Norm, I have to agree with you ... any structure needs to be engineered ... whether through calculation , or practical experience... The engineering becomes more important depending on the level of Stress you plan to put the structure through ...
One thing we must consider is the original intent of a Foamie ... Viz. a simple, Light , rot -resistant structure that most anyone can build, and without having a shed full of woodworking tools at their disposal .. That alone does not excuse us from the engineering .. or careful construction.
Since building with foam since the early ‘70s , I’ve become accustomed to the properties of Foam and know what to expect from building with this material ... and what I’ve learned from experience is ... the bigger the structure , the More Foam you need (thicker ) ... The more Stress you plan to put it through , the more reinforcing you need ... better skins and perhaps some framing .
It was never the intent of your humble narrator to build Giant RV style trailers with foam , although it would be possible , using the appropriate sized , attached , and placed (design) materials .
To dispel any further myths , we need a Structural Engineer to calculate and confirm our building method , depending on our intended use ...
I can accept that there are many that doubt this construction because it is a new method , previously untried by trailer builders .. but then , the experiences of the few that have tried and used this would tend to confirm that it works ...
The real question is , what type of structural failure do you expect ??? Do we think it will blow off the trailer as Conedragger originally suggested ? Can we dance on the top ? Will it delaminate and fall apart in the driveway ? If it gets hit by a semi , will it crush ? (likely)
All factors that affect most any trailer construction... I have a commercial RV in the driveway, made of the more standard materials , that I dare not stand on the roof , Is delaminating and falling apart after 5 years , and I still wouldn’t camp with in bear country .... and a crash would totally destroy . Yet thousands of those are on the road ...
Basically , the bigger the Foamie , the thicker/stronger the materials have to be ... and now with the prospect of a really Good glue ... the rest just falls into place ...