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Shear at the base of concrete walls

Shear at the base of concrete walls

Shear at the base of concrete walls

(OP)
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

RE: Shear at the base of concrete walls

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

RE: Shear at the base of concrete walls

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.

RE: Shear at the base of concrete walls

(OP)
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.  

RE: Shear at the base of concrete walls

(OP)
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.

RE: Shear at the base of concrete walls

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

RE: Shear at the base of concrete walls

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.

RE: Shear at the base of concrete walls

(OP)
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

RE: Shear at the base of concrete walls

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" or 16" 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...

RE: Shear at the base of concrete walls

Sorry... inner surface...

RE: Shear at the base of concrete walls

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.

RE: Shear at the base of concrete walls

(OP)
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
"...the member is part of a redundant structural system that allows substantial redistribution of load and will therefor display adequate ductility."  It is pretty apparent that a long wall poured with the base slab would meet that parameter.

later

RE: Shear at the base of concrete walls

The PCA publication is "Rectangular Concrete Tanks" 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 "Moments and Reactions for Rectangular Plates" 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.

RE: Shear at the base of concrete walls

(OP)
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

RE: Shear at the base of concrete walls

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 "tank" so to speak?

RE: Shear at the base of concrete walls

(OP)
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

RE: Shear at the base of concrete walls

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, "Moments & Reactions for Rectangular Plates" 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 "balance" 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

RE: Shear at the base of concrete walls

(OP)
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 "Moments and reactions for Rectangular Plates" 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

RE: Shear at the base of concrete walls

(OP)
thanks unclesyd,

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



RE: Shear at the base of concrete walls

------"the member is part of a redundant structural system that allows substantial redistribution of load and will therefor display adequate ductility"------

If the entire wall sees the maximum load at the same time (and in a constant fashion) is it really a redundant system? If the loads are from fluid contents rather than earth pressure then I don't think you have a redundant system. There are no lesser-loaded areas which can assist those areas seeing some overload. Especially with such a great aspect ratio.

RE: Shear at the base of concrete walls

(OP)
Subject to fluid load, and assuming a fixed base, the max moment occurs at the base (obviously).  We have reinforced the walls for that max moment, therefor they have that capacity up the full height.  In reality, the base is not fully restrained against rotation, and the actual condition will be somewhere between fixed and hinged.  This moves the maximum moment up the wall closer to the middle.  In either case, there is room for redistribution of the moment, either further up the wall or further down the wall.  for that reason, I believe that the quoted statement does apply to this case.  I would relate it to a slab subjected to snow load.  this load can be present in it's full capacity for some time, yet the slab assumptions still apply.

that's my two cents, any other thoughts are welcome...

dutchie

RE: Shear at the base of concrete walls

I would agree regarding redistribution if there is a lid on the system, but you mentioned that the top is free.

How does the moment redistribute up the wall? If there is less than fixity at the base, the wall is not stable.

As for the analogy to the snow on the slab - I've never really understood that one either! With concrete there is a certain element of "it's always worked when we did it like this in the past".

RE: Shear at the base of concrete walls

(OP)
The top is free, but the slab is restrained on either side (considering a 1m strip), so the moment just becomes more My instead of Mx.  As far as less than fixity at the base is concerned, what i'm referring to is fixity against rotation.  The wall is still stable.  

the reason the slab analogy is valid is that slabs have such low percentages of tension reinforcement, therefor behave is a ductile fashion, which is what leads to moment redistribution.  In a beam, reinforcement ratios are higher, and ductility is not as great, therefor, redistribution does not occur before concrete crushing and eventually failure.  

cheers,

dutchie

RE: Shear at the base of concrete walls

On November 3, 2003 tincan opined:

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 agree completely.  Early in my professional career (~1984), I assisted in the evaluation of a failed 15 ft wall of a sewage treatment plant in Houston.  In that case, the designer used USD (14" wall) rather than ASD (20" wall).  What was relevant to this discussion?  I modeled the wall using DSLAB30 and got excellent match between the predicted maximum moments and the locations of the cracks in the wall.  In fact, the cracks were orthogonal to the orientation of the maximum moment values.  The transition from a restrained edge to full cantilever (for both the actual wall and the computer model) occurred within 20 to 25 feet of the edge restraints - roughly 1.5 times the wall height.

How did a geotechnical engineer get so involved in what was clearly a structural issue?  The designer claimed that the edge of the foundation had settled, causing rotation of the wall.  (It had not - we did a full scale load test to confirm.)  After I ran DSLAB30 to evaluate the shallow foundation system's performance, evaluating the wall was a relatively straightforward matter.  I also have a lot of structural engineering in my background -



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RE: Shear at the base of concrete walls

(OP)
Nice post Focht3,

I should clarify a few things here, as the discussion has wandered somewhat from the initial post.  The initial post was an effort to validate a thought that in a tank, Vf<Vc/2 is not required and in fact, Vf<Vc is suitable.  We have wandered into discussions on restraint and moment distribution which have been very helpful.  These principles are in use only where applicable, i.e. where length to height ratios in the tank are within acceptable boundries for the model to be effective (ie. cantilever action within 1.5x wall height from restrained edges).  Otherwise, in the middle of a long stretch of wall, cantilever action is assumed for sure.

Also, just a note, I have noted before that I am using Robot Structural software to model the tank using plate FEM analysis.  I have done some more verification against PCA published coefficients and have found excellent correlation (which I would hope).  Also ran some models against CSA Appendix B (coeffecients for flat plates supported on 4 sides) with similarily good results.  These models are the basis for design, and do predict the cantilever action at the same points discussed above.

One more note, the latest version of ACI350 publishes load factors (sanitary co-efficients) that are to be used with USD, with the intention of getting the design stresses to better match the working stress values.  My experience is that it does ok, but crack width still ends up determining wall width

thanks for the input everyone, keep it coming if there are more thoughts...

dutchie

RE: Shear at the base of concrete walls

I couldn't let this pass by without getting more info on this..

JedClampett said:
<<The PCA publication is "Rectangular Concrete Tanks" and it's HORRIBLE.  There's the small matter of the "ERRATA" >>

There exists such a thing, an errata?!?... If I could get my hands on a copy. It would be great! I notice that in the PCA Book there are no shear values for Case 5 of the Rectangular Plate. Along with a whole host of other errors...

Mark

RE: Shear at the base of concrete walls

I'll fax you the Errata if you give me your fax number.  I'm not aware of a problem with the shear values, but it wouldn't surprise me.

RE: Shear at the base of concrete walls

Thanks Jed.

Email me at xhare2000 at yahoo dot com and I'll email you the fax number here.

Thanks.

MBruggink

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