Mobile Home Foundation Engineering Design CriteriaRelated Links ->: Major Design Factors | Design Criteria | Best Design Practices | Foundation Types
Decisions about mobile home foundation systems are based on many factors. This webpage presents the major considerations that our engineers consider when designing and planning your mobile home foundation system, although not all factors relate to every installation. Issues such as frost depth, seismic activity, and presence of flood plains have regional importance.
To help organize the issues and navigate through the myriad factors that impact foundation design and selection, this webpage is divided into several parts.
Part 1, site conditions, describes features and issues that are characteristic of the building site. These are factors that shape decisions about the foundation and over which the installer, designer, builder, or developer have little or no say. Each site comes with its own set of such conditions and these features vary from site to site and region to region. These "givens" begin to suggest the better foundation systems for a specific site.
Part 3, basic design practices, is a summary of best design and construction practices that can be applied in nearly all installations although their relative importance can vary by type of foundation. A review of these factors during the design process will help avoid costly mistakes.
As opposed to the HUD code for the construction of manufactured homes at the factory, which is preemptive of local codes and creates uniformity of manufactured home construction across state lines, foundation systems are subject to state or local building codes. So, while the homes themselves may enjoy consistency of design and construction, foundation plans are subject to review by the local code enforcement authorities.
2. Soil Types
Major Soil Types
1. The percolation rate for good drainage is over 4 in. per hour, medium drainage is 2 to 4 in. per hour, and poor is less than 2 in. per hour.
2. Soils with a low potential expansion typically have a plasticity index (PI) of 0 to 15, soils with a medium potential expansion have a PI of 10 to 35, and soils with a high potential expansion have a PI greater than 20.
One important measure of the ability of soil to support the weight of the home is its bearing capacity, a value representing the weight that one sq ft of earth surface is capable of supporting without risk of subsiding. This information may already be available from the local building department or from a local engineer. Values range from less than 1,000 lbs per square foot (psf) to more than 4,000 psf.
Other problems can arise when foundation systems are placed on soils that contain a high percentage of organic matter or on fill soil. Excessive organic matter should be removed and fill properly compacted.
Special mention should be made of a broad group of "expansive" soils. Expansive soils significantly change volume as they absorb water. Highly active soils (see map below) are particularly prone to shifting as water content rises and falls. Special building practices have been developed for homes located in areas with expansive soils. These are areas where local engineers should be consulted before designing a foundation system. Slabs-on-grade foundation systems are often desirable in areas with expansive soils.
4. Frost Heave
Frost is a critical element in foundation system selection and design in many parts of the country. Frost depth is simply the known depth in the ground to which water in the soil is known to freeze. The maximum frost depth (also known as frost line) is often displayed on isobar maps (see map below). Local building codes generally indicate the frost depth to which a foundation must be excavated to reach below the frost line.
5. Flood Hazards
Unless proper precautions are taken, homes located in low-lying sites near waterways or along the coasts are at risk of flood damage. Riverine flooding takes place when excessive runoff causes a stream or river to overflow its normal channel. Coastal flooding normally is the result of ocean storms, which can be severe.
The southeast coast of the United States is prone to tropical storms and hurricanes. Foundation design and selection in these areas is often subject to local code wind speed minimums (see map below). Where hurricanes are common, the selection of a foundation system must take into consideration its ability to hold a home down in hurricane winds.
Parts of the West Coast, and certain other mid-continent locations are subject to earthquakes that can move a home off its foundation (see map below). Since HUD-code homes are engineered to resist the severe forces and stresses that occur during transportation at highway speeds, they are particularly well suited to survive earthquakes with little damage.
Snowfall in northern and mountain regions can impose a heavy load on a roof and through a structure to its foundation (see map below). The locally mandated roof snow load requirements should be checked. The foundation selected must support the home with of the specified extra weight from snow and ice.
Termites represent a threat to wood buildings almost everywhere, although the problem is particularly acute in some areas of the southeastern states and Hawaii (see map below). The use of steel, concrete, and pressure-treated lumber can minimize the damage caused by these pests. There are good termite shield designs that can be incorporated into most foundation systems in areas of especially high infestations of termites. Most shields are constructed from lengths of galvanized steel inserted between the concrete and wood portions of the foundation. The steel protrudes outward and downward, creating a barrier to termite mud tubes.
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In Montana, all projects are performed by Harrison Engineering Services our affiliated company.