Impulsive Natural Period, Ti

[CaliEng]

The formula(s) for the impulsive period of the tank E.4.5.1-1 (a & b).
There is a note stating: "Substituting the USC units specified above: Ti = 0.128 sec"
However, I do not see any tank dimensions provided above to put into this formula. Putting in values for tanks I am considering, i get a Ti in the ball park of this, but I am a bit thrown off by them providing a certain value as it depends on specific tank dimensions.
Is this an error in the code?


Also, I notice that the value for Ci is truncated at 1.5 H/D. It is approximately linear from 1.0 to 1.5 H/D, is it unfounded to extrapolate linearly to 2.0?

Thanks

 

[JStephen]

In the latest revision, that "Substituting" note has been deleted. I think it was just extraneous stuff that found its way in there.
See this paper for background:

https://pdfs.semanticscholar.org/9351/cd259d0ae43223eb16c1d0205a4042f50bd8.pdf

Not sure about the H/D limit. One thing is that most "large" tanks where seismic design is most critical will fall well below this limit.
Note that in the standard procedure, period is taken as zero/rigid for the impulsive mode, so trying to calculate an impulsive period is an option, but not a requirement.
I believe that paper is based on the Veletsos method which is not limited on the H/D ratio. But when I have worked through the Veletsos method in the past, there are some questions as to how certain terms in that paper are defined, which leaves the answers less definite than is desired.

 

[HTURKAK]

There is a note stating: "Substituting the USC units specified above: Ti = 0.128 sec"
However, I do not see any tank dimensions provided above to put into this formula. Putting in values for tanks I am considering, i get a Ti in the ball park of this, but I am a bit thrown off by them providing a certain value as it depends on specific tank dimensions.
Is this an error in the code?

Although design methods in API 650 do not use the value of impulsive period of the tank, I think this empirical formula given for information only..
Regarding the value Ti=0.128 sec. can not be valid for all H/D ratios but should vary between 0 to 0.3 sec...

The shell thickness vary with height and for higher H/D ratios, flexural deformation of shell dominates and in this case you may use Housner formula..

In order to understand better the behavior of storage tanks during seismic event , I will suggest you to look New Zealand code, Seismic Design of Storage Tanks: 2009 and Eurocode 8 Part 4: Silos, tanks and pipelines.