Flexural Reinforcement Termination in Stem of Retaining Wall 1

[PEVT]

I am designing the tension reinforcement within the stem of a cantilever retaining wall. This wall, like most walls, is tall enough that it would be very inconvenient to require that the bars be placed full height at the time of the footing pour. I would like to provide a lap splice of the tension reinforcement within a reasonable distance of the top of the footing and so I think I must meet the provisions of ACI 12.10.5. Is this correct? Most designs I have seen for this scenario provide for the wall's moment and shear requirements, specify the general tension development length but do not address the fact that there is a termination in the tension zone.

 

[nongamer12]

you state that you're splicing the reinforcement. ACI 318-05 section 12.10.5 deals with terminating bars when they're no longer needed for strength. in my opinion, you're not terminating the reinforcing and don't need to satisfy 12.10.5.

 

[JAE]

I don't agree with that nongamer12. 12.10.5 specifically says not to terminate flexural reinforcement in a tension zone....it doesn't say "when they are no longer needed for strength.

Keep in mind that only ONE of the three requirements of 12.10.5 need to be met.

Section 12.10.3 must be met as well for the stem reinforcement.

 

[dcarr82775]

I read PEVT's question to be a lap splice question. If all the vertical steel is simply being spliced than 12.10.5 doesn't apply IMO. If he is terminating every other bar (or some other method of stopping some steel short of full height) than 12.10.5 would apply to the terminated steel but not the full height steel.

 

[msquared48]

I guess I do not get the question here. Tension lap splices are done all the time at the base of retaining walls where the stem meets the footing.

In addition, for taller walls where the moment in the stem drops to 50% (or a portion thereof) of the maximum, the tension rebar can be terminated - like every other bar or so. Even CRSI has details for this.

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

 

[JAE]

I took it to mean that the desire is to terminate some of the vertical bars (instead of carrying the maximum base As all the way to the top). This would mean you are terminating some of the tension bars and thus 12.10.5.

But if the question is to just use lap splices on all the bars to avoid having long dangling vertical bars sticking out the footing - then nongamer12's comment does make sense and 12.10.5 would not apply.

 

[hokie66]

12.10.5 doesn't apply to tension splices...the commentary says so. It does apply to terminating part of the bars, but in a retaining wall, I doubt the shear would ever be high enough so the clause would control.

 

[PEVT]

PEVT Here,
Just to clarify, I intend to provide a lap splice at EVERY vertical bar along the entire length of the wall. I have attached a typical sketch of this type of configuration. One tension bar is cast and developed properly into the footing during the footing pour and then another bar lap spliced to that one for the wall pour. The back face of the entire wall, by virtue of its design as a cantilever, is in the tension zone. This will allow for the reinforcement cage to be constructed more readily, however I wanted to be sure that 12.10.5 applies. You can assume that I have provided the proper development length. There will not be any stirrups (12.10.5) so it would seem that I need to check for compliance with 12.10.5.1 or 12.10.5.3. Thank you all. Your posts have been very helpful. Thank You.

 

[JAE]

I think you changing from 25 mm to 18 mm bars means that the splice must be positioned high enough to ensure that the stem moment can be resisted by the smaller bar starting at the bottom of the splice.

So find the point up the wall where 18 mm bars work (Point A). At that point would the bottom of the 18 mm bars. The top of the 25 mm bars would be your lap splice length above point A.

 

[hokie66]

As depicted in PEVT's section, I think there is plenty of length in the splice so that the 18 mm bars can be counted on as working fully at the top of the splice, and the capacity increases below that, until the 25 mm bar controls at the bottom. Where I see the weak link is the development of the 25 mm starters. I would make the cog longer, and place it below the longitudinal footing bars. I would also cog the top bars in the footing on the right end, and maybe the bottom bars on the left end.

 

[JAE]

hokie66,

I'm not sure about that. If you put the 18's as working fully at the top of the 25's, - if you go say 1/4 of the way down the splice, technically your 25's still have zero capacity as they aren't yet developed enough from their ends to really do anything.

So you'd have larger moment than the 18's can take, and the 25's aren't able to help any.

Not sure about your concrete code but in ACI, you must have a minimum length of development beyond the point where the bar begins to "do work".

 

[hokie66]

JAE,
This old thread had a discussion about this issue, which was possibly inconclusive. I agree with what miecz said "If you have the required reinforcing developed at both ends of the splice, you'll have enough developed in between."
thread507-233274

 

[TXStructural]

CRSI Design Handbook chapter 14 covers good detailing practice for cantilever retaining walls.

As you show, Class B tension laps are acceptable. I would hook the footing-to-wall dowels to the bottom mat rather than sit them on top of it, to improve development and make placing more secure. Show the 18mm bars extending all the way to the top of footing or provide a dimension. Best practice also includes "D" diagonal bars across the footing to wall interface.

For crack width control in tall walls (over 10 ft) we recommend that only half of the flexural reinforcement be lap spliced at the base, the other half should be extended to mid height of the wall. Consider that the change in bar size and the large area of steel in the lap region create a discontinuity in stiffness, which this practice is intended to mitigate.