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]

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.2011.108.115

"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/S0141029606003701

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

http://www.concrete.org/PUBS/JOURNALS/AbstractDetails.asp?ID=9944

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

Please share others you see too.

 

[itez]

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??

 

[itez]

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.

 

[wannabeSE]

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.

 

[itez]

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?

 

[hokie66]

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

 

[itez]

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?

 

[BAretired]

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

 

[itez]

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??

 

[itez]

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.

 

[BAretired]

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

BA

 

[itez]

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?

 

[BAretired]

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

 

[itez]

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.

 

[itez]

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.