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Tension-only reinforced concrete members with lapped splices

Tension-only reinforced concrete members with lapped splices

Tension-only reinforced concrete members with lapped splices

(OP)
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?

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

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

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

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!).

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

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?

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

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

You should treat this column as a tension tie?

RE: Tension-only reinforced concrete members with lapped splices

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.

RE: Tension-only reinforced concrete members with lapped splices

I've got a working theory on this that I'd like to throw out to the group for discussion. Consider:

1) Apparently you're off the hook if you've got a bunch of extra cover. Why? How much cover is required to be able to take advantage of this?

2) If you're not fully utilizing your tension rebar, you're also off the hook. Why? How low do your rebar stresses need to be?

In any tension tie member where the rebar is lap spliced, there will be some degree of eccentricity between the rebar on one side of the splice and the rebar on the other side. And that implies a moment in the tension tie member. If some of your tension tie rebar needs to be used to develop this moment capacity, then you would technically not have enough rebar left over to deal with your tension load. This assumes a very tight design for the tension tie reinforcement of course.

If you weld or couple your tie rebar, you accrue two advantages. Firstly, by virtue of the installation procedure, it's much less likely that there will be a significant eccentricity between the rebar on opposites sides of the splice. Secondly, if all else fails, the member can fail in bending and the tension tie can straighten out to safely satisfy equilibrium requirements. You'd still have all the required tension capacity. Presumably, the longitudinal axis of the straightened member would then be at a slightly different angle than the original tension tie member. No big deal.

Attempting to use my theory to explain the two points mentioned above:

1) If you've got enough extra cover, then you've probably got enough "outside the tie" flexural capacity to deal with any eccentricity induced moments. Consider the example of a major drag strut in a concrete floor slab. In the in-plane direction, you've obviously got plenty of moment capacity. In the out of plane direction, as long as the adjacent slab has a decent moment capacity, you should be okay there too. A way to apply the "extra cover" out might be to assume a conservative amount of eccentricity in the tie member and ensure that there's enough moment capacity in the adjacent concrete framing to deal with it.

2) If you supply enough rebar to resist both the direct tension and a conservative estimate of the eccentricity moment induced in the tie member, you should again be okay. Maybe one could assume an eccentricity of half the tie member depth etc. Again, this would provide a means of determining how much extra rebar you need before you can go back to using a lap splice.

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

RE: Tension-only reinforced concrete members with lapped splices

There is no global moment in the member if the laps are in the same place on either face. There is no localised moment in the lap if you have detailed additional stirrups at the lap cranks in accordance with code requirements. In New Zealand we are required to supply additional reinforcement to deal with the change in direction equal to 1.5 times the nominal capacity of the bars (i.e. With no strength reduction factors. If you do this there is no moment as the forces are dealt with as part of a strut and tie arrangement across the section, provided that the laps are balanced on either side of the member which is usually the case.

Either way I am sure there are plenty of structures out there that are standing with laps in tension!

RE: Tension-only reinforced concrete members with lapped splices

Agent,

Do you not agree that there will be an internal moment in the tie if the centroids of the bar groups on either side of the slice do not coincide?

Also, could you supply the NZ code reference that you mentioned above? You've piqued my interest. I'm going to see if I can find that.

KootK

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

RE: Tension-only reinforced concrete members with lapped splices

Will scan it for you. I agree with the moment is there. It's just taken out by the ties in a tension and compression force at the bend/crank in the reinforcement (which is resisted by a few dedicated stirrups).

RE: Tension-only reinforced concrete members with lapped splices

Methinks you are overthinking this. Tension tie members are not used frequently, and mechanical splices are readily available.

RE: Tension-only reinforced concrete members with lapped splices

@ Agent: the attached sketch shows an exaggerated version of the moment that I see in a lap splice. It would not be resolved by ties in an STM fashion. That's basically the crux of the difference that I see between mechanical couplers and laps. With mechanical couplers, the tie can fail in bending and you can sort of get rid of the moment and still be left with the tie. With lap splices, you're stuck with the moment no matter what.

@ Hokie: I'm sure that I am one of the, if not the, principal "over thinkers" on this forum. Like Lady Gaga, I was just born that way. If I had my druthers, code commentaries would be an order of magnitude longer than they are now. I think that it's pitiful that I have to wonder about stuff like this. This code cause is particularly bad. They supply a bunch of situations where the clause applies without ever actually communicating the fundamental intent of the provision.

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

RE: Tension-only reinforced concrete members with lapped splices

If the laps are not symmetric then there would be some net moment. Reality is they are mostly placed symmetric. Without doing any numbers I would have thought the moment is fairly small as the eccentricity is one bar diameter which in the big scheme of the force required to yield the bars results in a fairly small moment in relation to overall flexural capacity of the member.

We never account for example in design that there is a reduced effective depth at the exact location of the crank in a beam or column bar. It's just one of those many secondary things we ignore in practical design.

RE: Tension-only reinforced concrete members with lapped splices

Agreed. The moments would be relatively small and largely dependant on construction quality control. Thanks for taking the time to post the code clause for me Agent.

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

RE: Tension-only reinforced concrete members with lapped splices

A similar clause exists in ACI 318 under 7.8.1.3. Slightly different wording but the same intent.

RE: Tension-only reinforced concrete members with lapped splices

In the British code there is no mention of "tension ties". There are however requirements for lap lengths for members in tension. There are also lap lengths requirements for compression members and members in flexure. So for beams for example, the lap length for tension bars would be taken from "members in flexure".

I have a need for a concrete hanger to pick up a slab edge. Cannot have column below. So I have designed the beam above to carry the force from this tension tie. The bars in this column then go down and bend into the bottom of the slab it is supporting. I have allowed double the amount of tension bars required in the column, and as the British Code does not require couplers to be used, I made use of splices, but increased splice length by 1.5 of that prescribed in the code (40 x 1.5 = 60 x bar diameter for 30 MPa concrete. Also I provide closed stirrups at the lap zone.

I am confident that the spice connection is fine as it is overdesigned by effectively 100% to British Standard requirements ( I am aware this is not what ACI prescribes)

My only concern is at the connection where the vertical tension steel goes down into the flat slab that is hanging off the column. I have detailed the column bars to go down into slab with hooks.

If I calculate the bond stress from the straight section of bar anchoring as well as that generated by the hook, I have sufficient capacity. I have added top steel in the slab at this point (hogging moment reinf.) as if there is a column below the slab.

I have checked punching as if a column is below the slab. The difference being that the force is not transmitted from bearing of a column below pushing up, but by the stresses that come out of the column bars anchored to the slab concrete.

Has any one actually designed a concrete hanger column that supports a flat slab and how do you deal with the connection of slab to column. It is similar to a point load on a flat slab I would think.

RE: Tension-only reinforced concrete members with lapped splices

I can't locate the thread but this exact issue has come up here at least once before. You're on exactly the right track in terms of the failure modes that you're considering. Punching shear is the really tricky bit.

How thick is your slab? What are the dimensions of your columns?

I would favour a solution that gets as close as possible to a bearing style punching shear resistance as you mentioned above. Some options, listed in order of preference (for me):

1) Get a beam or some nib of concrete cast under the column deep enough that you can develop the tie bars in it. Your architect and contractor may not thank you for this.

2) Terminate your tension tie reinforcing at a weld plate cast into the underside of the slab and sized for bearing.

3) Use ACI Appendix D provisions for post installed rebar. It's not as inspiring as bearing for punching shear but at least you'll have addressed concrete breakout in some fashion.

As you've already intimated, this is a good connection to design conservatively.

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

RE: Tension-only reinforced concrete members with lapped splices

I think Sybie99 is touching on the truth here, and all this talk of moments strikes me as a little silly, and also of the human tendency to "run home to Momma". All of us try first to make a new problem fit the mould of what we expect, prior to trying to think of a new reason for what we see. I'll give it a go as well...

I'm a fan of reading the ACI reports (and other code committee background research) that goes into making the codes. The ACI people have got to be the most transparent, and actually it is an ACI report on laps in bars that I want to talk about.

ACI 408.3R-03 (still need to purchase and read 09, *blush*) specifically addresses in detail tension splices... More specifically it talks in detail about how splices are actually believed to work, and suggests what approximations may be appropriate to go into the codes. In terms of the transfer of a tension load between two bars, the load acts as a stress on the concrete through the mechanical deformations on the bars. That concrete then transfers the load to the adjacent bar through both the tension capacity of the concrete and a keying effect. The next bar over then adopts this load in reverse to the first (load originating) bar. There is no discussion of any internal moment, and I believe the reason for that is simple...

The eccentric effect of the bars being next to each other but not colinear is minor. For a normal bar (say 15M) for a standard lap distance, the confining force of the concrete is applied over the full length of the lap, and the eccentricity over approximately 16mm. As the gap increases, so does the length of the required splice. There is no comparison, and I think this is the major reason why there is nearly never an issue, except for where there is insufficient cover. The minor applied moment is overwhelmed by the tension capacity of the confining concrete, and so long as this very rare failure mode does not occur, the loads will transfer as through a standard stress lap effect in the PCC.

RE: Tension-only reinforced concrete members with lapped splices

KootK,

The scenario is as follows, also see attachment: 300mm thick slab, 35MPa concrete, hangs off a 450 x 230mm column (edge column).

There are eight 16mm bars in column. Yield stress of bars is 500MPa. The bars go straight down into slab for 150mm before bending with a 120mm radius bend to join into bottom mat reinforcement in slab. The horizontal part of the bar is 1000mm.

The tension force is 190kN (ultimate limit state force). The top steel in the slab at the column location is 565mm2/m perpendicular to slab edge and 1000mm2/m parallel to slab edge. I have checked punching using the following input: 300 x 200 column area, 35MPa concrete in slab, 250mm thick slab (actually 300mm thick but force is not applied at slab soffit. No punching rebar needed.

RE: Tension-only reinforced concrete members with lapped splices

You might want to move this question into its own separate thread Sybie99. We're drifting a bit from the OP's original question.

What you've detailed is spot on for dealing with punching shear. The question here, however, is whether or not punching shear is applicable to your situation. Were you able to employ methods #1 & #2 that I mentioned previously, I think that you could argue that punching shear applies.

As you've detailed your situation, unfortunately, I don't think that punching shear is the appropriate way to look at things. It leads to a check based on the wrong shear crack in my mind. Check out the attached sketch for my thoughts on the matter and a proposed solution. Do the British codes have hanger steel requirements for beams supported by other beams? Canada's code does and I see that as being the applicable design check here.

Another thing to consider is integrity steel design. In both Canada and the US, you have to have integrity steel installed that would provide an alternate load path if punching shear doesn't get the job done. With your detailing scheme, I don't think that you'd have anything to satisfy that requirement. Since you're using BS that may not be an issue for you.

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

RE: Tension-only reinforced concrete members with lapped splices

KootK is correct about the critical crack plane.

RE: Tension-only reinforced concrete members with lapped splices

KootK,

A23.3 has the same clause regarding tensions ties and mechanical splices. In the 2004 version of the spec it's 12.15.5.

RE: Tension-only reinforced concrete members with lapped splices

Thanks TLHS. For exposed tension ties, I've also applied clause 8.12 of the Canadian Highway Bridge Code (crack width).

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

RE: Tension-only reinforced concrete members with lapped splices

KootK

I have decided to also have a steel plate with 4 No 20mm bars butt welded to the plate, to sit below the slab. These bars are then lapped into column. The plate pushes up against slab soffit and then gives a scenario similar to a column that sits below slab.

RE: Tension-only reinforced concrete members with lapped splices

I'm glad to hear that sybie. I think that the plate is a good solution. Bonus tip: in my neck of the woods, that plate would have to be fire proofed.

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

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