Reinforcing steel beam
Reinforcing steel beam
(OP)
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.
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.






RE: Reinforcing steel beam
RE: Reinforcing steel beam
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.
RE: Reinforcing steel beam
I think if the beam can somehow be restrained against torsion, that it could be designed as Class 2 section.
RE: Reinforcing steel beam
RE: Reinforcing steel beam
RE: Reinforcing steel beam
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: Reinforcing steel beam
RE: Reinforcing steel beam
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: Reinforcing steel beam
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?
RE: Reinforcing steel beam
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.
RE: Reinforcing steel beam
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: Reinforcing steel beam
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.
RE: Reinforcing steel beam
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: Reinforcing steel beam
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
I
RE: Reinforcing steel beam
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
RE: Reinforcing steel beam
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.
RE: Reinforcing steel beam
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.
RE: Reinforcing steel beam
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...
RE: Reinforcing steel beam
I a starting to think adding a column and footing might be as good a way as any.
RE: Reinforcing steel beam
When your steel beam deflects there will be a couple of parallel waffle ribs that will go along for the ride and assume more or less the same curvature. And that means that they'll be sharing the load. Waffle slabs are usually fairly thick so I'm guessing that you might have a pair of 12"-ish continuous concrete beams sharing the load with your 16" simple span steel beam. That sounds like meaningful help to me.
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: Reinforcing steel beam
Also, I have found that while smaller diameter bars are easy to weld to the beam, the welds become more difficult with larger diameter bars as you end up welding in a deep groove which is both difficult to complete as well as review/inspect.
Great spirits have always encountered violent opposition from mediocre minds - Albert Einstein
RE: Reinforcing steel beam
I've been noodling on this a bit too. I was wondering if one might groove weld the rods to a small sized angle in the shop and then weld the angle legs to the beam in the field (clean fillet welds). You'd loose a little effective depth but gain some extra area so maybe that's a wash. Not sure how one would accomplish a splice if that's necessary.
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: Reinforcing steel beam
RE: Reinforcing steel beam
to Fishthestructure - very interesting advice about using round bars. I'll keep that in mind.
to structSU10 - what you suggest of making a box of the angles was my very first idea (although I did not mention it here) but our chief engineer was not enthusiastic about it because he said it was not an efficient way to do it (since the centre of gravity of the angles was closer to the beam mid-depth).
New Information:
I was on site again this morning and found that the block wall is not directly under the beam but about 2" clear of it. This suggests to me that the best thing would be to weld a tee section to the soffit of the beam. It is I suppose overhead welding, but is that really so difficult/expensive? The welding would have to be done from one side only, but if a continuous backer bar were welded to one side of the web of the tee before it was lifted into place, then I would think that a full penetration butt weld could be made from one side only. What do you think?
RE: Reinforcing steel beam
RE: Reinforcing steel beam
RE: Reinforcing steel beam
RE: Reinforcing steel beam
RE: Reinforcing steel beam
RE: Reinforcing steel beam
RE: Reinforcing steel beam
RE: Reinforcing steel beam
RE: Reinforcing steel beam
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: Reinforcing steel beam
to Kootk: I agree with what you say. It will apply to a number of beams and may happen at other floors up the building. Anyway, if that is what has to be done, that is what will have to be done, but we will have to check the situation on the other floors. Maybe the beams are not as heavily loaded. It would be interesting if anyone has ever done a load test for this condition to see if the beams buckle laterally.
RE: Reinforcing steel beam
RE: Reinforcing steel beam
1) The attachment beyond the theoretical cutoff point should be adequate to develop the tee's portion of the flexural strength in the beam at the theoretical cutoff point.
2) If the tee will be stressed beyond fy, it's tricky and you can approximate this as the the axial yield strength of the tee.
3) If the tee will not be stressed beyond fy, you can use the formula M/QI.
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: Reinforcing steel beam
Thanks. I have some follow up questions:
1. I am checking the reinforced section by limit states design which assumes that the entire portion of the reinforced section each side of the plastic neutral axis (PNA) has yielded. How do I determine if the tee will be stressed beyond fy?
2. For a plate used as reinforcement, there is, in addition to the requirement for the weld to develop fy times the plate area, a requirement that the length be not less than 2 times the plate width (if there is no transverse end weld). For the Tee section, is there only the requirement to develop As x fy?
RE: Reinforcing steel beam
2) Rules like that are basically addressing shear lag I think. As such, I'd apply the shear lag provisions for tension members to the tee considering the "connection" to be the portion of the tee extending beyond theoretical cutoff.
What percentage of the span do you plan to reinforce?
For what it's worth, I usually specify stabilizer plates at the ends of the tee. My understanding is that they are required sometimes. Unfortunately, I haven't yet figured out how to assess that need so I've been specifying them all the time.
When using plastic moment capacity, I'll do an extra check of my own. I'll ensure that the reinforcing can develop it's yield capacity on either side of the peak moment with a 2x safety margin. It's not been an issue yet.
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: Reinforcing steel beam
ok, thanks. Much appreciated.
I have not yet determined what percentage of the span to reinforce. That's my next step.
If the TEE is welded intermittently along its length (say 2" every 12"), and at each end to develop tee area x fy, can I take it that for service load deflection calculation the inertia to use is the composite section of beam and tee, or is it something less than that due to the intermittent welding?
RE: Reinforcing steel beam
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: Reinforcing steel beam