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Re: Concrete T beams/ ribbed slabs

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Andrew,

It would appear that the difference in "full" width vs. "effective" width
would depend on the type of beam in question.

As per the code, the full width is used for an isolated beam.  This means
a T-beam that does NOT have a slab "connected" to it.  An excellent
example of this would be precast double-tee beam or something similar.  In
otherwords, there is not slab placed monolithically with the T-beam
flange.  As a result, the cantilever length would be the distance from the
web of the T-beam to the tip of the flange of the isolated T-beam.

The effective width would be for when you have a slab that is placed
monolithically with the flange of the T-beam but is not necessarily
designed with reinforcement to span perpendicular to the T-beam.  Thus,
you don't really have a nice clear cut cantilever, and so you must use the
effective width of the the flange per section 8.10.2 or 8.10.3 for the
cantilever length.

As far as how wide of a strip to use, I would suggest that would draw two
45 deg lines from the location of the application of the load (i.e. full
width for cantilever or effective width) to the web.  Then use the width
of the triangle created.  Or you could use yield line analysis as Peter
suggested.

FYI, a good concrete book is MacGregor's book (Reinforced Concrete:
Mechanics and Design).  It has a section on yield line analysis if you
need it and also gave a rather good explanation of the full vs. effective
width (although my edition is a little dated...uses the '89 code if I
recall correctly).

BTW, I would suggest that you at least use fy=40 ksi for 1930's
construction.  It may even be slightly smaller, but it is highly unlikely
that it was 60 ksi.  It was not until the 80s and 90s that Grade 60 steel
became much more prevelent.

HTH,

Scott
Ypsilanti, MI


On Tue, 3 Dec 2002, Andrew D. Kester wrote:

>  I am doing an analysis on an existing building and I have one last check
> that is giving me fits, and the ACI is a little unclear. What I am trying to
> do is check the bending and shear stress in the flange of a T beam, that is,
> the stress transverse to the web. There is a point load at the very end of
> the flange, and the joint where two T beams meet, and this cantilevered
> moment causes a bending stress at the face of the web where the flange joins
> the web. I hope this is clear.
>
> ACI 8.10.5
> Where primary flexural reinforcment in a slab is considered as a T-beam
> flange is parallalel to the beam, reinforcment perpendicular to the beam
> shall be provided in the top of the slab in accordance with the following:
>
>      8.10.5.1   - Transverse reinforcement shall be designed to carry the
> factored load on the overhanging slab width assumed to act as a cantilever.
> For isolated beams, the full width of overhanging flange shall be
> considered. For other T-beams, only the effective overhanging slab width
> need be considered.
>
>
> I do not know what they mean by the full width or the effective width of the
> overhanging slab. The way I assumed you were supposed to check it was from
> the edge of the slab to the face of the web where the flange cantilevers
> from. A bigger question for me is how wide of  a strip to use as a model for
> the beam width of the flange. Since the transverse reinforcement is at 8"
> o.c., and this seems like a reasonable width, this is what I used. The
> flange fails, it is only 50% of the required factored moment.
>
> Another question is do you know anything specific I should be aware of in
> concrete construction from the 1930s? I was assuming 3000psi concrete and
> fy=60000psi, but I am thinking at least the steel is probably less. It does
> not say on the drawings I have. The transverse reinforcement is called out
> as 1/4" diameter bars.
>
> The specifics if you want to know:
>
> T-beam
> web width= 5"
> depth total= 10"
> flange thickness= 2"
> flange width=10" from face of web, or 25" total (10" x 2 flanges + 5" web)
>
> Transverse/temperature reinforcement: 1/4" diameter bars @ 8" o.c. (in a 2"
> flange/deck)
>
>
> Thank you in advance for any suggestions/ advice.
>
> Andrew Kester, EI
> Longwood, FL
>
>
>
>
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