Acceptable deflection of beam

[peroande]

Considering tearing down a load-bearing wall and replacing it with a beam (glulam or steel). I'm surprised at how much deflection of the beam is acceptable according to design programs and tables. My opening of about 3m would "allow" a deflection of 11mm. I think that's a lot. The replacement will be done on the ground floor. If the beam bends, I'll have a dip in the floor above that would be much greater than acceptable. A bathroom above would be at risk of cracking. Am I thinking incorrectly?

 

[klaskarlsson]

Thinking about tearing down a load-bearing wall and replacing it with a beam (glulam or steel). I'm surprised at how much deflection of the beam is considered acceptable according to sizing programs and tables. In my opening of about 3m, a deflection of 11mm would be "allowed." I think that's a lot. The replacement will be done on the ground floor. If the beam bends, I would get a dip in the floor on the upper floor that would be much larger than acceptable. A bathroom above could risk cracking. Am I thinking wrong?

But that deflection would be static.
That is, when the beam is in place and loaded, it can bend down a maximum of 11mm, then it won't move (except for the wood itself).
If you already have, for example, tiles on the upper floor, you probably need to set the beam in such a way that when loaded, it aligns with the upper floor's current floor as the wall provides.

Or you could oversize so the deflection is smaller.

 

[Gabbe1]

Traditionally, L/300 is accepted as deflection in residential buildings. However, it should be remembered that this deflection is calculated from the "worst case", in practice structures are rarely subjected to such high loads and thus the deflection becomes less.

If you plan to use a steel beam, it is possible to prebend it slightly before installation, thus achieving a straighter beam under load.

 

[peroande]

If you already have something like tiles on the upper floor, you probably need to install the beam in such a way that when it's loaded, it aligns the same way with the upper floor as the current wall does.

That's exactly what I'm wondering how to do.

 

[myrstack]

In principle, you support against the overhead structure to prevent exactly what you're concerned about when switching off. So the deflection of the beam doesn't mean that the deflection is noticeable in the overhead structure. In some cases, you may need to guess what deflection you will practically get, and then use some shims of equivalent thickness. Unfortunately, in this exercise, you have to guess the deflection that will occur after the beam takes the initial load, so with a bathroom directly above, it's still tricky.