I just remodeled a kitchen, adding granite countertops, a heavy, commercial-style range and ceramic-tile floors. I was concerned about the floor's flexibility. If it was too bouncy, the grout would eventually loosen and pop out.
There was a finished basement below the kitchen, so adding posts to bolster the floor wasn't an option. But the basement's suspended ceiling was easy to remove, which gave me access to the 2x10 joists under the kitchen floor.
As shown in the drawing, I used continuous steel wall-bracing straps as tension rods to stiffen each joist. I wrapped the joists from the top of one end, around the bottom at midpoint and back to the top of the opposite end. I nailed the strap to the joist every few inches with joist-hanger nails. The floor was substantially stiffened by this procedure, and so far, the grout has remained intact.
Tim Brigham, Koloa, HI, Kauai