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.

 

[TehMightyEngineer]

Before I dive into this, can we get high resolution images of this? I want to put these into my collection of engineering nightmare images.

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

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

 

[KootK]

I can't get the PDF or the photos to take. Here's a Dropbox link:

https://www.dropbox.com/sh/5wrynk1gust3ua9/AAB0k8y6k3ZJrduSunmgSh50a?dl=0

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.

 

[KootK]

Damn it.. just realized detail C is not going to fly geometrically.

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.

 

[oldestguy]

While one is up there with a welder, how about incorporating the decking to the upper chord with welds there also.? Belt and suspenders.

 

[TehMightyEngineer]

Thanks! Also, wow! Did the coring bit operator fall asleep or something? I've run one of them before and it's really obvious when you're cutting through steel and not concrete.

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

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

 

[TehMightyEngineer]

My first thought is that epoxying overhead is difficult even with proper training. I'd swap those out for a screw anchor from your favorite flavor of supplier.

The other thought is have you actually lost camber? I would hazard a guess that the truss being screwed to the metal deck is limiting deflections in the truss at least enough that you simply need to shore up the truss while welding but not actually jack it up any further.

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

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

 

[WARose]

Maybe I've missed the solution.....but what bothers me the most with this is doing this welding to the chord when it is already missing a portion. Is there a shoring/bracing plan for this while this work is happening?

 

[KootK]

While one is up there with a welder, how about incorporating the decking to the upper chord with welds there also.? Belt and suspenders.

Very thin deck on this. I'd expect any welding to just burn it through.

Did the coring bit operator fall asleep or something? I've run one of them before and it's really obvious when you're cutting through steel and not concrete.

Sounds like it was a young guy showing off for his boss. Just wait 'til his boss gets my bill...

My first thought is that epoxying overhead is difficult even with proper training. I'd swap those out for a screw anchor from your favorite flavor of supplier.

I explored that a little. Wanted Kwik - HUS. Not many good anchors that would yield a suitable embedment depth while also accommodating the standoff dimension. I'm saving asses here. If somebody has to be blinded by a little dripping epoxy adhesive to get it done, so be it. Amenhotep wouldn't have been bothered.

Maybe I've missed the solution.....but what bothers me the most with this is doing this welding to the chord when it is already missing a portion. Is there a shoring/bracing plan for this while this work is happening?

There is a plan. It's mentioned in my details above / on drop box.

The other thought is have you actually lost camber? I would hazard a guess that the truss being screwed to the metal deck is limiting deflections in the truss at least enough that you simply need to shore up the truss while welding but not actually jack it up any further.

Yeah, there is that. I'm asking for the truss to be jacked up to restore any eroded camber. If there is none of that, so be it. Contractor is worried about cracking the slab which is valid so he's just going to use his judgement in that regard. I'm okay with such a losey goosey approach here because:

1) At such a tight joist spacing, I feel that the stake here are pretty low.

2) I mostly just want the shoring/jacking to ensure that the thing doesn't collapse while they're messing with 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.

 

[KootK]

Revised detail C.

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.

 

[WARose]

[blue](Kootk)[/blue]

There is a plan. It's mentioned in my details above / on drop box.

Do you mean the statement: "PRIOR TO REINFORCING JOIST, JACK@ APPROXIMATELY 1/3 POINT TO RESTORE INITIAL FABRICATION CAMBER"?

Seems a little vague to me. (I.e. Jack it where? Top chord? Bottom? "1/3rd point"? What does that mean? Should it be at a panel point? Etc. Etc.)

I walk away from that with more questions than answers. (I (of course) realize this is a work in progress at this point.....but you asked for feedback.)

 

[KootK]

Do you mean the statement: "PRIOR TO REINFORCING JOIST, JACK@ APPROXIMATELY 1/3 POINT TO RESTORE INITIAL FABRICATION CAMBER"?

Yup.

Seems a little vague to me. (I.e. Jack it where? Top chord? Bottom? "1/3rd point"? What does that mean? Should it be at a panel point? Etc. Etc.)

Noted, thanks. That can certainly be made more specific. It's something that's been evolving with the contractor. I may actually opt to jack the neighboring joists instead of the damaged one. There's a panel point and bridging line near the 1/3 points which makes those locations particularly suitable. It's a weird thing that way. The damaged joist has had it's top chord completely severed and, as such, isn't really a truss any more in the classic sense. So I'm not sure what there is to be gained from jacking it. If there's no truss, wheat the point of trying to undo the truss action strains? My thinking is that it would be best to attempt to arrange things such that the neighboring joists are not seeing an undue share of the load at the end of the day. Of course, as TME intimated, the top chord may well still be effectively continuous by virtue of some ability to transfer compression through the deck slab. The jacking/shoring part of the equation isn't something that's going to be able to be handled with a great deal of accuracy.



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.

 

[EngrSTR]

Jack/ shore it up to compensate deflection, safety during repair and to reload joist effectively, Why not weld 2x2x1/4" angles for the damaged portion of the joist plus 6" longer each side on both sides ?

 

[KootK]

Why not weld 2x2x1/4" angles for the damaged portion of the joist plus 6" longer each side on both sides ?

1) Weld access seemed like a problem unless I went with end welds. Usually I avoid transverse end welds in loaded things but, here, perhaps that wouldn't be a problem.

2) Angle in angle scenario creates an interference between the new angle heel and the existing angle fillet does it not? I thought about doing something similar with bent plate but I wasn't convinced that a 2x2 bent plate was a practical thing to request.

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]

KootK: Mulled this over after sleeping on it and other than sizing welds and members I couldn't come up with anything cleverer.

About the only thing I'd check is you have the existing web members remaining plus your new web members you're putting in. Have you modeled any unusual load paths if your web members take only partial load and the original web members also take partial load; potentially putting some weird bending moments into the chords? Any thoughts on cutting out the existing unneeded web members once yours are in?

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

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

 

[KootK]

Mulled this over after sleeping on it and other than sizing welds and members I couldn't come up with anything cleverer.

Thanks for the extra effort, it is appreciated.

Any thoughts on cutting out the existing unneeded web members once yours are in?

That is something that I'd considered a bit. In terms of primary, pure truss action, I'd agrue that once the web is severed all of the adjacent, untriangulated webs would morph into zero force members. That's not perfectly true, of course, given the flexural stiffness of the chords and other mitigating factors.

The contractor would prefer not to remove any more material than necessary. There concern pertains less to cost than to his general feeling that what's hanging in there at the moment shouldn't be compromised any more than necessary. I think his impression is that the remaining truss is what's important and that my repair will be the supplementary suspenders. I see it reversed but respect his wish to do the right thing herre. I convinced myself that the vestigial webbing does no real harm and may help a bit by way of additional chord bracing and potential for V/H shear transfer.




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]

I convinced myself that the vestigial webbing does no real harm and may help a bit by way of additional chord bracing and potential for V/H shear transfer.

Yay ductility. I've also done some fun things with steel trusses thanks it's ability to redistribute forces.

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

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

 

[Huevo]

Square bars in lieu of round bars for the webs may give better access to the welds.

 

[Celt83]

could you do a inverted WT slotted between the top chord angles then provide spacers for the attachment of your chord bars?

Open Source Structural Applications:

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

 

[KootK]

Square bars in lieu of round bars for the webs may give better access to the welds.

I like it. Probably more reliable welds than the flare bevel too. Presumably, this kind of material is commonly stocked for architectural metal stuff.

could you do a inverted WT slotted between the top chord angles then provide spacers for the attachment of your chord bars?

I could, at least between existing panel points. What benefit do you see the tee providing? You seem to be proposing it in addition to the rounds rather than instead of them.

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.