I believe if you know the maximum amount of stress(internal force) that is to act on the U-shaped bar and also the area of the bar,you can be able to calculate the max. allowable load for the material.
That is since
Ss= P/A where Ss is the max. allowable stress on the material,P=? is the max. allowable load on the material which is to be determined,A= area of the material.
Making P subject of formula we can be able to obtain the max. allowable load on the material. That is,
A= P/Ss.