If you wanted an ‘L’-shaped section using solid wood, you would need to fashion two pieces and join them together. With any joint, there is inherent weakness: it’s weaker than if it were a single piece. With plywood of course, you can make this a single piece as it’s a sheet material.
It seems to me that when working making an ‘h’-shape with solid wood you need at least two joints. Chairs which include this as part of their design often have extra bracing members between the front and rear legs for extra stability.
When I was designing my second plywood chair, I decided to challenge my thinking as to how thin the ‘h’-shapes in the frame could be, particularly around the intersections and corners. The thickness I chose was about 45mm, which I was not sure was going to be wide enough but I needn’t have worried - it is easily sturdy enough, and I possibly could have gone narrower.
This leads me onto the other aspect of achieving rigidity - how to efficiently ensure there is no wobbling, warping or changing shape. I wanted my chairs and couch to be strong in resisting any reasonable attempt to force them out of shape.
There a few ways to try to explain this using the chair as an example - choose the one which makes sense to you:
- I need to resist warping along each of the X, Y and Z axes of a 3D assembled object
- I need to prevent the back legs moving sideways relative to the front legs, the whole chair leaning sideways and the whole chair leaning forwards/backwards
- I need to make it impossible for the front and rear parts of the seat to be out of alignment, for the back and seat to be out of alignment with each other and for the legs to change their angles with the floor
A good way of showing how I did this is by looking at the frame of my couch.
A similar design for achieving rigidity is used in the back of the couch. I have used two protruding arms here for symmetry but it strictly only needs one.