Structural modeling

[slickdeals]

Greetings,
I am seeking opinion on whether it is a good idea to model 2 buildings separated by an expansion joint in one model. Will it truly capture the dynamics of the building?

Assuming one building was weak torsionally and the other was not, would the first mode show the true first modes of each building or would it be the first mode of one of the buildings?

What is the best way to go about modeling such a occurence?

 

[JAE]

Two separate buildings = two separate models in terms of analysis - I think having two models in one program file might get confusing with any dynamic analysis - not sure which programs, if any, would be affected by this though.

 

[slickdeals]

That was my line of thinking as well, but wanted to make sure that this was right. I don't know if sophisticated means are present to account for such effects.

 

[271828]

I think you'll be better off with two models.

That being said, why not do some experimentation to see what happens? For example, create two vertical sticks side by side with different properties and run the modal analysis. See if the model can come up with the right modes for both of them.

 

[Tomfh]

What sort of restraint does the expansion joint provide?

 

[prsconsultant]

It depends on the software you are using for analysis. If you have an option define each building with its own diaphragm, the independent behavior of the two bldgs can be captured. I bet the buildings are tied together at some lower levels.

We normally model the two together.

 

[slickdeals]

The buildings are separated by a true isolation joint. Two reasons for doing that are:
1. Length of the building is about 400 ft. and a notch in the diaphragm. Imagine 2 200'x100' buildings connected by a 50' x 25' bridge.
2. Inflexible locations of lateral system making it better to separate than try to keep together.

I am using ETABS to do the analysis. I will try defining separate diaphragms and see how it works.

In addition, I would appreciate if I could some more responses on my previous question on a related note (thread507-210196)

 

[slickdeals]

I tried doing 2 diaphragms in ETABS. It only captures one buildings behavior in one mode. However, if I assign the same diaphragm definition to both the buildings, then I get them to have their true first modes reflect on the global first mode. Could others please verify and chime in?

 

[prsconsultant]

Yes that is a true behavior. You should see independent modes for each building. If the buildings are identical then you may see modes very close to each other. Lower modes may exhibit combined behavior if the two portions are tied at some level.