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I have tried on a number of occasions to use the flexible floor modelling
feature in ETABS, and I have to say (no offense to CSI) that it is a real
pain in the butt!

If you are planning on using the floor elements to model the slab, I would
suggest that you take an alternate route by:

1) Using SAP (90 or 2000)
2) Use beam elements with effective widths per a reference which you trust.

The reasoning behind the above statement is that it is pretty difficult to
extract useful shell information from ETABS when using the flexible floor
elements.  I personally would opt for 2), since using SAP for full scale
building models can be a chore (although SAP2000 certainly makes it a lot

If, on the other hand, you simply want to account for floor in plane
flexibility, then ETABS should be OK to use with the floor elements.  The
procedure I would recommend is to use the floor elements only for mass
distribution and floor flexibility, and still use beam elements for the
slab out of plane stiffness.  Of course, you'll want to set the slab-beam
elements to zero mass or else you'll be doubling up on their mass

As far as the actual procedural aspects of using floor elements, I think
you would need to just look at the manuals.  A word of caution, though,
concerning aspect ratios of membrane elements: in order to effect as
accurate a solution as possible, it would be advisable to stick to 1:1.  In
doing some parametric comparisons, I have found that the answers for ETABS
can vary quite a bit depending on the aspect ratios of mebrane elements
(IOW, the calculated stiffness of the elements changes depending on the
aspect ratio).

Does this answer your question?

T. Eric Gillham PE

> From: John Alcaras <john_e_alcaras(--nospam--at)>
> To: seaoc(--nospam--at)
> Subject: ETABS Modelling
> Date: Friday, September 25, 1998 2:52 PM
> Currently I am modeling a 10-Story RC Structure of Flat-Plate Slab with 
> Shear Walls and Columns in UBC Seismic Zone 4. Could somebody
> give me an idea or a procedure to model the said structure in ETABS
> version 6.2 with flexible floors.
> Thanks,
> John E. Alcaras PE
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