CRSI Retaining Wall Manual Tables Deciphering

[Sokka10]

I've been doing a lot of reading and studying up on cantilevered retaining walls lately. One of the sources frequently mentioned here is the CSRI Retaining Wall Manual. As I've been perusing the information in the manual I've come across some recommendations in their tables that don't make sense to me so I wanted to see if any of you could help me decipher some of these things:

1) See the table below. CSRI has 12 tables for level backfill where they use various values for different soil properties. On all of the tables they recommend a footing depth of 12" for all walls up to 14' tall. For many of these walls they recommend using "0" bars (the dowel bars from the stem into the footing) that are as large as #6 and #7 bars. My question is how are they proposing to develop the dowels into the footing for bars this large when they only have a 12" thick footing? For a #6 bar I calculate that a development length (ldh) of 18" would be needed, and for a #7 bar it would be 21" based off of ACI 318-14 Section 25.4.3.1. Are they simply reducing the required hook embedment by the stress ratio? Seems like some massive reductions they're taking for the dowels if that's the case...

2) Note in the table that for the "0" bars on the walls that are 6' and under they have the Hk tag in front (eg. a 6 ft. tall wall needs Hk #4 @ 9"). Footnote 2 says that Hk means that the bar should be hooked into the toe of the footing. That implies that all of the "0" bars that don't have 'Hk' in front of them do not need to be hooked into the toe. They aren't actually proposing that the "O" bars for walls over 6' tall just stab straight into the footing without any sort of hook into the toe....right? Every diagram they have in the manual shows the "O" bars hooking into the toe of the footing. There's got to be some kind of mistake here or I'm reading that footnote 2 incorrectly.

 

[Celt83]

1. They use all the possible reductions allowed to minimize Ldh, cover and As,req/As,provided ratio.

2. For walls less than or equal to 6'-0" in height the Hk on the stem bars indicates that the toe and stem reinforcement are the same bar. For taller walls the stem bar laps the toe bars within the stem.

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https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

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[Sokka10]

Thanks Celt83. That makes sense for the Ldh reductions, cover, etc.

I had considered what you're saying about the walls taller than 6'. That makes sense for the walls that are 11' or taller because the table shows the length that the "O" bars should dowel into the stem, but what about the walls that are between 7' and 10' tall? No dowel length is called out for those "O" bars which leads me to believe that the stem and toe reinforcement should be the same bar as they are for the 6' and shorter walls, yet there is no Hk designation for those wall heights. In fact in Section 5.3 CSRI says, "For walls not over 10 feet high, the "O" bars extend to the height of the wall less 3 inches."

So why don't walls from 7' to 10' have that Hk designation for the "O" bars?

 

[Celt83]

Ah I see what your getting at now.

It looks like perhaps the toe is to short to develop the #4 "O" bars for those shorter walls so there is a vertical hook at the face of the footing.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[ATSE]

Sokka - good questions.
CRSI publishes many great engineering references.
However, for the retaining walls, they could have done better with the tables and figures.
Their figures and tables regarding the "O" and "M" bars (O1 and O2) and the different conditions for lapping, staggering, and alternating short dowels heights are not clear, and certainly lead to confusion. They tried to cram too much into one table. One or two extra columns in the tables, and a better figure would increase understanding and lead to fewer mistakes.

 

[Sokka10]

It looks like perhaps the toe is to short to develop the #4 "O" bars for those shorter walls so there is a vertical hook at the face of the footing.

Which leads to another question. ACI Table 25.3.1 shows the standard hook geometry for development of tension bars. For all bar sizes the L[sub]ext[/sub] is to be 12 bar diameters. The footnote below the table notes that "A longer extension shall not be considered to increase the anchorage capacity of the hook." Just looking at the CSRI tables I can see that they should definitely have enough toe length to achieve 12 D[sub]b[/sub] for their hook extension. For example, basically all of the "Hk" bars they call out are #4 bars which only need an L[sub]ext[/sub] of 6" according to ACI (12 x .5 = 6).

So why are they hooking the bar vertically at the face of the toe if, as ACI says, the longer extension doesn't do anything?

However, for the retaining walls, they could have done better with the tables and figures.

Glad you agree. Took me a while to figure out the O1 and O2 situation as you mentioned.

 

[Celt83]

It sounds like you may be only looking at the vertical stem reinforcement hook, but the toe itself requires bottom reinforcement and if the distance from the stem face to edge of footing is less than Ld then the toe bar also needs a hook.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[dhengr]

Sokka10:
You say…, “…walls over 6' tall just stab straight into the footing without any sort of hook…” They, or you, should never be recommending ‘stabbing’ as a means of placing rebar, however convenient and easy it might be for the contractor. It doesn’t work to develop the bars, worth a damn.

 

[Sokka10]

They, or you, should never be recommending ‘stabbing’ as a means of placing rebar

Exactly, thus my original question: why do the CSRI tables appear to suggest that the "O" bars for walls taller than 6' don't have a hook into the toe? They have the "Hk" designation for the "O" bars on 6' and shorter walls, but not for anything taller than 6'.

the toe itself requires bottom reinforcement

That's what I would normally think, but oddly in the CSRI manual there is never any mention of having a separate mat of toe reinforcement. The toe reinforcement in all of their diagrams is either provided by the stem bar itself or by "O" bars that are doweled to the stem reinforcement. I'm not sure if you have the current CSRI retaining wall manual to look at, but that's how things appear to me