re_bar couplers use in R.C.C 1

[pdprao]

WE ARE PLANNING TO USE REINFORCMENT BAR COUPLERS FOR IMPORANT RCC STRUCTURES, FOR CONNECTING BEEMS AND COLUMNS . THE COULPERS ARE PLACED JUST ON FACE OF COULEMN JOINING BEEM REINFORMENT.

IS THERE ANY LIMITATION IN USING REBAR COULPERS WHICH ARE THREADED TYPE LIKE BARTEC COUPLERS, FOR BEEMS AND COLUMNS?.

WHAT IS CODE POSITION IN USE OF THESE COUPLERS FOR CRITICLE STRUCTURES?.

ANY EXPERIENCE AND OBSERVATIONS ARE WELCOME

 

[rethornton]

The ACI provision for bar couplers is that they meet 125% of the yield strength of the bars on which they are coupling.

 

[ishvaaag]

Respect position, I don't think to put them at beam-column face in critical structures is good. There the stresses are to become maximum, and adding an irregularity there doesn't seem good. Theory would recommend that the connections be placed at midheight of the columns, where hinges are surmised to appear. Yet this too airy placement, if better, is neither appealing to the common sense, one almost sees the concrete spalling.

I remain musing if, where it feasible, placing them at the center of the joints themselves wouldn’t be better. Main gain would be a complete concrete entourage, sides, top and bottom.

In any case, and in whichever the position you place the welded rebar connectors, I recommend to use stirrups in such disposition and as close as to ensure such confinement that the gain of concrete strength for confined concrete is standing. This may mean placing stirrups thicker than usual and at 3 to 5 diameters separation there. Extend such stirrup reinforcement for say a total height of thrice the height of the connectors.

Another practice one might consider is not to make 100% of the connections at the same station. This however may be unconstructive and against the understanding of that being the connection mechanically ensured, such precaution becomes unnecessary.

 

[CBII]

As I read the question it appears that the column is planned to be poured above the joint of the beam to the column. If ACI 318 is the code basis, the options discussed meet the code. Another option to the stagger of the splice proposed(which is probably not feasible if you're using the coupler as a form saver) would be to increase your reinforcing by 50% of the area of steel required at the connection. In any case, the coupler manufacturer has test data as to if the coupler has been qualified as a Type 1 or Type 2. You probably need to determine what type of coupler is required then find one that meets that requirement.

 

[PXC]

I have often specified a particular type of coupler and only recently came across what could be a problem with it in some situations. It has built-in slack due to the coarse threads so that when it goes into compression, there is potentially up to about 6 mm movement before the bars carry any load. There is no problem with tension. Column design theory takes into account the compression reinforcement but this could be quite ineffective over a length = coupler length plus development length of the bars. I have not tried to quantify the effect this might have on column design.

 

[RiBeneke]

PXC
A good bar coupler should certainly not have and noticable slack when properly installed. Maybe you have a problem there.

In any case, it is preferable to avoid bar couplers if possible in any location where stress reversals in the bar are likely, particularly under dynamic loading or in earthquake-critical zones. Even a good correctly installed thread-type coupler could develop slack due to thread deformation or bedding-in under stress reversals.

 

[BryanStein]

RiBeneke,

A Linton Threaded coupler is the only threaded coupler that does not have any "slack" in the threads because the threads are tappered ... similar to iron pipe threads. Any non-tappered threads will have some slack as a result of the tolerances required to thread a female and a male thread together.

I've performed seismic researsh using reinforcing couplers. If you're in a seismic zone, a coupler that develops 150% of yield is required (Linton threads won't develop 150% of yield), but the codes make no comment on the "slack" often inherent with threaded couplers.

Two things that won't have any slack:
1) Crimped on couplers. I found these to be the best for areas of high seismic risk.

2) Applying epoxy to the threads of standard couplers. They must be upset threads (fatten the rebar and then cut the threads to keep from decreasing the Xsectional area of the rebar by cutting the threads directly into the rebar.

Hope this helps.

 

[pdprao]

I WANT INFORM MR RE BINIKI, ABOUT OUR USE OF COUPLER. WE ARE USING BARTEC MAKE COUPLER. THESE ARE BEING USED FOR DYNAMICALLY QULAIFIED SAFETY RELATED STRUCTURES. WHAT DO U SUGGEST ABOUT SLACKNESS IN COUPLIG BEING DISCUSSED?. HOW MUCH TO PERMIT. THE COUPLERS ARE BEING USED IN TENSION AND COMPRESSION ZONES , AND DYNAMICALLY QUILIFIED STRUCTURES.SHOULD WE DISCOTENUE THIS PRACTICE?

 

[PXC]

RiBeneke, that was good feedback. I am particularly interested in your experience with epoxy to prevent slack in threaded couplers. The ones I have experience with are a very coarse thread, similar to dywidag bars, in which the normal bar deformations are the thread. eg a 20 mm dia bar has about 10 mm thread pitch about 1 mm depth/height. The strength of epoxies is typically about 80 MPa compared with 500 MPa for the bars. I have just done some approximate calculations based on bearing stresses, ignoring shearing stresses, and the epoxy looks like it would be overstressed at ultimate load by a factor of about 4 times. Do you have any experience with how this sort of connection performs with epoxy?

 

[RiBeneke]

BryanStein,
I have seen the Linton (or is it Lenton?) coupler and it is a very good no-slack design provided it is properly installed. I would only recommend for bars where the extension piece is straight, to avoid the chance of a rebar worker only screwing the bars together up to the point where a bent end points in the correct direction.
And, as you say, because the threads are cut into the rebar, it will be almost, but not quite as strong as the original bar.
I do not know of any research into the notch effect of the threads under long-term cyclic loading, such as on a highway bridge. The mechanical engineers may have some guidance here, based on the sharpness at the bottom of the thread root.
Crimped-on couplers are good if you can get the machine into the space. There was a brand called Camtak / CCl if I remember correctly, but I do not know who does it now.


pdprao,
I do not know the details of your structure or the Bartec product or the design codes in your area, but it does sound as though you should investigate the slack problem further, and perhaps ask your supplier to produce some design information and perhaps certification.


PXC,
It was BryanStein who suggested the epoxy.
I agree with what you calculated on the epoxy stresses, and doubt that epoxy would work reliably. Epoxy strengths vary depending on the formulation, and the epoxy is in a confined compression area, but even so, I would hesitate to expect it to carry more than about 150 MPa.
The contact stresses between threads can be somewhat higher than the tensile strength of the bar, hence the 'bedding-in' effect. Therefore, for example, cylinder head bolts are re-torqued after running a piston engine.

 

[RiBeneke]

pdprao,
I have now found some product information on the Bartec coupler at

http://www.ancon.co.uk.

It can be tightened to the extent that it will be free of slack, but they do not recommend the use of a torque wrench for this model.

I would consider it to be slack free for static loads or for dynamic loads that are always in the same direction (always tension or always compression in the bar).

I would not consider it suitable for dynamic loads where there is stress reversal (sometimes compression and sometimes tension in the bar).

 

[BryanStein]

Just for completeness, let me add that the "slackness" issue with many threaded couplers (especially with load reversals) has not been addressed by any governing body. It is an area that needs some research to determine the "real world" affect on structures. The intuitive affect would be a softening of the moment curvature relationship (flexural stiffness) for beams.

I have used all these types of couplers in seismic research projects. The crimped on is the best, and I have used epoxy in the threads of upset thread couplers with great success ... yes, I know the epoxy is overstressed when the numbers are checked.

 

[pallencn]

I am considering using these threaded bar couplers (Lenton or BarSplice) in a project. We are designing a "Phase 1" building which will likely have a "Phase 2" connected to it 3-5 yrs down the road. The couplers will be at the face of Phase 1, for future rebar to splice into and connect the phases. The couplers will have to transfer horizontal shear forces from the Phase 2 to the shared shear wall, and will also support gravity loads from the adjacent floor bay. Are these Lenton-type bar couplers the appropriate solution? They seem to have the load capacity. Reading the above has helped some, but I'm wondering if anyone has had a similar application? Thanks.

 

[spinspection]

Our project has used Lenton couplers to connect reinforcing steel. Basically, the project consists of a bridge type structure that had to be constructed in 2 phases. The bridge decking used two rows of #5 rebar and the connecting girders used #10 rebar. We are about to begin phase II and will connect the rebar. For quality control purposes, the question has been raised regarding testing the coupling with a torque wrench? Is this the best way to check the proper installation? What would be a acceptable frequency? What problems might arise? What if the existing coupling in the concrete had not been fully tensioned/threaded? Can anyone help? Please respond as soon as possible, thanks for your imput!