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
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
RE: Shear at the base of concrete walls
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
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
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
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
RE: Shear at the base of concrete walls
thanks for your input, it has been helpful
RE: Shear at the base of concrete walls
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
RE: Shear at the base of concrete walls
Best, Tincan.
RE: Shear at the base of concrete walls
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
RE: Shear at the base of concrete walls
Once again, you all have been a big help. Thanks
RE: Shear at the base of concrete walls
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
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
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
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
http://www.cervenka.cz/Web/
RE: Shear at the base of concrete walls
I am going to download a demo version and check it out.
RE: Shear at the base of concrete walls
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
that's my two cents, any other thoughts are welcome...
dutchie
RE: Shear at the base of concrete walls
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!
RE: Shear at the base of concrete walls
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
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 -
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Shear at the base of concrete walls
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
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
RE: Shear at the base of concrete walls
Email me at xhare2000 at yahoo dot com and I'll email you the fax number here.
Thanks.
MBruggink