Most of the doorframe pull up bars the force goes into the wall above the frame, not the frame itself. Doors have studding around them and that is what is taking the weight.
I want to get a few woody bands to go with mine. I find the chair support awkward and my shoulder isnt repaired enough to do max weight. I can do about 2 full pullups or 3-4 negatives, but my healing shoulder does NOT like it. At the gym I am doing 50% bodyweight on the gravitron and that is about appropriate.
MTA: because it is rare I dont have more to say
"When I got too tired to run anymore I just pretended I wasnt tired and kept running anyway" - my daughter Age 7