One of our goals in the upgrade is to modernize the trailer and improve component reliability. One element of this upgrade effort was to improve the lifespan of the flooring. The old sub-floor had been replaced at least once and we were, I think, looking at the 3rd sub-floor when we started the rebuild.

Note: As you read this, please keep in mind that it is unique to a 1963 19′ Airstream Globetrotter trailer / sub-floor combination. Other materials, frame sizes and geometries will have different mechanical properties.

As you can see, it was rotted and in pretty bad shape.

We determined early in the rebuild process that the original subfloor/fastener combination had a life span of something less than 57 yrs (see above).

For the new floor, we had three primary goals:

1. First we wanted it to last pretty much forever
2. It had to be cool . . . composite flooring sounds cool
3. It had to last pretty much forever!

Putting the cart before the horse, We went after goal #2 first and bought 4 each 4×8 sheets of Composite boards for the new sub-floor. . . .afterwards we started to wonder if composite boards were such a good idea.

Now that we had leapt into the unknown and bought the board, we set about classic confirmation bias to prove to ourselves that the board we bought was indeed up to the job. Unfortunately, there was no readily available engineering analysis to reference, so Pete set about doing his own analysis using techniques he had learned while pursuing his Mechanical Engineering master’s degree at Texas A&M.

Side note:

MEEN 688 Advanced Solid Mechanics was a crushing but strangely enjoyable course. Imagine doing partial differential calculus with 3rd order tensors; all while 32 years out of your engineering undergrad! Pete had to re-learn how to use a calculator . . . much less all that math stuff.

The picture below shows the view of the sub-floor/frame combination showing areas of concerned where the subfloor is suspected of being insufficiently supported, either due to a lack of frame support or a panel joint between the sub-floor pieces.

Questions to answer:

1. Will it last longer, or is it susceptible to mold, water absorption, UV radiation, . . .? (i.e., what are it’s corrosive properties?)
2. Is composite board strong enough for this use?
3. Is composite boarding a better choice than plywood?
4. Do we need to install additional outriggers or cross members to the frame?

To answer #1 Pete did a little research. Plywood is actually stiffer (i.e., should deflect less), but it is susceptible to water absorption, rot and mold. Composite boarding is susceptible to UV radiation, but won’t absorb water, rot or mold. Conclusion: composite board will last longer if you can protect it from UV radiation, edge effects and if it’s adequately strong . . . which leads to question #2.

To answer #2 Pete did multiple iterations of engineering analysis. First by hand-calculating modeling analysis of small sections of the floor and then validating those calculations using engineering modeling software.

Hand Calculations: Fitting for a 1963 Airstream, we found an engineering reference from 1959 entitled Theory of Plates and Shells, which, albeit in a circuitous and confusing manner, provided sufficient technical guidance to model the sections of the floor encircled in red above. In the absence of computers in 1959, problems like this one were tackled piecemeal using finite sub-sections, formulas and look-up tables. One such analysis was the modeling of the center, left rectangle above (scenario 1). This section was modeled as a plate, clamped on 3 sides with one free edge.

The snip below shows an excerpt of the types of formulas contained in the 1959 reference.

Scenarios 2 and 2a were modeled using a method described in Theory of Plates and Shells to model overhanging floors and resulted in only deflection calculations (i.e., how much will it move up or down), but not stress. Stresses were approximated by correlating deflection with principal stresses, but this was done with low confidence.

Computer Calculations: A computer modeling simulation was next performed on the same geometries as the hand calculations. Scenario 1’s results were very close to the hand calculations. . . 🙂 . . ; while scenarios 2 and 2a results were of the same order of magnitude . . . meh.

That’s cool. It looks like it will work. The final step in the engineering analysis was to model the entire assembly (4 plates on the frame) to determine if a laminar coat of fiberglass was need for stiffness.

The final analysis of the sub-floor was done with computer modeling to determine the maximum stresses and deflection with and without a re-enforcing fiberglass layer. The modeled deflection went from about 1/2 ” at the cantilevered overhangs (scenarios 2 and 2a) without re-enforcement to nearly zero with a re-enforcing fiberglass layer.

Bottom line: The sub-floor should pretty much last forever as long as it is not exposed to UV radiation for extended periods of time AND the sub-floor will be sufficiently stiff if we use 3/4″ composite boarding with fiberglass re-enforcement.