Floor diaphragms questions. 5

[engrmarks]

Hi!
Do I need to model the Slab on Grade (slab on ground floor) in ETABS? I have a two-storey structure. If so, do I need to use it as part of my diaphragm? Currently I only modeled my slab at second floor and assigned the diaphragm.

Thank you!

 

[r13]

You need to review the definition of "floor diaphragm" and how it acts in the structural system. Will your footings walk toward/away from each other due to lateral load? If there is such potential, you need to provide grade beams, ties/struts, or utilize the SOG to prevent/restrict such movement, but not a "diaphragm", although the concepts overlap certain features for elements mentioned above. You sure can model those elements in your structural model, but usually not necessary.

 

[Blackstar123]

Are you providing connection between slab on grade and your frame?
Usually slab on grade is not connected to the structure and we provide isolation joints to make sure forces are not transferred from the frame to the slab on grade.

 

[XR250]

Sounds like if you are asking this question, you shouldn't be using software to perform the analysis.

 

[EireChch]

XR250 pink star

 

[Blackstar123]

I don't find anything wrong with this question. These kind of questions used to pop in my mind too when I first graduated. All collage/university taught you, is theory and no practical application.

From question, it seems to me he's a fresh engineer and trying to learn how to properly model a true behaviour of a structure. If he's trying to learn then why shouldn't we encourage questions like this?

 

[r13]

We shall all recall the day we got start. But I agree with others that OP will be benefitted more by reading/review literatures before dive into computer method without understand the structural system he's dealing with.

 

[engrmarks]

You need to review the definition of "floor diaphragm" and how it acts in the structural system. Will your footings walk toward/away from each other due to lateral load? If there is such potential, you need to provide grade beams, ties/struts, or utilize the SOG to prevent/restrict such movement, but not a "diaphragm", although the concepts overlap certain features for elements mentioned above. You sure can model those elements in your structural model, but usually not necessary.

Thank you. Will do that. From my initial understanding it acts as the stiffness of the structure. I just wonder if the slab on the ground floor is needed to act as a diaphragm. Will read more on this.

 

[engrmarks]

Sounds like if you are asking this question, you shouldn't be using software to perform the analysis.

Right. Sorry. In a general sense as well not just from software application.

 

[engrmarks]

I don't find anything wrong with this question. These kind of questions used to pop in my mind too when I first graduated. All collage/university taught you, is theory and no practical application.

From question, it seems to me he's a fresh engineer and trying to learn how to properly model a true behaviour of a structure. If he's trying to learn then why shouldn't we encourage questions like this?

Thank you for understanding. I'm fairly new to industry (less than 2 years) and I'm only doing structural engineering on the side. My main work is on project management/execution. I just want to learn more on structural engineering.

 

[atrizzy]

I personally would try to make any design work without relying on the slab on grade. There may be situations where you have to use the SoG to tie together columns and walls, etc, but I wouldn't start with this assumption.

KISS principle! Keep it simple stupid! The simpler the structural system is to 'model' and understand, the more reliable it is due to all of the additional unforeseen bits of redundancy. If for force reasons, code reasons, etc, you need to end up utilizing the SoG, then so be it.