shear at the base of concrete walls

huangyhg

shear at the base of concrete walls
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
check out our whitepaper library.
are these walls uniform in thickness or are they tapered? tapering the wall (e.g. thicker at the bottom) may be one option.
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.
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.  
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.
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
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.
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
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...
sorry... inner surface...
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.