Dynamic Loads and Mass Participation

[mecheil2000]

I know now, why I got zero drift for some concrete towers by robot.
The reason is due to mass participation ratio of dynamic(seismic) anlysis
if participation ratio does not exceed some limits we will not get a good result from program.

My question is: can any one who has experience with dynamic analysis can tell me some tips to solve this prroblem.
I know some tips
1- we have to use pesudo modal analysis
but this also sometimes will not be enough...

Hoping I will get help.

[mecheil2000]

I ask if member " Zero Length" can help

[length zero]

I' v never seen that robot has give me this kind of error ( zero drifts ),
Finally they are an good control of seismic lateral response of structure, ( in this case FEM Model).
As we know drifts are nothing more than relative story displacements and if this values of displacements are to small, you must chek your model, maybe modal mass is less than 90 %, base shear force is to small, ....
This can happpen if you have very stiff structure, and story displacements are to small.
Also controll the values of free vibration periods and comparasion with aproximately hand calculated is an good parameter to check your model.

[mecheil2000]

I' v never seen that robot has give me this kind of error ( zero drifts ),
Finally they are an good control of seismic lateral response of structure, ( in this case FEM Model).
As we know drifts are nothing more than relative story displacements and if this values of displacements are to small, you must chek your model, maybe modal mass is less than 90 %, base shear force is to small, ....
This can happpen if you have very stiff structure, and story displacements are to small.
Also controll the values of free vibration periods and comparasion with aproximately hand calculated is an good parameter to check your model.

I thank you zero Lenght ,Really you have a very good experience
But I am using etabs, and as per the static of seismic load I can get the base shear,
can you help me how can I apply the base shear by robot

I have many issues I would like to discuss with you if it is possible please tell me
and I will be very appreciated

[troy_the.best]

dear frineds,

dynamic mass like mass source in Etabs. This picture explain mass in robot will using Modal and dynamic analysis. Analysis parameter assign loads convert to mass. for dead load scale factor 1, for live load scale factor is 0.3 for normal buildings. absolutly mass direction include X,Y not Z. try this.

falconado_turkey

[length zero]

You must select checkbox z at mass direction before you add.
And also be careful during conversion DL 1 ( self weight ) into mass because robot convert it automaticlly or you must disregard density to modal analysis. In other case you will convert double self weight.

Regards
Z.G.

[mecheil2000]

You must select checkbox z at mass direction before you add.
And also be careful during conversion DL 1 ( self weight ) into mass because robot convert it automaticlly or you must disregard density to modal analysis. In other case you will convert double self weight.

Regards
Z.G.
Hi Z.G

I got the reason why robot did not solve the tower properly
it is a commont bug in Robot When the User Use a RIGID DIAPHRAGM

Thank you

[troy_the.best]

Hi Z.G.

What is problem. İm not checkbox tick in Z mass. is this correct? İn modal analysis masses only lumped X and Y, or am i wrong? thanks.

falconado_turkey

[troy_the.best]

You must select checkbox z at mass direction before you add.
And also be careful during conversion DL 1 ( self weight ) into mass because robot convert it automaticlly or you must disregard density to modal analysis. In other case you will convert double self weight.

Regards
Z.G.

Hi Z.G.

Please look my verifications please. link in other thread.
Etabs and Robot in modal analysis give same results. İf check Z mass in parameters, robot calculate double mass in Z direction.

http://forums.augi.com/attachment.php?attachmentid=76605&d=1282437621

Best regards.
falconado_turkey

[length zero]

troy_the.best

I mean if u want to addd mass oscilating in z you must chech z in check box and than press add.

Maybe i have understand wrong your thread

Regards
Z.G.

[length zero]

As for reason that you must or not must to define mass oscilating in Z direction , i think that is other issue.
I tell my opinion for this issue:

When you have defined load participating factors to mases in etabs by default you have on "Include lateral Mass Only" , this mean that u have says to program that mass can oscilate in X and in Y only. If you have make this into robot also yo must have same modal response and values also.
But in this case you can't generate spectrum case in Z if you want. In this case if u want to use spectrum case or dynamic analysis in Z direction ( we know codes recomend it for defined reason and type of structure, ex EC - for structures with span >9m, and other reasons ;;;) you must define mass oscilating in z also.
But spectrum datas are difFerent from X,Y and Z.


regards
Z.G.

[mecheil2000]

I' v never seen that robot has give me this kind of error ( zero drifts ),
Finally they are an good control of seismic lateral response of structure, ( in this case FEM Model).
As we know drifts are nothing more than relative story displacements and if this values of displacements are to small, you must chek your model, maybe modal mass is less than 90 %, base shear force is to small, ....
This can happpen if you have very stiff structure, and story displacements are to small.
Also controll the values of free vibration periods and comparasion with aproximately hand calculated is an good parameter to check your model.

The rigid diphragm is the problem
rigid diaphragm is the bug in the program that creates this type of wrong analysis
Thanks "Lenght Zero" for your helping
mecheil

[troy_the.best]

As for reason that you must or not must to define mass oscilating in Z direction , i think that is other issue.
I tell my opinion for this issue:

When you have defined load participating factors to mases in etabs by default you have on "Include lateral Mass Only" , this mean that u have says to program that mass can oscilate in X and in Y only. If you have make this into robot also yo must have same modal response and values also.
But in this case you can't generate spectrum case in Z if you want. In this case if u want to use spectrum case or dynamic analysis in Z direction ( we know codes recomend it for defined reason and type of structure, ex EC - for structures with span >9m, and other reasons ;;;) you must define mass oscilating in z also.
But spectrum datas are difFerent from X,Y and Z.


regards
Z.G.



Hi Z.G.

first . thanks for advice. then actually responce spectrum analysis in Turkey not use Z direction and not add masses this directions. You are right. Thanks your opinion. its right.

falconado_turkey