Simply Supported Beam Model As Shell Element

[Saiful Islam Zaber]

I am trying to understand Shell element behavior . I have modeled a simply supported beam of 24 ft with 1ft thick shell element (Depth  2ft). Meshed Along length 24  and 2 along depth. Applied 3 kip point load at  Node along top surface to create a 3 kip/ft loaded simply supported beam of  1 ft Width X 2 ft Depth. Assigned the beam as spnadrel. 

In my opinion Moment wrt to major axis should be = 3*24^2/8 = 216 kip-ft/ft 

But according to etabs moment is 138 kip-ft moreover the diagram is rectangular .

Can anyone explain please??

 

Edited February 18, 2019 by Saiful Islam Zaber

[UmarMakhzumi]

Saiful Islam,

I like your curiosity and your question is good too. Generally for beams, if you wan to do 3-D modelling, solid elements should be used.The definition below would help you understand the difference:

Quote

Shell elements are used to model structural elements in which two dimensions are much greater than the third one and when the change of the analysed feature across this third direction can be neglected. It is reasonable for static analysis of panel/planar elements such as slabs or walls as well as thin-walled spatial elements such as shells. The advantages of the use of shell elements, as you said, results mainly from time-saving due to reduced number of finite elements (and consequently the equations to solve). So, anytime you can reduce the problem to planar problem and neglect what happens at the thickness of the element I don't see the reason to use solid elements. 

The results from the following simple analysis: http://www.javelin-tech.com/blog/2012/10/shell-vs-solid-elements/ suggest that it is when the third dimension is at least 20 times smaller than the other ones.

However, if the change of the analysed feature is on a comparable level in all directions of the analysed element, solid elements should be used. It is the case e.g. in block elements, like massive concrete elements which are among my interests. In such elements, the observed thermal and moisture fields are highly non-stationary and the resulting thermal and moisture gradients lead to formation of the internal restraint in the elements which, in turn, is the origin of (self-induced) stresses in the element. If that fact was neglected, we would not be able to analyse this phenomenon.

 

To simplify, shells elements are best for walls, slabs and modelling 3D elements where the thickness is smaller like surface elements. 

Hope this helps.

Thanks.

 

[Saiful Islam Zaber]

@UmarMakhzumi   Thanks a lot. 

As there are two type of shell in Etabs - horizontal and Vertical ( Slab and Wall ) , Modelling the simply supported Beam  by Slab Shell shows 216 kip-ft moment which is correct but Wall type shows 138 kip-ft  which is not expected.

That brings the question " How should I model link beam between Shear wall or How should I model Deep Beam or swimming pool hanging from a large depth beam (but not Deep beam) bottom, as Slab or Wall?"

 

 

 

    

[UmarMakhzumi]

33 minutes ago, Saiful Islam Zaber said:

As there are two type of shell in Etabs - horizontal and Vertical ( Slab and Wall ) , Modelling the simply supported Beam  by Slab Shell shows 216 kip-ft moment which is correct but Wall type shows 138 kip-ft  which is not expected.

I am not aware of the difference.

33 minutes ago, Saiful Islam Zaber said:

How should I model Deep Beam or swimming pool hanging from a large depth beam (but not Deep beam) bottom, as Slab or Wall?"

You can model a beam/ deep beam using beam elements and then mode the swimming pool as slab?

Let me know what you think. The picture above isn't clear. If you can explain what the framing and intent is, that would be great.

Thanks.