I am building a two level center island in my kitchen. It is a straight run (no legs and not l-shaped). The kitchen cabinets are 36" tall and I am going to build a half behind the cabinets (out of 2x6) at a height of 42" (This will be the heigh of my second tier). From that I am going to be L brackets to support the counter top to be put on the second level. My question is how do I anchor the half wall behind the cabinets? Is screwing it to the floor enought? I plan on covering the half wall with 1/2 ply wood (the same material that the cabinets are made from). I am just not sure that screwing the half wall to the floor is enough. Any help would be greatful!!

Consider using pieces of steel angles, maybe 6x6x0'-5", on at least both sides of a couple of the studs. The upright legs should be through-bolted with two bolts, and the flat leg should be lag-screwed with two lags through the sole plate and ideally to a joist below or to wood blocking between joists. They should keep your half-wall from flopping back and forth.

Anchor your base cabinets solidly to the floor at the toe kicks and to your half wall. They will then serve as lateral bracing to prevent movement of the counter. Some good heavy shelf brackets hidden under the base cabinets will help too. 2x6s sound like a bit of overkill to me. I built the same thing in my own kitchen and it's solid as a rock. I used 3" drywall screws to assemble the wall instead of nails. I didn't use the steel bracing though. Be careful of the width of the counter as the leverage of a wide counter will play into this. Mine is 18" wide.

I saw, somewhere a while ago, an advertisement for a floor bracket which consisted of a piece of steel tube and a base plate which was bolted to the floor. Your studs would drop into the steel tube, and a stiff connection was achieved that way, but I'll be darned if I could find it now.

Anchoring to solidly-anchored cabinets is a good idea.

All you need is a 2x4 wall that are put together with torque screws, then torque screw it to the floor, that is plenty. Drywall screws will work too. The screws to the cabinet will brace like mentioned above and the plywood or sheetrock will prevent any side to side movement. We've got about 15 houses in the last 5 or 6 years alone that has it done this way. Once the cabinet and plywood or sheetrock is in place, there is nowhere for the wall to go.

Thank you. I built the wall out of 2x4s and used the steel angles with lag screws into the floor joists. I also screwed the wall into the base cabinets. The thing is like a rock. Again, thanks a million!!