Shear at the base of concrete walls

[dutchie]

I am looking for some ideas how to cut down the thickness of concrete walls I am designing for a rectangular concrete tank. The walls are about 5m high and are to be poured integral with the base slab, free at the top. Because of the height, the thickness of the wall to resist shear is quite large, based on CAN A23.3 requirement that Vc/2>Vf to avoid placing shear reinforcement.

I am not averse to placing shear reinforcement, but have heard various thoughts on it's effectiveness. I have not been able to find any documents that relate directly to this situation, and was wondering if anyone else out there has any thoughts.

Thanks

 

[ERV]

Are these walls uniform in thickness or are they tapered? Tapering the wall (e.g. thicker at the bottom) may be one option.

 

[JedClampett]

We design these type of structures every day. I'm guessing your walls are about 16 inches (excuse the units) thick. I'd say that's not at all unreasonable. Changing thickness of walls is incrementally pretty cheap. The labor is practically the same no matter what the thickness, so you're only paying for the batched concrete. It's easier to get vibrators in the thicker wall resulting in denser concrete. Thicker walls are less likely to leak.
I've used shear reinforcing in walls. They consisted of #5's tying together the vertical bars spaced at 6 inches in regions of high shear. But we very seldom do this. Our company policy is to let the base shear govern the wall thickness.

 

[dutchie]

I have always figured that it is easier to pour walls of consistent width, this eliminates different form ties etc. as you work up the wall.

One thing I have done is staged the pours so that once I am approx 1.5 - 2m up the wall can be reduced in thickness due to lower shear. This works well on the exterior walls, but on the interior dividing walls, they have to be of uniform thickness from bottom to top to accomodate equipement.

 

[dutchie]

thanks to jedclampett for the reply. that is our policy as well, but in this case, the wall will be 30" thick at the base. Our retained fluid of 18 feet results in a factored shear of 18000lbs/ft (i'm converting from metric, so apologies if there is a mistake here). According to ACI (CSA A23.3 is basically ACI written in metric), that results in a d=27" and therefor a wall of approx 30" thick. One thing that will end up controlling thickness i suppose is ACI 350 recommendations for max z value, so as i am reviewing this i am realizing that i may only be saving 4-6" of wall thickness. Anyway, just wanted to see if the shear reinforcement thing was being done and how.

Thanks again for the reply, it reinforces what i am doing. always a good thing in this business.

 

[dik]

For consideration:

A23 in definitions defines a wall as a 'vertical slab element'
11.2.5.1 states that the requirement for ties if Vf exceeds 0.5*Vc may be waived for (a) slabs and footings

 

[JedClampett]

dlk has a good point. I get a required "d" of about 14 inches. You might have to thicken it up just to make sure it's not too heavily reinforced.

 

[dutchie]

that's a good thought, i had always looked wishfully at that exclusion, but never thought of confirming it with the definitions.

thanks for your input, it has been helpful

 

[dik]

Jed... how's Jethro? <G>

If the design can be treated as a two way slab, then the reinforcing may not be that great and 15&quot; or 16&quot; should be OK (Without considerations of long term deflections; I've never cambered a wall <G>)

With the load distribution, you may have to be concerned with 'corner levers' and the outer surface reinforcing at the bottom corners may have to be a little longer.

I also seem to recall an ACI or PCA publication on the design of bins, as well as circular silos...

 

[dik]

Sorry... inner surface...

 

[tincan]

What is the length and width of the tank? If L < 2W you may be able to consider the walls as rectangular plates. This could considerablly reduce the thickness. The Portland Cement Assn. has a bulletin concerning the design of rectangular tanks. Might be worthwhile checking out. Its not one of their better publications, hard to follow but it serves the purpose.
Best, Tincan.

 

[dutchie]

The tanks are 60m long and about 5m high, so the tables in the PCA book do not apply. And I agree, that book is not easy to fight through, although the first chapter is somewhat useful.

Actually, dik was onto something with this whole definition thing. The idea of the wall as a vertical slab is further reinforced in the commentary to the CSA A23.3 N11.2.8.1 which allows a designer to waive the requirement to provide minimum shear reinforcement provided that
&quot;...the member is part of a redundant structural system that allows substantial redistribution of load and will therefor display adequate ductility.&quot; It is pretty apparent that a long wall poured with the base slab would meet that parameter.

later

 

[JedClampett]

The PCA publication is &quot;Rectangular Concrete Tanks&quot; and it's HORRIBLE. There's the small matter of the errata changing every stinking number over 52 of the pages. A better publication is &quot;Moments and Reactions for Rectangular Plates&quot; by the United States Department of the Interior, Bureau of Reclamation. Unfortunately, it's out of print. Maybe there's a copy in your office or another engineer has it. If you can get a copy, hold on to it like grim death.

 

[dutchie]

i'll keep my eyes open for that one. i have been using Robot to model the tank walls as plate elements. just simple models, but pretty effective to predict moments and shear. the results have been comparable to the PCA publication where applicable.

Once again, you all have been a big help. Thanks

 

[jheidt2543]

dutchie,

I am wondering how your contractor intends to pour a 5 meter (+/- 15') tall wall integral with the base slab. I do realize that the base slab will be past its initial set by the time the walls are starting to pour, but (15')*(150pcf) = 2,250 plf at the base of the wall. I don't think the base slab concrete will have attained that strength by the time the wall is poured. How does the contractor keep the base slab from blowing out around the base of the wall. That is also a lot of wall form to hange from one side. Or, am I in the &quot;tank&quot; so to speak?

 

[dutchie]

i'm figuring that the pour will be staged, likely an initial pour of 2m or so, then the remainder. also, given the size of the tanks, the slab will be poured checkerboard, so by the time the slab is finished and the walls are formed and rebar tied, we are likely talking 2-3weeks min between base slab and wall pour.
also, the base slabs are quite thick, and even assuming 60% strength gain in the first three days, there is sufficient shear capacity in the slabs (600 - 800mm thick) to accomodate the dead weight of the wall. backfilling doesn't take place until much later as the tanks need to be leak tested first.

good point though, and a precaution that will be identifed pre-construction

dutchie

 

[tincan]

A 5M X 60M wall integral with the bottom slab would be more apt to be under cantilever action at a distance of 7.5M to 10M from the ends of the walls. At the ends one wall supports another and there is considerable horizontal moment and horizontal restraint at the wall juncture. As you move away from the end of the wall the horizontal moment lessens and the restraint lessens. The wall then tends to act more and more as a cantilever.

I would consider the 60M wall as a cantilever under hydrostatic loading and design for same, shear & moment. At the end of the wall I would consider the horizontal moment at the wall juncture and design for same for a distance of 7.5+M.

JedClampet is correct, &quot;Moments & Reactions for Rectangular Plates&quot; by the Bureau or Reclamination is worth its weight in gold. I've got a copy and will not allow it out of my sight.

5M (16.4') is not an unacceptable height of pour for retaining walls. It takes a good forman and a lot of ties. I would prefer one pour, that eliminates a horizontal joint in the wall. I'm assuming that you will waterstop all joints?

Another point, 60M(197')long wall, will it require any expansion/contraction joints?

NOTE TO JEDCLAMPET; THE PCA IS HORRIBLE. I don't get tanks that often, usually forget what I did the last time and have to re-muddle through. I use the Bureau of Reclaimation data and &quot;balance&quot; the moments. I have tried developing a spread sheet, to no avail and I am not familiar with any program for the tanks. How do you approach your solution? Any help appreciated.
Tincan

 

[dutchie]

Thanks for the input tincan,

you are right, 5m is not to high a wall, the staging was more an idea if the wall thickness was to change along the height. All joints are to be waterstopped, and we are using shrinkage compensating concrete, so our expansion joints are 60 - 75 feet apart.

I highly doubt I will ever find a copy of the &quot;Moments and reactions for Rectangular Plates&quot; in my neck of the woods, but there must be something else out there to show us the way. As I stated before, a simple finite element model does a good job of predicting the moments in shears in a rectangular plate with T-intersections. I am presuming the difficulty lies in predicting the moment redistribution through the plate, something the FEM does not do.
Good luck to all

 

[unclesyd]

Take a look at this site

http://www.cervenka.cz/Web/

 

[dutchie]

thanks unclesyd,

I am going to download a demo version and check it out.