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Structural Design for Modular Homes/Buildings

A modular home is created when one or more modular units are transported to the building site and assembled. Most modular homes are two or three units deep and one, two, or three units high. Few items affect the orientation of the units/modulars in a modular home:

  • The layout of the floor plan.
  • Allowable width, length, and height allowed for transportation.
  • The modular home manufacturer’s preferences.

The units/modulars can be orientated to the long dimension of the lot, most likely perpendicular to the street, but some oriented parallel to the short dimension of the lot, parallel to the street. It is not necessary to orient all the units/modulars necessarily in the same direction. Units/modulars can be oriented to be perpendicular to each other to form T, L or H shaped homes.

Sections/units of modular homes vary in size. It depends mainly on the maximum size allowed by the Department of Transportation, DOT.

Length section/unit/modular

In most areas, the maximum length is 60 feet.

Width of section/unit/modular

The maximum width is 14 to 16 feet in some locations.

Height section/unit/modular

Federal, state and local regulations limit the maximum height of any vehicle and its cargo to usually 13 feet 6 inches.

Since modular homes are shipped on flat-bed trucks with a height of 2 feet 6 inches, the maximum allowed height for the unit is 11 feet. If shipped without the roof, the wall height could be up to 14 feet.

If shipped with the roof, the wall height may be 9 feet, and the remaining allowable height is for the roof.

Planning for the orientation and number of units/modulars is usually a coordinated effort between the designer and the manufacturer.

Structural Design

The structural designing of a modular home will consist of two steps:

  • Structural design of each unit as an independent structure.
  • The entire house is structurally checked when all the units are assembled together.

The structural analysis and design for the modular home is similar to the design of the site built home/building with the following exceptions:

Notes for Analysis/Calculations:

Some corners of the units/modulars do not occur at a matching corner of the units/modulars below. Since the rim joists support the loads, provide the column or multiple studs at the corner, end of the rim joists when the above occurs 1-Lifting Forces: Each unit/modular should be checked under dead load, increased by some percentage at points of lifting.

Flat Foundation top for modular homes:

Many modular home builders like the top of the stem wall to be flat, without anything sticking out.

  • Sill Plate:

Many builders like double sill plates so the anchor bolts are completely hidden within the second sill plate.

  • Holdowns:

Many builders do not want the anchor bolts of the holdown to be sticking above the sill plate. Specialty detail for the holdown is usually used in this case that does not require anchor bolts to project above the sill plate.

First Story Framing

You can significantly reduce the first floor deflection on the first floor for modular homes, (especially for a span of 13 feet or more), by using site built pony walls to support the floor.

Use one holdowns at each corner of the entire perimeter of the building, whether the calculations need it or not. Use holdowns at the first and second stories.

When a modular unit and a site built portion meet at a corner, provide a holdown for both the modular unit, and the site built unit at the same corner.

Tie the Entire Modular Home Together

Modular units should be tied together to form a single building by one of the following means; straps, bolts, lag bolts or screws.

Location of ties for modular homes

  • At the marriage/mating lines, in the long dimension of the units/modulars. At each level, first, second, and roof.
  • At connection lines between the units/modulars, in the short dimension of the units/modular, at all levels; first, second, roof.
  • If the roof trusses are made in different sections, you need to provide straps there as well.

For Two Stories Modular Homes

  • Ceiling joists are used with the first story units. Floor joists are then used with the top/second story units. It is a good idea to show two framing plans for the second story.
    • The first plan is the Ceiling Framing Plan that represents the lower unit’s ceiling. Give the title “Ceiling Framing Plan, Lower Units.”
    • The second plan is for the Second Floor Framing Plan that represents the floor of the upper units. Give it the title “Second Floor Framing Plan, Upper Units
  • As the second story units/modulars come with double rim joists, they can support second story and roof loads. So, there no need for the headers in the first story units/modular. However, columns or multiple studs must be provided to support the loads in the first story of the unit/modular.
  • If the second story unit/modular is shorter than the first story unit/modular, you may not have the double rim joist that is usually included in the second floor unit/modular. In this case, you have to use a dropped header in the first story, at the opening to support the loads.
  • If the capacity of both of the double rim joists of the modular is needed at any opening, show this on the plan the note “No splice in the rim joists along this opening is allowed.”

Roof Framing for Modular Homes

Top units of two story modular homes and the first story unit for a single story modular home can come with or without the roof framing. Please note that if the roof framing is not included, ceiling framing is always included with the top modular units.

If the height of the above modular unit is less than what the Department of Transportation allows, then the roof will be installed in the factory. If not, the roof will be installed on site. Another way to overcome the height issue is to use hinged/folding trusses, also known as Cape Cod trusses. Some roof trusses come with hinges so they can be shipped flat and lifted to the correct slope on site.

Hinged/folding trusses are also used to overcome the width of the unit/modular. The overhang could be hinged and folded back on the top of the unit. The roof can also be made of several sections, fabricated on the factory, transported, then assembled on site.

Some types of the roof for modular homes have double rim joists, 2-1 ¾ x 11 7/ 8 or 2- 1 ¾ x 9 ¼ LVL, at the perimeter of each modular. These double rim joist act as a continuous header along the perimeter of each unit/modular and there no need for additional headers unless the load is heavy. In all cases, columns or multiple studs must be provided for the support of the vertical loads. In this case, the roof framing most likely is not riding on the wall. Trusses or rafters may need opening hangers to transfer their load to the double rim joist.

If the roof trusses consist of main trusses and cap trusses, the top chord of the lower main trusses must be laterally braced against buckling by:

  • 2x6 straight at 2 O’clock.
  • 2x6 diagonal 45 degree if you don’t have hips at both ends.
  • Show both the above bracing on the roof framing plan.
  • Show sectional details.

If you have main trusses whether they are hinged or not, the county requires that you show the connection at the ridge/mating line where the two trusses meet from the right and left units for modular homes.

Details

  • Do not use the detail you used for site-built projects. Developed special details for modular homes are usually used.
  • Make a key plan with at least two different hatchings to show which portion is modular and which portion is site-built, and then add a few text notes under the key plan to describe the site built items and factory built items to the layperson on site.
  • On each framing plan, for any item that needs to be installed on site, add the following note, “Site installed”