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I’ve got some problem during calculating 2-storey building on seismic forces. I created modal case, but the “Rel.mas.UX(%)” and “Rel.mas.Uy(%)” are too low to add seismic case. I tried to increase number of modes up to 50, but I can’t even reach 70% to do residual mode as it was mentioned in one of the Autodesk presentation. I’ve added “Load to Mass Conversion” too, but “Rel.mas.UX(%)” and “Rel.mas.Uy(%)” isn’t rising anyway.
P.S. In RSA there isn’t my country’s seismic codes, so I’m trying to calculate model with the help of spectral method.
Can someone help me solving this problem?
There are some screenshots below.
Hi @Anonymous
Without Active Mass Direction Z you need 8 modes.
Otherwise 226.
Rafal Gaweda
Hi @Anonymous
Is it reasonable to turn off Z direction, in this calculation?
NO, because you have very heavily loaded beams in case 2, so you can't neglect the vertical vibration of these beams.
Moreover, you already reach 90% of mass in mode 4 for Z.
M. Agayr
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@Anonymous
your structure is very steep in the first level.
If you consider that the other floors are embedded in the first level you already get 82% in X from mode 2
M. Agayr
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Hi @Anonymous
I just wanted to point out that in very stiff structures at the bottom, the masses of these levels are mobilized only at very high frequencies, sometimes exceeding the cutoff frequency of 33Hz.
In this case a good opinion on the seismic behavior of the structure can be obtained by considering the flexible part of the built-in structure in the rigid part.
Looking at the mobilized masses in X and Y below 33 Hz, you are not so far from the 70%, so in my humble opinion you can use the residual mode.
M. Agayr
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Just to complete my point of view, in the substructure model we see that the mobilized mass is 82% at mode 2.
So in the complete model you are sure to have mobilized all the mass of the upper part of the structure before 33Hz. This upper part being the sensitive part of the building.
M. Agayr
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@Anonymous
Maybe I was not clear enough, sorry
Firstly i'm relay interested, if it is necessary to have 60 or more modes for 2-story building?
it is not a matter of number of modes, the goal is to know if the number of mode retained is sufficient to estimate the efforts in the part of the structure which interests you.
In this case, it is the upper part of your structure that is sensitive to earthquakes. In the model (sous-structure) of this part alone, you can see that mobilized mass is almost 100% in X and Y with only with 7 modes.
It would be good if you compare the efforts in some elements of the high part obtained with 7 modes in these models with and without the lower part.
Secondly, i can't find mobilized mass (82%) in mode 2, and which direction we're talking about? Can you show me where is it, please.
The 80% in mode 2 are related flexible structure only without the lower part, see attached file in post 7.
It'd be great if you advice me some manuals about this topic, so that i could read it.
see attached file (in French :0( )
M. Agayr
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