by KCStudly » Thu Jul 26, 2018 2:45 pm
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Seems like a good deal to me, too.
On structural vs. not structural. It depends. Generally if the main frame rails are a smaller/thinner angle section, then I would assume that the top rails are structural. If the main rails are deeper box or channel section, or thick wall, then it is more likely that the top rails play a lesser structural roll.
Either way, if you replace the uprights and top rail with something even more substantial (like a well constructed box... not necessarily heavy, but rigid and deep in section... like a standy), and competently join them to be unitized, then removing the "structural" upper frame becomes a none issue. Heck, there have been wooden campers built with no metal frame, just a tongue and axle mounts.
So, whether they are needed to meet the specified weight capacity or not, those upper rails are structural due to their geometry. They form a truss. The structural form of a truss spreads the tension and compression fibers out, while minimizing the amount of material required to resist the loads. In a bending moment it is always the outer fibers that see the highest loads, so by moving those fibers further apart the "leverage factor" (if you will allow the simplification) takes affect; the outer fibers see less stress on a longer lever. Shorter lever, more stress. (More properly it has to do with mass centroid vs. cross sectional area placement, or "moment of inertia", but the lever analogy is easier to understand.) Because the load on a flat bed trailer is essentially trying to bend the front and rear decks down over the axle, the top rail is mostly affected by tension, and is therefore less susceptible to buckling so can be a relatively light cross section; and therefore even a relatively small/light section of metal will add substantial strength (just like the tension cables or rods of a railroad truss look minuscule compared to the larger compression members).
Another way to think of it is bending a piece of paper vs. bending corrugated cardboard. One paper ply (or light frame rail) bends easily, but intact, with the outer skins spread apart by the wavy uprights, a corrugated cardboard "truss" or unitized structure, is much more rigid (like a double rail trailer). The truss allows all of the members to be smaller/lighter w/o sacrificing overall load capacity.
KC
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