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Slab modelling in ETABS -membrane, plate or shell ??
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h_rushikesh_patel
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PostPosted: Mon May 09, 2011 11:38 am    Post subject: Slab modelling in ETABS -membrane, plate or shell ?? Reply with quote

Respected Sirs and Dear friends,

I have a question regarding modeling of RCC slab in Etabs software. It has got 3 options to define slab type 1. Membrane, 2. Plate 7 3. Shell.

As I understand,

Membrane type slab = only in plane stiffness and no out of plane stiffness
Plate type slab = only out of plane stiffness and no in plane stiffness
Shell type slab = both in plane and out of plane stiffness

so, when we assign slab type as membrane, it only transfers forces to beams supporting it and does not take part in load taking along with beams, but when we go for shell or plate they have out of plane stiffness as well, so they take part in load caring along with beams (as if T beam is put) and sagging moments in the beams get reduced significantly. (Which I feel will be the actual on site condition).


My questions are as follows:

1. What I understand is correct or not?

2. The other thing is that which is ideal for modeling of RCC slab in Etabs? And what cares should be taken in design etc while assigning any particular type to slab?

3. Generally people donít use shell or plate and use membrane as it matches results with Staad frame analysis (without slab modeling) with which they are used to, but Etabs does have facility to analysis beam slab floor acting together (T -beam analysis - not design) then can I use that and design beams for reduced moments I get by this analysis??



Please comment.

Thanks and Regards.
Hrushikesh
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thirumalaichettiar
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PostPosted: Mon May 09, 2011 12:36 pm    Post subject: Reply with quote

Er.Hrushikesh,

Answers are :

1. What I understand is correct or not?

Correct.
Assign as shell element.

2. The other thing is that which is ideal for modeling of RCC slab in Etabs? And what cares should be taken in design etc while assigning any particular type to slab?

For RCC shell element assignment is best since it will take care of bending in two direction which is for 2 way action and it will of advantageous in lateral analysis.

3. Generally people donít use shell or plate and use membrane as it matches results with Staad frame analysis (without slab modeling) with which they are used to, but Etabs does have facility to analysis beam slab floor acting together (T -beam analysis - not design) then can I use that and design beams for reduced moments I get by this analysis?

If you need bending in one direction say for one way slab it is OK but usually slab is assigned as shell element even if it is exported to STAAD since there are options in Etab while exporting to other software. Check with that.

T.RangaRajan.
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suresh_sharma
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PostPosted: Mon May 09, 2011 3:03 pm    Post subject: Reply with quote

Dear Hrushikesh,
By and large you appear to be correct in understanding membrane, shell and plate in etabs. You have raised something about T beam. T beam may be useful for very very simple structure viz wall, column and beams. But in frame structure with lateral load no component will work as T beam. Now I have almost forgot about T beam. I feel you can not model T beam for frame analysis with lateral load. In frame analysis design support moment is always higher than mid point moment. Am I correct?
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h_rushikesh_patel
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PostPosted: Tue May 10, 2011 6:36 am    Post subject: Reply with quote

dear sirs,

thank you for sharing your views, as u said that concept is understood correctly by me but if we use shell as slab type can we design slab using co-efficient method given in IS 456. or need to design as per the stress contour obtained from etabs analysis? and which stresses to be checked ?

is there anything else to be taken care for slabs or beam design ?

and talking about T beam action i meant stresses will be shared by slab along with beams for which we generally dont design slab if used IS 456 co-effi. method. so what to do ??

please comment.

regards and thanks,


hrushikesh
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Kumar_Abhishek_Singh
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PostPosted: Tue May 10, 2011 9:39 am    Post subject: Reply with quote

You should model slabs as shell elements.

Membranes carry load by tension along the plane only. They are thinner than plate elements and cannot carry bending. For detailed reference please refer the book by Salvadori.

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suresh_sharma
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PostPosted: Tue May 10, 2011 3:06 pm    Post subject: Reply with quote

Dear Abhisek,

I beg to differ from you. What ever definitions about plate, shell and membrane are mentioned in Text books do not necessarily compare with the software. The definitions as applicable with etabs are as under:

Membrane: used for conventional slab with beam.

Shell: is used for flat slab and wall.

Plate thin:  is used for raft foundation where bending is predominant.

Plate thick: is used where shear is predominant.
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rajancivildesign@gmail.co
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PostPosted: Wed May 11, 2011 2:59 am    Post subject: ETAB Step by step example with wind and seismic loading Reply with quote

Dear SAFIANS,
I have a soft copy of  ETAB example step by step procedure  for Indian code with seismic, wind loading. Those who have interested can receive the same through email.
My email Id is: rajancivildesign@gmail.com.
V.M.RAJAN.
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dibesh2063
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PostPosted: Wed May 11, 2011 5:06 am    Post subject: Reply with quote

Dear Mr. Rajan,

please kindly mail the example pdf file to the following address:
dibesh2063@hotmail.com

with thanks in advance,

dibesh
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manjunath_chinni
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PostPosted: Sun Jun 05, 2011 8:35 am    Post subject: Reply with quote

Dear Mr. Rajan,

please kindly mail the example pdf file to the following address:
manjuiitb@gmail.com

with thanks in advance,
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ajit_g_g
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PostPosted: Sun Jun 05, 2011 12:43 pm    Post subject: Re: ETAB Step by step example with wind and seismic loading Reply with quote

rajancivildesign wrote:
Dear SAFIANS,
I have a soft copy of  ETAB example step by step procedure  for Indian code with seismic, wind loading. Those who have interested can receive the same through email.
My email Id is: rajancivildesign@gmail.com.
V.M.RAJAN.



dear sir pl send me the file

my email id is ajit_g_g@yahoo.co.in

thanks

ajit
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