Lateral bracing to existing steel beam

[ajk1]

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.

 

[DaveAtkins]

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

 

[ajk1]

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.

 

[gagne73]

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.

 

[KootK]

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.

 

[BAretired]

See attached pdf for another suggestion.

BA

 

[ajk1]

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.

 

[hokie66]

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?

 

[KootK]

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.

 

[hokie66]

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.

 

[KootK]

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.

 

[Brad805]

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.

 

[hokie66]

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.

 

[KootK]

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.

 

[ajk1]

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?

 

[KootK]

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.

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.

 

[ajk1]

ok, thanks.

 

[gagne73]

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

 

[Tmoose]

"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

 

[ajk1]

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.