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Support For Modular House

by John F Mann, PE

The buyer (owner) of a modular house is generally responsible for providing the necessary foundation support.

For a completely new house, a qualified professional engineer should prepare design plans for the new foundation, including interior columns and footings. The engineer must have accurate plans from the modular house manufacturer.

Even if existing foundation walls are to be used, various new foundation elements will very likely be required. Existing foundations should be evaluated by a qualified professional engineer to determine if any remedial work is necessary.

New Foundation Walls

Design of foundation walls is essentially the same as for any new house. However, the following issues should be given special consideration;ar.

(1) Adequate lateral support along top of foundation walls

(2) Adequate tiedown for wind

In general, standard code provisions (tables) that specify design requirements for foundation walls (thickness, reinforcing if necessary) are based on the key requirement that top of foundation wall has complete lateral support from first floor.

Although often overlooked by architects and builders, connections between top of foundation wall and first floor should be designed for the actual lateral force applied by soil backfill (and any surcharge) for the specific house. This includes; (1) floor joists to sill plate and, (2) sill plate to foundation wall.

For a site-built house, these connections can be designed by the architect or by a qualified engineer.

However, for a modular house, connection between floor joists and sill plate is designed by the modular manufacturer. For completely proper design, the foundation engineer should either determine that joist-sill plate connections are adequate or foundation walls should be designed based on the condition that top lateral support is not available. The engineer should then include piers to brace foundation walls, so that the wall spans horizontally (not vertically) between piers.

Even if joist-sill plate connection is adequate, or can be made adequate on site, the following additional issues must be addressed;

(1) Lateral support for foundation walls parallel to floor joists. Unless lines of solid blocking are provided (by modular manufacturer or by builder, on site) there is no effective top lateral support.

(2) Bolts connecting sill plate to foundation wall. Consideration must be given to how bolts will be installed.

Standard building code requirements for sill plate anchor bolts are not based on lateral support requirements for foundation walls and must not be used as a basis for top lateral support.

Modular plans may show "braced wall" panels for wind resistance. However, requirements for tiedown to foundation walls must be carefully checked, especially in areas where design wind speed (per governing building code) is 100 mph or greater.

The foundation must be designed to resist uplift force from tiedowns.

Existing Foundation Walls

When a modular house is to be installed on existing foundation walls, the condition of such walls must be carefully assessed. The following issues require special attention;

(1) Indications of any settlement problem

(2) Evidence of excessive inward lateral pressure from soil backfill

(3) Water infiltration into basement

Installation of anchor bolts, including tiedowns for wind uplift, may be difficult at best. Engagement of sufficient foundation mass may not be practical, such that new foundation elements are necessary to resist wind uplift. 

If top of foundation wall is to be raised, by installing new block or concrete on top of existing walls, there will be increased bending stress (on existing wall) due to inward lateral pressure from existing soil backfill. If grade is also to be raised, increased bending stress can be very high.

In general, if top of existing foundation walls is to be raised, new bracing elements (inside basement) will very likely be required.

Thorough evaluation of existing foundation walls may result in the conclusion that a completely new foundation system is warranted. 

Support At Center Of House

Support requirements at center of house must be carefully designed. Major problems with lack of adequate support at center of house can occur when proper design is neglected.

In general, a modular house will be manufactured in two or more sections that are shipped separately and then assembled on site.

Standard industry code provisions require that the modular manufacturer provide "suggested" details for foundation support. However, the engineer responsible for foundation design must provide final design of all supports.

For each separate section, at or near center of the house, the manufacturer generally provides a "mate band", which is essentially the same as a rim joist across ends of floor joists. Each mate band may be a single or multiple member, with same depth as floor joists, such as single or double 2x10.

When two modular sections are assembled on site, each mate band from each section will be adjacent. The manufacturer specifies a gap of about one inch between mate bands to allow for installation tolerance. Bolts are also typically specified to connect the mate bands.

The manufacturer often considers the assembled mate bands to provide support, as a girder, for first floor joists and for the double bearing wall that also supports second floor joists, attic floor joists and roof framing (due to modular construction). Suggested support details typically show maximum spacing of columns in the basement.

However, the engineer should not use such suggested details without carefully considering locations of joints in the mate bands. The manufacturer generally does not specify locations of joints on their plans. Even if they did, as-built joint locations would have to be site verified unless a very conservative design approach is taken.

Since there is a wide gap between mate bands of adjacent sections, the two separate mate bands should not be considered to act as one unified member unless connections (between mate bands) are designed to transfer force across the gap.

Typical bolted connections do not have adequate design capacity to transfer design loads, from one modular section, across the gap to the mate band of the adjacent section.

Bolt holes are all-too-often drilled near bottom edge of mate bands, resulting in high stress concentrations, causing severe splits. 

Each mate band must therefore have design capacity to support all design loads from one section, which is most often half the house. This requirement makes consideration of joints (in mate band) essential.

Net result is that a completely separate girder is very often a much better design approach to support loads at center of modular house.

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