Tension-only reinforced concrete members with lapped splices 1

[StrucCivEnv]

Has anybody ever designed a a tension only concrete member without any mechanical or welded rebar splices? I'm designing a building without an interior concrete slab, so I can't fan out rebar from the column bases to resist the spreading forces from the roof load. The idea is to pour a grade beam from column to column with adequate reinforcement to resist the spreading load. ACI covers lapped rebar design for tension reinforcement, but it seems to be implied that this is for tension in beams. Obviously I can weld or mechanically fasten the rebar together to make a continuous reinforcement, but is it necessary?

 

[KootK]

I've done what you're suggesting and I know of no limitation prohibiting it. I wrapped the lap joint nice and snug inside some tightly spaced closed stirrups for good measure but I don't know that even that was necessary.

I remember a thread a while back that asked this question in the context of an axially loaded column that was being used as a hanger somehow. The consensus seemed to be that mechanical coupling was the way to go. So for many engineers, I think the answer will depend on the application and the nature of the consequences of failure.

With field welding of rebar, I worry about quality control. So I guess that would be my last choice of the three options.



The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[StrucCivEnv]

Thank you for your input! I'll add some additional stirrups and go with the lapped design.

 

[jt12]

If you put in a ACI tension lap splice, this should transfer the tension load. I've never seen anything implied in ACI that this tension lap splice is only valid for flexural members. That would seem to go against how the ACI code works. You can add the closed ties if you like, but if there is enough confining concrete surrounding the bars, it likely isn't necessary. If it was compression rather than tension these ties might be more necessary.

 

[hokie66]

To the contrary...tension tie members are treated differently. ACI318-08, 12.15.6, requires mechanical or welded splices for tension tie members, and I believe that this has been the case for many years. The same requirement is in the Australian Standard.

 

[KootK]

Nice one hokie. I`ll have to check to see if we`ve got a similar provision in Canada. Does the AU standard say any more about the mode of failure that we`re trying to prevent? The ACI standard does`t elaborate much. Would ties in in strut and tie model count as truss tension ties for example?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[hokie66]

I don't think the Australian code is more specific, but believe the failure mode contemplated is splitting of the cover concrete due to eccentricity. I have seen (written, not built) examples of this being addressed by closely spaced ties or spirals around laps, but don't think this strictly complies. I think ties in strut and tie models are different, as they aren't normally spliced, and the emphasis is on anchorage at or beyond the nodes, rather than development.

 

[hokie66]

According to this reference, the prohibition against lapped splices in tension tie members was first introduced in the 1995 ACI code.

http://www.concreteconstruction.net/Images/Mechanical vs. Lap Splicing_tcm45-343411.pdf

 

[Canuck67]

I have had discussions with other Canadian engineers over the years that the Canadian concrete code requires mechanical splices for pure tension members, but do not have a specific code clause to reference.

 

[KootK]

Thanks for the article hokie. Did you notice that it mentioned that lap splices might also be unwise for freeze thaw or marine environments? Apparently, Arizonans are the only North Americans who should use lap splices outdoors.

When you mentioned splitting of the cover due to eccentricity, did you mean:

1) Eccentricity of the non-contact lap splice variety?
2) Or overall load eccentricity in the tension member?

Interestingly, when I`ve added ties in the past, I proportioned them such that the splice would conservatively work as a strut and tie assembly. Lap splices in limited cover applications utilize concrete in tension. For a non-redundant, important member, I always felt better about replacing the concrete tension with steel ties. Hopefully I`ve satisfied the intent of these code clauses if not the strict verbiage.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jt12]

Since I didn't previously know (or forgot) that 12.15.5 existed, I guess I'm going to dig myself a deeper hole. The commentary says that a tension tie A) has axial tension over the entire section B) a level of stress in the reinforcing causing every bar to be fully effective C) limited concrete cover on all sides. In a fully tension member, it seems reasonable that A is satisfied. B & C however are designer dependent items that could easily be slightly changed to allow a pure tension member to be able to be designed with tension lap splices rather than mechanical splices.

 

[Agent666]

I don't really see how it's fundamentally different to a beam in flexure, one Side is in tension for flexure. For tension both sides happen to be in tension. You are free to lap the bottom bars in most codes when they are on the tension side of the beam. Some cases require a staggering of the bar laps or some additional stirrups, but you can do it, the same would apply in a tension member as a good detailing practice.

 

[hokie66]

The difference is that in a flexural member, you don't lap bars where the moment is greatest. In a tension tie member, the force is constant over the entire length. I didn't write the provision, but it seems plain to me that lap splices in tension members are prohibited.

 

[Agent666]

Hokie66, with the codes I work with there are only restrictions on locating laps when flexural yielding under earthquake loads are considered (keeping them clear of potential plastic hinge regions). Under static loading you can locate the lap at the point of largest moment if you wanted to, obviously better to locate clear of this as you note but there isn't a restriction as long as you follow a few rules on closer stirrups and the like and provide reinforcement to deal with the forces resulting from the cranked bars.

Our code (NZS3101) is based on ACI318, so I would have guessed similar applied in the US with no restrictions that I know of like others have mentioned (not withstanding that I haven't looked up the clauses being referenced!).

 

[Agent666]

Ok, had a read of clause 12.15.6 and I guess you can't really argue with what it states!

The last paragraph of the commentary sort of implies a staggered lap is an alternative which can be used with some engineering judgement. Based on what other international codes allow I would definitely allow this approach, a tension member is likely to be governed by limiting crack widths so is likely to be over reinforced from an ultimate strength perspective which would make me feel more comfortable about lapping the reinforcement.

 

[Tomfh]

Australian code AS3600 allows tension laps for columns in tension (10.7.5.3), but forbids it for tension ties (13.2.1).

How are these cases different?

 

[Agent666]

I think a column is under compression predominantly from gravity loads with the odd excursions into tension under lateral loads, whereas a tension tie is under constant tension under gravity loads. The ACI code mentioned hangers in arch bridge structures specifically as an example where the member would be classified as a tension tie.

 

[hokie66]

Agent666,
I agree with your interpretation and explanation. The Commentary gives leeway in defining a tension tie member, and one criteria is that the bars are fully working stresswise. In many cases like ties across a portal frame building, elongation should be the controlling factor, so if the stress is relatively low, splices could be permitted, with appropriate means of preventing splitting under those stresses.

 

[Tomfh]

What about a column that's always in tension? e.g. at a short backspan?

You should treat this column as a tension tie?

 

[hokie66]

I think not, at least for your example. If the end column at a short backspan fails in tension, the compressive force on the next column decreases.

A reinforced concrete hanger, on the other hand, is critical and would be considered a tension tie.