kennyrayandersen
Senior Member
- Joined
- Apr 29, 2007
- Posts
- 1,750
QueticoBill":1q2tcx25 said:
Stressed skin panels are just panels. The core prevents the facesheets of the panel from buckling so that they can react more load. If you the use the stressed skin panels to make a box, the dimensions of the big box are now what reacts any torsion that is being applied. The stressed skin panel is generally used to react in-plane shear and axial loading as well as they can be good at reaction bending loads due to things like localized pressure, or some other distributed load. Once the big box is built they act together and each panel then would react the shear and axial loads and the overall torsion is reacted by the box, not the panels.
It's easy to do a kind of demonstration by using a shoebox. Without the lid on, the applied torsion is being reacted by the area you get when you make a section cut through the lid-less box (a thin-walled 'U'). We refer to this as an open section. Open sections are very weak in Torsion. Put the lid on and the torsional area becomes the whole area circumscribed by the bottom, sides and lid and the box is now very rigid in torsion (like a wing on an airplane), Each side of the box is then only reacting shear, but the box overall is reacting the torsion. By comparison each side's contribution to the torsional stiffness is miniscule (there is some, but it's tiny).
Now, if I built the same shoebox out of 1/4" corrugated cardboard, the box without the lid would be stiffer in torsion compared to the lid-less thin-walled box because the area I get when I make a section cut is thicker and it has more area. However, if I then put a lid on the corrugated box the stiffness would be very similar to the standard shoebox (assuming the total thickness of the panel skins are the same). The one difference would be that because the skins are stabilized in the corrugated box it would ultimately be able to carry more torsional load before failure (the box cold also carry more bending and axial loads as well).
So, we see that the floor of the tear reacts a few kinds of loading. It transfers load from the chassis to the cabin, it distributes the local loads to the chassis (people laying on the floor), and it acts as one of the walls of the box. As such, it carries very little torsion load because that is carried by the overall box. This assumes that the floor is strong enough to get the loads into the overall box (or cabin). Now, if there is a square door and it spans from the top all the way to the bottom so that there is no wall, then the torsion would be carried by the area of the floor, the roof, and chassis at that point. By retaining area above the door, and making the top of the door round, the side would still function as part of the torque box (although the overall stiffness has been reduced a bit since the side with the door is less stiff than a continuous panel).
This is why, IMO the design of the frame must extend aft past the cutout for the door, so that the frame can help with this torsion. It's also why a box-section (square or rectangle) is a better choice for the frame as 'L's' or 'T's' are open sections and less capable of reacting torsion.
Currently, in my cabin sketch I have crossmembers at floor locations with higher local loading like at the front and back of the cabin, the door entrance (where you sit), between the door and the front cabin wall (where your torso would be located when you lie down), and at the axle location. These locations would also be used as attach location to join the cabin to the chassis with the exception of the aft most crossmember as there is no frame there.
x2” frame-to-cabin clips) - .79 lb
