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Concrete T beams/ ribbed slabs

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The concrete is probably 2,000 psi or 2,500 psi at the most unless this was a 
monumental structure requiring going to, !!!3,000 psi!!!  Do the plans call 
for a "class" of concrete?  If so, this should give you the 28-day 
compressive strength.  If the concrete was made using Florida beach sand, be 
ultra conservative.  (I think I may have an old Urquhart & O'Rourke concrete 
text around somewhere and I will try to find it.)

The steel was probably 33,000 psi yield or 30,000 psi yield with an 
allowable stress of 16,000 to 18,000 psi.  Are you sure that they were 
*round* bars?  1/4" round bars were not deformed, IIRC.

You *are* using working stress design to check it, aren't you?


A. Roger Turk, P.E.(Structural)
Tucson, Arizona

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:

. >   - 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

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