Mojave Bob wrote:It is not really intended to provide the strength in the joints. The joint should be strong in itself, and the fabric just adds to that.
On the contrary. Any joint with foam is only as strong as the foam; which is in fact very weak in tension and bending. Most foam joints that I have personally checked (either in testing or when checking off cuts to see how well they had bonded) have failed at the foam just beyond the glue seam.
The foam is just there to separate the outer fibers (the ones that do the work, in our case the fabric skins) and to hold them far enough apart. It is this separation of the outer fibers (a longer lever, if you will) that determines the strength of a material in bending (look up moment of inertia in static analysis if you what to learn more). It's why box tubing is hollow, and why I-beams are shaped the way that they are. The reason the foam and canvas works so well is because the foam is not easily compressed over broad areas. This allows it to keep those all important outer fibers separated. All of the tensile strength comes from the skin, and for a given skin strength, the farther apart you place them, the more the panel will be able to resist flexing.
So it is absolutly the skin and reinforcements at the corners that are doing the work. Scientifically, mathematically proven.
Here's where it gets a little tricky, it is not just the strength of the fibers, it is also the unity of the composite, so the glue bond between the layers is very important.
Think about it this way, if you have three thin boards and you stack them one on top of the other in a span, they will bend more when a load is applied than the same three boards that have been glued and screwed together. The strength of the individual members doesn't change, but the strength of the composite does, due to unity.
So even if the house wrap fibers are stronger and lighter than canvas, if the bond to the foam is inadequate, the unity may be compromised and the structure may not be as strong.