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Moment along a rebar lap splice
2

Moment along a rebar lap splice

Moment along a rebar lap splice

(OP)
A cantilevered retaining wall that I am designing has vertical bars, lapped with vertical footing dowels.  The wall height from top of footing is 8-feet.  There are footing dowels (#4 @ 9") which extend from the footing, 3-feet into the wall.  They are lapped with vertical wall reinforcing (#4 @ 18") which is 8-feet long; therefore, the lap is 3 feet at the bottom of the wall.

The footing dowels are designed for the moment at the top of the footing.  The vertical reinforcing bars are designed for the moment at the top of footing dowels - at the 3-foot height.  But the moment changes pretty rapidly along the length of the lap, because it is a cantilevered retaining wall.  

How do I know that I don't have a development deficiency at any particular cross section along the lap?



 

RE: Moment along a rebar lap splice

I can see that you are trying tosave steel here, but atwhat cost - your confidence in your design?  Why not just design both bars to the moment at the top of the footing and be done with it?  Peace of mind is a good thing.

Mike McCann
MMC Engineering

RE: Moment along a rebar lap splice

I agree with Mike--however, to answer your question, unless the moment diagram is convex (which it isn't on a cantilever), the bar will always be "more" developed than the moment in the wall.

DaveAtkins

RE: Moment along a rebar lap splice

(OP)
Dave, but the loading is triangular for a cantilevered retaining wall, so the moment is indeed "convex," isn't it?  That was actually the source of my concern, as I imagine a critical section at various positions along the lap, I wondered if moment could increase faster than the gradual development (starting from zero) of the footing bar side of the lap.

Thank you both for your input.

 

RE: Moment along a rebar lap splice

No, a convex moment diagram bulges outward, like the moment diagram for a simple span beam.  Since the moment diagram for a retaining wall is linear, and development occurs in a linear fashion, your bars will always be developed at least as much as the moment in the wall.

DaveAtkins

RE: Moment along a rebar lap splice

(OP)
Sorry Dave, I think I meant concave.  The momenent diagram for a linearly-varying load is parabolic, not linear, right?

RE: Moment along a rebar lap splice

JSA2:

Randomly check required reinforcing at two locations, say 1' and 2' above the footing. For each location, calculate the reinforcing required and make sure the reinforcing ratio is maintained wthin the range of one development length above this specific location/plane. Done.     

RE: Moment along a rebar lap splice

i agree with mike.  some extra bars doesn't hurt anyone.   

RE: Moment along a rebar lap splice

Yes, the moment diagram will be parabolic, and concave.  My mistake.

But, in any event this is better than a linear moment diagram, in terms of development of reinforcing.

DaveAtkins

RE: Moment along a rebar lap splice

The moment diagram for a linearly varying load is concave (cubic, I think).  For a uniform load due to surcharge, it's also concave (parabolic, I think).  So if you have the required reinforcing developed at both ends of the splice, you'll have enough developed in between.  The problem with adding reinforcing when it's not necessary is that, after a while, people start to think that it is necessary.  Besides which, somebody has to pay for that extra reinforcing.

RE: Moment along a rebar lap splice

(OP)
Kslee, this was my thought too.  I haven't done that yet, but I suspect when I do, I will concur with Miecz too. Thank you all for your input.   

RE: Moment along a rebar lap splice

assume your strees varies linearly from zero at the end of the bar to full strength at the end of the required lap length.

Now you can create a reinforcing force diagram and compare

RE: Moment along a rebar lap splice

You don't need to explicitly check the forces between the ends of the lap. As Dave and miecz point out, the anchorage builds up faster than the moment.

RE: Moment along a rebar lap splice

JSA2:

Experience can be gained either by open discussion, or self persuasion, depending on the nature of the topic in concern. For this case, I think a quick calculation may provide better understanding and strengthen your conviction for other matters that share the similarities. Have fun.
 

RE: Moment along a rebar lap splice

That's fine, but do it on your nickel, not the client's.

Mike McCann
MMC Engineering

RE: Moment along a rebar lap splice

I think an engineer shall find time to settle his curious mind, trivial or not. None of my clients argues how I spent my time, as long as I deliver the products that fitting their's interests within agreed time frame and budget. Fact of matter is, I would not hesitate to apply the cook book solutions only if there is no doubt. Otherwise, find time and ways to settle it.   

RE: Moment along a rebar lap splice

Man, you guys are confusing me.  And I thought I had a good grasp on retaining walls.

JSA2, I think you need to carefully consider your design.

In cantilever retaining wall design, the dowels coming out of your footing, serve one purpose and one purpose only.  That purpose is to transfer the moment from your wall, to your footing.  The wall itself, basically creates a vertical force couple.  Tension in the steel pulling out of the footing, compression in the concrete pushing into the footing.  Never the less, it should transfer the moment into the footing.  Most of us turn the bend of the dowel into the toe of the footing.  These dowels should have a Class B lap splice (1.3ld) into the wall, and be fully developed in your footing.   

The primary reinforcing in your footing should be in the top portion of your footing on the heel side.  If reinforcing is needed on the toe side, it should be in the bottom portion of your footing (which is why most of us turn our dowels that way).

If a key is required, the vertical reinforcing should be placed towards the toe of the wall.

I say all of this because your original post doesn't make sense.  Hopefully, your footing is thicker than your wall.  I can't figure out why you would need reinforcing @ 9" in your footing, and @ 18" in your wall.  It doesn't add up.  As others have stated, don't make it by such a gnat's behind.  There are very few times where I have spaced reinforcing greater than 12".

RE: Moment along a rebar lap splice

(OP)
Chip,  My question which started this ONLY refers to the vertical lap splice between the wall dowels and the footing dowels.  

The lap is 3-feet and it is at the bottom of the wall.  The dowels are designed for the wall moment at the top of footing.  The wall verts are designed for the moment at the top of the lap (3-feet above top of footing).  The footing dowels are a greater area of steel, say per lineal foot of wall than the the verts.  

As you imagine the moment increasing below the top of the lap, the bars which are there to resist this increasing moment (the footing dowels) are not yet developed.  But the moment is increasing in what has been described here as a concave cubic shape.  My inquiry was for the purpose of understanding whether I only had to check the reinf quantity at the top and bottom of the lap, or if there could be a critical, governing section somewhere along the lap.   

RE: Moment along a rebar lap splice

Well, I may be a dinosaur, but this is a very straight forward question.  To terminate a bar in a beam (every other bar in your example is terminated), you need to check three things.

The bar termination has always been the tough part of solving this problem in my experience.

In my copy of ACI 318 (don't ask what year) it appears you should check article 12.10.

Good Luck

RE: Moment along a rebar lap splice

Nope.  Therein lies your problem.  The way you have described your design, the wall verts should be designed for your maximum moment, at the bottom of the wall.    

I understand what you are trying to do, but, I think you are going around it from not quite the right angle.  I'll try to explain but as you already know, it can get confusing.

You're trying to treat it as a stepped wall, w/ your step being three feet from top of footing.  When you do this, you have to lap the reinforcing at the step.  If the wall was being formed in two pours, the first of which is 3 feet from the footing, and your design shows that at this location, you need #4@18, then every other one of your #4@9 dowels have to extend out from the top of that 3' pour, whatever length is required (I think around 30" for f'c=4000).

Does that make sense?

RE: Moment along a rebar lap splice

(OP)
Chip, I don't think you're correct.  All the laps, I believe can be in the lower 3-feet of wall, regardless if it's one or two pours.  The upper verts (#4@18) are developed above and below the 3-foot height for the moment at the 3-foot height.

RE: Moment along a rebar lap splice

Assume the wall is 8' tall with linear increasing earth pressure behind the wall, and assume the required development length is 2'.

The #4@18" bars in discussion satisfy both strength & development requirements for the plane at 3' above the footing. Similarily the #4@9" bars satisfy both for the plane at the wall-footing interface. Now let's check the plane at 2' above the footing, in which the amount of reinforcing lies in between #4@9" & #4@18". By inspection, the #4@18 bars are still ok, since it has 2' length below this plane. However, the #4@9" dowells, while meeting strength requirement, apparently do not satisfy the development length requirement (3'-2' = 1' too short), a further check using ACI "Excessive Reinforcement" provision is required to justify the shortcoming (note that the #4@18" bars do not meeting strength requirement at this plane while meeting development length requirement).

You may check the plane at 1' above footing, for which both #4@18" & #4@9" are ok by inspection. If I didn't make any mistake in example provided, your reinforcing plan obviously is questionable.  

RE: Moment along a rebar lap splice

(OP)
Kslee,  Your "interface" hotlink sent me to a MS Office promo site??  Am I doing something wrong?

RE: Moment along a rebar lap splice

(OP)
Kslee,  These posts seem to make their own hotlinks.  I did not try to make one for MS Office, but the post did it for me.  ??  Can we post documents on this site, such as a pdf in order to illustrate?

RE: Moment along a rebar lap splice

JSA2:

I didn't link anything in my discussion.

I think you can upload certain types of document throuh the link provided by this forum located at the very bottom of this page (next to "Step 3 - Attachment"). I have not tried so far.

For your question in this post, you can simply draw 4 horiz. lines representing elevation 0', 1' 2' 3' above the footing. Then draw 2 vertical lines at a distance apart - one for #4@18" extended from top of wall to top of footing, and one for #4@9" dowells that are terminated at 3' above footing (ignore the splices). Now exam each bar at 2' level, and ask the question "Does each bar passing this plane satisfys both strength and development criteria in both directions"? Well, simple question can be quite confusing, in fact it is really not that simple if you chose to think about. Again, have fun, and nice weekend.

RE: Moment along a rebar lap splice

JSA2:

I am correct.  Pull out your reinforced concrete text book and study lap splices.  You're developing the bars (dowels) going into your footing, to transfer the moment into your footing, therefore, your vertical reinforcing in your wall should match your dowel spacing.   If you do not understand this, seek your senior engineer's advisement.

Kslee1000's post confirms what I'm telling you.  He is describing the theory behind it.  

 

RE: Moment along a rebar lap splice

Quote:

You're developing the bars (dowels) going into your footing, to transfer the moment into your footing, therefore, your vertical reinforcing in your wall should match your dowel spacing.
I don't follow your reasoning, ChipB.  Where exactly do you believe JSA2's wall will fail?

RE: Moment along a rebar lap splice

check as(req)/as(prov) if you're having ld issues.

RE: Moment along a rebar lap splice

miecz:
Ugh!! I did not use the "F" word!

 

RE: Moment along a rebar lap splice

So what was the consensus on this?  JSA2, we do exactly what you described, it saves our clients money.  BUT, I would agree with ChipB and KSL.  It has to be checked as they described which we were not doing correctly! Its funny cause we had a ton of discussions here in our office about it, sucks.  Hopefully none of our walls will fail b/c the verts were not lapped properly or as most masons do, placed a whole heck more reinfocing than they need :)

RE: Moment along a rebar lap splice

(OP)
ruready,

I have not "run this to ground" by checking the moments verses the laps at a few different sections along the lap length yet.  My sense so far is that the development of both bars of the lap, when added together will be adequate for the increasing moment at any cross section along the lap.  If I do this computation, I will come back to post my results.  Thanks again for everyone who contributed to this.

RE: Moment along a rebar lap splice

ruready:

Only a personal opinion, your walls will not fail if it (splice) is the only item deviated from code requirement. However, it could be one of the reasonS that contribute to a failed wall.

RE: Moment along a rebar lap splice

JSA2, we finally looked into this more, from what we understand from ACI 318, you have to extend the bar that is not needed for flexure anymore, 12db or d past the point of where you are calculating the moment and of course you have to have your required lap lengths as well with your upper and lower reinforcing. Check out 12.10.3 of ACI 318

RE: Moment along a rebar lap splice

And these are the criteria I mentioned in my post.

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