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Re: Two-way flat plate moment frame question

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Cliff,

The provision that you highlighted is there to make sure that you account
for cracked sections when doing your lateral analysis.  To my knowledge,
beyond that it is up to you as the EOR to determine a reasonable effective
width to use.  I don't recall much in the way of guidance on the issue
from either 318 or other sources.  The best "guidance" that I can think of
is to use the effective width provision for doing a T-beam or L-beam
analysis (see section 13.2.4 and in particular, figure R13.2.4).  There is
no direct "pointing" to this section from where you were, but it would be
something that would likely be considered reasonable.

As to the level of "cracking" to consider (i.e. what percentage of Igross
to use), again I don't believe that 318 give specific requirements or even
specific guidance.  The nearest thing would be section 10.11.1 as you
pointed out.  However, it should be noted that section 10.11.1 points out
that the values given are alternative "typical" values that can be used
instead of a more detailed analysis/determination of cracked values.

HTH,

Scott
Ypsilanti, MI


On Tue, 21 Oct 2003, Clifford Schwinger wrote:

> I have a question about “slab-beams” in non
> post-tensioned two-way flat plate moment frames.
>
> Section 13.5.1.2 in ACI 318-02 has this somewhat vague
> statement: “For lateral loads, analysis of frames
> shall take into account effects of cracking and
> reinforcement on stiffness of frame members.”
>
> The commentary doesn’t really clarify anything except
> to say “For nonprestressed slabs, it is normally
> appropriate to reduce slab bending stiffness to
> between one-half and one-quarter of the uncracked
> stiffness.”   One-half to one-quarter the stiffness of
> what? The full tributary width of the slab framing
> into the columns on the moment frame?
>
> The commentary statement seems to imply that when you
> have building with square column bays (let’s say 30’ x
> 30’) then the moment of inertia of the equivalent
> “beam” in my moment frame is equal to 0.25 x “I”gross
> where “I”gross is the gross  moment of inertia of
> entire tributary width of the floor slab framing into
> the column (i.e., a 30’ wide “slab-beam”).
>
> This seems to agree with section 10.11.1 where
> simplified approximate moments of inertia for various
> moment frame components are listed.  For flat plates
> and flat slabs “I”effective = 0.25 x “I”gross.
>
> What isn’t stated is a clear definition as to the
> width of the moment frame “slab-beam” member for which
> “I”gross is computed.  Is the width of the “slab-beam”
> equal to the full tributary width of the slab that
> frames into the columns in the direction for which the
> moment frame is being analyzed – or is it something
> less?
>
> If the full tributary width of slab is considered as
> the “I”gross beam width then how is the column
> stiffness (or beam stiffness?) modified to account for
> the torsional flexibility of the slab-to-column
> connection?  Is the torsional flexibility accounted
> for in the “0.25” factor that’s applied to “I”gross?
> I’m figuring that maybe the “0.25” factor is comprised
> of the product of two numbers – 0.5 x.0.5 = 0.25.  The
> first “0.5” being an adjustment that modifies the
> effective width of the “slab-beam” to one-half the
> actual slab width (this modification reduces the beam
> width to account for the torsional flexibility of the
> slab-to-column connection).  The second “0.5” factor
> may be an adjustment for converting “I”gross to
> “I”effective.
>
> I don’t want to overestimate the stiffness of the
> moment frame beams, but then again I don’t want to
> underestimate the stiffnesses of the slab-beams
> either.  If you underestimate stiffness of flat-plate
> moment frame slab-beams, the column k-factors will
> quickly go through the roof (due to the wimpy beams)!
>
> Does any of this make sense?
>
> TIA,
>
> Cliff Schwinger
>
>
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