Make a separate Rib for each piece of pipe that makes up the rollcage.

(you might want to make each pipe(rib) a different size (different OD and ID))

Use the CORNER and JOIN only for a series of lines that define the shape of a bent piece of pipe.

So, when 3 pieces of pipe end at the same point, you need to make 3 different RIBS in CATIA.

Depending on how detailed you want to get, you might put all the ribs into a single PartBody, or you might want to put each rib into it´s own PartBody, or...

In Industry, most companies will make a separate CATPart file for each piece of pipe and then use the Assembly Design workbench to bring all the parts into the rollcage assembly as a CATProduct. The Welding workbench can be used to define how all the pipes are welded together.