I have two doubts in seismic analysis

1. In our past and current projects, in seismic analysis we are using ZPA as 0.385. but if i change that value to any other number (example ZPA=1000) even though the reults are same, then what is the use of that ZPA in seismic analysis

2. In ceasar II config file, MISSING MASS ZPA there are two options extracted and spectrum. but if i select extracted or spectrum, the results are same.

these are the errors in CEASAR II or I did any mistake ????
please clarify my doubts..

In my opinion if you are not using predefined response spectra i.e. reg. guide 1.60 and UBC spectra in built in Caesar data then the value of ZPA will not have any effect on the result of response spectrum analysis.This ZPA value is only valid for predefined response spectrum.

OK THANK YOU trips

But In ceasar II config file, MISSING MASS ZPA there are two options extracted and spectrum. but if i select extracted or spectrum, the results are same.

this is also for predefined spectrum

waiting for ur replyy

i send a mail regarding this same issue about ZPA value 0.385.
but intergraph people saying that ZPA is compulsary. whether it is compulsary for pre-defined resonse spectrum or for user defined dpectrum also ?????

Richard can u please help me...

Durga,

1. I have replied for ZPA (Reg. Guide 1.60/UBC -- g's)as given in control parameter. This factor is used for predefined response spectra.As per Caesar help
When used with certain pre-defined normalized response spectra, it is used as the acceleration factor (in g's) by which the spectrum is scaled.Both the Reg Guide 1.60 and the UBC curves are normalized to represent a ground acceleration (ZPA) of lg; the true value is actually site dependent. Therefore, entering ZPA value here appropriately scales any Regulatory Guide 1.60 or the Uniform Building Code response spectra.

However if you want to consider any scale factor for your response spectra , you can consider it as a factor in spectrum load case tab of caesar II.

2. The ZPA value in your second option is related to missing mass components ( response related to non extracted modes). You can find its detailed explanation in the caesar help (c2TR) technical discussion.In my opinion if the acceleration value for both extracted and spectrum option are different in your inputed response spectrum then the result should not be same. Please note to compare the effect of included mass correction you need to chekc the stresses /forces , however the displacement values may be not much affected by non extracted modes (higher modes) because it gets from lower order modes.

I hope i have provided you some idea about your questions , however the best response can be get from earthquake analysis expert.

TRIPS,

thanks for ur reply..

whether this statement is true or false????

1. for user defined spectrum , before cutoff freqency we know accleration value from our user defined spectrum so no problem . but after cutoff frequency i.e usally 33 HZ what ever ZPA value we entered is used as acceleration value

F=ma , we know mass value and after 33 hz this ZPA acts as accleration.. so force is calculted and applied at that node.

if it is used for predefined spectrum, then how force is calculted after 33 hz

After cutoff frequency , if we select the option Include Missing mass then only the quasi static response of non extracted modes to be combined with modal response to get the total system response .The ZPA used for missing mass is not the ZPA as entered in the control parameters.This is automatically selected from the spectrum based on the option selected by you in configuration file.This ZPA is used to calculate the missing mass residual load (load related to non extracted modes). This residual load can then be applied statically to get the resulting displacement,stresses and reactions representing the net response of all rigid modes.This response is then added to the response of the extracted low order modes to get the total system response.

You may also say that extracted modes response are dynamic(or amplified response) and non extracted modes response are approx rigid (non amplified) range. The response of non extracted modes are rigid based on the rigid ZPA value as selcted by Caesar II for missing load calculation.
I suggest you to read the description regarding include missing mass as provided in Caesar II help.

Trips,

thank u for ur reply

1. can u give me any reference for ur statement "The ZPA used for missing mass is not the ZPA as entered in the control parameters". because for our past and present projects we are giving ZPA( for user defined spectrum also) even PMC specifies that use zpa=0.385 (for user defined spectrum also)

2. can u pls do a small activity.

use ur own spectrum , and in config file MISSING MASS ZPA put spectrum ( it means CAESAR II use the largest entered frequency corresponding acceleration value as ZPA)check the include missing mass data.

change that largest frequency corresponding acceleration value and check the missing mass data NO CHANGE??? why ..

even if u change to extracted mode(CAESAR II will use acceleration associated with the frequency of the last extracted mode. So this value depends up on Cut‐off frequency.) still included missing mass data is same... what it mean?????

neither it is changing for spectrum nor it is changing for extracted mode??? i didnt understand what ZPA value CEASAR is taking for calculating the included missing mass....

If you change the last acceleration in the spectrum input, then you change not only this acceleration at the last frequency (cut-off).
You change then also accelerations until to frequency before.
If you have in this range eigenvalues then you becomes other modal answer and other missing mass.

The entered value for ZPA in the Control Parameters is only used with the canned shock spectra shown in CAESAR II dynamic data - Reg Guide 1.60 and UBC. The entered value for ZPA in teh control Parameters has no effect on user-defined shock spectra.

Thank you for ur reply Dave Diehl

1. In Caesar II config file, MISSING MASS ZPA there are two options extracted and spectrum. but if i select extracted or spectrum, the included missing mass results are same. what it means????

2. In spectrum we are entered largest frequency is 10 HZ ( spectrum range shown below) so corresponding acceleration as ZPA and it calculates force It is clear..


* RANGE
* FREQ (Hz)
10.0000
5.0000
3.3333
2.5000
2.0000
1.6667
1.2500
1.0000
0.8333
0.7143
0.6250
0.5556
0.5000
0.4167
0.3571
0.3333

3. but if i select extracted and i keep the cut off freq 33 HZ how Caesar know what is the acceleration value at 33 HZ ???? & in what way Caesar is calcating force then.....

Dear all

please help for the above question......

Cut-off say only calculate frequencys until this value (and the next higher value). If your spectrum input not change , then you get no other results.

And here the Help-Text from config-file :

"The default for this option is extracted, which means that CAESAR II will use the spectrum value at the last "extracted" mode. Changing this value to SPECTRUM instructs CAESAR II to use the last spectrum value as the ZPA for the missing mass computations."

I also dont no what the different is between use last extracted mode and use last spectrum value.
Its the different "extracted" means without missing mass calculation and "spectrum" with missing mass calculation ?

Ohlinger,

ZPA what we are entering in control parameters is for predefined spectrums.


For user defined spectrum, ZPA required for calculating force (F=MA) (FOR INCLUDED MISSING MASS) after cut off frequency (mostly we will give 33 Hz in the control parameters)

Now, we have choice to select ZPA either from spectrum or from extracted mode.

A.SPECTRUM instructs CAESAR II to use the largest entered frequency of the input spectrum.

Example: take this spectrum :

RANGE
* FREQ (Hz)
10.0000 ------------ Corresponding to 10 hz we have one acceleration
value, that value is taken as ZPA for calculation of force
5.0000
3.3333
2.5000
2.0000
1.6667
1.2500
1.0000
0.8333
0.7143
0.6250
0.5556
0.5000
0.4167
0.3571
0.3333


B.Extracted instructs CAESAR II to use the acceleration (that acceleration =ZPA) associated with the frequency of the last extracted mode

For example, if the user specifies cut‐off frequency 33 Hz then CAESAR takes related to 33 Hz what is the acceleration value that acceleration considers as ZPA

This is the concept for calculation of included missing mass

But my doubt is

1.If i select extracted and i keep the cut off freq 33 HZ how Caesar know what is the acceleration value at 33 HZ???? & in what way Caesar is calcating force then…..

2.RESULTS: Neither it is changing for spectrum nor it is changing for extracted mode???

i didnt understand what ZPA value CEASAR is taking for calculating the included missing mass....

The cut-off frequency concept is :
After this frequency -in the earthquake case 33 Hz- no more important dynamic effect.
Thats right.

My standard spectrums input ends at cut-off frequency.
So i dont have this problem.

In your case spectrum input end at 10 Hz.
Nevertheless Caesar calculate eigenvalues until 33 Hz.
Our question are :
Which acceleration take Caesar to calculate missing mass response by extracted or spectrum in your example ?
More then this which accelarations take Caesar for the eigenvalues greater 10 Hz until 33 Hz ?

Dear all

if any one know about this topic please share with us ??


thanks....

I believe CAESAR II uses the last entered value for all additional modes calculated beyond the last spectrum value. That configuration switch affects only the missing mass term.

Dear Dave Diehl,

I didn't find any change in included mass data for both spectrum as well as extracted.

If I change the last frequency corresponding acceleration value, for example: user defined spectrum highest freq 10 HZ corresponding acceleration value I changed. still the results are same.

I didnt understand exactly what CAESAR is doing, can you please elobarate...

Do you have an acceleration defined beyond your last extracted natural frequency?

Also, do you run the static error check after updating the Configuration file? That's required to register the changes made.

Dear Dave,

I tried for Different Cases, and I followed your statement also I made static run for every time when I change the config file... But I didn’t find any changes in INCLUDED MASS DATA RESULTS.

1.In Config file for missing mass ZPA I selected spectrum. And I checked for extracted also.

In spectrum I changed 33HZ corresponding acceleration value to 1000, 2000, 3000 very big values. So CAESAR should takes the ZPA as 1000 (I think).
But there is no change in included mass data results. I checked the % Force added.

2.In spectrum if I give up to 10 hz but my cutoff frequency is 33 hz, what CAESAR do for calculating forces in between 10hz to 33hz (what are the acceleration value CAESAR used from 10 hz to 33 hz)??

Whether it Extrapolates and calculates the ZPA for 33 HZ or
it takes same value of 10 hz, for in between 10 hz to 33 hz to calculate forces…??

please help me....

DAVE/RICHARD can you please clear this doubt??

DAVE/RICHARD can you please clear this doubt??


THANKS
Durga...

Dave/Richard,

If i change the missing mass ZPA(IN CONFIG FILE) to SPECTRUM or EXTRACTED..

In which RESULTS i can see a change??

The default for this option is "extracted", which means that CAESAR II will use the spectrum value at the last "extracted" mode. Changing this value to "SPECTRUM" instructs CAESAR II to use the last spectrum value as the ZPA for the missing mass computations.

You won't see any differences in your static results, this configuration directive affects dynamic runs only. Even then you may not see a difference:
- the spectrum values could just happen to be very close for both settings
- your missing mass component could be very minor, so changing this value has no real effect.

If the missing mass is a significant component of the response, and if the extracted and spectrum settings correspond to different spectrum values, then you would see a difference in the results of displacements, forces, restraints, and stresses.

Richard,

Thank you for ur reply

ZPA is used for calculating force after 33 Hz, then How can I see the results of displacements, forces, restraints, and stresses after 33 Hz.

up to 33 hz we can see the results but after 33 Hz how ??

Can you please explain..

The response is the "total response", from all contributions - this includes the "missing mass" contribution after your cut-off.

Good morning Rich,

thank you for your valuable information,

After changing ZPA spectrum to extracted( spectrum ZPA & extracted ZPA are different), iam checking included mass data option in that iam checking force added%..

Is this a correct check,, or any other results I have to check...

Richard,

can you please clear this Doubt...

What are you checking this against?

In config File MISSING MASS option : i changed SPECTRUM to EXTRACTED.

(Note: ZPA value for SPECTRUM and ZPA value for EXTRACTED are different.)

Now,

which results i have to check to feel the change of ZPA???

You don't have to check anything. You want the included mass/force as small as possible. When in doubt, extend your frequency cut-off. It is wise to have spectrum values that go out as far as your frequency cut-off. If you set this up right, this setting shouldn't make that much of a difference in your results.

Mr. Ay concluded the practical aspects of the problem. In my opinion it's all you need.

I would add some theoretical aspects you may detail yourself.

There are modes with frequencies higher than the frequency at which the spectral acceleration returns to the Zero Period Acceleration (ZPA).

As frequency limit, 33Hz is included in REGULATORY GUIDE 1.60 "For frequencies higher than 33 cps. the maximum ground acceleration line represents the Design Response Spectra."

NUREG/CR-6645 says "The contribution of modes to the total response is most accurately and efficiently treated by static analysis of the "missing mass" (i.e. system mass not participating in the modes with frequencies below f_ZAP) multiplied by the ZPA".
However, it's not a simple math multiplication; APPENDIX I of NUREG/CR-6645 presents a mathematical descriptions of the "missing mass" contribution to total response. Studying also the references presented in CR-6645 would give you more knowledge about the subject.

Hi,

Rich & Mariog

now its clear

thank you for your reply....

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DP