Regarding Dynamic Analysis

[MOHAMMEDIDREES]

When we perform dynamic analysis in the out put of building modes we have Modal Participation factors and Modal Participating mass ratios what are thsese and where do we use them in our calcs

 

[drile007]

In short: Modal Participation factor means what percent of defined mass is vibrating in particular mode shape.

Drile007

 

[Diquan]

be carefull if you're not familiar with "Dynamics of structures". I recomend reading books from Clough, Penzien, Chopra, Paulay and Priestley, they have really good books to introduce you into dynamics of the structures

sorry for my english

Diquan

 

[Whiffle]

You should really understand what you are doing before you just "jump" into dynamic analysis in ETABS (or any program). Dynamics of Structures by Chopra is a very good textbook with good examples. He goes through a Response Spectrum Analysis and a Response History Analysis and explains the steps very clearly.

 

[civilhak]

Hi,
Modal Participating mass ratios indicates how much mass is activated in your modes...according to EC8 this value should be atleast close to 90%...if you are undir this value, you should run more modes undir til you reach this value.
Lets say your MPMS is 60%, then your resulting forces are to low, if you increase the number of modes then the resulting forces increase...this 90% is a good indicator in showing that you are running enough numer of modes.

 

[Diquan]

Correct.

Most of the programs of analisys reach 100% very easy, because is something of equations and the response spectrum it's using.

The number of modes are very easy to obtain, in a building each floor have 3 DOF (degrees of freedom), 2 displacements 1 rotasional, so let's say a usual building of 8 floors or storys is going to have 24 DOF that is 8 floors * 3 DOF = 24 DOF

Again, sorry for my english, i speak spanish.

Diquan