Foundation Analysis
Course Notes August 18, 2025The foundation of a manufactured home is comprised of four categories: piers; straps and anchors; longitudinal stabilization devices; and lateral stabilization devices.
Piers counter gravity forces (live and dead loads) and transfer the weight of the home to the ground. Straps, anchors, longitudnal, and lateral stabilization devices counter horizontal window forces as well as uplift wind forces.
Analysis Purpose
The analysis of a manufactured home foundation is for documentation and educational purposes only. The analysis shall produce multiple, visual views and formats (in 2d layouts and 3D models) of the components used to counter the forces acting on a manufactured home's foundation.
For example, piers are constructed and positioned under a manufactured home. Piers are commonly constructed with the following components, listed from bottom to top:
- A footer made of ABS or concrete.
- Two or more concrete masonry units CMUs that adhere to the ASTM C90 specification.
- A cap made of approved ABS material or 2" x 8" x 16" pressure treated wood.
- Two shims made of approvide ABS material or pressure treated wood.
The result of stacking the components listed above appears as:
A constructed pier with an ABS footer pad
The purpose of these stacked products is to transfer concentrated loads from a home downwards through the concrete blocks, distribute the load through the wider-dimensioned footer, and ultimately distribute the load onto the underlying, at grade soil.
The description above is a common text-based description of the purpose of piers constructed under a manufactured home. The structural analysis is to produce visual presentations of the load bearing process.
Equally important is the visual presentation of the result of not adhering to the manufactured home installation standards. For example, the installation standards requires the following:
- CMUs must be centered on the footer.
- For piers less than 36" in height, the CMU stack must not be out of plumb by more than 0.5". For piers between 36" and 67", the CMU stack cannot be out of plumb by more than 1".
- The long dimension of the CMU must be perpendicular to the I-beam of the steel frame under the home.
- The contact between the I-beam and the CMU should be "cushioned" by a cap or shim. There cannot be direct contact between the I-beam and the CMU.
The analysis should reveal the negative outcome of not adhering to the above requirements.
The analysis purpose is to generate multiple presentation formats and views to help facilitate the understanding of how live and dead loads, along with wind forces, are acting upon a manufactured home, and most importantly, why acutely adhering to the Model Manufactured Home Installation Standards (MMHIS) – also referred to as the HUD Code – results in a safer, longer lasting foundation of a home.
The analysis is not to verify or validate existing reports and engineering analysis used in the creation, development, and evolution of the two federal standards: Manufactered Home Construction and Safety Standards (MHCSS); and Model Manufactured Home Installation Standards (MMHIS). These federal standards are administered by the federal Department for Housing and Urban Development (HUD) of the U.S. government.
In addition, the analysis is not to evaluate and test products from foundation product manufacturers. Product manufacturers use independent consulting firms to evaluate and test their products across various soil classes and load capacities.
Measuring Value-Add
The measure of the value-add of this documentation is in the reduction of the number of installation code violations, reducing the cost to state agencies that must inspect potential code violations reported by homeowners and mobile home park administrators.
Target Audience
The target audience for this documentation are newly licensed and experienced installers of manufactured homes. However, the documentation should be available to homeowners (individual or corporate). It's to ensure that the foundation of their home installation meets standards, barring extreme weather events like hurricanes and tornadoes, or drastic shifts in the ground below a home caused by earthquakes or sinkholes.
Manufactured Home Construction and Installation
Manufactured homes are constructed in one or more sections, giving rise to the phrases: single-wide, double-wide, triple-wide, and even quadruple-wide.
The wood framing of a manufactured home (floor joists, walls, and roof trusses) is constructed on top of a steel chassis, consisting of two steel I-beams running the length of a home. Several steel crossmembers connect the two I-beams. Gussets added to each I-beam provides additional structural support for the home.
The I-beams, crossmembers, and gussets are here on in referred to as the frame.
The frame is connected to the home through several rectangular steel "tabs". One half of the tab is welded to the frame and the other half is bolted to the underside of the floor joists.
Double-wide manufactured home - bottom view with chassis
The design, engineering, and construction of a manufactured home/dwelling unit, as shown in the image above, is governed by the Manufactured Home Construction and Safety Standards (MHCSS).
The installation of a manufactured home at a geographical location, as shown in the image below of the foundation components (including piers, anchors, and stabilization devices), is governed by the Model Manufactured Home Installation Standard (MMHIS).
Double-wide foundation
The image below shows the combined home and foundation components of an installed manufactured home.
Installation of a double-wide manufactured home
Double-Wide Models
Foundation
The following 3D model of piers, anchors, and longitudinal stabilization devices (LSDs) enables full interactivity with a pointer device (mouse, pen) and touch gestures. Pinch to zoom. Press and move to rotate. Two-fingers (or right-mouse-click and hold button depressed) to pan.
Home and Foundation
The foundation with the manufactured home.
Piers
Unlike site-built homes, or modular homes affixed to site-built concrete foundations around the perimeter of the home, the piers of a manufactured home consists of dry-stacked, open-cell 8"x8"x16" concrete masonry units (CMUs) and solid block 4"x8"x16" CMUs.
The term dry-stacked means that CMUs are stacked on top of each other without the use of poured concrete and rebar within the open cells and without the use of a bonding agent between the stacked CMUs.
Double-stacked concrete blocks
Pier Construction
The following table lists each component used to construct a pier with an ASTM C90 Concrete Masonry Unit (CMU). The components in the table are listed in the order of construction: bottom to the top. The first component laid down by a manufactured home installer is the footer — commonly, an ABS pad — followed by concrete blocks (CMUs), a pier cap if necessary, and shims.
Each row in the table features an interactive 3D model of the component.
| Description |
|---|
| Pier |
|---|
Footer. The pier footer may be either concrete or an approved ABS footer. ABS footers come in a variety of sizes. The dimensions of the smallest ABS footer pad allowed in Florida are a 16" x 16". The ABS footer size will determine the amount of weight that can be applied to the ABS footer and will determine the pier spacing. The main factor that governs foundation size is the soil bearing under a home.
CMUs. ASTM C90 load-bearing concrete masonry units (CMUs) or more commonly referred to as concrete blocks come in a variety of sizes. There are only two sizes approved the HUD's Model Manufactured Home Installation Standards (MMHIS) for constructing dry-stacked piers under a manufactured home: (1.) 8"x8"x16' open cell CMUs; and (2.) 4"x8"x16" solid concrete CMUs.
The nominal (listed) dimensions of a concrete block is a whole number (e.g. 8) – an integer number without a decimal or fraction. The actual dimensions, however, are always 3/8" less than the whole number. Three-eights of an inch (3/8") is the amount of mortar used when CMUs are stacked for the construction of a wall, for example.
Cap. A single 2" x 8" x 16" pressure treated, or other approved materials, shall completely cover the top of the pier. Plastic ABS cap blocks have been approved in some states (e.g. Florida).
Shims. In new home installations, the dimension of shims are 3 1/2" wide x 6" long x 1" high. Shims can occupy no more than 1" in vertical height in new home installation. In used home installations, shims are the same except for height. Shims 1 1/2" high can be used and, therefore, shims can occupy no more than 1 1/2" in vertical height in used home installations.
The result of stacking the components listed above appears as:
A constructed pier with an ABS footer pad
ASTM C90 Specification
Concrete blocks used in the construction of dry-stacked piers are manufactured to the ASTM C90 specification. The specification title is Standard Specification for Loadbearing Concrete Masonry Units. ASTM refers to the standards body Advancing Standards Transforming Markets (ASTM). C90 is the specification ID.
The current specification is identified with the last two digits of the year in which the specification was modified. The complete identifier of the current specification, made available on Jan 20, 2025, is ASTM C90-24.
A printed copy of the ASTM C90 specification is available for purchase at: https://store.astm.org/c0090-24a.html
Compressive Strength
The minimum net area compressive strength of concrete masonry units complying with ASTM C90 is 2,000 psi (13.8 MPa).
There are three classes of concrete masonry units: Normal Weight, Medium Weight, and Lightweight. These units are suitable for both loadbearing and nonloadbearing applications.
https://www.masonryandhardscapes.org/resource/cmu-tec-001
Availability of ASTM C90 CMUs
Concrete blocks are readily available at local hardware stores. Blocks sold at Home Depot are ASTM C90 certified. The description listed in the product page for concrete blocks on HomeDepot.com, states the blocks are manufactured by a company called Titan Block. The online product page includes the following:
Titan Block is 1 of the nation's largest producers of concrete block. Titan Block's products are found in all types of construction projects from homes to schools, factories to office buildings. The 8 in. x 8 in. x 16 in. concrete block is most commonly used in the construction of concrete walls for structures of standard size.
The Product Brochure, linked in the 'From the Manufacturer' aside section of the online product page, states the following at the bottom of Page 2 of 4:
All Titan Block units meet or exceed the ASTM C-90 specification for concrete masonry units.