topic Re: Modal Analysis Parameters? in Robot Structural Analysis Forum

Modal Analysis Parameters?

Μηχανικος — Fri, 29 May 2015 12:03:41 GMT

Hello everyone, i am trying to run a modal analysis on my model and i need some help with the parameters

Please have a look on the picture below. i have numbered the sections for which i need advise.

Number 1

For the mass matrix i know the definition of the three options and i know that consistent is more accurate but needs more computational effort.

However which one do you usually choose when you have to design a RC structure in a seismic region?

Number 2

For mass direction i am pretty sure that to get accurate results you need to tick all three directions.

Number 3

Ok so if i convert load cases to masses do i have to tick disregard density or no?

one more thing, do you always convert load cases to masses?

what about selfweight? do you also convert selfweight to mass or is it already taken into consideration?

Number 4

I have no idea about this....so which method do you use?

Number 5

Do you think its a good idea to use limits with percent of mass participation (90%) as it is required by Eurocode? or shall i leave it inactive and just increase the number of nodes until i get the appropriate mass part.?

Number 6

I know from dynamics of structures in order to calculate the dynamic response of a structure we used mass matrix, stiffness matrix and damping matrix. therefore i think i have to tick the "Include damping in calculations"......what do you think

thank you, your help is greatly appreciated

Re: Modal Analysis Parameters?

Μηχανικος — Thu, 04 Jun 2015 12:23:16 GMT

1a. Modal Analysis Parameters 2015

1b. Modal Analysis Parameters 2011

2. Modal Analysis

3. Modal Analysis Assumptions

4. Modal Analysis - Precision of Calculations

5. Modal Analysis Parameters - Iterative Solver

6. Available Solvers

7. About the block subspace iteration

8. Block subspace iteration with shift application

Re: Modal Analysis Parameters?

Μηχανικος — Tue, 09 Jun 2015 13:08:54 GMT

Can someone please confirm if this is the correct procedure for carrying out the modal analysis?

1. Below are the parameters used for the modal analysis

i tick the disregard density box and

2. then i convert all deadloads to dynamic mass as shown below

is this the correct way to deal with the mass of the structure?

Thank you

Re: Modal Analysis Parameters?

dimogrec — Tue, 09 Jun 2015 14:54:03 GMT

A small note aboute mass and disregard density

You should tick disregard density only in case you convert the case 1 into mass (selfweight of structure DL1) something that you did not as shown in the second picture.

Self weight of the structure (weigth of columns beam and slabs (slabs only if defined with thickness FE (and of course in case your material has a defined density)) is automatically converted to mass

So for better understanding there are two cases

1) if you add loads on case 1 (DL1) (for example dead load of perimetrical walls and non structural dead load on floors) and you want to convert them to mass (as EC8 requires) you have to use "load to mass conversion" convert case 1 with coefficient 1 (as EC8 requires). In this case you have to tick disregard density because if you don't tick it, Robot will convert the self weight of your structure into mass twice.

2)if you use an other load case for your non structural dead loads for example case 2 DL2 you convert only case 2 into mass (again with coefficient 1 (as EC8 requires). in this case you don't tick disregard density tab on modal analysis parameters

Now for live load cases depending on your building class (residential, shopping mall or whatever) conversion coefficient is 0.3 or 0.6 ecc...

So change the coefficient of cases 8to12 in your second picture (unless they are all dead load cases...)

Re: Modal Analysis Parameters?

Μηχανικος — Tue, 09 Jun 2015 18:00:41 GMT

Once again thank you for your time and effort, i totally get it now. I really appreciate it this. thank you

I want to ask you one more question regarding modal analysis

which method do you use?

block subspace iteration, subspace iteration, Lanczos algorith...etc?

Re: Modal Analysis Parameters?

dimogrec — Wed, 10 Jun 2015 14:27:39 GMT

Subspace iteration