Aggressive Steel Joist Repair

[KootK]

A steel joist on one of my projects found itself on the losing end of some plumbing work. We'd originally wanted the joist supplier to tend to the repair but their opinion was that a joist this badly damaged couldn't be repaired. Since I've gone and attempted to repair it anyhow, I thought that it would be beneficial to post the scheme here for critique. See attached PDF, replicated below. Some deets:

- 24' long
- 3" non-composite slab above
- 14" deep
- Uncommonly tight, 32" joist spacing
- Residential floor space above
- No new capacity required. Simply the reinstatement of the original capacity.
- Existing chords are 1.5" x 1.5" x 0.125" angles.
- Existing webs are 1/2" dia rods
- Panel point spacing = 24" classic.

It's tempting to explain my rationale here but:

1) Much of it should be self evident (I hope) and;
2) I don't want to taint the critique with a bunch of upfront "selling".

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[NS4U]

I assume the purpose of Detail C is to provide a means to support the cut ends of the joist TC, is that right?

 

[KootK]

I assume the purpose of Detail C is to provide a means to support the cut ends of the joist TC, is that right?

Right. I'm not convinced that the localized rod reinforcing restores the flexural stiffness of the undamanged chord about either axis. I could likely spend an afternoon justifying the chords acting as cantilevers for buckling and make it work. Even if I did though, I probably still wouldn't trust it.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Celt83]

my thinking is it would restore some As between the cut regions and maybe get rid of the need for the HSS's, the rounds would serve to bring the centroid of the area back up closer to inline with the existing joist chord and transfer force across the top where you won't get any weld on the T stem.

Edit: yeah this wouldn't do anything at the high panel point locations since the existing bar would be in the way.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[KootK]

Yeah, I think that the tee could only go a) where no existing web panel point was and b) where no new web panel point will be. Which pretty much gives the tee nowhere to be. My original version of this encompassed similar thinking regarding trying to give the restored chord some flexural stiffness across the repair. See below

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Agent666]

Honestly I'd just abandon the joist and try throw a new beam next to it to re-support the slab (jack load into the beam with grout jacks and grout gap once beam is preloaded).

Its too far gone, let it die....


EDIT - or under it if ceiling space permits?

 

[KootK]

Abandoning the joist is pretty undesirable as there's already services running through it. I actually don't feel that the reinforcing scheme has turned out to the all that onerous for something that was supposed to be "impossible". I've done more extensive work just to get new RTU's supported. And this can be done from a small step ladder.

As a matter of policy, I almost never say never when it comes to strengthening steel joists. Worst case scenario, "reinforcing" the joist just morphs into effectively building a new one using the old for scaffolding. I will, however, happily attempt to deter folks with the price tag.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

Agreed with KootK, this looks more favorable to me than installing a new beam (I've done both). Not by much, but definitely not "impossible".

Then again, I think KootK and I are similarly minded that if we can justify having fun with engineering then all things equal we go that route rather than the "easier" way.

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/