column rebar splices through slabs 1

[lshort]

I have a querry on column rebar splices through slabs. Is there a necessity for the column offset bars from below to almost have contact with the column bars above as they pass through the slab. The column can have bending moments which require a structural depth. It is my feeling that the detailing manual says they should be in close proximity but would like to get other interperation.
LloydS

 

[syr]

ACI 318

 

[ishvaaag]

For splices it seems that tests show bigger strength when there's no contact, but some distance, between the rebar.

However, splices at floor level or so use to be made by some bending of the rebar below, and many -I would say most- times with scarce or nil care about if the main rebar are kkeping some contact or not. This is compounded by the disgust of the designers and inspectors on losing mechanical depth to the rebar there where the moments are higuer in the column. This in my experience, that only includes a barely notieceable earthquake bout over the code mandated lateral forces causes no problem for what at service level happens for buildings where earthqauke does not dominate the design.

Fortunately, when earthquake dominates the design, you will be keeping bigger attention to the splicing detail both as a designer and inspector, and you can resource even to mehanical connections instead of splices to deal with these issues...for the rebar is thought having a preferred position anywhere, splices being a hassle to deal with.

 

[JAE]

ACI 318-99, Section 12.14.2.3, allows some tolerance in having non-contact splices - bars must be spaced not farther apart than 1/5 of the splice length nor 6 inches.

 

[lshort]

Jae
Aci318-99, section 12.14.2.3 is not applicable to columns

 

[lshort]

Ishvaaag
I agree that sometimes there is little regard by the contractors for the splice. However that does not justify the contractor detailing and placing rebar in that joint haphazardly. This joint is a very important for moment transfer due to any lateral loads and also due to bending moments from unbalanced live loads. In designing any flat slab structure we take into account the stiffness of the column above and below the slab at the joint. As Engineers we design that column above for the worse load combination due to bending moments and axial loads. Those combinations can at the bottom of the column. Proper placement of that reinforcement is important as the bending moment capacity is totally dependant on the location of the rebar.

 

[ishvaaag]

Sure, I agree in that if people like we design something for the better it must be built so. I was just indicating that some general miscare exists in what I have seen, and also noted that the safety factors -plus of course secondary paths of loads, I add here- mean that it also are being non-consequential -the buildings standing on good condition shows this- till the test comes.

In Spain as of now, the general buiding ruins use be mainly misguided and badly executed reforms, many times combined with recent rains, so the detrimental effects of some bad practices may not show.

The problem of losing mechanical arm is more general and I have seen it a lot as well in beams or mats foundations by walking over or misplacement. (Slab or plate floors are less frequent here). Normally however the loss of mechanical arm I would say is (when so there being) in the range of 10%.

In any case, yes, if someone was thinking in becoming carefree as an inspector on my words, don't do, for it is your duty to ensure the things are built well.

 

[lshort]

Ishvaaag
I agree with what you are saying. But as Engineers and inspectors we state that the rebar is to be placed in accordance with codes and certain standards. We look to those codes for guidance to ensure a sound product at the end of the day.
What I would like to know is what other people look for in this situation. There are dimensions given in RSIO for Radial offset bars. I would assume those are the standards.
Those values give an offset that places a space between radial offset bars of 10 mm+-. If bars are placed to this value then I do not see much of a moment reduction and the value would certainly be no more than 10%. It's when those field offsets become 2 times the value in RSIO and the fabricator refers back to sec 12.14.2.3. as 150 mm spacing allowed.

 

[JAE]

Ishort.....please explain why you believe that Section 12.14.2.3 in ACI 318 is not applicable to columns - this is a general section on splices and I don't see anywhere where it denies application to columns. A splice is a splice no matter where it is located within the structure.

If you can point to where it specifically negates columns here I'd like to learn. Thanks.

 

[lshort]

Jae
Thanks for responding. I asked that same question to the aci technical committee and they came back with a response that they did not consider this section to be applicable to columns.The comment from ACI reads;
"> > ACI Member Services.
> > In my opinion ACI 319-99 section 12.14.2.3 is not applicable to the
> > radially offset column bars. For splice reinforcement details please
> > refer to ACI 318-99 section 7.8.1.
> > We appreciate your interest in ACI and hope that we can be of service to
> > you in the future.
> > Sincerely yours
> > Technical Staff "
Do you realize that we are referring to radial offset bars and not longitudinal offset bars?
If you were splicing bottom bars in a beam would you still try to maintain the same structural depth or would you consider it appropriate to decrease the depth by 6 inches and offset the splice towards the top of the beam. The code doesn't specifically say that you cannot offset bottom splice bars in beams 6 inches towards the top but I would think that is understood. I do not have that code section with me tonight but I think within that code section it refers to longitudinal splices. I will get that section tomorrow and reply with it.
If the code doesn't specifically place a limit does that necessarily mean there isn't one.