Using PCA Rectangular Concrete Tank tables when water surface does not extend to top of wall

[jjezmarlo3]

How have all of you concrete tank designers out there been modifying the tables in PCA's Rectangular Concrete Tank publication when the operating water surface is well above the top of the tank wall (i.e. significant freeboard)? Do you just assume that the top of the concrete tank is down at the operable water level, or do you modify the tables somehow? I would appreciate your insight.

 

[amec2004]

I would suggest you build a FEM model to check it if your conditions is different from the table’s assumptions. It won’t take that much time.

anchor bolt design per ACI 318-11 crane beam design

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[jjezmarlo3]

Sorry about the wording in my post, I obviously meant when the operating water surface is well below the top of tank wall.

 

[ukengineer58]

Well the pressure for normal depth is what it is for Sls checks and full for uls checks. From memory the tables show forces for height, it is safe to reason the height is related to the water height. What would the effect of a freeboard be to the forces? I have verified pca and other tables using fem and other sources and everything points me to this conclusion.

 

[ukengineer58]

Just to reiterate the tank normally should be full for uls checks as if it floods, something gets blocked etc you don't want it falling down.

 

[BAretired]

I agree with ukeng58. Assume that water level can be as high as the wall.

BA

 

[jjezmarlo3]

Thank you for your comments.

amec2004-
I am not fully trusting my FEM model, which is why I would like to verify its findings with those in the tables, appropriately modified of course.

uk-
While the moment and shear at the base of the wall would likely not change much with the additional tank freeboard height, I would think that the horizontal wall moments at the corners, and possibly the wall shears at the corners could be significantly lower if there is freeboard above the water than the case where the water is to the top of the wall. I was just wondering if people were modifying the tables in some way to take into account this reduction.

uk and BA-
I agree that the tank should be designed with water to the top of the wall in the rare case that the tank is flooded, but applying the additional environmental/watertightness (Sd) factors to this extreme condition seems overly conservative. I would like to design the tank assuming the water is to the top of the wall with an Sd=1.0, and then also design the tank with the water at its operating level with the appropriate Sd factors. I am unsure though how to determine the moment and shear coefficients from the tables when the water is not to the top of the wall during this operational level.

 

[amec2004]

>>I am not fully trusting my FEM model, which is why I would like to verify its findings with those in the tables, appropriately modified of course.

Those tables were created by running FEM model using SAP90, see introduction on page 1-1 and 1-2

If don't trust your FEM model, you shoul not trust those tables either.


anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com

 

[jjezmarlo3]

I understand. I was just trying to find a way to back check.

 

[JedClampett]

As I've said in many posts before, I don't trust the PCA tables. When they published them a while back with a gross error (the numbes were wrong on every sheet) and corrected it with one loose errata page, I got turned off. Plus, I've never liked their presentation. Their pamphlet on circular tanks, on the other hand, is pretty good.
Get a copy of the Bureau of Reclamation's "Moments and Reactions for Rectangular Plates" It's not in print, but either someone around you will have a copy or you can find one online. It has partial loads and you can interpolate.
But, unless there is a passive (no operator action required) overflow, I would follow the advice of designing the liquid to the top of the tank.

 

[slickdeals]

The reference that Jed mentioned can be downloaded from here

http://www.usbr.gov/pmts/hydraulics_lab/pubs/EM/EM27.pdf

 

[ukengineer58]

I don't understand this must be the simplest fem model you could do, ignore the freeboard, you could even model the wall have one load at Sls checking I.e. crack control and the other for uls failure I.e. to the top. Design the wall at top of wall results for strength and at normal operating height for cracking.

 

[amec2004]

>>I don't understand this must be the simplest fem model you could do

Agreed.

>>the wall have one load at Sls checking I.e. crack control and the other for uls failure I.e. to the top. Design the wall at top of wall results >>for strength and at normal operating height for cracking.

Agreed.

The SLS for slab crack control check is important and neglected by most people.

A while back I did a buried concrete sump design using both FEM and PCA table, the comparision is quite interesting
See attached PDF file : Conc-Tank-by-FEM-and-PCA-Table.pdf

I hope this sample calc will help you to resolve the issue and don't forget to to the slab crack control check.

anchor bolt design per ACI 318-11 crane beam design

http://www.civilbay.com