[GregH]

First, the flanges are not meant to hold any amount of weight and will be guaranteed to break as they are brittle.
Not only could they break but the screws holding them up could pull out.
Secondly, the galvanizing on the pipe is toxic and will be forced into your skin.

You need to make this out of black iron, either all pipe or a pipe grab bar and structural steel attachments.
It will also have to be welded as you will not find a screw-it-together-yourself solution that will be safe.

Attaching blocking between joists like OhioDraft suggested but with 3 1/2" screws instead of nails is the first step but where the bar fastens to the blocking should be from the side so that the force is on the shear and not the pull.
You would also want to reinforce this with a diagonal brace to prevent it from swaying.

You should look at a commercially made one and the instructions for installation to see how others do it or to be safe just buy a good quality one.
A fall would make the exercise benefits pointless if you wind up in a wheelchair or worse.

Let us know how you make out.