New hole near composite steel beam 3

[phuduhudu]

We are putting a new hole through a composite RC slab on metal decking adjacent to an existing composite steel beam. The hole is within the effective concrete flange width but not right next to the beam. I can frame the hole with new non-composite steel members but I can't find any guidance on the effect on longitudinal shear in the slab. Presumably if the flange is now not symmetrical with a wider flange one side of the beam, this means that the longitudinal shear will now be correspondingly less on the side with the narrower concrete flange. I am asking because the longitudinal shear is critical here and obviously I can't do anything to the concrete or the shear studs on the beam. All the guidance I can find relates to either an internal beam with symmetrical flanges or an edge beam where the flange is all on one side.

 

[mike20793]

Can you conservatively assume the effective flange width is decreased over the entire length of the beam and compare the shear flow and number of studs?

 

[KootK]

I don't think that the hole will affect your longitudinal shear situation at all. I see the only real issue as being a reduced compression flange width. If you're at mid-span, it might be an issue and I'd run some numbers on the revised compression block depth. If it's a low moment location, I'd just frame it out and move on.

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.

 

[phuduhudu]

Thanks for the responses. I don't have a problem with the reduced flange width and it is not at midspan but there are a lot of shear studs which gives me a lot of longitudinal shear to deal with and this is a problem for the whole length of the span.

 

[KootK]

I stand by my original comment that this probably doesn't affect shear flow through the studs in any meaningful way. If anything, it might reduce demand on the studs if more of the section compression is moved into the steel section. If you provide a plan drawn roughly to scale and showing the opening, I'd be happy to try and prove my point.

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.

 

[msquared48]

How big is the hole, and what shape is it? How thick is the metal deck + concrete infill?

Mike McCann, PE, SE (WA)

 

[phuduhudu]

The concrete is 130mm thick on a 1.2mm metal deck with 50mm ribs. I have attached a drawing of the beam and shown the hole. I am not concerned about the shear studs. I know that these can take the shear. In fact it would be good if there were less because then I would have less shear to transfer into the concrete flanges. The issue is the longitudinal splitting shear in the concrete at a line just above the first metal rib either side of the beam. I think I have now satisfied myself that putting a hole in does transfer shear from one side of the flange to the other as it becomes asymmetric. As I don't know what reinforcement is in there I have to assume the longitudinal shear was already at the limit and this would make things worse. I am happy to be proved wrong. The question is what else can I do? Perhaps I need to strengthen the steel beam by adding on a welded or bolted plate to the bottom flange? The other thing is that I know the steel beam would work as a non-composite beam but the concrete shear connection would have to fail first and that isn't necessarily a ductile failure.

 

[KootK]

Ah... it's an enormous opening. That changes things. I thought that we were talking about a mechanical shaft etc.

First off, if the beam works as non-composite, I believe that most engineers would be satisfied to leave it at that. I know I would. If it were necessary, I'd just count on one sided composite action like an edge beam.

Other thoughts:

- when the opening is cut, the flange compression forces will redistribute. You may well get some concrete cracking as that happens.

- failure of the studs is pretty ductile actually. Usually it takes the form of localized concrete crushing low on the studs. Were it not somewhat ductile, we wouldn't be able to space the required studs uniformly.

- for a variety if reasons, many beams in a composite floor are often designed to work as non-composite but are provided with a nominal amount of studs anyhow. Nothing ever seems to go awry so, at least, there's precedent for this not being a problem.

- have you calculated the longitudinal shear capacity of the concrete either side of the deck? It should be greatly in excess of the demand. I wouldn't assume that it was anywhere near maxed out originally.

- even with the new opening, you haven't actually changed the longitudinal shear capacity of the deck right beside the beam flange. It would still be your allowable shear stress multiplied by the distance from the peak moment to the end of the beam. Or something similar. You'd just reduce longitudinal shear demand on the opening side of the beam as there would be less compression in the flange on that side after installing the opening.

You've raised some interesting points here that we rarely consider in these situations.

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.

 

[phuduhudu]

Thanks KootK. I have picked up that often beams are given a few studs even when they are non-composite. My problem was that the beam has many more studs than required for full shear interaction. That is good in that the studs won't fail but the program that checks the longitudinal shear at the nearest lap joint calculates that shear based on the number of shear studs times their ultimate capacity and this causes the concrete to fail in shear but of course that is way more than the actual longitudinal shear so I will do the calc by hand and let the software guys know about it!

 

[KootK]

Hey, were it not for software error messages, where would we get our mentoring nowadays?

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.