by mezmo » Sun Nov 27, 2011 2:41 am
Just some "ponderings and conjecture" on the question. So watch out any noses or toes
or knickers. 'Not intending to get anything out-of-joint, stepped on, or in a twist. This is
just discussion.
First off, I think the question is quite germain but not alarming. It is a serious consideration
for building any type of vehicle/enclosure. We need to consider all possible forces it has to
survive. More 'thinking like an engineer' in-so-many-words. This doesn't mean we amateurs can't
do this - build foamies of larger dimension - , just that we just need to think about what we're
doing when we do it - so we can eliminate or lessen the possibility of structural failures -
consider it getting good value for ourselves from our efforts.
I think the crux of the "problem/thing" thing is to consider just what is expected of the fabric
and glue - and to consider its origins. It is an old but practical techique, that to me seems to
have been used as a water-proofing measure or to provide a water-proof outer skin.
Our "sock-effect" phenomenon is a latter-day extension/discovery of/about it.
So, to me, the question we need to consider is: Is the fabric and glue 'matrix' really structural
or is it just a water-proof finish ? And, can it be both? I'd think it can be both - if only based
on our anecdotal/empirical evidence of the small trailers that have been built so far and used
successfully. BUT, I'd guess there is a diminishing strength type of phenomenon as the size
and spans of a unit grows.
"The strength is all in the skins" rule of thumb/statement needs to be determined for the fabrix/
glue matrix. How does it compare to the fiberglass & resin matrix in strength? I should think it'd
have to be similar to the fiberglass in that the more layers/plys used, the stronger that total 'skin'
is. [Of course this entails more glue a/o resin too.] And we also need to consider how rigid the
fiber and glue matrix is. Is it self supporting ? And is it really strong enough? I ask that because
fiberglass shells are when made correctly, and are usually @ 1/4 inch thick usually, from my observation.
Let me be clear that, that is a generalization, and that is for the shell only as the structure. That shell
strength can also be affected by factors such as the shape involved [thinking of boats and 'eggish style'
trailer bodies etc.] of course. We also need to consider how heat affects things. Will fabric and glue
hold up to heat as well as the usual fiberglass?
Now all that leads to the consideration that we are - as it has been pointed out many times - building
a composite structure - not a shell only. So we assume the 'skins/shells' held apart by the foam core
do not necessarily need to be as thick as single self supporting shells need to be - think of a single
thickness self-supporting shell as two shells with zero core/separation that thus 'appear' as a single shell.
Since we are trying to keep our two shells apart to take advantage of the strength benefit that gives,
I do think it's important to have what ever core that is separating the skins be kept attached to the skins,
especially as size/area increases. So far the tests done by forum members on the adhesion of various
glues and fabrics combinations show that they all vary in adhesion [no surprise] and usually end up
separating eventually at some time or stress level. The "sock-effect" is thought to contain any problems
with that as long as the skins/shells are still kept apart. But, we must be losing some strength and
possibly overall structural longevity when the skin separates from the core. Perhaps the solution would
be to attach the initial fabric layer to the core [foam] with the newly [to us] discovered 3M 30NF contact
cement [Or we should really find out what all the SIP manufacturers use to glue their foam core to their
shell layers.] and then coat the outside of that initial fabric layer with the TB II & III a/o resins and continue
on from there to as many layers as desired?
That being said, we also need to view the structure overall, and to consider ways to reinforce it. Sides and
roofs can be buttressed/strengthened by partitions and shelves and cabinets, as well as how they are
attatched to each other. Wall and roof shapes can also affect their strength. Trying to make it a unitized
structure/entity is an often used strategy for this and is really one of the unconscious ideas behind the
"sock-effect" too I think. Other approaches may be to use primeter framing of some sort around the foam
panels/sheets, or to build a space frame to attach our foam cored panels to. All sorts of 'hybrids' or kinds
of building techniques are possible I think. One thing I'm thinking about is doing a 'stitch-n-glue'
style [borrowed from plywood boat building] inner shell [i just love birchwoood interiors.] and applying the
foam core to that [I like the idea of good insulation too] and the fabric/glue or fiberglass/resin outer skin to
that. And we also need to keep in mind that curves impart strength. It's not as easy to build curves
compared to straight/flat but why not use all/any structural enhancements or methods for their beneits
if it fits our needs or design ideas?
Well it's getting late and my 'thinking cap' has fallen off so I'll trundle off now.
Cheers,
Norm/mezmo
If you have a house - you have a hobby.
