I have a 100 +/- year old oak dining set with chair seats that have become quite brittle.
Over the years planks on the chairs have split and I have glued them back together. At one point I added two 1/8" x 1 1/4" aluminum bars screwed to the underside to provide additional support, but this has had limited success and the small screws have popped out at places over time. Recently, two seats broke and I am now seeking a better way to reinforce them. My first thought was to provide a wood cross member support at the metal bar locations secured to be part of the seat. In this approach, the reinforcement pieces provide additional stiffness to the seat, but do not connect directly to the legs. For this reason, the stiffeners must be well attached to the seat to transfer any loads back to the seat at the corners where the seat rests on the legs. Although this can be accomplished with continuous gluing, there is the problem with the different expansion and contraction due to grain direction of the seat and stiffeners. If I attach the stiffeners to the seat with screws in slotted connections at each end of the stiffener, I am placing all the weight from the center of the seat on the two end screws that secure the cross member. The seat is 7/8" (or less at seat impression) thick and so a lot on load is placed on a small area of a very old oak seat where the screws penetrate.
How can I refine this concept or is there an alternative means of support that I have not considered?
EDIT: Issues raised in several answers led to adding more info.
I double checked and three chairs have cracked and not one crack is at a glue line. I added a photo of another chair to illustrate the cracking. In addition, these chairs have been well treated. No standing, limited use and I try to use other chairs with heavier guests. The wood has split itself. I believe the screws in the aluminum straps may have failed because they do not provide adequate vertical support. As the chair is loaded, the seat sags and places lateral load on the screws causing them to work loose over time (they only penetrate the wood about 1/4"). Ast Pace suggested that the triple leg braces might make the legs more resistance to movement in the seat. That's interesting, but I'm not sure how to determine if it is so and would be substantial enough to cause the wood to split.