"How much deflection can be allowed? "

Basically zero.

"Basically zero"

Depending on ones definition of "basically," nobody would have any granite counter tops if that were true. Certainly true for my definition of zero. Basically zero is the value of your comment.

That is why you do not see a lot of wood framed tables with granite tops.

Even cabinets with sides all the way to the floor exhibit essentially zero deflection under load.

Stone is a brittle material, and cannot survive significant movement in its support.

You would likely require massive hardwood aprons (think both thick and tall) or metal under structure (think how a large fish tank is supported).

Why do you think the overhangs for granite counter tops are so small?

Metal rods cut into the underside or other metal supports are often required past about 12 inches (or less depending on the exact type of stone).

The longest counter dimension with 3 cm granite that is not supported is a DW, 24 inches wide.

Apron height helps a lot (doubling decreases deflection by 8x), but to be able to sit at the table, one needs to get one's legs under there, which limits apron height to about the typical 4" for a 29" leg. Doubling the width of the apron only decreases deflection by a factor of 2. A fifth leg near the center of this size of table would stiffen it considerably. Metal is surprisingly bendy, such that adding angle iron to the aprons would not have enough effect to bother. I may have to go to a glass top over wood instead of granite slab.

Now you know why you do not see a lot of stone slab tables.

You have to pick the correct metal shape.

You are incorrect about iron.
The flex of the metal can easily be removed as an issue with correct design and construction.

When chosen and used appropriately is will significantly stiffen wood.
You could cover the inside of the apron with about 1/8 in steel and have very little deflection (look up 'flitch beam').
It will take a number of screws and barely interference sized (about 0.001 or so) holes for the screw shanks
through the metal.

A piece of angle wrapping around the inside to the bottom edge would also add significant strength, as would just a flat strip on the bottom of the apron for its length.

A huge amount of flitch beam performance comes down to hole size in the metal matching screw shank diameter tightly, and enough screws and resulting bearing area to make the two material behave as a single unit and the wood does not crush under loading at the screws.

The screws limit buckling of the metal away form the wood so it stays flat in place with very little deflection for the composite assembly.

Angle are channel (open side down) are common since they have the vertical dimension required to get decent strength.

Just like strapping the bottom of joists with 1/8 in steel strap (plenty of flex there) stiffens the joists greatly The method relies on the fact that the bottom of the joist MUST get longer as the joists sag allowing even a floppy piece of 1/8 thick x 1.5 in wide strap 12 feet long on the bottom of the joist to stiffen it very significantly (the same as putting a huge wooden lip on the bottom of the joist)

Any suggestions on how to closely size (barely interference) the screws and holes for the added metal plate/angle? If not readily done, is there some sort of tapering ring that would fit inside the hole, to provide a compression fit as the screw is tightened?

"Any suggestions on how to closely size (barely interference) the screws and holes for the added metal plate/angle? "

Use number size drill bits that match the screw shank.

Remember the hole is ALWAYS a few thousandths over the bit size.

You might even need to try metric bits.

The most accurate holes are reamed after drilling them as close as possible (but still undersized).
Reamers come in 0.0005 inch steps (or even smaller).

I've decided to just go with wood to beef up the aprons on the long sides. This gets the deflection to under L/1300 with me standing on the center of one edge, which I don't intend to do and hope that nobody else does. I will also add a third rail longways, teed off the two shorter apron sides, 1-5/8" wide x 4" high. I'll also put some blocking in the short direction at a couple of locations.

For beefing up the aprons -- is it best to just glue the extra board to the inside of the existing apron, or should I also screw it?

I'm using the legs in the link. My table will be a foot shorter and and inch narrower than the one in the link. Cherry, stained almost black.

Here is a link that might be useful:

"For beefing up the aprons -- is it best to just glue the extra board to the inside of the existing apron, or should I also screw it? "

Make sure you use a glue that will not exhibit creep.

ALL the polyvinyl acetate glues have creep.

Elmer's, Titebond (in all it grades).

You need a resorcinol glue or hide glue (even the liquid hide glue).

Thanks for the clue on glues -- I hadn't realized the creep issue. Makes for interesting reading.

The Osborne base also uses a screw to help draw it into the mortise/tenon joints -- pocket hole in apron, screw into the leg. The wood corner braces have screws into apron, and an anchor bolt from leg. They don't say to also glue the braces, but I'm considering doing that. Should I peg the mortise / tenon joints, or is the improvement not worth the risk of splitting a tenon?

I went with just adding two longwise rails (1.625"x4", 54" long), plus some cross bracing. Still to be added is 5/8" plywood, then the 3'x5' granite slab. Worst case is that I sit on the edge of the long side, but still should be less than L/1300 deflection. Standing in the middle will give less than L/2000.

This post was edited by attofarad on Wed, Mar 27, 13 at 13:02