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Hi Artur and Rafal,
we have to prepare a course on Structural dynamic by using ARSA instead of SAP, and we have started naturally the process by the validation of systems of 1 degre of freedom on a set of excercices made with succes by using SAP. So the goal tounatly show the benefit of using ARSA as an powerful academic. Unfortunatly, he have found some difficulties with Robot for the determination of the natural period of 2D steel frame for for 4 scenarios of the transverse beam. So hope that you could help me to go
1-for the first scenario we have found that robot not allow the user to put a point mass on the beam as SAP allow it. So we have use the option of load on bar, and we convert this load on mass during the modal analysis, and we find a lower period than the one found both by hand calculation and SAP.
2- in the second hypothesis ( rigid diaphramg), wich are suppose to eleiminate the axial load in the beam, we have use the ''rigid link'' option in robot without succes.And there is no way in Robot to remove a rigid link and go to an other option. Robot is not really flexible with his tools.ee it weell
3-In the third case SAP allow the user to amplify the inertia of the section whitout changing it with the option of ''modify stiffness'' . It is impossible to do it in Robot wihtout changing completly the member. So team develo taking notes to insert this feature for next version of ARSA. We have also triy the compatiblity nodes tools whitout any succes because we don't have enough demos or examples to manage it weli, but it was very easy todo it with SAP
4- I did not go to the fouth case because I want to understand how to use the tools provide RSA to perform those appar case is provideently simple analyis.
The RSA file for the first case is provided in attachement same as the link in order to follow the video where the 4 snerarios have been acomplished with full succes by SAP :https://www.youtube.com/watch?v=O1rZRojOf4c
Hello, please check the model for the first case. Robot allows to user to put a mass to the node.
Moreover, in your model self weight has been taken into account.
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Roman Zhelezniak
Robot Evangelist & Passionate Civil Structural Engineer
Hi Romanich, Rafal and Artur,
I did again the first step by creating the node in the midle of the transverse beam, by using the command create node whitout divided a bar. After doing itaht, I have maded a verification of the structure and Robot send me a warning about a isolated node that I have ignored. I have created a load case in order to assign the contrated mass on the node, and after I have deleted the load case, in order to eleiminated the influence of the self-weigth. After making an analysis, I have still find the same value for the period wich as Romanich which are lower than the one made by hand calculation and SAP.
At this point, I think that there is two ways to do the first step, and I think that the self-weigth will not have any influence in the response of the system because his density is iqual to zero. So the question is why RSA gave this lower value for the first step and how to find the continue the process to find the same results as SAP in a easy way. So hope that Rafal or Artur will give me a support on it.
Hi Patrick,
Results from the first post was obtained in SAP2000?
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Roman Zhelezniak
Robot Evangelist & Passionate Civil Structural Engineer
.
Hi Romanich,
yes Romanich, in a easy way I did all the fouth steps before doing it with Robot, SAP has a lot of tools which are make dynamic analysis in an easy way. I can post you also all the foutth steps in SAP.
By the way you can do all of them and in a same file, as in the video. Did you fallow the video. I can post the video tutorial and the sap files for you.If you want.
Patrick, it will be great if you share SAP2000 models and video. I can't use youtube (now I'm in China and lot of useful services are blocked)
Regards,
Roman
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Roman Zhelezniak
Robot Evangelist & Passionate Civil Structural Engineer
I used your Robot model , modified what was wrong and found the following results for the 4th case( no rotation on top beam)
Is that good enough??
Rafael Medeiros
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Just created a node on the top beam, applied the 30000kg mass on X direction and created a Ry rotation support on the 3 top nodes.
Robot file is attached
Rafael Medeiros
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I made a video the same way you did.
- Rigid links can be deleted from the rigid links assign window , just selecting the nodes . Watch in the video. It can also be deleted from rigid links table.
- To increase the Iy ,I created a generic (Ay,Iy, Iz... tab on "Bar sections") section , copying the W310x74 properties. Watch the video
http://screencast.com/t/tOH2vqS0
all 4 models attached. Rename to rar
Rafael Medeiros
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There are still significant differences in vibration periods and frequencies between the SAP2000 demo (full video with link given by asab1409 or some reference values from it given by PatSt-Luis1804 in the first message of this discussion) and the values obtained in Robot for the files supplied by Rafacascudo.
I have made some more detailed investigations and the conclusion is that the main difference is caused by not mentioning (deliberately or not?) in SAP2000 example that the modal analysis is run WITH considering shear deformation in bars.
Only at the very end of the video (about 8min20se) it is mentioned that the value close to the theoretical reference value can be found in SAP 2000 with "(...) one more adjustment(...)" and this adjustement is not shown in the video.
Shear deformation can be activated for bars in Robot using this setting:
There were also some other reasons of differences related to models of Rafacascudo:
1/ when creating in Robot the model analogous to "Use of Diaphragm" in SAP2000 the Membrane rigid diaphragm, connecting only horizontal displacements should be used and not Todos rigid link related to all displacements and rotations,
2/ when creating the section label with increased stiffness and assigning it to the roof bars the material was changed from weightless ACIERO to STEEL A572-50 with density specific for steel. It was necessary to restore the weightless material
Basing on models of Rafacascudo (thank you
) I have made files showing the results for various ways of modeling. I have contained several models in one file to compare it more easily. I have used the models analogous to SAP2000 (normal, diaphragm, increased stiffness, no rotation) and additionally 2 models very close to theoretical reference.
The screen captures below show the results for them both when considering and when not considering the shear deformation in bars:
As it can be seen when considering the shear deformation the results are very close to results obtained by SAP 2000.
Comparing vibration periods:
SAP2000 Robot
normal 0.284162 sec 0.28417 sec
diaphragm 0.28251 sec 0.28252 sec
increased stiffness 0.234911 sec 0.23492 sec
no rotation 0.221894 sec 0.22190 sec
Moreover the results of Robot are perfectly coherent with theoretical references when considering or not considering shear deformation - respectively 0.21978 sec and 0.20094 sec
I have attached the models corresponding to these tests.
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Regards,
Pawel Pulak
Technical Account Specialist
Great detective work , Pawel!!!
In the rehearsal video of increased stiffness model I remembered to change the material to aciero(weightless) .Forgot to do that in the final video .I could also had ticked "disregard density"
I never understood that "magic" in the end of PatSt-Louis1804 video
Rafael Medeiros
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