MEMBRANE OR SHELL IN ETABS ,WHAT IS BETTER MEMBRANE OR SHELL,HOT TOPIC

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membrane and shell are the property in the ETABS "slab section". Before modeling the slab we have to assign a property 

let's see

MEMBRANE:

the membrane doesn't give the real stiffness behavior of the slab. Load distributed as per yield line theory. load distribution shapes are triangular, trapezoidal, and rectangular. still, it is widely used in small projects in zone 2 and 3.slabs and beams didn't interact in analysis.
In  ETABS  slab doesn't show deflection, moment, and many other valuable results. These results are very important for detailing slab manually



for yield line their see my another blog: copy link and paste it in google browser 
https://civilimp.blogspot.com/2021/05/how-load-distributed-from-slab-to-beam.html  


WHERE TO USE MEMBRANE:  

1) where  the slab is in regular shape like rectangular and square  (means no complex
 shape) 

EXAMPLE :



S1 ,S2 ,S3 ,S4 are the regular shape slabs

2) surrounding beams having the same sizes  (means don't have too much difference in stiffness), the minor difference  in sizes  allowed but the large difference isn't acceptable
 

EXAMPLE: 

All beam "A", beam "B", beam "C" and beam "D" has the same size of 230× 400 mm in the

the figure is given below


 3) membrane   is generally used  in smaller projects 

4) membrane is  not used in seismic prone areas ( for example in zone 4 and 5)






SHELL:

shell brings the real stiffness behavior of the slab into the model. Load is distributed as per stiffness.
Empirical T beam action is actually generated in the analysis 

WHERE TO USE SHELL:


1) we can use the shell for real behavior of slab in  the structure, with the help of shell

EXAMPLE:
                        
these are some examples of irregular shape where load cant be distributed as triangular, trapezoidal, and rectangular




2) when the surrounding beam having  different sizes (means  have too much difference in stiffness)

beam "A": 300×400 mm2
beam "B":230×300  mm2 
 beam "C": 230×300 mm2
 beam "D":300×450  mm2
clearly in the dig stiffness differ









3) we can use the shell  in seismic prone areas like zone 4 and zone 5 for capturing real behavior

4) we can use the shell  in a bigger project  because it gives better results

5) shell is preferred for irregular shape of the slab 



HOW TO USE IN ETABS:

(A)MEMBRANE: 

a)simply give the property membrane to the slab while defining the section

b) you have to define 2 types one for " one-way slab" another for "two-way slab"  (because  we have to specify if the slab is one way)

c) for defining one-way tick mark the one-way option. shown In the figure below




d) we don't need to mesh the slab in case of membrane

e) draw slab by check whether it is one way or two-way 




(A)SHELL: 


a) give the property shell while defining the section of the slab.

b) we don't need to specify the slab is one-way or two-way because the shell distributes load as per the stiffness.

c) now draw the slab then mesh it properly.



RESULT DIFFERENCE IN BEAM:


1) WHEN THE SLAB IS MODELED AS AN MEMBRANE :

when  the slab is modeled as a membrane in the structure than the moment in beam shown in the figure below
FIGURE 1


here 180 is the negative moment in the beam at the face of support (at column)

and 150 is the maximum positive bending moment  in the beam

and see the circle at support there is no moment but in actual, there is slighter moment developed, we will see in the shell model 


1) WHEN THE SLAB IS MODELED AS SHELL :

the same model is drawn in ETABS with the same section of the column and beam. But the difference here is slab is modeled as a shell. Now result
    
FIGURE 2



here 150 is the negative moment in the beam at the face of support (at column)

and 110 is the maximum positive bending moment  in the beam

and see the circle at left support there is some  moment that moment we cant capture in membrane model 
  


RESULT : 

in figure 1 the bending moment value is high because -
*  membrane doesn't give the actual behavior of the slab
*  membrane doesn't distribute the load as per stiffness.


in figure2 the bending moment value is high because 
*  shell gives the actual behavior of the slab
*  shell distributes the load as per stiffness



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