What happens in my FEA analysis when I pin the top of walls from 1st floor?

[rte4563]

I need to check the capacity of the deck and walls in this building. My senior supervisor told me I can just put pinned at top of the walls from the 1st floor. So I only need to model the 2nd floor deck and walls since everything below has enough capacity. I wonder what happens then in my FEA model when I only model deck and walls on second floor - I get increased moment in the walls on 2nd floor? Im guessing this is a conservative way of doing it when it comes to 2nd floor?

 

[BAretired]

It's as clear as mud.

 

[271828]

Yep. Mud.

What type of system is the floor? Steel framed, wood, reinforced concrete, ...

What type of walls? CFS, wood, concrete, CMU ...

Check the deck and walls for what, exactly, and by what standard(s)? A particular building code?

 

[bowlingdanish]

Your senior is telling you modelling assumptions to achieve the holistic design approach of the building. They sound like conventional idealisations that are sound and proven, but we don’t know the building or anything affecting it.

To take a random stab at what you’re asking - it’s common to have ‘pinned’ connections to floor-wall concrete as the rebar detailing doesn’t necessarily provide full continuity, and it is conservative both in deflection analysis and mid spans reinforcing to assume a rotationally free joint. It’s also not necessarily a realistic assumption, but conventional wisdom shows it is a safe one

 

[SWComposites]

And why have you not asked your supervisor these questions??

 

[rte4563]

It’s also not necessarily a realistic assumption, but conventional wisdom shows it is a safe one

Is there any way this isn't a conservative assumption when you have RC concrete? I know it depends on a lot of factors but could you think of an example in a building like this that is RC concrete.

 

[milkshakelake]

I'm not sure what the question is. But if it is what bowlingdanish is saying, your slab will impart forces on columns and shear walls that cause out-of-plane bending in them. For columns, I'd keep them fixed, because pinning them would be unconservative in terms of transferring slab moments. For shear walls, making them pinned can be unconservative for the shear wall because of the minor axis moment, but we've been pinning it for a long time and had no issues, though I've never actually done the calculation to see if it makes sense. It is conservative for the slab design itself.