How is the U shape loaded ie where does the force act and how is the U supported.I may be able to help but it will depend on what other information you can give.
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,
In continuation,
P= A.Ss
where P is the allowable load you are looking for,A= cross-sectional area of the material and Ss the allowable stress on the material which you must know before you can be able to get P.I believe you can be able to calculate the area and get it on your own.
The stress needs to be a factor-of-safety away from the yield strength.
Other factors:
fatigue loads
dynamic loads
reduced area at threads
heat affected zone if welded
stress riser at thread root
corrosion
what is at risk (human life / water presure)
expected lifetime
