Retaining Wall - Flexural Reinforcement from Stem Into Footing 21

[CWEngineer]

I am trying to get some clarification regarding the flexural reinforcement of the stem of a retaining wall into the footing.

Does the flexural reinforcement in the stem of a wall, need to be developed into the toe, such as show in Figure 1 of the attached document. Or is providing a standard hook (12db), sufficient, such as that show on Figure 2 of the attached document? If providing a standard hook is sufficient, can the hook be turned towards the heel?

Thanks in advance

 

[KootK]

It's a shame that it's divided up into smaller sections.

Oh... don't get me started. I own every version of the CRSI manual ever printed. The new format messes with the continuity of my collection terribly.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Deadblow]

KootK, where did you find the figures 13-27, 13-28, and 13-29 that you posted on January 14th? Thanks

EIT

 

[KootK]

@Deadblow: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Deadblow]

Thank you!

EIT

 

[RFreund]

I'm glad this resurfaced. I've always followed the CRSI detailing but I just don't get how you can have a joint (stem to footing) that has bars developed on both sides of the joint and should satisfy your basic design requirements (all seems perfectly acceptable) but yet it only achieves 70% of the design capacity. I must be missing something.

EIT

http://www.HowToEngineer.com

 

[KootK]

I've always followed the CRSI detailing but I just don't get how you can have a joint (stem to footing) that has bars developed on both sides of the joint and should satisfy your basic design requirements (all seems perfectly acceptable) but yet it only achieves 70% of the design capacity. I must be missing something.

Are you really looking for additional clarification here are you just venting a little concrete joint design frustration? If you need more than the miles of posting above to convince you, I can give it a whirl in more general terms. I'd need to take the kid gloves off though.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[DaveAtkins]

KootK is the "Muhammad Ali of concrete rebar detailing."

DaveAtkins

 

[KootK]

Rope-a-dope. Come 'git some.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Trenno]

Great thread and contributions by KootK. Certainly some great info here to take on board.

 

[RFreund]

Missed some of these responses...

I think there is plenty above that gets the point across (great job by the way) but I guess one question I have:

When you presented the case of the wall turned 90 degrees with the cantilevered beam case. If you develop the bars does your beam have the calculated capacity (per code)? Or would it be 70% of calculated strength?





EIT

http://www.HowToEngineer.com

 

[Tomfh]

At these closing joints, does a large radius bend (which limits bearing stresses) give higher capacity in practice than a normal bend? Any good test results out there?

Also, where a normal cog/bend is used we generally put a crossbar in the corner of the cog, whereas most of the images above don't bother. Any thoughts on cross-bar vs no cross-bar?

 

[CELinOttawa]

Tomfh: The presence of the bar has either no effect, or negligible effect, from what I have seen in papers on the matter. It is a convenient bar to have for tying steel, nothing more. Somewhere I have a copy of an ACI report that talk about it... *looks suspiciously at structural library*

EDIT: I was wrong; See below. Don't let this be your take-away.

 

[CELinOttawa]

I am too busy for a thorough search (hopefully someone else will have something difinitive), however have a look at this:

 

[CELinOttawa]

Well am I ever glad I couldn't put my hand on that old ACI report. Apparently I was dead wrong:

The latest research I can find is out of South-East Asia and Australia. "Standardisation of hooks in AS 3600" by A. Wheeler, R. Bridge and W. Marsden has a specific section on this exact matter. I quote: "The effect of the transverse bar at the internal diameter of the bar is significant with higher failure load observed in the 10mm bars. For the 12mm and 16mm bars, only the short hooks with no transverse bar demonstrated a pull out failure. In the corresponding tests where the bar was present the bars fractured in the 12mm and 16mm tests."

From my read of their paper, it seems that the concrete crushing limit state governs for small diameter bars (with their correspondingly small hook diameter). As a result, the presence of the transverse bar within the hook spreads the forces out, lowers the stresses, and potentially prevents the initiation of the crushing which otherwise would unzip the bond and allow the hook to pull out.

I suspect this has never become an issue in practice because these hooks often occur at supports, where secondary forces (bearing stresses, etc) act to confine the hook and increase the capacity. That, and I think most practitioners have been detailing the transverse bar within the hook from either their training or gut feel. I certainly do, even after I thought I had learnt it was useless.

Take way: Small bars with tight diameters really need this transverse bar within the inside diameter included. We need to watch out for this detailing in corbels, trusses, and other small but highly stresses bar locations. You really do learn something every day; This makes me very glad to be back here poking around once again...

 

[KootK]

When you presented the case of the wall turned 90 degrees with the cantilevered beam case. If you develop the bars does your beam have the calculated capacity (per code)? Or would it be 70% of calculated strength?

Frankly, I don't know. I raised that as an interesting counter example to my own arguments in this thread. If you flip through this ACI doc, it's hard not to come to the same conclusion as we have for the retaining wall joints: bar development is usually necessary but not always sufficient to constitute an adequate joint. In practice, this is what I have been doing:

1) Moment frame joint = detailed design.
2) Cantilevered beam with no back-span = detailed design.
3) Roof beam with no columns above = detailed design.
4) Most other cases = provide Ld/Ldh and hope for the best.

I've always thought it interesting that, if you examine a typical joint from an STM perspective, providing only Ld/Ldh into the columns would almost always result in problematically steep strut geometry. As I mentioned previously, I figure that the beneficial clamping that one gets from having a column above improves that situation.

Along the same lines, I see incompatibility with how we design our columns. We design them assuming the smallest possible sliver for the compression block because that yields the most favorable results. Logically then, again from an STM perspective, one would think that beam bars would need to be developed over the column compression zone rather than across the entire width of the column. But that is not what is done.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

At these closing joints, does a large radius bend (which limits bearing stresses) give higher capacity in practice than a normal bend? Any good test results out there?

A gentler radius does improve matters. This harkens back to two items that I mentioned above:

1) European provisions where bend radius is sometimes "designed" and'
2) The curved bar node STM method where, again, radius is a design parameter.

Also, where a normal cog/bend is used we generally put a crossbar in the corner of the cog, whereas most of the images above don't bother. Any thoughts on cross-bar vs no cross-bar

My thoughts:

1) Almost everything above is concerned with opening joints. Closing joints are much less problematic.

2) I believe that the cross bars do in deed help. That said, large flexural bars would need correspondingly sized cross bars to have the same proportional benefit. A 10M cross bar won't do much for a 30M flexural bar etc.

3) In my opinion, the cross bars essentially improve bar development/anchorage. As discussed above, development/anchorage tends to be just one of several possible failure modes that need to be addressed in the complete design of a concrete joint.

4) Particularly with larger flexural bars, I worry about the accuracy of placement with the cross bars. For obvious reasons, they tend to wind up at the point of curve tangency with the horizontal leg rather than at the curve mid-point.







I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[RFreund]

@KootK - thanks for the response! When you say detailed design do you use STM?



EIT

http://www.HowToEngineer.com

 

[KootK]

I use the ancillary ACI docs whenever applicable. STM is neat but it takes too damn long unless I have a spreadsheet to lean on which, usually, I don't.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Tomfh]

A gentler radius does improve matters. This harkens back to two items that I mentioned above:

1) European provisions where bend radius is sometimes "designed" and'
2) The curved bar node STM method where, again, radius is a design parameter.

Do you know of any practical tests? Intuitively I'm not totally convinced a gentle curve would be that much stronger. Yes you get lower bearing stress, however you need to start the bar bending much closer to the surface, where confinement wont be as good. I'd love to see some destructive tests of the two cases.

1) Almost everything above is concerned with opening joints. Closing joints are much less problematic.

I was thinking of the closing joint case, e.g. your post CURVED BAR NODES 14 Jan 16 21:42, with the strut tie model.

In my opinion, the cross bars essentially improve bar development/anchorage. As discussed above, development/anchorage tends to be just one of several possible failure modes that need to be addressed in the complete design of a concrete joint.

This is how I imagine it. I think the bar would rather get its fingertips around a bar, but again I haven't seen any real test results.

 

[KootK]

Do you know of any practical tests?

I know of this one. It requires one to read Swedish however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.