I hadn't seen a cantilever retaining wall with ties until today. Unsure what to do about it. 3

[OFabianTowers]

Hi everyone,

I am reviewing a design for a cantilever retaining wall from an external consultant for my company.

The design has several unusual aspects (large footing length, almost as big as the wall height, 20 ft, and a deep shear key at the bottom, for example), but what strikes me the most was the fact that
the stem has ties as, per the calcs, the shear at its bottom is larger than the concrete shear strength (as per the common ACI formula).

After a quick review, I contacted the consultant who asked me this simple question: what is the problem with using stirrups? I reviewed all the literature I know (Bowles,Brookes, ACI design handbook) and I could not find a clear answer myself.

Now, I asked the consultant to double check all forces, talk with the geotechnical consultant, and enlarge the stem base such that the ties are not needed.
The concrete capacity is less than 7% lower than the demand, so I estimate that the change should not be a major variation.

Still, I had never seen this situation, so I am asking for any thoughts from you guys.

Thanks in advance!

 

[bugbus]

I see this more and more, especially taller walls. I suspect it comes from MCFT forming the basis of more design codes these days, where thicker elements are penalised to an extent when it comes to shear strength. In many cases, thickening the stem only gives you a modest increase in shear strength. Adding even the minimum shear reinforcement will add a whole heap of shear strength and is not necessarily difficult to install.

There is this pervasive idea, at least in my office, that anything which is not a column or beam (i.e. retaining walls, footings, culvert base slabs, etc.) simply must not be provided with shear reinforcement. In my opinion it's one of those old-school rules-of-thumb that simply don't work any more.

 

[wrantler]

Can you post a pic of the detail. I am curious.

 

[JAE]

I would think the labor involved in placing "gobs" of ties in a wall would far outweigh the cost of a thicker stem.

I've not ever seen ties used in a wall like that.

 

[HTURKAK]

I would prefer to satisfy that shear strength of concrete is greater than factored shear force.. and if necessary , i would prefer to increase the stem thick. I did not design , witnessed the use of shear reinf. for stem of the retaining walls ..






I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[TLHS]

You guys are seeing this more because I think ACI got around to putting the size effect stuff into the code. We've had it in Canada for a bit longer and even then there's still some feeling out about when it makes sense to thicken vs reinforce. It very quickly starts getting into diminishing returns once you're in the 18-24inch range. That being said, 7% isn't a lot.

There's a good paper here that has a bunch of test results and other information on the concept:

https://www.rjc.ca/uploads/research...hear-behaviour-of-thick-slabs-v117-issue4.pdf

This graph is the real meat of it, though.

 

[canwesteng]

Are you constructing the wall, or providing independent technical review? If the former, you should know which is cheaper and can ask the consultant to make the change. If the latter, there is no technical reason not to, and you shouldn't have a comment on the design.

 

[wrantler]

I guess my only gripe is we design these walls with pretty conservative assumptions and they have performed well as-is.

 

[dik]

Part of RJC's website... they were always up to date with concrete design, going back half a century. Going through their personnel and there were only a couple of names that I was familiar with.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik