flat slabs limited edge column moment transfer

huangyhg

flat slabs limited "edge column" moment transfer
hi!
i have a question on this subject.
when calulating this problem by hand, you divide flat slab into 2 strips, one is the column strip and the second is the middle strip.
when calculating negative moments at the "edge column" we had a limitating moment trasfer to column that was expressed as mt=0,18*b*d^2*fck. this expression limited the slab section of becoming over-reinforced.
now, if i calculated this using quivalent frame method, i had an option of reducing negative edge moments by not more then 50%, if calculated moments where higher then this, but then i had to increase the postivive momets in the span by the same precentage..
im now using structural analysis program that uses fem.
my question is, should i also limit the values of momet transfer to edge column, and also reduce -moment/increse +momets if necesary or not?
thank you
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what were you using to get that equation (.18bd^2)*(fck)?
i've never seen that before.  
aci's approximate method works well by hand.
that the expresion dictated by eurocode.
do you have something familiar in aci?
if that is giving you so high of a moment that your seections is overreinforced for a slab i think you may be calcing something wrong.  the moment at the end bay of a flat slab should be low (certainly lower than at the first interior column).  if you are getting an over reinforced section at the exterior column and not at the interior columns, i would look at your analysis again.
is you slab thickness constant?
yes, the thickness is constant.
but you got me wrong.
does your aci code limits the value of the moment that can be transfered from slab to column?
it depends a lot on the geometry, but there is no way that the exterior column would ever see more negative moment than an interior column struip.  it may be posible with some really funky geometry, but then you would'nt use the approximate method, you would need to use something else.
can you correct the issue by increasing the slab by 1/2" or maybe even an inch?
yes.
although depending on your shear, you could theoretically transfer all moment into a column.
i think that mt is the limiting expression of how much moment can be transfered to the columns due to punching shear restrictions.
if this is the case, then to increase moment transfer you need to provide spandrel beams along the edge to transfer moment to the column via torsion.
check out:
csd72 , egsaclty!
basicly, what this expresion is doing is limiting your neutral axis depth to 0,5d insuring that the stell in your  section (slab) would yiled.
if this is not satisfied, your slab would probably fail due punching mechanism.
but my question is, should i check what is the value of mt, after my software provides me with the results of bendnig moment isocurves, to insure that the punching shear restrictions are satisfied???
eg. look at the picture,
i got some strange slab moment results at edge columns
i modeled a simple structure, flats slabs thicknes 20 cm, rectangular column array 6,00 m spans, column 40/40 cm and storey hight 3,00 m.
loads where: self-weight and 10kn/m^2 on slab deck.
heres the structure: