UKC

I've got one of those doorframe pullup bars that cam themselves into the doorframe producing a horizontal force on the frame itself. 

I'm currently doing some weighted pullups and I'm currently hitting around 110kg. 

Obviously it's massively dependent on quality of the frame and construction but how much weight would it take for a well built doorframe to break? Would it just go with a bang or would there be signs ? 

Aled

It depends is probably the best answer.

For the trim at worst two little brad nails (1.2mm thick, 15-30mm long).

The frame itself is probably pretty strong if the wall framing is good quality.

There could be subtle aural or visual signs, but ususally if it´s going to go, it goes with a bang. Similar feeling to your toe suddenly parting company with that crucial microedge on a slab: absolutely no warning, and possibly dire consequences. Speaking from bitter experience.

There was an article on here a while back about a girl that was paralysed from one failing. Made me never want to use one!

Use the proper tool for the job. You are risking injury as well as inviting diy which is possibly worse.

Was this with an over the doorframe bar or one of the flimsy in-doorframe expanding bars that have cups you can mount?

Over the door frame, it was an old house and the wooden lintel suddenly gave after a few months of regular usage with no signs of weakness (the lintel, not me). 

Doorframe strength depends on a lot of factors none of which you will be able to judge until you risk an injury. Older houses tend to have a reasonably substantial wooden frames, newer houses it could be a few panel pins and coats of gloss emulsion holding the Architrave on. Its not a structural feature and was never designed to take these loadings, a better option is the screw in type that expands into fixed cups 

I have seen pull up bars that mount within the door frame, that have a kind of parallelogram set up at each end, so the more weight added the more force it pushes out with (bit like a cam). 

I cant seem to see to find a link now, but could be good alternative? 

I think this was with one of the expandable in-frame ones without using screw-cups.

A friend ended up on his back under the broken door frame lintel and pile of plaster! 

Just a reminder for anyone using one of these. They CAN be used safely but they DO need very careful attention. In particular, because they expand by rotating the outer bar wrt the inner one, always make sure you're doing pull-ups on the correct side, i.e. so that your pulling acts to tighten the grip rather than slacken it. Also, I've found it's very hard to tighten the bar really well as the metal surface is quite slippery, so adding a bit of fingertape (or any other tape that isn't slippy) to at least one section of the bar makes a big difference to how tightly you can secure it. Never a bad idea to put a cushion underneath as well while in use, just in case.

... ... I'm currently doing some weighted pullups and I'm currently hitting around 110kg....

You must be an absolute BEAST. I'm in awe, I've never seen (IRL, not YouTube or somesuch) anyone doing sets of weighted pull-ups with more than 50kg (saw that a couple of weeks ago with someone doing 3 sets of 5 with full RoM and perfect form. But 110??

I think (presume?) that's including bodyweight but still impressive (to me at least)