirements can't be rule-of-thumb applications because the soil bearing loads ultimately assigned must be similar to those assigned to all other footings, to avoid differential settlement of the building. Individual footings are subject to bending forces in all directions, which must be calculated to determine necessary thickness and the amount of steel re-bar required.
(CAUTION) Footings should always be reinforced with deformed steel bars (commonly referred to as re-bar), made especially for reinforcement of concrete. In continuous wall footings, there should be a minimum of two #4 bars running continuously along the length of the footing longitudinally, plus #4 bars at some selected spacing across the longitudinal bars, to properly space and position the long bars. Bar numbers refer to the number of 1/8th’s of an inch in their diameter; therefore, a #4 bar is 4/8th’s or 1/2 inch in diameter. Re-bars serve to strengthen the concrete by giving it ability to resist tension—or pulling— forces, and thus create beam action so that footings will span soft spots or voids in the soil without breaking or causing differential settlement. In column or pier type footings, bars should be required at certain spacings across both the footing width and length, based on calculations made for bending stresses placed in the footing by the column or pier loads, resisted by the resulting forces being pushed back by the soil. The bars in footings must be placed so that no face of any bar is closer than 3 inches to faces of dirt at the bottom or sides of the footing. In order to hold the bars in position, special metal support or chairs are installed at specific spacings. These rest on the soil and support the main bars. An alternative method of bar support often used by builders, is to drive short sections of re-bar into the soil at the bottom of the footing, attaching the main bars to them. Special plastic caps, or separators, must be used to prevent metal-to- metal contact. This prevents water from migrating up the support chairs, or bars, and rusting out the main bars.
(CAUTION) Pieces of masonry, brick, concrete block, stone or broken concrete must not be used to support or position re-bars. This is because all masonry materials will absorb water from the underlying soil, which will in turn be drawn up to the re-bar, rusting it out and destroying its value.
(CAUTION) All bars should be new, clean and free of oil, grease, rust, scale, dirt or any other material which would interfere with their ability to bond to the concrete.
Wherever possible, footings should be cast into side forms of wood or steel surfaces. This assures accurate and consistent dimensional control of the elevation, width and thickness of the footing. The forms are later removed. See ( ill. 14-A). Occasionally the soil is stable enough to be suitable for using the profile of the dug trench as the form ( ill. 14-B). This is less desirable however, because of the less uniform dimensional control, the rough and uneven nature of the earth faces, and the tendency for earth to loosen and fall into the concrete during the pouring process.
(CAUTION) Prior to pouring concrete, it’s essential that the soil at the bottom of the footing trench be level, undisturbed virgin soil, or soil which has been properly compacted to high acceptable densities, and be completely free of loose soil, debris, and water. Any violation of this requirement will impact directly on the integrity and quality of the footing.
Some regions of the country, due to local code requirements, require masonry foundation walls to be reinforced both horizon tally and vertically with steel reinforcement. In order to tie these walls to the footings, vertical re-bars must be placed in the footings at intervals, and of sizes, to match those required in the walls above. These bars should be bent at their base to provide a foot—or hook—for proper embedment and bond into the footing. These bars should be wire tied to the main footing bars, and be temporarily aligned and held in proper vertical and horizontal position until the concrete is set.
In order to maintain the required minimum depth of footings below ground surfaces which slope, footings are stepped down in elevation, not unlike stair steps. The maximum incline or ratio of rise-to-run of such steps is 1 to 2. Any less ratio or incline is O.K., For example, 1 to 3, 1 to 4, etc. See ( ill. 15). Footing bottoms should never be sloped to accommodate changes in depth.
ill. 15: Stepped Footings: Stepping Of Adjacent Wall Footings; Side Elevation of a Continuous Stepped Footing
Concrete is a mixture of stone, sand, portland cement and water. The proportions of each material are critical to the strength and workability of the concrete. The mix hardens by absorption of the water into the portland cement to hydrate it, as the mix sets up. Development of full working strength takes a minimum of 28 days. The most critical factor which affects strength is the ratio of water to cement. Higher cement content for the same (or less) water, increases strength.
(CAUTION) For this reason, additional water should not be added to already mixed concrete. If for some reason the mix is just too stiff (or dry) precluding its proper flow into the forms, or is necessary to slightly improve workability or plasticity, only an absolute minimum quantity of water should be added. Small variations in water content make large differences in workability, but also in strength. Concrete should never be so wet so as to flow like soup or water. It must, however, be able to fill all voids of the forms without the ingredients being segregated. A field measure of the stiffness, or water content, of concrete is the test for slump. This tests how much a cone of fresh concrete will sag (slump) under its own weight. The maximum allowable slump should be from 4” to 5”.
(CAUTION) Concrete must not be dropped from great height. 3 to 4 feet of drop is the maximum. For placement from greater heights, troughs, chutes, tremie or mechanical pumping must be used. Dropping causes separation of the stone aggregate and sand from the water-cement paste.
The strength required of concrete varies for different uses. For typical residential footings, concrete which has a minimum design compressive strength of 2500 lbs. per square inch (psi) usually adequate. Higher concrete strengths are possible and are, indeed, required for many other applications. Concrete strength can reach as high as 8000 psi, and more.
(CAUTION) For consistency in quality control and reliability of mix strength, it’s recommended that concrete be supplied by
ready-mix firm which specializes in this business, and delivers the pre-mixed material to the jobsite in the familiar concrete trucks, ready to pour. These firms furnish job delivery tickets which show the mix design strength, and can be relied on reasonably well to provide the specified mix strength and assure consistency between loads. You can request to be furnished copies of delivery tickets; and , if doubts arise concerning concrete quality and /or strength, insist on the taking of test cylinders to be tested by a laboratory, with the results furnished to you.
(CAUTION) Concrete must not be used if more than approximately 1½ hours have elapsed since the water was added to the mix. After that time, especially at higher air temperatures, the initial set or first hardening stage begins, and concrete should not then be placed or disturbed.
Hand or jobsite mixing of concrete should not be allowed. It’s not unusual on very large projects requiring large quantities of concrete that a batch plant might be set up to handle materials and mix the concrete at the jobsite, in order to reduce costs and speed up delivery times. However, this would almost never be the case at a typical residential or sub-division site, and any other type of hand or small machine mixer would not normally produce the quality and consistency required.
Concrete which has been placed during very hot and /or dry weather conditions must be protected and kept moist after placement, to prevent premature evaporation of its water, which will result in impaired strength and hardness Concrete placed in cold or freezing conditions must be kept warmed to at least 50 degrees F by protection, including covering and heating if necessary. This process of protection is called curing. Curing must continue well beyond the final set period, which begins after about 8 hours.
These are the below-grade walls which transmit to the footings the loads of the superstructure above. They are usually of cast-in- place concrete or of unit masonry such as concrete block. Since foundations extend below ground, they are subject to moisture and other factors which would deteriorate less durable materials. Codes stipulate minimum wall thicknesses for various foundation materials, which vary dependant on height of the wall, the material it’s constructed of, whether or not it’s reinforced, whether of solid or hollow units, and how many stories above it’s supporting. The minimum thickness is usually 8 inches for cast in-place concrete and for masonry.
If constructed of masonry, and if required to be reinforced (common in the west and in areas subject to seismic activity), vertical re-bars are installed at certain intervals, fully embedded in cement grout poured into the open cells, or spaces, of the masonry. See ( ill. 11). Horizontal masonry reinforcement is also required, being placed in the mortar of the horizontal bed joints at frequent intervals, such as every 2nd or 3rd horizontal layer, or course. The vertical bars are not as frequent a requirement in the east and mid-west; however, the horizontal reinforcement is. See (Figs 11 and 12).
All unit masonry walls should have full mortar-filled horizontal and vertical joints. Vertical joints should not occur directly over one another in successive courses, but rather, should be staggered in alternate courses so that the blocks develop a natural bond ( ill. 16).
ill. 16: Common Running Bond (Elevation)
Other than for the sake of appearance, the joints of masonry which is buried in the ground and exposed only to soil on both faces, need not receive special treatment. However, mortar joints of basement or cellar walls, and masonry exposed to inclement weather should be compacted and made more resistant to wind and water penetration by a process called tooling. Excess mortar is removed and the joint is shaped and smoothed into various profiles by special metal joint tools. See ( ill. 17).
ill. 17: Details Of Masonry Joints: Tooled Joints; Raked Joints; Struck Joints
Before earth backfill is placed against any kind of basement wall, the walls should be braced either temporarily or by the permanent structural floor system, in order to prevent their movement, cracking or tipping-over from the forces exerted by the earth. See (ill. 2).
it is important that all adjacent parts of a building be adequately anchored or connected to each other for several reasons:
1) To resist up-lift due to wind action.
2) To resist lateral (horizontal) movement or displacement of components due to wind or earthquake forces.
3) To prevent displacement of components during the con struction process.
4) To provide strong connections of dissimilar materials.
The first location requiring strong anchorage is between the foundations and the floor and /or wall systems which rest on the foundation. If wood-framed floor or wall systems are to be constructed, steel anchor bolts at least 1/2 inches in diameter should be set in the foundation so that the threaded ends rise up sufficiently to pass completely thru the wood sill or plate member which will be placed and anchored. The sill will then be secured with large steel washers and nuts drawn tight against the sill. If cast-in-place concrete is the foundation material, the bolts should be long enough to have at least 7 inches of embedment in the concrete, the bottom end of the bolt bent in an “L” shape for withdrawal resistance. If masonry units are the foundation material, bolts should be long enough to extend at least 3 courses deep into the block masonry, and have large plate washers on the bottom end which are embedded in a horizontal mortar joint. Cement grout is then filled into the open cores or cells of the masonry, encasing the bolts. Bolts should be located at approx. 4 ft. intervals, (some codes allow 8 ft. max), with a minimum of two bolts required for any single piece of plate or sifi member being anchored. See ( ill. 18).
ill. 18: Anchorage Of Sill To Foundation; Typical Anchor Bolts
DAMPPROOFING and WATERPROOFING:
As a minimum precaution, all below-grade portions of basement walls should be dampproofed. This is done with special bituminous or asphalt materials of heavy consistency made especially for this purpose, applied by brush or trowel. 2 separate coats are recommended, so that skips or thin applications of a first coat are covered by the second. Carry dampproofing down and across the top of footings. See ( Figs 12 and 19).
ill. 19: Piped Foundation Drain System
As pointed out previously, concrete floor slabs resting on the ground may or may not be anchored to the foundation walls. In warm climates where footing depths are shallow, and especially where the exterior walls are wood framed, it’s not uncommon for the floor slab to be poured integrally with the foundation ( ill. 4). However, this is not usually recommended in colder climates, where an insulated separation is desirable ( ill. 5).
If it’s expected that wet soil conditions outside the foundation walls will prevail continuously, or be present in significant amount. Periodically, or for extended periods of time, then a membrane type of waterproofing system should be installed. This utilizes a type of special waterproof sheeting—or membrane—which is adhered to the wall and then covered with a layer of hardboard or other protective material to keep backfill operations and materials from damaging it. One membrane system consists of alternate layers of asphalt or coal tar pitch and impregnated roofing felts, very similar to the make-up of built-up roofing. 2 plies should be the minimum application. This membrane should be carried down the foundation wall to the footing, across the top and down the outside face of the footing.
If severe standing ground water is expected, a foundation drain system may be required to relieve pressure and build-up of water. This is a pipe system which runs around the outsides of the footing of all excavated basement spaces. Special pipe is used which has small perforations, or holes, for the water to enter, or is special porous cement pipe which allows the ground water to seep directly thru the walls of the pipe. This piping system must be pitched to carry the collected water to dry wells or into a storm water drain system. Gravel should be backfilled around and over the top of this drain system, and the gravel covered with a waterproof felt or other covering which will prevent soil from seeping thru it into the gravel and then into and clogging the pipes. ( ill. 19)
(CAUTION) Wood in direct contact with the earth should always be avoided, to prevent rot and infestations by termites, carpenter ants and other destructive actions of the soil.
Prev: Site Preparation