Seismic Parameter, Xis, issue in Appendix E
Seismic Parameter, Xis, issue in Appendix E
(OP)
The problem I'm facing is in regard to equation E.6.1.2.2-1 which gives the height from the bottom of the tank shell to the center of action of the lateral seismic force related to the impulsive liquid force for the slab (pile cap) moment, Xis. When I calculate this distance, I get a value that is larger than the liquid level. Also, I noticed that when I vary the liquid level in the equation while holding the tank diameter constant, the stability factor, J, decreases (the tank becomes more stable) with an increase in liquid level where I would assume the opposite to be true.
The values I am using for the equation are:
D = 127 ft
H = 44.5 ft
The value I calculate using E.6.1.2.2-1 for Xis:
Xis = 50.21 ft. > H
&
J = 3.11 ---> unstable
This is also greater than the height of the tank which is 49.5 ft.
Arbitrarily setting H = 60 ft (the tank is now more slender and should become less stable)
Xis = 50.37
J = 2.78 ---> more stable than before
Please advise.
The values I am using for the equation are:
D = 127 ft
H = 44.5 ft
The value I calculate using E.6.1.2.2-1 for Xis:
Xis = 50.21 ft. > H
&
J = 3.11 ---> unstable
This is also greater than the height of the tank which is 49.5 ft.
Arbitrarily setting H = 60 ft (the tank is now more slender and should become less stable)
Xis = 50.37
J = 2.78 ---> more stable than before
Please advise.





RE: Seismic Parameter, Xis, issue in Appendix E
The height for the impulsive loading for the slab moment is the combination of moment due to pressures on both the bottom and the shell, but is only divided by the horizontal force to get the equivalent height, and can in fact be higher than the shell height. The equations could have been formulated differently to make this less confusing.
On the other item, I'll have to look at it when I have more time. Stability should be reduced with a higher tank. Make sure you're using the right equations and right factors in there.
RE: Seismic Parameter, Xis, issue in Appendix E
I do not understand how the line of action of a mass that is at the bottom of the tank could be higher than the shell height. Please explain more so that I may understand. Values obtained for Xi and J when using a ring wall foundation are consistent with my understaning of the physics involved.
RE: Seismic Parameter, Xis, issue in Appendix E
RE: Seismic Parameter, Xis, issue in Appendix E
RE: Seismic Parameter, Xis, issue in Appendix E
RE: Seismic Parameter, Xis, issue in Appendix E
RE: Seismic Parameter, Xis, issue in Appendix E
RE: Seismic Parameter, Xis, issue in Appendix E
RE: Seismic Parameter, Xis, issue in Appendix E
RE: Seismic Parameter, Xis, issue in Appendix E
The "ringwall moment" is used for the tank stability and anchorage because the interior of the tank has no strength - all the overturning resistance is at the shell. So, a ring analysis is appropriate.
When designing a ringwall foundation we use the ringwall moment. Because it is a ring, it does not directly resist forces and moments inside the tank shell.
When designing an slab foundation we use the slab moment. Because it is a slab, it must directly resist forces and moments inside the tank shell.
RE: Seismic Parameter, Xis, issue in Appendix E