What EXACTLY is a lap splice?

[Lion06]

One thing I've always wondered a little about is what exactly is a lap splice? I mean I get that we're transferring load between bars, but why are there requirements to make sure the bars are within 6" of each other?

What I'm picturing is a straight up tension member (See attached sketch). Say it's a 30" x 30" tension only member that is 40' long. Say you need (16) # 9 to take the tension. Let's say I have (16) # 9 (around the perimeter similar to a column) from the top of the member down 25', then I have (4) # 18 bars that are centrally located below that. Why can't I take the tension out of the rebar into the concrete to the point where the concrete stress reaches fr, then develop (not splice) (4) # 18 above that line? Is that the definition of a splice.

 

[rb1957]

in my world a lap splice is formed by overlapping (typically) two sheets, so load is transferred directly from one to the other.

a butt joint is made by butting the two sheets together, and joining them with a separate doubler.

 

[msquared48]

Personally, I would not ever put a #9 bar in a tension transfer position with anything but a Cadweld, or something similar.

I choose to directly tell the forces where to go. Not leave it to chance. But, that's just me.

In theory, your scheme could work. I just would not trust it as well as a direct link, particularly if it is a critical connection.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Lion06]

ACI prohibits splices for anything bigger than a #11. I'm trying to determine if what I'm suggesting is, in fact, a splice. I'm having a hard time seeing otherwise, but wanted to ask the question.

 

[msquared48]

I realize that I am not directly answering your question here, but if this is a column, then you also have to comply with a uniform distribution of steel to control cracking, Placing the #18's as you have done does not satisfy this.

Also, in your sketch, you are showing a lap splice of the #18 bars, even though they are "lapping" with #11 bars, outside of the ACI requirements for the #18's even though the #11's are compliant.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

SEIT...if I understand your premise correctly, you have not described a splice but strategic placement of tension reinforcing. A splice, in ACI/CRSI vernacular, is the attempt to achieve continuity of a single reinforcing bar.

The 6-inch max. spacing requirement is to prevent a zone of unreinforced concrete and is supported by the available data as that was the spacing in the testing used to develop the criteria.

 

[Lion06]

Mike-

I generically showed them in the center only for illustration purposes. I wanted to make clear what, in my mind, was not a lap splice.

Ron-

Correct. I have no need or desire to make a "lap splice", just provide a load path. I know that splices of #18 bars are prohibited, which is why I was asking the question of what exactly constitutes a splice.


Just to look at it a little differently. Let's say there is a cold joint at the top of the #18 bars. If I provide a length of the #11 bars that is equal to the development length of the #18 into the lower portion, what, if anything, is unacceptable about that? I've provided sufficient strength for the upper portion, the lower portion, and a load path between the two.

 

[JAE]

StructuralEIT,

Take your sketch - draw a line across the concrete section halfway down the l_d length.

This is a potential crack failure plane.

Your bar tension for your 16 #9 bars (full capacity) is 864 kips (phi x As x 60).

The bars would have to be developed their full ld value past that center failure plane. In your sketch, you only have (l_d/2).

Same goes for the #18 bars developing enough l_d past the failure plane.

This essentially requires a total of l_d(for #18) PLUS l_d (for #9).

Even still, you have a potential failure surface that isn't a plane but a meandering surface that winds its way across the section over the bar ends in a back-and-forth pattern. Not sure if this would actually fail this way but I could see it.

 

[msquared48]

SEIT:

OK... but you said " If I provide a length of the #11 bars that is equal to the development length of the #18 into the lower portion, what, if anything, is unacceptable about that?"

The problem is that you are still including a #18 bar in a lap splice situation, and as you pointed out, it is not accepted by the ACI. Are you questioning whether that restriction should be in force too?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Lion06]

JAE-

Is there a reason you picked halfway? I was thinking (even though I didn't draw it that way), and said in an earlier post, that the #18 should be developed above the line defined as follows: the line that marks where the tensile capacity of the concrete is exceeded as the tensile force sheds from the steel to the concrete.

I had a two step thought process in my head. First, if you have this column that is anchored at the bottom and the # 11 bars stop short of the bottom by 20'....... a tension failure would look something like a tensile failure of the concrete section but above the bottom of the #11 bars. The remaining length of bar would just pull out of the parent member because the remaining embedded length is just so small. If, however, there are #18 bars developed above and below that plane then all was good in my mind.

Point is well taken about overlapping the bars by ld of the #11 plus the #18.

The more basic question about this, however, is does this constitute a lap splice per ACI?

 

[Lion06]

Mike,

I'm questioning whether that condition constitutes a lap splice. If the definition of a lap splice is anytime bars overlap then it would be extremely difficult to use a #18 for a caisson or in any application at all over 60'.

 

[msquared48]

OK:

Then my answer is, yes, it does constitute a "Lap Splice".

However, it does not meet the requirements of a structurally acceptable and functional "lap splice" as defined by ACI.
\
As for the longer lengths of #18 rebar, that 's what Cadwelds are for.

Pardon my frankness here, but it seems like you are trying to complicate a simple situation. No offense intended.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

SEIT...to answer your bottom line question, in my opinion, No.

Think of it this way...could you reduce the cross section of the column below the #11's? If you can reduce the cross section to the point that the #11's would extend outside the perimeter of the reduced section, then logically you have no lap splice.

You have an overlapping tension stress path, but not a reinforcement lap splice.

JAE..nice explanation.

 

[Lion06]

None taken. We were asked to make a significant change to major portions of a building very late in the game and I'm just trying to make sure I'm not missing anything when we explain why it can't/shouldn't be done.

 

[BAretired]

SEIT,
As a matter of interest, the Canadian code does not permit lap splicing in tension tie members. It requires a fully welded splice or full mechanical connection with splices in adjacent bars staggered at least 800mm (32").

For bars in compression, the rules are different. Bar sizes No. 45 and No. 55 corresponding to your #14 and #18 may be lap spliced with No. 35 (#11) and smaller bars.

Personally, I have never lap spliced such large bars.

I don't think your sketched lap can be justified using strut and tie model.

BA

 

[rapt]

StructuralEIT,

The detail is requiring that force be transferred from one group of bars to another through the concrete, so it is a lap splice.

The further apart the bars forming the splice the more severe the stress condition in the concrete.

ACI limits the spacing between the bars forming the lap to 150mm (12.4.2.3) because of this. The Australian code allows greater than 150 as long as the lap length is increased by 1.5 times the spacing. Except in Tension Tie members, where lap splices are not allowed. Mechanical or weld splices must be used in this case.

 

[slickdeals]

What I'm picturing is a straight up tension member

You are not allowed to use transfer force between reinforcing bars using a lap splice. It will have to be only through a mechanical splice.

See Section 12.15.5

 

[KootK]

SEIT,

Great question. I've often wondered about this myself. Here's my opinion:

1) The situation that you've described does not constitute an ACI lap splice.

2) With the appropriate detailing, you can do what you've proposed in your sketch.

My reasoning is as follows:

Any time that you are transferring stress from one rebar segment to another via a "lap", there is concrete compression involved. This is true even when the bars are in direct contact with one another since there is no inherent shear connection between the bars. Since we're dealing with two tension forces connected by a concrete strut of some sort, I believe that strut & tie methodology is a reasonable way to approach this problem.

I believe that a code defined "lap splice" is simply a version of this strut & tie model where we have deemed that the offset between bars is small enough that it is not necessary to provide the "ties" in the strut and tie system. Based on testing, judgment, or both, it seems that ACI has decided that 6" is the appropriate limit.I think that it's perfectly acceptable to "splice" bars that are offset more than 6" so long as you provide the ties required to justify the strut & tie model. In your case, this probably means:

1) A group of stirrups and/or ties at the top and bottom of the pseudo-lap splice. In reality, you were probably going to do this anyhow as a matter of good judgment.

2) Worked out on a strut & tie basis, your "lap splice" length will likely be substantially longer than a code lap splice. Again, this is a good thing. I certainly wouldn't be shy about exaggerating the lap splice length in this situation.

I know of three similar situations that we encounter routinely:

1) The design of shear walls where concentrated vertical reinforcing is interrupted by door openings etc. and must be offset. In these situations, we typically "lap" the offset bars by an entire storey.

2) The design of footing extensions where the new rebar is drilled and epoxied into place and assumed to lap with the existing (offset) rebar. This situation concerns me as the standard design methodology doesn't seem to consider the need for "tie" elements.

3) The design of trim reinforcement around openings in reinforced concrete slabs. Typically we extend the trim reinforcement beyond the opening so that it can be consider "lapped" with the reinforcement interrupted by the opening.

In practice, we frequently count on tension getting transferred between offset rebar segments. If we can't rely on strut & tie type mechanisms to reliably achieve this, we're in trouble.

Also, good luck with the exam this Friday!

KootK

 

[rapt]

Error in my last post, the ACI reference for 150mm was 12.14.2.3

KootK,

The force in each bar is being transferred to the concrete over the length of the bar, not at the free end. So you would have a series of compression struts over the development length of the bar, not just at the end. So you would need your ties spread over the length of the lap, not just at the ends.

I think I will stick with the mechanical splices that other codes require!

 

[slickdeals]

http://www.utexas.edu/research/ctr/pdf_reports/index.html

Look for publication series 1855.

Anchorage Behavior of Headed Reinforcement: Literature Review
Anchorage of Headed Reinforcement in CCT Nodes
Anchorage Behavior of Headed Reinforcement in Lap splices
Behavior and Design of Headed Reinforcement in Concrete Structures