×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

hook within hoop

hook within hoop

hook within hoop

(OP)

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?

RE: hook within hoop

The additional bar increases the shear carried by the steel by 50%. Omitting the central bar and reducing the stirrup spacing by one third achieves the same result and is the more typical detail.

BA

RE: hook within hoop

(OP)
Did you mean the additional bar increase the shear resistance or increasing the shear itself (meaning making the shear worse)??

If it increases the shear resistance. Can you create the same result by wrapping the external beam by U shaped carbon fiber reinforced polymer?

RE: hook within hoop

(OP)

I think you meant the additional bar increased the shear resistance of the beam. Now my problem is. I'm only used to closed hoop and didn't notice the additional bar in the plan. I'm the project manager and engineer. And the beams are already casted with stirrup spacing not reduced by one third. In cases like this. How do you fix it... where you missed to add the additional bar. Would drilling the beam from bottom and adding the bars vertical at center do it? Or retrofit with carbon fiber reinforced polymer. What is usually done in the field in cases like this.

RE: hook within hoop

Shear resistance of a reinforced concrete beam consists of two parts, Vc and Vs. Adding a bar as shown on your sketch increases Vs but has no effect on Vc, so the total shear resistance is not increased by 50% only the resistance of the steel.

I would rather not comment on carbon fiber reinforced polymer as I have no experience with it. I presume it would increase shear resistance but would be concerned about it effectiveness during a fire.

BA

RE: hook within hoop

(OP)

I'm talking about diagonal cracks being restrained by hoops or stirrups. The number one purpose of these transverse steel is on this diagonal cracks. concrete alone would be poor in shear. Vc is diagonal shear capacity of the concrete? But it's so poor (because concrete very poor in tension which is related to the diagonal crack) that is why tranverse steel needed. So Vs (diagonal crack shear capacity of the stirrups and hoops) are much more than Vc (diagonal shear capacity of concrete alone). Do you agree? I think Vs handles 80% of the diagonal shear and Vc only 20% (?)

RE: hook within hoop

Quote:

Do you agree? I think Vs handles 80% of the diagonal shear and Vc only 20% (?)

It depends on the steel diameter and spacing, and the concrete grade. The contribution of the concrete may be more than 20%, but certainly reducing the shear steel by 33% will significantly reduce the shear capacity.

You need to tell the engineer responsible for the design and ask for a solution.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: hook within hoop

(OP)

The designer said to decrease the live load and don't have experience with carbon fiber reinforced polymer wrapped in U-shape beneath the beam. How much would this increase the Vc of the concrete? Or V(CFRP). Anyone has used this? What else did you do to increase the shear capacity?

I suggested to the designer about drilling hole beneath the beam and inserting bars to increase the Vs. But he has not done this before. Has anyone tried this or have seen this elsewhere so I can discuss with him about this by showing it is plausible?

RE: hook within hoop

Quote (itez)


The designer said to decrease the live load and don't have experience with carbon fiber reinforced polymer wrapped in U-shape beneath the beam. How much would this increase the Vc of the concrete? Or V(CFRP). Anyone has used this? What else did you do to increase the shear capacity?

Is the designer prepared to decrease the live load to the point where the existing stirrups are adequate without the central bar? I have no experience with carbon fiber reinforced polymer, but U-shape suggests you are talking about a Tee beam. I seriously doubt whether polymer could adhere to the sides of the beam enough to generate any useful shear strength.

Quote (itez)

I suggested to the designer about drilling hole beneath the beam and inserting bars to increase the Vs. But he has not done this before. Has anyone tried this or have seen this elsewhere so I can discuss with him about this by showing it is plausible?

This idea might work, but 500mm is a pretty deep beam and you would not want to drill through any existing stirrups. How would you anchor the bars...with epoxy? Would it be easier to drill from the top and anchor the bars with a nut top and bottom and forget about the epoxy? I have never done it before, but I would think it is a possibility.

BA

RE: hook within hoop

Drilled in bars may well be the best option. I'd suggest talking to one of the companies that produce products for post-installed shear reinforcement.

I don't know the commercial or legal arrangements on this job, but surely the original designer will have sign-off on whatever is selected as the remedial work.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: hook within hoop

You said.... “I'm the project manager and engineer.” It’s a shame the project manager and project engineer didn’t check the shop drawings or inspect the rebar in place, prior to the pouring of the conc. Those are both normal activities and responsibilities of those parties on a construction project. They also have to read and study the plans for the job.

BA’s method of anchoring the added shear steel with nuts and a washers would work Start with a 1.5" deep +/- cores, then drill the hole, and maybe epoxy the rod in place. Then, once the nuts are tightened the cores can be grouted full (patched) around the nuts. You will have to map all the existing steel so you don’t cut it in the drilling process. You might also want to test the actual conc. strength and tie steel strength, and the original shear steel calcs. to see if you can’t get a bit more cap’y out of what’s already there. This might allow you to use a smaller rebar and drill for the added shear steel.

They do use FRP to do what you are talking about in the way of increasing bending and shear cap’y. of conc. beams. It is a very expensive repair method, and not something that anyone without considerable experience in design, material selection, surface preparation and application should try on their own. You should contact a specialty contractor who does this kind of work if you want to know more about it.

RE: hook within hoop

(OP)


Ok. Just for my knowledge in further discussions with the designer. The ACI requires stirrup spacing of d/4. How many of you follow the d/4 code or use wider spacing? If the beam depth is 500mm. ACI requires 500/4 = 125mm or 4-5 inches spacing. The structural plan of the project has 4 inches stirrups spacing. Is 4 inches typical in your plans? Of course I'd discuss any further shear strengthening with the designer, but just need to know the ACI side on this.

RE: hook within hoop

This is an international forum. We are not discussing ACI requirements. We are discussing an issue which has developed on your jobsite as a result of your failure to follow engineering drawings. We would like to help, but if you want a sensible answer to your questions, you must give us a little more information. There may be other ways of looking at the situation, but we are not clairvoyant, so help us along. What is the load on the beam and how does it relate to other beams and columns?

BA

RE: hook within hoop

(OP)

It is a girder and I will really put the shear bars or retrofit after the designer calculated how many spaces apart tomorrow.

Please see attached picture of the beam an hour before concrete was poured with longitudinal reinforcements in place and closed hoops (stirrups with 135" hooks). The top bars are 8 pcs of 20mm grade 60 layered in 4s and bottom bars are 4 pcs of 20mm grade 60. The beam width is 300mm and depth 500mm. The slabs are connected to it. Close hoops (stirrups) spacing is 100mm.

Do you call it a T-beam? The slabs are just connected to it with no longitudinal bars extended outside the rectangular beam. So technically it is not called a T-beam which has more reinforcements. True?

I'm wondering if carbon fiber can just be wrapped around the beam. The slabs are light at only 100mm thick and it is 4 way so other beams around it can carry it (said the designer). You agree that carbon fiber completely wrapping the beam can increase shear resistance and this can be a good solution (instead of just U-shape wrapping below the slabs)?

Also alternatively, do you know of drill machine that has least 400mm drill in length? We can see only 6 inches drill and not 16 inches drill. Is there such?

Thanks a lot.

RE: hook within hoop

I don't see how carbon fiber can be wrapped around the beam, but you may want to discuss it with someone who has used that technique before. I have not.

Your photo suggests that missing the reinforcement while drilling through the beam is likely to be a challenge. You should discuss this with a concrete drilling company in your area. You may need to use GPR (Ground Penetrating Radar) to locate reinforcement before drilling.

BA

RE: hook within hoop

Your original sketch sure looks different than your latest photo, but never mind that, nobody ever pays any attention to that insignificant crap in commenting on an OP. You would be surprised at how an experienced engineer makes a pretty good judgement of the problem when he/she is shown a complete and properly proportioned sketch, and is given a meaningful description of the problem. And, three bars t&b is not quite the same as 6 or 8 bars t&b in the same size beam, on this kind of problem.

I’m also curious how bldg. design and construction works in your area/country? How many layers of design engineers, project managers, and project engineers are there, none of them knowing what the other is doing or how they go about doing it? Does the design engineer ever come out and look at the construction progress? Does a design engineer have to know anything about constructibility? What is the training and testing required to be called an engineer in your country or can anyone just print up a calling card saying ‘I are an ingineer’? Why are you coming to E-Tips for a situation like this, when you should be going directly to the design engineer, who can at least look at a sketch/set of plans which aren’t so drastically in error? Those must be some pretty scary bldgs. to live in and around. We all run into problems or errors on construction jobs from time to time, but some of you guys really seem to have a corner on that market.

RE: hook within hoop

(OP)

In my country. Designers said they only calculate for reinforcement ratios and constructibility have to be handled by contractor but coordinated with designer for alteration in plans if construction is not possible. So what we do is draw shop drawing and submit to them for approval (especially for complicated design). In the case of the building. I thought it was just typical and mostly I just use U shape stirrup or hoop in small beam like 300x500mm. In the past we only put middle stirrup in big beams So I missed the drawing on the single middle stirrup and it is my mistake.

The designer said they used the most conservative seismic codes such that even if calculations would show 10 inches spacing, they would use D/4 or 500/4 or 125mm spacing and even 100mm or 4" is specified. Also the designer won't show me any calculations except if I show it to him first so I need to know something before I discuss with him later.

BA, in the picture. Do you consider it as a tee-beam and what could be the effective width of it then?

I'm solving for Vc from Vc=2*rho*b*d*sqrt(fc)
given
b=11.8112 inches (300mm)
d=19.68504 inches (500mm)
fc=4000 psi
rho=0.75
Vc==2*rho*b*d*sqrt(fc)=22,057.26 lbs.

But it is a tee-beam (is it based on the pictures?). So what should be the effective width? If it's 15" instead of 11.8", then Vc becomes 29,527.21 lbs.

Now for Vs=2*rho*As*fy*d/s
given rho=0.75, As=2(0.11), fy=40,000 psi, d=19.68504", s=4 inches
then Vs = 64,940.63 lbs

and total shear capacity = Vu = Vc + Vs = 22,057.26 + 64,940.63 = 86997.89 lbs

Shear is related to moment. If shear = 86997.89 lbs. What is the corresponding moment capacity? What is the formula to relate shear and moment? I just want to verify his work by knowing some basic formula so I'd understand his after I discuss with him because I have to shoulder the very expensive carbon fiber wraps retrofit. Thanks.

RE: hook within hoop

This would be considered a tee beam, but that doesn't materially improve your shear capacity. I don't think there is a practical way of improving the shear capacity of this beam. Post-installed shear reinforcement requires anchorage at each end of verticals, and that looks impractical to achieve, based on your photo. I don't know much about carbon fibre wraps, but would be dubious of manufacturer's claims.

RE: hook within hoop

Quote (itez)

BA, in the picture. Do you consider it as a tee-beam and what could be the effective width of it then?
For purposes of shear calculations, b is the width of the web which is 300mm whether it is a tee beam or a rectangular beam.

Quote (itez)

I'm solving for Vc from Vc=2*rho*b*d*sqrt(fc)
given
b=11.8112 inches (300mm)
d=19.68504 inches (500mm)
fc=4000 psi
rho=0.75
Vc==2*rho*b*d*sqrt(fc)=22,057.26 lbs.
I do not use the ACI code. I use CSA A23.3 (Canadian code). The term 'd' is the effective depth from the c.g. of steel to the compression face of the concrete. It is less than 500mm, probably about 400mm. I do not know what 'rho' is. Maybe someone else can advise.

Quote (itez)

But it is a tee-beam (is it based on the pictures?). So what should be the effective width? If it's 15" instead of 11.8", then Vc becomes 29,527.21 lbs.
If you are drilling holes and inserting bars, it makes no difference whether it is a tee-beam or not. If you are wrapping with carbon fibers, the flange will get in the way, so I can't see that being possible.

Quote (itez)

Now for Vs=2*rho*As*fy*d/s
given rho=0.75, As=2(0.11), fy=40,000 psi, d=19.68504", s=4 inches
then Vs = 64,940.63 lbs

and total shear capacity = Vu = Vc + Vs = 22,057.26 + 64,940.63 = 86997.89 lbs
Again, d < 500mm. The term 2*rho appears wrong . By CSA code, your value of Vs is too high.

In my area, steel suppliers are not stocking 40,000psi steel any more. If you order it, you will usually get 60,000psi steel. This is something you should check with your supplier.

Quote (itez)

Shear is related to moment. If shear = 86997.89 lbs. What is the corresponding moment capacity? What is the formula to relate shear and moment? I just want to verify his work by knowing some basic formula so I'd understand his after I discuss with him because I have to shoulder the very expensive carbon fiber wraps retrofit. Thanks.

There is no simple relationship between shear and moment. In a simple beam, moment is 0 at each support and maximum at midspan while shear is maximum at each support and 0 at midspan (assuming uniform load). For a continuous beam, moment is negative at the support (tension on top) and positive at midspan (tension on bottom). Shear is maximum at supports and zero somewhere near mispan.

BA

RE: hook within hoop

(OP)

Quote:


Now for Vs=2*rho*As*fy*d/s
given rho=0.75, As=2(0.11), fy=40,000 psi, d=19.68504", s=4 inches
then Vs = 64,940.63 lbs

and total shear capacity = Vu = Vc + Vs = 22,057.26 + 64,940.63 = 86997.89 lbs
Again, d < 500mm. The term 2*rho appears wrong . By CSA code, your value of Vs is too high.

rho is the strength reduction factor which is 0.75 for shear calculations.. it is 0.9 for beam.
Why, what is the web reinforcement shear formula for Vs in your country? In the ACI code, it is really

Vs=2*rho*As*fy*d/s = 2*0.75*0.22*40,000*19.68504 inches / 4
Vs = 64,940 lbs

Maybe it's high value is the spacing of 4 inches. When the spacing becomes twice or 8 inches, Vs becomes half or only 32470 lbs. What is wrong in the calculation?

About the T-beam and the flank action on top. Well. I'd suggest to the designer if I can put some hole in the flank and do full carbon fiber wrap around the beam covering 1 meter from the column face. Remember stirrups only work after the diagonal cracking already occurs. It pins the diagonal cracking. So full CFRP wrap from Sika around the whole beam should prevent the diagonal cracks from fully opening and brittle failure (in addition to the existing 4 inches stirrup already present).

RE: hook within hoop

(OP)
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/ICCM12proc...

RE: hook within hoop

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.

RE: hook within hoop

(OP)


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.

RE: hook within hoop

(OP)
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?

RE: hook within hoop

(OP)

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.

RE: hook within hoop

(OP)

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.

RE: hook within hoop

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?

RE: hook within hoop

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.

RE: hook within hoop

(OP)

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...

"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.

RE: hook within hoop

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

RE: hook within hoop

(OP)

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.

RE: hook within hoop

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

RE: hook within hoop

(OP)

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.

RE: hook within hoop

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.

RE: hook within hoop

(OP)

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?

RE: hook within hoop

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.

RE: hook within hoop

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

RE: hook within hoop

(OP)

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?

RE: hook within hoop

(OP)

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?

RE: hook within hoop

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.

RE: hook within hoop

(OP)
After reading many journals about transverse post tension and they all report positive results (links below). I'll use it. Please see the PDF of the following first link and see page 2. Should my post tension be like that with one plate and 4 bolts or is it like the second URL in the following with only 2 bolts? Are these normal metal plates and bolts or special ones that must be ordered in another country? Tomorrow ill let the designer do this instead. Since its his first time to do this external post tensioning. All useful tips welcomed so I can inform him. Thanks.

Shear Strengthening of Reinforced Concrete Beam Strengthend by Transverse External Post Tension

http://thescipub.com/pdf/10.3844/ajeassp.2011.108....

"Shear strength of reinforced concrete beams strengthened by transverse external post tension at shear span is effectively improved over reference beam specimen"

http://www.sciencedirect.com/science/article/pii/S...

Behavior of reinforced concrete beams post-tensioned in the critical shear region

http://www.concrete.org/PUBS/JOURNALS/AbstractDeta...

Title: Shear Repair of Reinforced Concrete Beams Using Post-Tensioning

Please share others you see too.

RE: hook within hoop

(OP)

After reading the papers above in more detail. I noticed the plates were pressed by hydraulics with post tensioning of at least 0.04 Fc. How can I press the plates above and below the beam toward each other before putting the bolts and tightening the nuts??

RE: hook within hoop

(OP)

dhengr, I just realized you actually meant a horizontal post tensioning. I thought you were referring to option B with steel plate on top.

But horizontal post tensioning needs hydraulics and I don't know how you would do that to a finished beam without any holes made prior.. unless you would anchor it at the sides??

Also I wonder if an extra I-beam column and beam over the middle girder will do. But if the I-beam would be more than 4 inches as it would become very big.

Something puzzles me. Why is CFRP so popular when the plates on top and bottom with steel tendons at sides can do. Maybe it's because mostly old buildings are CFRPed and it's hard to chip the floor above to access the top of the beam? Also new beams would creep after 10 years so I wonder if the CFRP would become invisible by then because after creeping the fiber may become loose.

Btw.. my structural designer is just 25 years old. He uses maximum safety margin so the design is surely safe. And he is open to new insights or ideas. I'd discuss with him any good tips you guys can offer so we can solve the shear problem. Thanks.

RE: hook within hoop

I did not notice any shear walls in the framing plan posted on May 12th. The stirrups may serve more than one purpose if the beam is part of a moment frame in a high seismic area.

RE: hook within hoop

(OP)

There are no shear walls because it is a special moment frame and only columns and beams resist the entire seismic load.

I talked to the designer. He won't want to use the steel tendons at sides with plate above and below because he reasoned it is not connected to the concrete. He said CFRP at least is epoxied to the cement so there is composite action unlike the latter. But he gave me option whether I want to choose metal plates epoxied to the sides of the beam (without bolts) or CFRP epoxied to the sides. What do you guys suggest?

RE: hook within hoop

I wouldn't like to rely on glue to solve your problem.

RE: hook within hoop

(OP)

There are opposing views between two of the designers in the designing company, one said shear retrofit is not necessary, the other one said it is necessary. This may be because diagonal cracking and shear in beam is not totally understood, that's why we have different codes.

Anyway. Can you give examples of buildings where the beams collapse and fall to the floor from brittle shear failure (diagonal cracks)? One of the designer said it won't fall to the floor. Could it be the longitudinal bars remain and the concrete falls down.. or do the longitudinal bars hold the diagonal cracked pieces? Have you seen such failure?

RE: hook within hoop

itez,
There are many laboratory examples of concrete beams failing in shear (diagonal tension). Usually the beam does not fall to the floor because the load is applied by jacks and is released upon failure of the beam. However, failure by diagonal tension tends to be sudden and without warning whereas failure by flexure is preceded by excessive deflection.

Have I seen such a failure in a real building? No, but I have never inspected the remains of a building after a seismic event.

BA

RE: hook within hoop

(OP)

Designer has released the retrofit design. It's composed of Carbon Fiber 4 inch width distance 4 inches apart. They said it's up to me if I want it full wrap but they think U-wrap under the T-beam is sufficient with 2 dowels stick to each side and below. They haven't heard about internal and external stirrup reactions and think I'm overreacting and said there is no other solutions.

I talked to the carbon fiber installer. They said they can drill the slabs avoiding cutting any rebars with 4 inches space for the hands to work around. They they will put epoxy grout to the slab holes to bind it original strength. Will this work? I asked the designer, he hasn't done any carbon fiber before want me to inquire the installers on their technique.

Isn't it slabs are connected to beams by the rebars and the concrete can't take much tension anyway. So if you temporary put holes in the slab and then get it epoxy back to original strength with all rebars intact. This would be a good solution??

RE: hook within hoop

(OP)

Please see attached file of the crfp detail.

Next week. Something will be implemented that has never been done in the country. It will be first so I need some tips.

The crfp span detailed is from center column to right and center column to left.

The 100mm slab is 2.5 meters by 5.7 meters. The part to be holed alternatively (wrapped around fully) is on the short span. Do you think it will work? Or is putting a hole unnecessary and U-shaped below slab can hold (the drawing has the pointers of the Full Wrap and U-Wrap interchanged due to cad typo).

By the way. My designer doesn't know how to compute (or forgot all about it). He designed the building by using all outputs from ETABS and never any manual calculations. So if ETABS says Vu (shear) is so from the certain load combination, then Vc+Vs must be more than Vu or it needs extra Vf (shear resistance by cfrp). He doesn't know how to compute for cfrp too. He just used typical details. To those who have seen the layout of the building and the picture of the actual bars and stirrups posted earlier in the thread. Is there possibility no cfrp is needed? He welcomes peer review of it since he can't do any manual computations of any kinds and just based on outputs from Etabs. Is it possible Etabs can sometimes overdesign things?? I told him to use 2.4kpa office load but he said it would take him a week to rerun everything and doesn't have time. The default is 4.8kpa commercial load. So for those who have designed such before. Please tell if the stirrups provided in actual can take an office load. Thanks. If it can. I'll pay him more to recompute for office load (he is presently busy designing other buildings) and possibly just do U-shaped Wraps as drilling the slabs alternatively is not easy.

RE: hook within hoop

I suggest you obtain a peer review of the design, but not over the internet.

BA

RE: hook within hoop

(OP)

No other designer in my country wants to peer review another designer work. That is why I have to backtrack and reverse engineer the plan to do the manual computations. Now i'm doing manual calculations of the interaction between the crfp and the internal stirrups (designers in my country don't even know about this interaction).

Picturewise. After diagonal crack forms. The stirrups can yield. If stirrups uniform. Yielding is uniformed In case of cfrp. Usually it debonds before stirrups yield and the locations where it does that is not uniform. Therefore fabric with high axial stiffness (more material or higher modulus) can prevent the stirrups yielding because the confinement of the cfrp strain holding the diagnonal crack and hence suppressed the steel stirrups from contributing to the shear resistance. After cfrp debonds. The shear resistance will suddenly go back to the Vc and Vs. Has anyone analyzed about this too? This is reason why I need to wrap it around the beam by putting hole in the slab, to avoid debonding. But worried if the slabs would hold. It's 2.5 meters short span, 5.5 meters long span. The side where cfrp will be wrapped is the short span. Designer just told me to be careful not to cut any bars.

One installer said he will use chipping hammer on the edge of slab and beam to put hole in the part. One said he will drill it first to soften the part before using chipping hammer But do you think this would damage the beam upper compression section in case of midspan and upper tension bar-concrete bonding in case of support? Is this too risky? I have sleepless nights thinking about it so if you have any tips at least on the slab part, give some even if you can't comment on the cfrp principle itself. It will do much help. Any technique to put hole in the slab alternatively as illustrated in the attached illustration in last message?

RE: hook within hoop

One of your design team has stated that remedial measures are not required. Your design engineer does not know how to do manual calculations for the beam, relying instead on computer output. He has stated that the beam, as reinforced, is adequate for dead plus live load but not for seismic effects, yet you are asking someone from eng-tips if the stirrups are adequate for office load.

You need a knowledgeable engineer to review the structural design to determine what, if anything, is needed. If you can't find someone locally, then search elsewhere, but get on with it. The longer you dither, the longer the project will be delayed.

BA

RE: hook within hoop

(OP)

The designer originally designed the U-shaped carbon fiber wrap. It is I who insist that it be wrapped around the beam (full carbon fiber wrap) to avoid debonding. He said I can do either way. Just have to be careful not cutting any bars. So I solely decide whether to wrap full or U-shaped.

Reviewing what I learnt previously about compression block in interaction diagram strain/stress analysis. I learnt that in T-beams, flexural strength of the flange can be neglected but not compression advantage of the flanges in T-beams.

I found out in manual calculations that the stress block is within the flange.

a= Asfy/0.85fc'b
As = 2 (0.5 inch^2 (20mm bar) area x 4 bars at tension)
fy=60000 psi
fc'=4000 psi
b = 20 inches (flange width from the 12" original width)

a=1.764 inch

Since the flange is 4 inches, then compression block is within the flange.

Is the above calculations correct?

If I chip the flange, the compression advantage would be gone in midspan (because putting grout to connect new and original concrete may be poor). I know in support, the tension side is on the flange side so no advantage of the flange, but not in midspan compression side.

Please comment at least on the above. The contractor office won't recognize any other designer. The original designer forgot about the formula for compression block because they designed using ETABs for very complex buildings. So the above is my guide whether I chip the slab for full wrap or use U-shape carbon fiber. If I chip the slab, the compression block in the flange may be lost even if it would be grouted later because it is not longer continuous.

By the way, the slab is one way because long span is twice short span, hence designer doesn't mind chipping the short span side. But I'm concerned about loss of compression block in the flange.

I only have a day to decide so please kindly comment on the above and not tell me to find other designers because the office won't recognize them. Thank you.

RE: hook within hoop

(OP)

Oh, the reason for the above is because I decided to use shear reinforcement for maximum seismic loading resistance and it has to be done now before ceiling are put to cover the beams.

I'm getting convinced T-beams have advantage in the compression strength at midspan. I think it doesn't have any advantage in the negative moment at support with tension at flange, it may even avoid plastic yielding and ductile mode.

Another idea. Carbon fiber has different strain than the strain of rebars. Do you think metal plates epoxied to the sides of the beams would be better option since at least it has same strain as bars and the superposition of Vc+Vs+V(reinforcement) would more coincide?

Ordinary structural engineer in my country doesn't know what I'm talking about, so hope you guys can comment.

Since I have only one or two day to go before let the subcontractor do the retrofit. Let me share my real situation. I'm the owner of the building, the contractors and their engineers don't even understand what is shear and can't grasp the situation. If I'd sue them for not putting the middle stirrup. It would take years, and if I told them to rebuild the buildings, they can't just do it because they have to demolish everything to foundation and court hearing would take long. So I have every means now to let them do any shear retrofit possible. If metal plates more expensive than CFRP, it's ok too since they would shoulder it. Do you think metal plates is better since it's strain is more compatible with that of rebar? My designer forgot about strains and stress calculations so don't know what I'm talking about, and the contractor won't recognize other designers so I'm on my own and hence needs your tips for just deciding whether to use metal plates or cfrp. I already decided for shear upgrade since the contractor will shoulder everything and I can't let it pass now. Thanks.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources