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Lateral bracing to existing steel beam

Lateral bracing to existing steel beam

Lateral bracing to existing steel beam

(OP)
I am designing lateral bracing to several existing steel beams that were installed many years ago, at the underside of a waffle slab. Attached are 2 options, but both require a fair amount of field welding which is expensive and time consuming. The first option probably does not work because the minimum required concrete thickness for the KWIK bolt is 3" and the waffle slab is only 2½" thick (although perhaps the topping could be considered as adding to the slab thickness, but probably not prudent to do so).

Does anyone have any ideas for a better brace, with less field welding? Note that the position of the steel beam varies relative to the rib of the waffle, but it never is directly under the rib.

RE: Lateral bracing to existing steel beam

I would consider a third option--fasten a WT to the underside of the concrete joist, and weld the brace to that.

And I wouldn't worry about the welding. A lot of times field welding is preferred by contractors to field bolting, because there is more tolerance in the field.

DaveAtkins

RE: Lateral bracing to existing steel beam

(OP)
Can't really fasten anything to the bottom of the rib because of the narrow width (5") and the congestion of bars within the narrow rib (#8 and #6 bars - although that would not meet spacing requirements today, it did at the time this was constructed in the 1950's). I realize that field welding allows for more tolerance than field bolting, and that some field welding is required, but the contractor wants to minimize it because of time and cost. That is in accordance with my experience as well. I think it is generally accepted that field welding should be minimized, but sometimes it is required. anyway, irrespective of that issue, even if everything is filed welded, there is still the question of how to connect to the concrete.

I have an idea though...use a "U" shaped plate say 4" long and 4" wider than the concrete rib, to fit under the rib and extend say 4" up each side of the rib. The bottom of the U shaped plate would be shop welded to a small L strut extending to the flange of the steel beam that is to be laterally braced. The spaced between the U shaped steel plate and the concrete rib would be filled with grout. I will try drawing it up and see how it looks.

Thanks for the comments.

RE: Lateral bracing to existing steel beam

if you can do #2 from both sides, no tension is required from the anchors, although that increases the welding more.

I cant tell what is directly above the steel beam, but below the underside of conc rib. if there is space there, you could use a single angle on top, exting out in both directions, and then add some type of stub up between each rib, using compression only, would avoud you having to drill anything into the concrete.

RE: Lateral bracing to existing steel beam

A couple more ideas.


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.

RE: Lateral bracing to existing steel beam

(OP)
To Gagne73, Kootk and BARetired; Very good suggestions.
I am a bit reluctant to field drill a hole thru the beam flange. Can they do that neatly without weakening the flange? I like the BARetired idea, but I am not sure if it may be grouted above the beam, all the way up the waffle void to the underside of the 2½" slab. It is currently all covered with spray fireproofing so I can't see until removing the fireproofing, but I suppose could break any grout out locally. Is a bit confined to reach in to weld on top of the beam, but we can ask the welder if he can do it.

Thanks for the time and thought you all put into this. It is very much appreciated.

RE: Lateral bracing to existing steel beam

Just a thought on KootK's second suggestion of making the beam torsionally resistant...I assume that is what it is for. Why not avoid the cut and put the HSS all on one side?

RE: Lateral bracing to existing steel beam

Quote (ajk1)

I am a bit reluctant to field drill a hole thru the beam flange. Can they do that neatly without weakening the flange?

They can. As long as flange net rupture strength exceeds gross yield strength, you're good. We do it all the time for wood nailer attachment.

One attractive feature of BA's scheme is that all of the assemblies would be identical.

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.

RE: Lateral bracing to existing steel beam

ajk1,
The ability to drill the flange acceptably is just dependent on the equipment. The dimensions of the drill, i.e. length including bit, and clearance from web, need to be checked. Overhead drills like this are heavy, and magnetic is best. You probably don't need a 3/4" bolt.

RE: Lateral bracing to existing steel beam

Quote (hokie66)

I assume that is what it is for. Why not avoid the cut and put the HSS all on one side?

That's exactly what it's for. I considered one side -- and even bottom -- but the asymmetry gnawed at me. In the interest of avoiding the cut:

1) Channels if they'd work.
2) Two 4x4 angles, diamond like. Same torsional value.

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.

RE: Lateral bracing to existing steel beam

I would break a U shaped profile like you suggested and bolt thru the thick region of the waffle. We have done this before. You can weld whatever you want to the U/S of the u shaped profile. This makes drilling thru the concrete easy and you have a simple overhead weld + side weld to the beam. Welding to the top flange is a good idea, but I am not sure you will have space for the welding rod.

RE: Lateral bracing to existing steel beam

KootK,
You would have to have two 8x8 angles to achieve the same torsional profile.

The asymmetry would not bother me for torsional resistance, but I like BA's simple top flange brace better.

RE: Lateral bracing to existing steel beam

Quote (hokie)

You probably don't need a 3/4" bolt.

Heck, could probably be unistrut and some screws for all the difference it would make.

U-shoe thing would be extra slick if the bottom of the shoe were level with the underside of flange. You could use a horizontal angle brace and conceivably bolt all of that together.

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.

RE: Lateral bracing to existing steel beam

(OP)
Is torsional strengthening of the web alone sufficient to prevent lateral buckling of the flange?

For how much of the length of the beam would the torsional strengthening be required? How can that be determined?

Yes I can I believe make the bottom of the "U" flush with the beam flange.

Thanks everyone for the ideas and time that you have put into this. It is very much appreciated. Now I have to think and decide.

Another matter - has anyone ever had a problem with this site, that after typing in a whole lot, it suddenly disappears, possibly due to inadvertently touching the wrong key I suppose? Is there any way of getting it back when that happens?

RE: Lateral bracing to existing steel beam

Quote (ajk)

Is torsional strengthening of the web alone sufficient to prevent lateral buckling of the flange?

It can be. You just improve the torsional parameters that go into your LTB bracing calc until you don't need bracing.

Quote (ajk)

For how much of the length of the beam would the torsional strengthening be required? How can that be determined?

It could surely be cut short a bit but I doubt it would be worth the hassle of trying to sort out the theory.

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.

RE: Lateral bracing to existing steel beam

(OP)
ok, thanks.

RE: Lateral bracing to existing steel beam

if you can have an angle across the top extending to both sides, you wouldnt need to worry about drilling. simply a fillet at the edge of the top flange on each side, welded to the underside of the angle. Then similar to kootk said, field bend the angle to bear inside the concrete drops, or field weld another small piece to bear up in that same spot.

if field drill is needed, it would be an electromagnetic drill likely, depending on flange size, may be a little tough to fit, bot not impossible

RE: Lateral bracing to existing steel beam

"Another matter - has anyone ever had a problem with this site, that after typing in a whole lot, it suddenly disappears, possibly due to inadvertently touching the wrong key I suppose? Is there any way of getting it back when that happens?"

what often happens to me on all forums, bulletin boards and documents is I highlight a word or short section with the mouse, then start to type what I want to overwrite the highlighted section. Apparently the un-tended mouse shifts a little bit, and highlights a huge alternate section when I'm not looking, so my new typing replaces it.
<CTR> Z often takes me back

RE: Lateral bracing to existing steel beam

(OP)
To Gagne73: thanks for the clarification. That seems a very good way of doing it, it the angle can be reliably shaped to bear against the sloped side of the concrete rib. The only problem may be that in some cases the top flange of the steel beam is about only an inch away from the side of the rib, so may be a bit of an issue to sneak the angle up in there, but perhaps not. Maybe I will give the contractor a couple of options and let him decide which to do.

To TMoose - very interesting. I will give CTR Z a try next time it happens.

RE: Lateral bracing to existing steel beam

I feel dumb. Why has no one asked why the beam needs new lateral braces, and for what purpose? Is there additional load being added? Why didn't the beams have this in the first place? What is the beam even supporting and how does everyone seem to know all of these things given no information? sad If it's for LTB it only needs 5%ish of the flange compression so im sure you could get a fixing to work within the slab comfortably. I'll leave now

RE: Lateral bracing to existing steel beam

No need to feel dumb. ajk1 will tend to split his questions up into logical chunks with each being a seperate thread. So, sometimes, there's some backstory to be had elsewhere. For example, we previously discussed whether or not the grout connection alone would be capable of bracing the beam which was, in all likelihood, the original designer's assumption. Beyond that, if ajk1 tell me that his beam requires some LTB bracing, I simply take him at his word.

One of the thinks that I enjoy about ajk1's threads is that the crux of them usually revolves around coming up with a creative, build-able detail for a tricky renovation situation. They're like clean little homework problems where you don't need to bother with any math. Plus you get to do some drawing and compare your answers with the other kids.

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.

RE: Lateral bracing to existing steel beam

Oh well that's a relief shadeshappy

Quote (KootK)

coming up with a creative, build-able detail for a tricky renovation situation.
heh well it'll be hard to top bolting on two 1/2 HSS sections to prevent LTB pipe

RE: Lateral bracing to existing steel beam

Quote (BD)

heh well it'll be hard to top bolting on two 1/2 HSS sections to prevent LTB pipe

Right? And you'll notice that, despite that solution being whacky and an unlikely final choice, ajk's still willing to give it some legitimate consideration. Safe space -- I love it. And, one time in ten, that whacky solution winds up being the dark horse best solution.

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.

RE: Lateral bracing to existing steel beam

(OP)
Kootk has beaten me to it with the answer and managed some very kind words for me in the process. So thanks to BowlingDanish for giving Kootk the opportunity to be so kind. Basically it was designed without lateral bracing, but the engineer who designed it 20 years ago now agrees with me that it should have lateral bracing. The whole issue was triggered during current garage repairs when I was reviewing the effect of the mastic traffic topping weight and whether the designer of the steel beams would have included that weight when he designed the steel beams in the 1990's, since the mastic was not indicated on the original 1956 drawings, and was added some time after the original construction. Now please don't ask me why the steel beams are there. If I give the whole background I will spend an awful lot of time of peripheral issues, though I do appreciate your comments.

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