Because the foundation transfers the weight of the house to the soil, prudent engineering principles and calculations are necessary. However, prescriptive code requirements often codify the worst case situation, adding unnecessarily to costs for all foundations built in an area. The optimum foundation will depend on factors such as climate, soil, topography, and building loads.
Basically, concrete footing widths are determined by total design loads in pounds per linear foot of footing and allowable soil bearing capacity in pounds per square foot. Column footing sizes are determined by total design load in pounds and allowable soil bearing capacity in psf. See Tables 1 and 2.
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Table 1 Footing Widths, in Inches, or Typical Single Family Dwelling Loads Design Load Allowable Soil Bearing Capacity, psf lbs./lf footing
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Table 2 Column Footing Size, in Inches, for Typical Single Family Dwelling Loads Design Load in lbs. Allowable Soil Bearing Capacity, psf
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As shown in the above tables, if soil bearing tests are made, footing widths may be reduced substantially thereby reducing costs, assuming local codes are performance based.
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The 1986 CABO One and Two Family Dwelling Code prescribes minimum footing widths and depths, but Section R-108, Alternate Materials and Systems, provides a mechanism for innovative design and material usage. All major model codes have similar provisions that should be used whenever soil bearing tests or engineering calculations are appropriate.
Reinforcing of concrete footings is required by some local codes or is routinely installed as "local practice." Footing reinforcement is seldom necessary for footings placed on undisturbed soil. Compacted fill often has sufficient bearing capacity that makes reinforcing unnecessary.
Footings in expansive soil conditions should always be designed by qualified engineers and will most likely require
reinforcing. Otherwise, elimination of footing reinforcing rods is a legitimate method of reducing costs in many cases.
As with footings, reinforcement in foundation walls is seldom necessary in nonexpansive soil and in areas outside of seismic zones 2, 3 or 4. If reinforcement is routinely installed in accordance with local code requirements or local practice, it will be worthwhile to examine soil conditions and work toward change.
Under stable base conditions, concrete slab floors do not require welded wire mesh. It is not recognized as structural reinforcement and provides no significant function. If installed correctly (in the upper third of the slab), welded wire mesh may be of minor value in limiting the width of cracks.
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Since mesh is seldom installed correctly, and since properly placed < control joints localize cracks, and ‘ since carpet or resilient flooring cover cracks, welded wire mesh is of dubious value in most cases. A survey conducted for NAHB in 1984 indicated that about 60 percent of the code jurisdictions do not require mesh in slabs.
Most major model codes require 3 1/2- inch or 4-inch thick concrete slabs — on-grade. Because the slab is 100 percent supported on compacted fill and because normal house loadings are relatively light, a 2 1/2-inch thick slab may be more than adequate on soils with high bearing capacity.
The pressure treated wood foundation
is a proven cost effective alternative
to masonry in some areas. As with all foundation systems, the realization of full performance potential requires proper attention to design, fabrication, anti installation.
Wood foundations, marketed under the name "Permanent Wood Foundations," have been used successfully in many areas of the country. They are built basically like exterior walls using lumber and plywood treated to American Wood Preservers Bureau FDN standard. Details of the system are available from the National Forest Products Association (NFPA), 1250 Connecticut Avenue, NW, Washington, DC 20036. NFPA is also investigating pressure treated wood for expansive soil applications, especially for crawl space foundations.
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Waterproofing |
Throughout the years, there have been many attempts to develop the definitive basement waterproofing method. Some methods have worked better than others, but basement leakage continues to be one of the most common customer complaints. Capillary water and condensation on comparatively cool walls and floors create dampness. Most capillary problems diminish or go away when concrete completely cures or when positive drainage steps are taken. A plastic film vapor barrier under the basement slab and on walls reduces capillary action through the concrete. Condensation dampness is a function of internal humidity and temperature difference between air and surfaces. When the dew point is reached, water vapor changes from a gas to a liquid. The most consistent and major problem is the hydrostatic lateralpressure of groundwater which seeks out cracks m concrete or mortar joints. Elastomeric compounds applied to the exterior surface of the foundation wall help reduce the problem. In addition, dram tile set in gravel around the exterior basement perimeter (French drain) helps to some degree. Parging alone is practically ineffectual. Substitution of plastic films for parging and many of the so-called "sealants" has, in some cases, been effective. Good drainage starts by keeping rain and melting snow away from the foundation by proper surf ace grading. A 1-inch rainfall on an 1,800 square — foot roof will generate about 1,125 gallons of water. Add the water that falls within a few feet of the foundation and the drainage from patios, porches, driveways, etc., and the potential for basement leakage is veiy high unless most of the water has been removed before it has a chance to percolate near the house. |
Gutters and downspouts are helpful if the downspouts direct water far away from the foundation. If not, water is concentrated in one area, increasing the probability of leakage at that point. In addition, settling of backfill allows water collection alongside the foundation walls. By paying careful attention to surface grading, foundation waterproofing is simplified.
Many new drainage boards, panels, fabrics, and plastic mesh products have been developed recently that are applied to the exterior foundation wall and relieve hydrostatic pressure by draining away water. Some were developed for highway and commercial building construction and have been proven effective. Some even have integral insulation laminated with filter fabrics and water retardant facings.
All these products are based on the fact that lateral water pressure is the culprit in most leaky basements. The major drawback of most systems is that water which has drained down has no place to go once it gets there. Drain pipe that simply circles the foundation perimeter can fill up quickly with water and then eventually silt. Water starts backing up the foundation wall, recreating hydrostatic pressure.
The NAHB/NRC developed a waterproofing method in about 1965 that still works. It relieves the hydrostatic pressure by allowing water to drain in the path of least resistance ~ down. The system does not stop at the footing, however. It provides a method which keeps the water away from the foundation wall permanently.
The system is used with the pressure treated wood foundation but was originally developed for use with concrete and concrete block foundations. The system lets water drain under the basement floor in a controlled manner where it can either drain into the soil below the slab or into a dry or wet sump. The system is described in detail in Basement Water Leakage.. Causes, Prevention, and Correction, available from NAHB, 15th and M Streets, NW, Washington, DC 20005.
Good supervision and construction practices are very important in waterproofing the foundation.
Cleaning the footing prior to placing the foundation wall; using care in placing concrete to prevent entrapped air or aggregate segregation; placing concrete at least 4 feet per pour; vibrating concrete; using low slump concrete; and providing drains for window and door wells all help to reduce leakage problems.
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