moments redistribution

huangyhg

moments redistribution
is possible to redistribute negative moments at the end of supports in a beam-frame structure?
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what type of structure do you have? the answer is different depending on whether it's steel, concrete, or wood.
14159
i'm sorry is a post-tensioned concrete structure
you should be able to reduce the end -ve moment; it will increase the end span +ve moment a tad so that if you don't develop the -ve moment (in real life), there will be less cracking in the bottom beam web (likely not the same issue with posttensioned stuff).  also less of a moment into the supporting column (smaller size and less rebar).
for parking garages, i've used a 20psf live load for determining col and endspan bm moments, then used regular 50 psf loading for the balance of the frames.
dik
also see aci 318-02 sections 8.4 & 18.10.4(if prestressed..i don't know if this would apply to pt)
just to throw in a little caution, years ago, some inexperienced engineer did a c.i.p concrete parking structure. he neglected to put in sufficient negative steel, and the structure was very long, without expansion joints.
a lot of cracking resulted - no fun at all.
sacrebleu... as long as it's tension, it would help reduce the negative moments <g>...
dik
dik,
how do you figure that? we figured the upper side of the beam was in tension near the supports, and the concrete shrinkage during curing aggravated it.
one major problem with all of this is if punching shear is a design criteria. the punching shear check must be done for the moment/shear that the column/slab joint can attract. punching shear is a brittle /non-ductile failure mode and arbitrary redistribution is not allowed for it. as the slab at the face of the end columns is not normally cracked under loading, the only redistribution possible would be due to cracking of the column. this is possible for very low rise buildings but for medium and high rise buildings the columns will be uncracked under vertical loading due to the large axial compressions involved and redistribution should not be done.
one computer program widely used for pt design immediately reduces the moment difference either side of columns to zero when redistribution is asked for. this is completely illogical when you think about punching shear design. redistribution should require that the moment attracted to a column remain the same no matter how much redistribution is allowed for either side of the column.
other things that need to be checked when redistribution is allowd for are increased ductility limits for strength design and also the effects on crack control. the deemed to comply crack control rules in aci cannot be used in this case as they underestimate the stress in the reinforcement in areas where moments have been distributed away from. add sacreblue's restraint stresses to this and you have real problems unless all of this is allowed for properly in design (and it is not by most designers, often due to a lack of understanding of the basis of code clauses).
sacrebleu... was thinking of an axial tensile load in the slab, for humour... hence the <g>... big grin!
dik