Minimum tank length for calculating Hydrodynamic forces (ACI 350.3)

[Drum1979]

I am designing a below grade concrete reservoir approx 70' x 68' x 20'. The reservoir has been divided into 4 cells of 16' x 68'x 20' using baffle walls.
Is there a minimum dimension recommended in the code for which we do not have to the calculate hydrodynamic forces as per ACI 350.3 ? I am just wondering if I should be including seismic forces in the 16' direction.

 

[msquared48]

If the water can flow between the baffle walls, equalizing the hydronamic pressure, then no differential lateral forcde need be applied.

If the water cannot flow between the cells, then I would assume one can be full while the adjacent is empty. Hence differential pressure to design for.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[JStephen]

Even if water can freely flow around the baffles, you could still generate pressure loads in an earthquake.

 

[SAIL3]

Interesting question...I believe JS is correct that there will be pressure loads generated by the seismic motion....how to calculate them is the big question..
API has addressed this in both elevated tanks and tanks at grade taking into account any sloshing affects...
I designed a liquid tank a few years back in a high seismic zone and after much thought and analysis finally decided to design the walls for the pressure resulting from the full mass of the liguid
bearing on that wall...this was conservative, but reflected the highly complicated motion of the liquid in a seismic event..
In your case, the 68' direction would have a long mass of water...how much of this has time to get activated before reversal
in direction occurs? ...
a WAG on my part, say 30%
anyway, you could do your own little experiment with a plastic container , put some baffles in it and water...then shake and observe the response...it's amazing how much one can learn from this.
There is also a vert component to consider..

 

[JedClampett]

My company does a lot of work in seismically active areas and our criteria is to design baffles for sloshing forces. Luckily I've not had to do a lot of these because it sounds like a major pain in the behind.

 

[ATSE]

Drum,
Not sure if ignoring forces is ever a good idea unless your baffles are flexible.
350 is fairly straightforward regarding hydrodynamic forces on dividing walls and baffles. Sames as exterior walls.
For a rigid baffle, one side has total effective lateral press higher than hydrostatic, and one side lower by the same amount (the vertical dynamic component can change this).
That is, convective and impulsive add to hydrostatic on one side and subtract on the other. Use some judgment on how to assign the vertical accel component pvy to each side. Worst case is to follow the signs of the convective and impulsive.
Put those two trapezoidal pressure diagrams on the wall (one on the left and one of the right) and construct your shear and moment diagrams.