I am about to build an 18" high by 5-foot long solid wood entryway bench that will have top storage accessed through the hinged top/seat/lid panels, similar to a piano bench. Storage under the seat will be about 8" deep. The front and back top rails/aprons will need to support the lid without much deflection when 2 people sit on it. I would really like to use solid 3/4" or 7/8" thick apron full 8" wide to reduce deflection to the minimum.
However, I am aware that just gluing the solid apron into the legs with mortise and tenon joints would likely be a problem due to moisture expansion. The way around moisture expansion/contraction would be to use narrow maybe 2 1/4" wide rails with floating solid wood panel or glued plywood panel. I don't think floating panel would be appropriate here because the top rail would probably deflect more than I would like when people sat on it. I am thinking of using plywood panel glued to the rails or just using plywood for the aprons but would prefer solid wood.
Another thought is to use a solid wood apron connected to the legs with tenon only at top 2 1/4" and let the remaining apron below that just float up and down against the leg or inside a shallow mortise in the leg. I think 2 1/4" tenon would be adequate for the shear but I expect there would be a problem with racking. It would be nice to connect the bottom of the aprons to the legs in a way that allows the bottom of the apron to float up and down on the legs but held tight against the legs to counteract the racking forces. I hope there is a simple solution to that detail. I am often amazed at how there are simple solutions to things I can't figure out.
I used on-line calculator to estimate moisture movement in 8" cherry and came up with about 1/16" for my location. I think that is enough to cause something to break but maybe the wood would compress and stretch to accommodate it. I don't think so though.
I can't make a decision on what to do.
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