Seismic force distribution to cylincrical vertical tank

[jmy]

I am designing a concrete slab foundation for a vertical cylindrical tank. The tank height and diameter are 38.75 ft. and 9.5 ft., respectively. I have calculated the seismic base shear, but since there are no individual floor levels, I am unsure of how to distribute the load vertically.

Do I apply the total force at 1/3 the tank height to coincide with the centroid of the triangular shaped force distribution along the height of the tank?

Also, I am unclear as to how to calculate the fundamental period of the tank.

Thanks in advance for any direction and input.

 

[OldPaperMaker]

jmy,
If you are in the US the short answer to your question is to follow the instructions in Chapter 15 of ACSE7-10 (or 05). If you are just designing the foundation and anchor bolts you might chack with the folks that designrd the tank itself.

As far as the location of the application of the load effect from the liquid it should be at the center of gravity of the liquid itself, not at the location of the ctr of pressure of the liquid. The tricky load effect is where to apply the convective (sloshing) load effect. I think that this is covered by equation 15.7-13. There are also equations for the various periods of the structure.

I hope that this helps.

 

[JStephen]

For typical municipal water tanks per AWWA standards, or for industrial tanks per API standards- the force distribution comes from the AWWA or API code. Generally, it'd be nearer the center of mass, not the 1/3 point. The allowance for sloshing usually represents a reduction in the force, not an increase. The tank is considered "rigid" for the fundamental period.

There is a section there in ASCE 7 that allows small tanks to be considered rigid, which is a simplification as compared to the tank standards. If seismic loading is low where it won't make much difference, that may be the preferred method.