Can you straighten a beam with an angle iron?

[Dragon85]

The angle iron should be bent the other way if everything is to be straight in the end.
I would suggest that you cut notches on the inside to make the beam a bit softer and lock it all with a plank on the top side.

Protte

The wood on the edges is quite worm-eaten, so there's some compression on the edges. However, the heartwood is entirely untouched by the woodworms because it's old heartwood that seems to resist most things trying to break it down. Cutting notches on the inside might be a bit wrong in that case. It's the outside that probably due to moisture from outside has swollen and needs more space, which stretches the outside as I understand it. The inside is then compressed and if I bend it back, the inside will be stretched again while the outside will be compressed.

 

[fribygg]

It’s not exactly the easiest thing to replace. Partly because I don’t have such timber readily available since it’s about a nearly 7 m long beam, and it’s difficult to replace because I’m basically on my own and such a beam is very heavy.
I also can’t drag it out for too long as I’m trying to get things in order so that I can store more timber which is currently lying outside under a tarp that I need to remove from the parking lot before summer.

You can screw and glue together a beam from regular planed two-by-fours; you can probably fix that this afternoon since you have wood at home. It would likely be both faster, cheaper, and simpler than buying angle iron.

 

[bygges]

Well, it's an option, but the ridge beam and the upper arms in the gable are not bent in the same way as the forearm is, which makes it a bit more difficult to build a straight wall that follows them. Some of the forearm beam will stick out beyond the wall frame later on when I replace the panel on the outside. Part of the wall frame will also hang over the inside of the beam, creating a void between the bottom of the wall and the brick wall below.

Maybe you have to accept some compromise? You are working in a structure that is anything but precise from the start. Even modern houses have, like, 120 studs that have warped over the years so interior walls become slightly banana-shaped, etc. It seems doubtful to aim for everything being completely straight and perpendicular.