Moment along a rebar lap splice 2

[JSA2]

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?

 

[JSA2]

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.

 

[kslee1000]

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.

 

[JSA2]

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

 

[JSA2]

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?

 

[kslee1000]

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.

 

[ChipB]

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.

 

[miecz]

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?

 

[swivel63]

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

 

[ChipB]

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

 

[ruready]

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

 

[JSA2]

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.

 

[kslee1000]

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.

 

[ruready]

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

 

[Dinosaur]

And these are the criteria I mentioned in my post.