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Too Much Reinforcing in Concrete Wall?
3

Too Much Reinforcing in Concrete Wall?

Too Much Reinforcing in Concrete Wall?

(OP)
I am working on a WWTP where the precaster would like to keep a wall thickness to 14".  I can make this thickness work for the loads, however, the reinforcing is becoming quite heavy on the inside face mat.  I would need to use #9 bar @ 4" o.c.  Although this still falls below the maximum steel ratio and the spcaing of the bars is wide enough to allow the largest aggregate to easily pass through I am having concerns about this configuration.  What are your thoughts?  Is there a good reason that I should insist on going with a thicker wall?

Thanks

RE: Too Much Reinforcing in Concrete Wall?

as long as you meet the reinforcing requirements per ACI 318, what's wrong?  can you use a larger bar size?

RE: Too Much Reinforcing in Concrete Wall?

(OP)
Before attending an ACI seminar not too long ago I would not have thought much of it.  During the seminar the speaker worked through a sample problem and the reinforcing came out to #8 bars @ 5" o.c.  Which he dismissed as unrealistic and went to a thicker wall.  This stuck in my head because I could remember occasions where I had used that spacing in the past on a wall which was the same thickness.  In fairness maybe he was thinking of a poured in place solution which would be much more difficult than in the controlled environment of a precasting plant.

I suppose I could go to a 10 bar but I understand that they are much harder to find in our area and the precaster does not stock them.

Thanks for the quick response!

RE: Too Much Reinforcing in Concrete Wall?

In my mind #9 @ 4 inches are too close.  Even if you meet the maximum reinforcing ratio in the old code of .75*rho balanced, you're probably not going to meet the strain restricted requirements of the ACI 318-02.  Not to mention, there's not going to be very much clearance between wall bars, especially when you put in the steel in the other direction.  We had a engineer in our office attend an ACI seminar and they said that you should try not to exceed 1.75 percent.

RE: Too Much Reinforcing in Concrete Wall?

how tall is your wall?  What are the load conditions that require a 14" thick wall?  Are you resisting both gravity and wind/seismic loads?  if you think about the wall being a simply supported beam 12" wide and 14" deep, would 3 #9 bottom bars be too close together?  If you think of the wall as a 12" wide x 14" long column, would 3 #9 bars on one face be too much?  Perhaps that's oversimplified, but as long as you satisfy all of the reinforcing criteria of ACI 318 for the design, which includes the strain criteria, the #9 at 4" o.c. should be ok.  

RE: Too Much Reinforcing in Concrete Wall?

(OP)
The wall is 18' tall.  The loading condition that is being designed for is soil pressure plus a large surcharge.

RE: Too Much Reinforcing in Concrete Wall?

I might not be as concerned about the #9 @ 4" in the wall as I would be in making sure the connection from the wall to the foundation is healthy enough to take what sounds like a very large moment.  I mean, the #9 @ 4" sounds excessive, but if it works, it works.  Can you increase concrete strength to get a little larger spacing?

RE: Too Much Reinforcing in Concrete Wall?

Although ACI 350 does not appear to have a wall deflection criteria, I would suggest that you calculate and be comfortable with your wall deflections.  PCA publication "Rectangular Concrete Tanks" has wall deflection coefficients.  

RE: Too Much Reinforcing in Concrete Wall?

They will have a lot of difficulty not getting voids - especially if the wall is tall.  

RE: Too Much Reinforcing in Concrete Wall?

#11@6" may be better for setting concrete - if it works numberwise.

RE: Too Much Reinforcing in Concrete Wall?

Just remember, if you start bumping up your Reinf. size that your d will get smaller.  If you are already maxed out, this could be problematic.

RE: Too Much Reinforcing in Concrete Wall?

(OP)
I have checked the deflection and I am ok there as well.  I do not think the height of the wall present an issue with voids since the wall is precast and will be laying flat on the beds during the pour.  Also the panel will not have any moment at the base.  It is going to be a simply supported wall, top and bottom.

After talking with the precaster it sounds like we have the option to go with a ribbed wall as well.  This may actually be a cheaper option than this heavily reinforced flat wall.

Thank you all for the input.

RE: Too Much Reinforcing in Concrete Wall?

If these are precast, why are high strength tendons not being used?

RE: Too Much Reinforcing in Concrete Wall?

From a liability standpoint, I think you're making a mistake to design a large tank for a precaster.  The precaster should have an engineer on staff for such a design, thereby accepting full responsibility for the tank.  I hope your insurer knows what you're up to.

RE: Too Much Reinforcing in Concrete Wall?

jmiec, I disagree.  The precaster does have an engineer.  He hired waytsh.  Why does the engineer have to be in house?  At my former employer, we worked for many precasters, though they were architectural precasters.  As an engineer, you are taking the liability of any of your work.  waytsh's insurer will be aware of his work as he renews his policy each year anyway.

Besides, in no different in you providing a design for a contractor to build.  The contractor has a responsibility to build it correctly, per your design, just as the precaster is responsible for his quality control.

PMR06, not all precasters have the ability to use prestressed or post-tensioned tendons in there products.  As an engineer though, you should encourage your precaster to explore prestressing and post-tensioning.  There are many benefits to using these methods.

RE: Too Much Reinforcing in Concrete Wall?

I dont have all the exact parameters, but how are you not over reinforced??  a #9 @ 4" oc produces a rho=0.021!

RE: Too Much Reinforcing in Concrete Wall?

Boiler-
For f'c=5ksi and fy=60ksi, I come up with a rho max (or 0.75 rho balanced) of 0.0252.  You are right, his rho is 0.021, but it is not over-reinforced.

RE: Too Much Reinforcing in Concrete Wall?

As I said before, you need to check Sections 10.3.3, 10.3.4 and 10.3.5.  If you're that close to rho max, you're probably going to violate 10.3.5 and certainly you'll reduce your capacity due to Section 9.3.2.

RE: Too Much Reinforcing in Concrete Wall?

My bad, I accidently checked it against rhoT, not rho balance.  Rho T = 0.02125...which proves Jed's statement that it's in the transition region that reduces phi per 9.3.2.

The tensile strain is 0.0051 so 10.3.5 isnt violated.

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