hook within hoop

[itez]

Please see attached illustration.

For a beam that is 300mm in width and 500mm in depth. How necessarily is it to add hook within a closed hoop? How would it affect the shear resistance? For a wider width or column. It is necessary to restraint the core from dilating. But for a beam with only 300mm width (or 220mm hoop since there is less the 0.40mm concrete covers), how would it exactly affect the shear resistance when closed hoop is already provided?

 

[itez]

BA, you are right, formula is wrong, there is no 2.

And beam depth, d must be deducted by 2.5 inches so


Vs=rho*As*fy*d/s = 0.75*0.22*40,000*14.185 inches / 4
Vs = 23,405 lbs

here is link to tests of sika full CFRP beam wrap

http://www.iccm-central.org/Proceedings/ICCM12proceedings/site/papers/pap878.pdf

 

[hokie66]

itez,
None of your calculations are going to make any difference unless you understand them fully. But one comment which BA made could possibly be your "get out of jail card". If the design is based on 40 ksi steel, and you actually have 60 ksi, that could mean your required capacity is satisfied. Or not. Not for us to say, it is the design engineer's responsibility to make that decision.

 

[itez]

The designer said the reduced stirrups can still support live and dead load and certain seismic thresholds. But the original full design can support higher seismic intensity. So even though the present stirrups is sufficient for normal activity. I want the strength of the full design and I decide to let him use full sika wrap. But since he hasn't done it yet. He will seek the advice of other designers how to design carbon fiber wrap.

The calculations can give me the justifications to spend much money from my own pocket to make the beam strong as it originally should be (to avoid future problems and being implicated later).


To determine shear at a certain distance from support, the following formula is used.

wu = 1.2D + 1.6L

But for seismic. I think the following needs to be used instead. wu= 1.1D + 0.5L +1EQ

To get the shear Vu

Vu = wu (L /2 – d)

Now Vc and Vs shear capacity must exceed Vu by a wide margin. Better shear resistance can resist larger seismic loading.

The sika representative in my country would only install after he got the design. And my designer hasn't designed one yet but seeking another designer advice. So if you guys have actually do a full carbon fiber wrap. Please share your experience and how well it behaves. I won't settle for U-wrap below slabs because it is poor and you have to bore holes across the beam. But with full wrap, no need to put any holes.

 

[itez]

Here's a puzzle.

It is said that stirrups and web reinforcement only work after the diagonal crack already formed. But how come in most beams in buildings. You can't see the diagonal cracks in the sides of the beam (like in the shearing tests of beams in labs that you can find many place)?

Or maybe the diagonal cracks are only hidden inside the beam? It is said after the formation of cracking, shear in an R/C beam is resisted by:

1. Shear resistance through aggregate interlock across crack surface (Va)
2. Shear resistance of uncracked concrete in compression zone (Vcz)
3. Shear resistance through dowel action of longitudinal rebars (Vd).

After more load, Vc and Vs resistance take over. At what point should the cracks appear inside and outside?

Can you visually see this diagonal crack outside the beam?

 

[itez]

Please see option 2 in attached file. Has anyone tried it? Or what could be its drawback?

The designer spent 3 days and night calculating every inch of the beam for shear and all load combinations and he concluded there is a need for retrofit. I'm not so keen on using carbon fiber as it's not widely accepted or the U-shaped may suffer debonding or anchorage failure and it is not good idea to cut the slab for full wrap.

Some of you suggest putting a bolt screwed on top and bottom and this is in option e in the attached illustration. But there are so many bars running in between. Please see option b for bolt on sides of the beam and comment on your experiences or what you heard about it or what you think about it. Thanks.

 

[itez]

The designer can't decide on what retrofit to use since he has never done any beam shear retrofitting. He is not confident on carbon fiber as he is not sure of the performance. We are discussing option b in the attached illustration above, but would it work? What is its drawback or to focus (brand)? Once we are convinced it's feasible. He would solve for the bolt and plate strain, etc. and apply this. So hope you guys can help. Thanks.

 

[JacksPanic]

We have used carbon fibre successfully in rural africa before, so it can be done. I suggest you employ a reputable company to assist with the design.

on a side note, why is all the top steel concentrated so heavily within the beam? and also, is there no top steel in the transverse direction required over the beam?

 

[Meesamali512]

Carbon Fiber wrapping is not something that every contractor can do. Our company specializes in the field of structural strengthening and we recently did a job for an O & G contractor where we strengthened their critical frame structure's beams using CFRP by manufacturer "Fosroc". Sika is also famous for CFRP and SikaWrap is most common wrapping system. For carbon fiber wrapping system, you need following.

1. Detailed design of strengthening system (it will be done by professional and experienced designer or consultant)
2. Specialty contractor that can actually apply the CFRP system. (most important thing in application is surface prep and adhesive application)

Again do remember that this system is extremely costly compared to common dowel insertion methods. Many a times it seems an overkill.

 

[itez]

Meesa, I have spent days and nights researching about it by reading any professional journals and I'm getting worried about it. In the paper:

http://repository.lib.polyu.edu.hk/jspui/bitstream/10397/5408/1/Process of debonding in RC beams.pdf

"Chen et al. (2010a) has recently demonstrated that adverse interaction between external FRP shear reinforcement and internal steel shear reinforcement may significantly affect the effectiveness of FRP shear strengthening for the debonding failure mode, especially when side strips are used. However, the effect of such shear interaction is not appropriately reflected in existing shear strength models for RC beams shear-strengthened with FRP (Chen 2010)."

The designer has never designed one before and not much in my country knows how. So I plan to avoid using carbon fiber as it's not widely accepted.

What if I just demolish the beam and recast the concrete after putting the right tranverse reinforcement? Is this sensible? Since it is a beam, pouring it with ready mix concrete would pose no problem. There may not be gap because gravity in the pour can level it. Can anyone recommend this??

About dowel insertion method. Problem is you can't hook the top bar and bottom bar this way. And there are many bars at top. Why. What other dowel insertion methods do you know so I can discuss this with the designer as he is out of idea for this unique problem he hasn't encountered before. If I can just recast the concrete. Let me know guys. I haven't slept for many days already thinking about this all and getting desperate. So appreciate any little insight you can share guys. Thanks.

 

[BAretired]

itez,

Have you checked with your supplier to see what grade of steel was supplied for the stirrups? Many suppliers provide Grade 60 even when Grade 40 is specified. In my area, Grade 40 is no longer readily available.

If the design is based on 40ksi yield and 60ksi was provided, there may be no need to take remedial measures.

BA

 

[itez]

It is just 40ksi. The designer has calculated the beam shear and he said it is not enough and needs to be fixed, yet he hasn't tried using carbon fiber before, and I'm afraid of using it after reading so many journals detailing the controversial aspects. The beam needs to be shear upgraded. I wonder if I can just demolish it, add stirrups and recast it? This may work better than the unpredictable carbon fiber reinforced polymer.

Please see attached picture. Is the steel tendons depicted possible? Can the screws and nuts resist the diagonal crack opening in the beam shearing? Has anyone used or heard of it before? Thanks.

 

[BAretired]

From a strength point of view, I believe that Detail (b) shown on your sketch would work. There are two problems with it, however:

1. There is no fire protection for the steel.

2. The nuts project a considerable distance above the floor. Do you have a topping thick enough to cover them?

BA

 

[itez]

Yes. We will put 2 inches cover and it is adequate and we can put fire retardant or protection to it. But is it normal metal plate and normal bolt? I'm worried the metal plate bolt hole may just give way. Remember the internal stirrups have to yield first when Vc and Vs resistance is exhausted. Then the metal plate steel tendons depicted in the illustration will be the one to confine it just like the case with carbon fiber where the tensile strength of 4000 MPA and strains of just 1.5% at Modulus of about 230 GPA is what does the confining action (not just superposition but have stronger role than Vc+Vs+Vt (tendons)). Do you think the steel tendons especially the bolts in the illustration have tensile strength of 4000 MPA too?? If Yes. Then I'll tell the designer to do this instead of crfp which he thought was the only solution and no other thing way to fix it as he has not encountered this before. So tips from you guys welcome. Thanks.

 

[hokie66]

I am in the somewhat the boat as your designer. I don't have any suggestions as to how to fix it. Agree that carbon wrapping for shear reinforcement is highly problematic. It has been used quite a lot for adding tensile reinforcement, but shear, I don't know.

The proposal to use vertical bolts or tendons outside the stem of the beam also involves lots of unknowns, as the system has no test results to rely on. The interaction of the external and internal reinforcement would be complex, and I wouldn't rely on it.

Before I did anything, if you have the authority to insist on it, I would ask for a peer review of the design. In my experience, it is very unusual for a beam of that size to require so much shear reinforcement.

 

[itez]

Please see attached layout of the floor which an office member posted before.

As you can see, it needs many reinforcements because it is a main girder in the middle with secondary beams at back and front framing into it. And in the plan, the stirrup spacings are 10mm bar 1 @ 50, 10@ 100, 8@ 150, rest 200. It has 60 ksi in the plan and middle stirrup leg. But we didn't have stock of 60 ksi bar earlier and so I asked if 40 ksi can be used. He said yes. Now the middle stir up is missed, so the strength reduction became significant with only 40 ksi used and middle leg missed.

The beam sizes of girder and secondaries are 300mm x 500 mm. The slabs are 100mm (2 way). Live load is 100 psf, Maximum earthquake load combination is used. This is why I saw his calculations have at least Vu=200 Kn in the girder.

Can I just demolish the girder at the center and put the right stirrups and cast the concrete again? Why won't this work? What are the reasons? Is it because gap may form and the rebars may have cement left? But we can brush and bars and make sure no gap is formed. This may be safer than the unknown carbon fiber or steel tendons.

About steel tendons. I don't think the entire beam bottom would be plated because it can affect flexure. I think it would be strip like too with 4 inches spacing and 4 inch plate just like stirrups. But can the steel tendons have tensile strength of at least 4000 MPA?

 

[hokie66]

So your maximum shear is in Beam 2B-10, just to the right of Column C-1. Again, I would want another opinion as to whether strengthening is required. I am not going to attempt it myself, as my opinion would have no standing, and with all the units mixed up, I would probably make an error anyway. Don't start chipping away at the structure until you know what is required, as you may only make it worse.

 

[BAretired]

I agree with hokie. Do not start chipping until you have a proper design evaluation.

The plan is one we have seen before on another thread. I believe the 100mm slab is too thin as a one or a two way slab, but I am basing this on CSA A23.3. I don't know what ACI requires. If you are using a topping, hopefully the surface on the slab is rough enough to develop bond so that it becomes a structural topping.

If you decide to use Detail (b), the bolts will not have a tensile strength of 4000MPa. Why would that be a requirement? And why would you space them at 100mm centers? It would seem to me that 200mm centers would be more practical, but that is for the design engineer to decide.

BA

 

[itez]

You will notice the shear is huge from the center C1 to the secondary beam framing into the middle of the right girder. Stirrup spacing should have been uniform 100mm or 4 inches. But the problem is we followed the 10 @ 100, 8 @ 150, rest 200. This means the 10 - 100mm spacing only reaches up to 3/4 or less of the big shear span (only 3/4 up to where the secondary frame into the right girder). If we used uniform 100mm the entire girder, no problem, but we didn't. Not only that. We didn't use 60 ksi (no stock so we requested 40ksi after approval) and we didn't put the middle leg. This is why there are 3 misses and that is why the designer recommends shear strengthening but he hasn't done CRFP before and the journals gave me concern.

The reason for adverse effect of steel stirrup and CRFP is because the debonding is not uniform, so the parts where debonding has not taken place, the diagonal crack is pinned, the one where debonding has taken place, the diagonal crack opens. After the CRFP snaps at its ultimate load, all the stirrups with no uniform yield would react suddenly. All this concerns only occurs because of debonding. If there is no debonding before the CFRP snaps, there is no problem because you will reach the beam shear limit before it fails if no debonding occurs.

Now in the steel tendons scenerio at sides with plates underneath. There is no debonding process. So if the steel tendons hold. Then it can be a solution. Can the bolts and nuts hold, or welding or any means to secure it?

 

[itez]

To continue with the above. Please see attached picture of the actual girder-secondary beam bars framing, you can notice the spacing near the girder-secondary is 150mm instead of 100 mm.

The floor will be used as office only. But 100psf is designed. But then.. even if you reduce live load to 50psf. Shear doesn't vary match. This is because load combination of 1.1 DL + 0.5 LL + 1 Earthquake load is used.

So this would fail in strong earthquakes. So we really need shear upgrade.

Now for the steel tendons. What must be its strength that can hold the diagonal cracks from opening?

I have paused construction for 2 weeks already and many people already mad because of delays. Today is holiday in my country. I'll talk to designer tomorrow who is still continuing with CFRP design. If I'll let him start on steel tendon design. He will discontinue with CFRP. So can steel tendon really work? We can't hold construction for another week so have to know if steel tendon would work or maybe I should demolish the girder. Based on the picture. Is it not easy to demolish and recast the concrete after putting stirrups at middle?

 

[dhengr]

I don’t mean to add more fuel to the fire in this soap opera, but why don’t you consider running a couple post tensioning tendons on each side of 2B-10, from left of C-1, harped under 2B-5, and to the right of C-2? Wouldn’t this take some load and shear off of/out of beam 2B-10? And, it would apply the upward force right where you wanted it. For the most part these could be fire proofed by just widening 2B-10 with shotcrete or spray fire proof after tensioning.