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Re: Flat Slabs

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First to Albert,

Flat slab design is NOT limited to a LL of no more than 3 x DL unless you
use ACI 318 section 13.6 (Direct Design Method).

For a flat slab design using ACI Section 13.7 (Equivalent frame, or equal)
you do not have the same restrictions.

Also, for the record, ACI 1995 changed the 3x rule to 2x (clause
for two-way slabs under Direct Design Method.

Actually, if you use section 8.3.3 of ACI 318 (alternative to frame
analysis) with one-way slab and beam system you have the LL<=3xDL rule

For most practical structures, [and certainly the pile supported slab in
question (with high loads and moving point loads)], they would NOT normally
be designed using ACI section 13.6 or 8.3.3.

Back to the original thread...

For Michael,

i have done two very large pile supported slabs with large imposed live
loads (>500 psf) for the US Navy. the conforming design was one-way beam and
slab which we value engineered to flat slab (ie with drop panels). the drops
help out for punching shear, but they particularly add stiffness and
increase negative moment capacity. in our situation, the ground could be
easily shaped/supported and no lost forms were required.

our particular slab was POST-TENSIONED (bonded) so we had to do some
particular detailing to handle restraint from piles and ground

in your instance the pile spacing is close, and depending on the pile
diameter i would also tend to use a thicker slab, rather than add caps or

i did a quick run using RAPT (2D computer software), and a 12" slab with 800
psf LL is reasonable for flexural rebar. deflection is okay by inspection. i
am not sure of your pile size but it will have to be more than 20" SQ (or
equal shape) for punching shear to be okay. depending on the pile size and
the cost of rebar/conc i would still tend have a thicker slab.



Mark Geoghegan

S T R U C T U R A L   T E C H - V S L
            GUAM  -  HAWAII

From: "Albert Chou" <ChouAS(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: Re: Flat Slabs

for a flat slab design, the live load is limited to no more than 3 times =
the dead load, because of pattern load concern. thus, you have to have a =
beam/slab system in stead.

>>> dparsons(--nospam--at) 09/20/01 02:47PM >>>
Michael Zaitz wrote:

> We are looking at a preliminary design for a flat slab floor (at grade)
> supported by precast piles at 12'-6" o.c. each way.  The floor must
> support a 800 psf live load.  We also have a forklift carrying a 7000
> lbs roll of paper (trying to get more information on this).  They have
> not informed us of any rack storage in the building.  At present we are
> looking at a 12" slab as a starting point.

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