joiststimber-framingwoodworking
I've been reading up on joints, particularly drop-in joints. Some of the drawings have a curved cut on the underside leading up to the tenon, sometimes labeled a "scoop cut".
Curves make construction more difficult and time consuming. How important are they to this kind of joint? Why aren't they ever used on through or blind mortise and tenon joints?
I guess I'm curious why you wouldn't simplify the layout like this:
Yellow = deck joists
Green = support beams
Your decking would run diagonally over the joists. If you use 2x boards for decking, that should be fine if the joists are 16" on center. If you want to use 5/4 decking, you should probably put the joists 12" on center so that the diagonal run is closer to 16".
It might be good to consider a sliding dovetail.
Edit 11/16/13 To assemble your bed frame, the M&T joints with captured bolts with nuts to hold the corners together with the legs.
Just as a mention, a good hardwood needs to be used for the assembly. regular 2X4, 2X6s and other similar materials used in the construction of homes will be too soft, and when the joints are put under stress SPF, Hem fir, etc is going to compress and loosen the joints. To use this material you need glue and screws to hold all this together. That is not an option for you I think?
To assemble this, white oak, ash, Black walnut and a dense variety of mahogany would be a good choice, if it is not cost prohibitive. There are other hardwoods too, just give a look. Red oak and Douglas Fir are hardwoods, but very prone to readily split under the stresses you are going to expose the joints to.
Another good thing about some hardwoods, is that you will not need the same dimension of material to have the same strength as 2X framing lumber.
The sliding dovetails are ideal for the cross members into the side rails. To use the same joint for the attachment of the legs would produce a weak joint in my opinion, I have seen it fail in pedestal table legs.
To dissemble this in time, and to ease the assembly, which sliding dovetails can be a bear to slide together, I suggest tapering the dovetail so as it goes in, the joint gets tighter, it doesn't need much, a 1/16th of an inch over 4 inches on each side would dramatically increase the ease of assembly/disassembly. A router jig with a bushing guide on the router would make the female cuts in the rails, and a router table with a tall fence and a shim on the appropriate corner of the support for the slats would cut the male dovetail. A push bock would be handy to control the top heavy stock for this cut.
Best Answer
Notches in structural members such as floor joists cause stress risers that decrease the strength greater than the material removed. A scoop cut reduces the strength only by the removed material and no more. The scoop can be freehanded if it does not show, so it may be faster than carefully cutting a notch.
In constructions where the member is significantly stronger than necessary the scoop is not required and a square cut will yield a neater, tighter joint. Even though there is overlap, the goals and techniques of building construction and rough carpentry are often much different than cabinetry and fine carpentry.