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Posted: Mon Dec 22, 2008 2:51 pm Post subject: flat slab span arrangement and design technique
I have one point to make clear from the experts on Flat slabs.In the
direct design method of flat slab design, one must have at least
three spans in each plan directions But in the equivalent frame
method this stipulation is not a must. Some architects make plans with
2 large spans and two cantilevers on both sides of the spans and in
some cases with a single span and two cantilevers and they want
structural design to be carried out with flat slab or flat plate.In
such cases, is it advisable to go for flat slab or flat plate design
for a building of 20 floors and more in an earth quake zone.Of course
these buildings have shear walls all around the lift core.For doing
the analysis of these buildings can we go for finite element analysis
with rigid diaphragm assumptions for the slabs?.In a previous
discussion in sefi forum about flat slabs some members were expressing
to assume 75% of the lateral loads to be taken by the shear walls and
the balance 25% by the available columns. I wish to know how this
apportionment is possible in the program input if we are analysing by
STAAD or ETABS. Please clarify this?.
Posted: Tue Dec 23, 2008 7:35 am Post subject: flat slab span arrangement and design technique
I am trying to answer second part of your query which is related to how to satisfy the clause of (25% +75%) base shear distribution using STAAD.Please note that this distribution is for dual system and not for Flatslab+Corewall type building. You can do this by three methods explained bellow.
1. Run the analysis checks the % bases shear taken by Frame. If it is less than 25 %.Calculate the scale factor such that after multiplying base shear contribution by frame will become 25%.Rerun the analysis using this scale factor.
2.If you are using plate element for shear wall modeling you can change thickness of plate element such way that frame will take at least 25 % base shear. Off course you have to attempt no of trials for this..
3. One more option is by defining different load combinations.
For this one has to calculate scale factor as explained in method 1 and has to define separate load combination by modifying basic load combination and by using load list command design frame elements for this combination.
E.g. suppose you workout scale factor 1.25 then the load combination 1.5(DL+EQ) will become 1.5DL+1.875EQ.
Dear expert please correct me if I am wrong in interpreting the past discussion on this Forum.
Thanks And Regards
Dipak D Gaikwad
Detailing in post-tensioned flat slabs is very critical. An interesting article can be found on detailing for bonded PT strands at
This may not be the practice followed in toto everywhere, but is worth reading
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