Increase shear capacity of KCS Joist? 1

[dylansdad]

I had designed KCS bar joist for future HVAC units. However, the HVAC units are now heavier and larger (changed Mechanical PE during a re-design). As such some of the KCS joists, installed, are overstressed in shear. Any suggestions about how to increase the shear strength? Thanks.

 

[phamENG]

Nope. Those are about as shear capable as a joist can get. Sounds like you need to tell the MEP the bad news. Let them and the architect and contractor know that the structure was designed and built to X. Now you want it to do Y. Either go back to doing X or you're going to have to rip out the roof and put in stronger joists or add intermediate supports (and all the mess that goes along with modifying joists).

Mechanical units are expensive, but so is the structure. Changing things has consequences. Be a team player (as you clearly are by asking this question), but the structure can only do what it can do. The MEP engineer wouldn't agree to cool 20,000sf with a unit only capable of putting out enough for 15,000sf, so you shouldn't have to make framing the can only carry 15kips carry 20kips.

 

[dylansdad]

phamENG,
Thanks for the reply. That is what I thought also.

 

[KootK]

I would think that one could reinforce the KCS joists for the additional shear just as one reinforces a non-KCS joist for addition shear. If you're unfamiliar with the techniques for doing that, let us know and we'll point you to the pertinent SJI documents. We'd need to see an overlay of your demand and capacity shear diagrams to get sense for the extent of the reinforcing required. If it's midspan point loads with larger loads, you may wind up reinforcing pretty much all of the webs. If it's point loads nearer the supports, you may only need to reinforce between the point loads and the nearest supports.

 

[phamENG]

KootK - I don't recall the exact circumstance, but speaking to an erector once he was cursing a detail for trying to do that. Kept complaining that there wasn't enough room to add the reinforcement. I think he eventually got it on there, but I'm not sure it was quite what the engineer had in mind. It may have just been that particular configuration - but I've avoided trying it ever since.

 

[KootK]

KootK - I don't recall the exact circumstance, but speaking to an erector once he was cursing a detail for trying to do that.

Me no understando... for trying to do what?? Joists get reinforced for shear all the time. I guess I'm failing to understand what makes this case so exceptional other than, perhaps, the extent of the reinforcing which we don't know yet.

 

[phamENG]

Trying to reinforce a KCS joist. They tend to use angles for web members and have vertical webs. There's a lot less space in there to make the reinforcement happen.

And the alignment of the reinforcement is a bit harder to get right. Regular K series it's just a matter of welding an angle to the top and bottom chord angles next to the round web bar. When it's an crimped or double angle already...it gets challenging.

 

[KootK]

Yeah, I saw the KCS part. That just doesn't deter me. If we're talking about the kind of KCS joists shown below, I actually think that it makes some things a bit easier. Consider:

1) A single angle can be made a double angle while keeping the web force fairly concentric to the truss.

2) Dealing with connection on the outisde of the chords is usually a boon for evaluation and potential reinforcing.

3) The verticals don't actually participate in the shear resistance of the truss anyhow, so that's purely a joint congestion issue.

I'd take a KCS with hot rolled angles over some stupid, cold formed hat section thing with snaking tube webbing any day of the week.

 

[phamENG]

so that's purely a joint congestion issue

That's my primary point. The congestion makes the new connections difficult to make and prone to mistakes or 're-alignments' in the field that may not quite get you what you expected or wanted.

Or maybe the guy was just a doofus and I fell for his shenanigans. Certainly wouldn't be the first time.

dylansdad - do you have the shear diagram with capacity overlay? If it's near the end with only a couple web members, maybe it's worthwhile taking a shot at it. Maybe you could just have them remove the webs and weld a steel plate in with some stiffeners on it? That could be neat.

 

[KootK]

This freebie doc shows some of the concepts that are commonly used for reinforcing joists: Link. They've got an answer for darn near every scenario. If this were my problem to deal with, my path would be to take a hard look at the details of the actual truss and loading before rejecting the reinforcement option out of hand. I'm not of the opinion that KCS = unreinforceable.

 

[dylansdad]

Thanks for the back and forth, something to consider. I have reinforced joists before, and thanks for the link. Worst case scenario is demand/supplied=4.39k/4.0k.

 

[KootK]

That's only 9% overstress. My first stop would be sharpening my pencil to see if I could plausibly get it down to 5% so that I could do nothing. If that doesn't pan out, there are a number of ways to shed 9% without too much fuss. Lateral distribution would probably be my first choice. How deep are these trusses?

 

[dylansdad]

Koot,
14 to 16" deep, webs are rounds. And my pencil is a nub already.
Thanks for the reply.

 

[phamENG]

Oh. Webs are rounds? Wow. I'm accustomed to seeing angles for those. Then you can probably do something with it with a bit more reliability, especially as close as your DCR is.

Here's another good doc if you need it: Link *Edit: this is essentially the same as KootK's post - his is a powerpoint, this one is a white paper.

 

[dik]

Compression or tension elements? With compression elements you can occasionally stop the reinforcing short of the panel point due to reduced slenderness increased stiffness in the middle of the web member. Can this help?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dylansdad]

pham,
Web rounds surprised me also. Thanks for the link.

 

[EngineeringEric]

Not to jump in late, but can't you just share the load more or add new joists into the mix that have a shallower seat/field splice?

It seems like its simple enough to say it doesn't work. We can change the unit or reinforce, here is the roof reinforcement ideas (load share or new joists) and here is my additional fee for that work

I usually don't like full length reinforcement of joists due to the construction costs, chances of error, the heat it produces in small chords, it usually only adds so much capacity, it isn't pretty, and it still takes a lot of steel. If timelines work, i'd rather a post-installed bar joist or load sharing to more joists when applicable.

 

[bones206]

I had a retrofit job recently where post installed spliced roof joists were specified to help support a new snow drift. The lead time came back as 8 months! I was forced into a much more expensive and labor intensive existing joist reinforcement scheme simply due to the current supply chain situation.