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Manufactured Housing Ground Anchor Systems

Ground anchor system that limits lateral building movement in manufactured homes.

Drawing shows ground anchors attached to the piers that make up the home's chassis.

Manufactured homes are not usually installed with permanent foundations, but instead sit on short piers that resist mostly gravity loads. Ground anchors are plates or augers imbedded in the soil that limit lateral building movement down through tension members tied back to the home's chassis. In this way, high wind (hurricane, tornado), earthquake and flood forces are resisted through the coherent mass of the earth acting with the anchors. Properly applied, these systems are a cost-effective way to limit structural failure compared to conventional manufactured housing foundations, saving lives and property.

In a complete ground anchor foundation system, the house is supported by several steel I-beams which are supported by an array of short piers. Tensile members, usually diagonal, connect the main beams with each anchor assembly. An anchor consists of a rod between the anchor head and anchor base, which is buried 3' to 9' into terra firma and allows the combined mass of the retained soil to resist uplift and lateral forces. Types of ground anchors include "manta ray" plate anchors, "rock anchors" for rock embedment, single and double helix anchors that are augered into place, and rods or chains that anchor into a poured structural fill. Tension members may be steel rods, straps or link chains. Most systems offer stabilizer plates to limit the anchor heads horizontal displacement.

The most common ground anchors are auger, or helix-type systems. They are applicable to several types of piers, and feature anchors with a single- or double-helix base and an optional lateral stabilizer near the anchor head. The helix is a 4" or 6" circular plate in the shape of a short spiral. Steel straps hold the home's chassis to the anchor heads. Variations include steel rod anchors which may be encased in a poured structural fill collar, or "rock anchors" which may be installed in solid rock.

Earth-Lok is a "Manta ray"-type anchor system designed for earthquakes. It features a large (up to 6" x 6") anchor plate that can be driven up to 18' into the ground. The plate is then rotated with a gad, or second steel rod, and pulled back to lock. A galvanized threaded rod holds the plate to the home's chassis, which is clamped to a series of steel pyramidal supports. Only certified crews can install the system.

Another system, Ground-Loc, makes use of link chains or allthread rods tying the home's chassis into an anchoring material poured into excavated cavities in the earth. Working with piers by others, the manufacturer provides pier-to-chassis clamps and allthread rods to tie the carrying beams together. One model converts the foundation to a permanent foundation, thus qualifying the home as real property, enabling a low-interest mortgage.


Quality and Durability

This type of system helps stabilize the house and make it feel more stiff, while not being easily prone to deterioration.

Safety and Disaster Mitigation

By its very nature, the system helps to reduce severe damage due to strong winds, seismic activity and flooding.


Easy

There are many factors that go into determining an appropriate foundation system for a manufactured home, including home configuration, anticipated loads, soil characteristics, first cost and maintenance. A structural engineer should always be consulted.


Initial cost can range from $500 to $1000, depending on what type of home is being anchored and labor costs, as well as manufacturer.


If the home is kept properly and well maintained, this system can pay for itself in just one storm.


Most states do not require lateral load resistance for manufactured housing. For those that do, wind load provisions are established by MHCSS (Manufactured Home Construction and Safety Standards). Performance standards for ground anchors are established by ANSI.


Not Applicable


Manufacturers make various claims on installation time varying from two to eleven hours. Most of these systems are designed for retrofit as well as new construction, and are relocatable. Sufficient preloading of each anchor is essential for adequate load resistance. Soil test probe kits may be available. Some of these systems can be installed on sloping sites.


Typically guaranteed for the lifetime of the supported structure, provided installation was properly done.


One distributor of auger anchor systems estimated a ground anchor system for a typical single-wide manufactured home to be $200 (including anchors, stabilizers, straps and bolts) plus about $500 for installation. A ground anchor system, in the proper application, may save lives and will reduce the cost of repairing a home after an earthquake, flood or windstorm.

The results of tests requested by HUD following the devastation of Hurricane Andrew in 1992 indicated that ground anchors perform to ANSI standards only in certain soil conditions. Failure results from the anchor head moving laterally, allowing the home to tip off its piers, or from the anchor being pulled up due to inadequate base plate resistance. Helical or auger-type anchors that are painted but not galvanized will lose their corrosion resistance during installation.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.