Compression vs Tension Lap Splice Development Length

[thoughtofthis]

I'm sure this question has been asked, but I did not find an answer that was satisfactory in the threads.

In a doubly reinforced concrete beam that is a part of a moment frame, does ACI 12.2 govern the length of splice of the top reinforcement or 12.3. The building is already built and uses 12" splices for all top bars at midspan. 12" does not meet 12.3 for the bar size used, but it is way off if 12.2 governs. I doubt that the intent of the top bars was to carry compression loads (most likely for ductility and to satisfy 12.11/12.12), but technically they could fall in the compression zone under most gravity loadings.

Thanks!

 

[thoughtofthis]

Also, does 7.11.1/7.10.5.3 apply for the ties of all top bars?

 

[JAE]

If the bar goes into tension (and is relied upon for strength) then 12.2 would govern. You have to fully analyze the beam under all code-required load combinations to see if tension is developed in the location of the splice.

 

[strucguy]

Top bars are often spliced at mid-span and typically don't require tension lap splice unless you are relying on the top bars to resist the bending moment at mid-span.

 

[rapt]

At midspan of a beam, a top bar may not be required for strength at all. In that case, as long as it is ignored in calculations, or only the capacity the provided lap can generate is used in the calculations, in both tension and compression (different amounts), then there is no problem with smaller laps. If only 50% of the compression lap length is provided, then the bar can develop 50% of its capacity in compression. If at the same time only 30% of the tension lap length is provided, the bar can only develop 30% of its strength in tension.
The bars may have simply been provided to support the stirrups.

 

[TXStructural]

Analyze per JAE's comment.
You are required to have full development of reinforcement at all sections where the reinforcement is required, with the exception noted in 12.2.5. If the beam is truly "doubly reinforced" as you state, the top bars at midspan do need to be fully developed because they do act as compression reinforcement at that section. If the beam was designed as only tension reinforced, with top bars carried for convenience, then they would not need to exist at all for design purposes, so a short splice would be inconsequential.

 

[thoughtofthis]

Thank you for your replies. @TXStructural The beams are continuous. Doubly reinforced = simply the presence of top and bottom steel continuous through the beam.

What about in the case of perimeter beams? 7.13.2.2 At least 1/6 of negative moment reinforcement continuous over the span length (2 bar min). If a splice was present in top bars at midspan, it must be class b splice, no?

Also, 21.2 is the minimum for every SDC, but A. It requires two longitudinal bars continuous top and bottom (min) developed at the face of support. If a splice was present in top bars at midspan, it must be class b splice, no?

 

[TXStructural]

My question about being doubly reinforced was to question whether the top reinforcement at midspan was used in computing capacity of a compression block and determining the neutral axis for tension reinforcement. If it was, then the splice would be required to meet the lap length in 12.3. Provisions of 12.3 apply, so if it is excess reinforcement, you get the reduction.

ACI 318-11 7.13.2.4 requires class B for the top bars required in 7.13.2.2.

In the context of 21.2, the requirement for continuous bars would appear to require full splice, based on 12.10.4. 12.15.2 requires all tension laps to be class B unless you have twice the required reinforcement. I'm not sure this could be construed to allow shorter slices even when there are plenty of bars. That said, if the distance between bars complies with the intent for non-contact lap splices, you could use adjacent bars which lap with the short splice to assure that the splice is adequate. (That isn't terribly clear: imagine a three-bar splice, where two bars are in a 12 inch contact lap, and the third bar overlaps each of these bars by class B length and is within the non-contact lap distance requirement.)