CQC seismic forces in seismic combinations
Hi,
here are some explanations to the appropriate parts of your post:
@Anonymous wrote:I've been reading this post and making some checking and I don't undertand some issues about combinations from a spectral case and the equivalent forces it uses
On one side, I have a combination of different cases including a seismic espectral case (which contains all the modes of vibration, or if you want a CQC combination of the spectral case) and on the other hand I have the same combination but with the CQC pseudostatic forces transformed into a static case I expected identical forces in bars but there are some differences I can not understand why. I am calculation static linear in this case.I thought that the seismic spectral case worked with those CQC forces to calculate the internal force but there is something I missed. Could you help me to understand the difference?
The explanations is similar to explanations given in this forum for requests related to base shear or sum of reactions for seismic cases - it is related to the order of operations related to calculating quadratic combinations.
In this specific request, related to pseudostatic forces, Robot performs analysis for seismic case in such way that pseudostatic forces are found for each specific mode and static analysis using these loads is made for each mode (each mode treated as if one simple static case).
Then, when displaying results for the CQC combination of the seismic case the CQC combination of corresponding results from composing modes is calculated.
It GIVES DIFFERENT RESULTS than running one static analysis using CQC combination of pseudostatic forces.
I have made a simple example with 2 concentrated masses and significant participation of both vibration modes to illustrate it on the screen capture below. It contains a seismic case with 2 modes and a static case 6 loaded by CQC combination of pseudostatic forces converted from the seismic case.
In the top row the pseudostatic forces are displayed for mode 1, 2 and CQC combination of pseudostatic forces and loads for CQC combination of pseudostatic forces converted to static case (case 6).
In the bottom row the shear forces and bending moments are displayed for mode 1, 2, CQC combination of modes and loads for CQC combination converted to static case.
When comparing results in the 3rd and 4th column very significant differences can be observed. It is so because although it is possible to display in Robot the CQC combination of pseudostatic forces this combination is not used in analysis - analysis is run only for specific modes and then CQC results are calculated from results for modes.
Additional remark related to attempts to physically interprete these CQC results - they should be interpreted as some envelope of expected values with the possibility that they can have also opposite sign (as usually in vibrations:)).
Robot gives the possibility to convert CQC combination of pseudostatic forces into static case but this feature should be used rather for selected mode. Of course such choice can be done easier if there is a single dominating mode.
394-98236683 wrote:On the other hand I've been reading about Non linear and P-delta calculations but those concepts are not the same as the books:
The P-Dalta calculation is something more diffrent than I thought.
The question is if I can make a non-linear calculation with the combination which contains a CQC case of the seismic case (created from the spectral case) can I make it with a P-delta calculation?
For the combination containing seismic case it is not possible to activate geometric non-linearity (inclusing P-DElta).
For the combination containing static case obtained from the conversion of pseudostatic forces it is possible to activate geometric non-linearity (inclusing P-DElta).
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Regards,
Pawel Pulak
Technical Account Specialist
Thanks for your quick answer.
I understood the calculation of the base shear because you calculate all the sum of reactions of each modes and then CQC of the total reaction of each vibration mode and I understand that a seismic case has a lot of information with each force and mode of vibration but I don't undertand what RSA do after that, when I combine the case with other ones. I've checked that I get the same results creating a case of the CQC forces and convert the pseudostatic forces (from CQC) to an static case when I combine them with other forces. Although I calculate the CQC forces, RSA only runs with some specific modes when I combine the with other forces? but whick ones? the main modes defined in analysys type? or every one I've calculated? does it calculate statically linear with each mode with the other cases I defined and displays a final result aplying a CQC final combination? Then I understand it can not be calculated a non-linear analysys because it would be necessary a multimodal non linear analysis (I don' know the technical name of that
)
The problem is that I saw that RSA ignores the spectral load case in a combination with other loads when we run a non linear analisis but I read the following and see the video:
There you can see how to create a CQC case with the spectral case in order to use the results to get a non linear analysis calculation and I thought it took the results of CQC forces to combine them in a non linear analysis but I get different results from expected.
For example I attach a structure with different combinations: non linear- linear-case converting CQC-case with CQC manually input with RSA 2015.
Checking the internal forces in elements you can find that:
- Combination 4=combination 11 because 11 its a non linear combination of a spectral case and RSA ignores it. Ok
- Combination 10=combination 13 because it's a linear combination one from a spectral case and the other one is the CQC of the same spectral case. Ok
- Combination 20=combination 30 because one is calculated creating a case with the Forces of the CQC manually and the other one is converting the CQC pseudostatic force to statical case. Ok
- On the other side I undertand that I can transform the forces from a mode i or a CQC to a statical case and then combine it (linear or even P-DELTA) as you can see in 28 or 31 or create one manually with the same values as 20 or 30 cases. Ok
- But 13 is different from 20 (or 30). If RSA calculate statically the forces of each mode with the other ones and shows the final CQC I undertand the difference becauseI thought that the case CQC case created (8) contained only the final CQC forces so I expected the same results. So due to the different results the case 8 continues having the same information as the spectral X case (6) and continues calculating with each mode statically when I combine them. If this is the reason, Ok. Then, creating a CQC case it's useless.
- Then I don't understand why in the case 14, which is a non linear calculation with the CQC case is not identical to 4 and RSA doesnt ignore it as I get in combination 11. What forces does it use? I thought that with this procedure I could work with the final CQC values and get the same internal forces as the case 31 but not. Have I missed something? What does it do then?
Thanks in advance