Concentrated Load Distribution on a Slab on form deck for a mezzanine

[wrxsti]

I am trying to model a form deck slab on typical mezzanine layout in etabs

see model with loads --->

https://imgur.com/a/z8Eca8i

(sorry was trying to post image was not working)

slab is thin shell 4" - f11 f22 f12 m11 m22 m12 stiffness modifiers 0.25 4000 psi

BM diagram as thin shell shown here -->

https://imgur.com/a/l1HYnbE

slab is membrane

BM diagram as membrane shown here -->

https://imgur.com/a/d3sXjK7

if you define a deck section it only allows membrane modelling which negates any out of plane bending
not sure if this is because of practice?

I am finding with the thin shell model the concentrated loads distribute more to adjacent secondary beams allowing smaller secondary beam sizes
this distribution changes with thickness of slab

Can anyone experienced share any light on the matter?
which model should be used ?


Also i was trying to verify the behaviour of shell elements

COMPARATIVE MODEL #1

slab thin shell connected to frame by links --->

https://imgur.com/a/at21THG

link only U1 (vertical) fixed in model

BM 3-3 ---->

https://imgur.com/a/9cOwYIO

BM 2-2 ---->

https://imgur.com/a/5V5tX7S

torsion ----->

https://imgur.com/a/OvESvh3

axial ---->

https://imgur.com/a/Mvbj3c3

COMPARATIVE MODEL #2

Slab thin shell drawn on top of frame ---->

https://imgur.com/a/npNMPyR

no release on shell objects

BM 3-3 ----->

https://imgur.com/a/Ogp6n3A

BM 2-2 ----->

https://imgur.com/a/hbaJugX

torsion ---->

https://imgur.com/a/zpmKH7Q

axial ----->

https://imgur.com/a/Nkf1AEw

Initially after reading about how the program uses shell elements
an alternative to creating simply supported slabs was to use links
which i tried to do in COMPARATIVE MODEL #1
What i really wanted to do is model the slab sitting on top of the beams
and not connected which wouldnt produce hogging moments in the beams ( seen as RED on BM 3-3)
links only see to work in the direction.

Is there anyway to model the slab as just resting on the frame in etabs?
i know usually in practice the form decks are secured to beams but was just wondering if there was any way
to do above mentioned.

 

[wrxsti]

I am trying to model a form deck slab on typical mezzanine layout in etabs

MODEL WITH LOADS

1st MODEL slab modelled as thin shell 4": f11 f22 f12 m11 m22 m12 stiffness modifiers = 0.25, material = 4000 psi

Deformed Shape

BM Diagram

2nd MODEL slab is membrane which means no out of plane stiffness as above, material = 4000 psi

Deformned Shape

BM Diagram

if you define a deck section it only allows membrane modelling which negates any out of plane stiffness
not sure if this is because of practice?


The deformed shape of membrane however looks more practical but does not allow much for load redistribution
as shown in the images above the thin shell modelling for same loads results in more distribution over adjacent beams.

In a case where concentrated load is directly above beam on slab load just transfers directly to the beam below it .

I am finding with the thin shell model, the concentrated loads distribute more to adjacent secondary beams allowing smaller secondary beam sizes.
This distribution changes with thickness of slab

Can anyone share any light on the matter?
which model should be used ?