Time period of structure 1

[compe_ad]

I am getting higher time period of the structure for my 3D model (T=1.2 secs). From 2D and hand calculation, time period is around 0.14 secs. I also think time period is really high from my 3D model. Is there is a way to check manually how ETABS is calculating time period. This is the second time I am facing similar issue. Can anyone please help me out on this?

 

[JoshPlumSE]

I like to look at the mode shapes and mass participation.... Those mode shapes will tell you if there is some incorrectly modeled in your structure. The mass participation will tell you if this is just a local mode.

Sometimes you may have to solve for a lot of modes before you get past the local modes. ETABS is usually pretty good about creating a model that isn't prone to local modes. But, this is a general principal of dynamics.

 

[compe_ad]

Thanks!
How can we figure out if it is just a local mode by looking at the mass participation?

 

[JoshPlumSE]

Well, it will have a very low mass participation.

If you're looking at the ASCE codes about the "fundamental" period of the structure in each direction, then you need to see something pretty significant. Like 60% or higher for a single mode.... Well, that's at least what I would expect to see for a mode to be considered the "fundamental" mode of the structure. I'll give you a screen shot of some ETABS output if I can. But, I have that program installed on my other computer.... So, it will take me a little while to get to it.

 

[JoshPlumSE]

Below is an image from the "modal participation mass ratios" output table. This was from an analysis example verification file included with the install.... Example 05.

It might not be the best example because the "principal" axes of this structure don't actually correspond to the X or Y direction. Regardless, you can see that the mass participation ratio for Mode 1 is 41.89% in both directions. For mode 2, it is basically the same (42.6% in both directions.

 

[compe_ad]

This was really helpful. I appreciate.