Good Day friend,

i have calculated cutoff frequency using the formula given in caesar ii application guide (water hammer chapter).

for max. cutoff frequency i have used the shortest elbow to elbow pairs and the value comes out like = 428 Hz..

But when i use this value of cutoff freq. in my caesar ii dynamic/waterhameer analysis; an error occurs as follows:

"strum sequence failure, extend frequency range".

even the error appears, still the anlysis continue... why?

i have now following questions:

1) why this error occurs and what is the remedy.
2) should i use max cutoff frequency or min cutoff frequecny. what is reason for choosing anyone of them
3) what is the actual purpose of cutoff frequency. i mean what role it palys in the dynamic/waterhammer anlysis.

Thanks

1) The "sturm sequence failure" does not mean there is anything wrong with your job, rather that there is at least one more mode close to where you stopped extracting frequencies.

Take as an example a system where you set the input up to extract all frequencies below 95 hz. The extraction routine will stop as soon as a frequency is found that exceeds 95 - for this discussion, assume this is 100hz. How do you know there isn't another frequency very close to this 100hz that may affect your results? You don't unless you rerun, changing the frequency cut-off. However, a better way is to implement the Sturm Sequence Check, which determines how many frequencies exist between 0 Hz and some other value (not what they are, just how many). So we take your last frequency (100 hz in this case) and add 5%, which in this case yields 105 hz. We then use the Sturm Sequence Check to determine how many frequencies exist between 0 and 105 hz. If the count is the same as the number of frequencies just extracted (between 0 and 100hz), then we report that the check passed. If we get a higher count, we report that the check failed.

There are some models that will always fail. For example, consider a cantilever running in the "X" direction. For every mode in the "Y", there is a corresponding identical mode in the "Z". Regardless of what your cut-off frequency is, the extraction will stop at the first mode of any given pair. The Sturm Sequence Check, by adding 5%, will always find (at least) the 1st mode of the pair, and therefore report a failure.

2) You would want to use the higher frequency, because the results then include the response from more modes. However, you don't want to get ridiculous here either - the more modes you include, the longer the job runs.

3) In general, you don't know how many modes you need to include in the analysis. For one job 10 may be sufficient, for another you may need 50. This is unknown, so a better way is to say "give me all the modes below 'xxxx' hz." You need to include enough modes so that your results include at least 90% of the system response to the applied load. There are various "rules of thumb" to guide you indetermining this value, such as the discussion you referenced in the Applications Guide.

Dear Richard,

Thankyou very much for your useful comments.

Today as i run my dynamic analysis there was no "sturm sequence failure" ; even the caesar calculated one frequency 476.933 Hz above the cutoff frequency of 462 Hz.

Why it is so?

Thanks

Because your system doesn' have another frequency within 5% of 476.933.