Mode Shape in Etabs

[Roy88]

Hi Guys,

I am curious why does ETABS report mode shape with units. Back in my uni day, I was constantly reminded by my professor that the mode shape is mathematically just an eigenvector...It does not have "any" real magnitude and only indicate the shape of the structure.

Is there any reasons why ETABS report mode shape with unit? Is it different compare to the classical modal analysis technique?

Thanks.

 

[saadpervez1]

Which unit are you talking about, there is only an absolute value, it doesn't change whatever unit you have. The only unit that is there for mode shapes is the unit for time period.

 

[Roy88]

Hi Saadpervez1,

When you look at eigenvalue (i.e time period or frequency) - Etabs provide them in terms of unit of time (say seconds for period and Hz for frequency) but my when displaying the mode shape in tabular form, it provides mode shape of every floor in terms of mm...Now the question is why? Magnitude of mode shape should not have "any" real magnitude as it is just an indication of the mode shape.

 

[saadpervez1]

Can you share a screenshot?

 

[jowook]

The components of the eigenvector must have a unit, so the etabs can draw mode shapes on display. However the value of components are all relative, so your professor is correct. If you don't understand, please see an engineering mathematics book for eigenvalue problem. You just forgot what eigenvalue problem is.