Reinforcing steel beam

[ajk1]

To strengthen a steel beam that has ready access one side only, I am considering welding in a 35M weldable grade rebar top and bottom, one side of the web only, where the web meets the flange. However that makes it a Class 3 Section, because it is not doubly symmetrical. Can adding lateral braces at close spacing to the compression flange allow the beam to be checked as a Class 2 Section? I am using CSA Standard S16.1.

I know that there are other ways to strengthen the beam, such as welding a tee to the underside, but that requires breaking out the block wall below, spray fireproofing the tee and rebuilding the block wall below.

 

[jayrod12]

I would be inclined to say no. Why not just size the reinforcing so that it works as a class 3 section?

 

[KootK]

I vote yes. Something like this:

1) Treat the rebar tension and compression as externally applied axial loads applied to the class 2 section. These will induce a torsion on the beam.
2) Design the remaining class 2 beam for said torsion or provide torsional bracing frequently enough that the torsion accruing between brace points is inconsequential. I'd probably go with the latter.

Note that it would not be sufficient to simply brace the compression flange laterally. You'd need bracing that provides effective rotational restraint.

While I believe that this strategy could be employed if desired, I'm not sure that it's the most expedient way to prosecute the design. The class 3 business might be simpler.

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]

I will look at that possibility, but that is a lot of reinforcing, since I would lose approximately 15% in strength in designing as Class 3 versus Class 2. I suppose the welds would have to be sized to develop all that reinforcing? I am trying to increase the beam strength by about 25%, so if it is Class 3, I would have to increase it by about 40%

I think if the beam can somehow be restrained against torsion, that it could be designed as Class 2 section.

 

[ajk1]

To Kootk - I did not see your response at the time I wrote my response to jayrod12. Thanks for your thought. Very interesting. I will do some calculation and see how it turns out as Class 3. Thanks to you both. I may have a follow up later.

 

[ajk1]

What is the best way to add reinforcing? A 35M bar top and bottom does not provide sufficient strength. If I use a square bar, say 1½" square, I think it will conflict with the radius of the beam web to flange junction, unless the corner on the bar is coped.

 

[KootK]

What's the bottleneck? Deflection? Straight bending stress? LTB?

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]

Resisting moment. But it might help if the deflection were reduced too, since the block wall that it supports has cracked. But the bottleneck is mainly resisting moment.

 

[KootK]

Any chance it's deep enough to make sense externally post-tensioning 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.

 

[ajk1]

It is 16" deep and spans 23 feet ±.
I find that the beam reinforced with 1 - 45M bar top and bottom one side only, and checked as Class 3 Section, has 25% more resisting moment (and 43% more inertia) than the unreinforced section checked as a Class 2 Section. So that might work, provided that 45M rebar does come in weldable grade, and that the welds can be developed where we weld it to the flange and web (web = 5/16" thick).

My gut feel is that it is not worth trying to prestress it. What were you thinking...Dywidag bars?

 

[KootK]

Dywidag or something similar. In retrospect, it was lousy advice. Post-tensioning isn't something that you want to do asymmetrically.

45M is one heck of a reinforcing bar. What is the source of the extra load on this beam? Is there a meaty slab above it that might be able to be made to act compositely?

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]

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]

I see that you are very innovative!
The extra load is from extra thickness of topping that was not recognized as being there when the steel beam was designed and installed in 1994, but which we found out about when we removed it to do structural repairs to the parking garage floor. Actually I don't yet really know the load that the beam takes because the 1994 design was very arbitrary in determining the amount of load that the steel beam would pick up. That is the next thing I will have to look at.

 

[KootK]

If there is solid slab above the beam, as opposed to metal deck and slab, you might be able to core some holes about the beam and post-install nelson studs to make things composite. Let that extra topping work for you!

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.

 

[EngineeringEric]

Hate to ask the obvious but can you not add a column or transverse beam lower? I have seen a W-section reinforced with a channel and/or angles before. Amount of steel from the channel was rather significant. I will not say it worked numerically or what the complications were but it was interesting none the less.

One other option, if you cannot make the beam a composite, can you remove the slab and weld a T-section to the top flange. The web if the T welded to Top Flange and then the flange of the T will be then encased in concrete. This will increase your depth for deflection as well as decrease your stress. And it is symmetrical and rather clean to install and inspect. Depending on thickness of concrete you can get a good amount of additional depth (6" slab can make you 5" deeper beam).

PS: And during install you may just relieve all your deadload-preload when the slab is jacked and supported. One more benefit in the composite or the T-reinforcement

T
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[ajk1]

To Kootk - it is a waffle slab, and there is a block wall above the beam so not the best for making it composite.

To Engineering Eric - I noted in my very first post of the question, that a T added to the bottom of the beam is a possibility. That would be easier than a T to the top, because only need to remove the top course of block, rather all the block wall above the beam, although admittedly it would be overhead welding.

Adding a column is a possibility (I too had thought of that), but would have to brace the top of the column, and cut thru the slab-on-grade and pour a footing, fireproof the column, etc, so not that elegant but certainly worth considering.

I think the 45M bar one side top and bottom is probably as economical as any method, provided the bar can be welded to develop it.

I'll give it some further thought.

Thanks

 

[KootK]

To Kootk - it is a waffle slab, and there is a block wall above the beam so not the best for making it composite.

The presence of the waffle slab may be a boon here. You should be able to say that at least two of the ribs running parallel to the steel beam non-compositely share the the load applied to the steel beam. And there's a pretty good chance that will get you back to a "do nothing" option.

If you feel like playing along, post a plan showing the ribs in relation to your beam and a cross section through the beam and waffle slab.

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]

How does that get me to a "do nothing" state? Half the column strip width was cut out in 1994, for a length about 10 feet parallel to the span.

Maybe the block wall could be packed up tight to the steel beam soffit and made load bearing, and rely on the 5" slab-on-grade to take the load. The top course of block is likely hollow, so not easy to pack up, but could take the top course out and pour it back as solid concrete up to the steel beam soffit. But pressure on soil under slab on grade probably more than can be justified.

I guess the next step is to make a better stab at what the load the steel beam has to take. May require computer modeling which may be a fair bit of work. Perhaps another way is to figure out how much capacity was lost by cutting out the half column strip and saying that is the strength that the beam should have.

 

[EngineeringEric]

My apologies, my mind somehow missed that part

If you are concerned about fireproofing and that, i think the removal of your top block (below the beam) is the way to go. Just remove that course, overhead weld, and replace block.

Or maybe add a steel plate 1.5" wider than your flange so you can weld the plate to the beam from above. you seemed to state that there is a gap between the steel and wall, maybe enough for 1/2" plate?. This may require additional compression flange bracing but you can probably make positive connections to the slab...

 

[ajk1]

yes that is possible.

I a starting to think adding a column and footing might be as good a way as any.