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  1. For stiffness modifiers to slabs, If your framing is such that you don't have Stiff Beams (beams normally > 3*Depth of slab), you will see a lot of difference when you apply and don't apply stiffness modifiers. Consider flat plates, where you have thick slabs on periphery beam. Now when you don't apply stiffness modifiers to such a case, you get less moment in you beam as more load is transferred by the slab to the column, and slab is considerably thick and has a reasonable stiffness relative to the beam which is supporting it. For the same case when you apply a modifier, the slab stiffness is reduced and more load in transferred to your beams which are now stiffer than your slab because of the modifier. Why should we use a stiffness modifier? for the above example, a flat plate would have some sort of cracking when subject to seismic loading and because of that the stiffness would be reduced, as a result the beams would be carrying higher load than they were actually supposed to take(considering if the original design was based on no stiffness modification for slab). Thus by using modifiers for this case, a structural engineer is able to consider the effect of heavy loaded beams and designs beams to a greater load. Therefore, using stiffness modifiers where there is a reason can help you design robust structures. What I am trying to say is that for slabs like 4", 5" the difference in moments is very small for the cases with or without modifiers if the beams are deep. Don't worry too much about using modifiers unless you have a condition that demands one. Just, plug your numbers in and do your analysis as for most of the general cases, the difference due to modifiers would be close to nothing as explained above. Every thread I go, people are confused about them. Dont worry that much, just use the default numbers and be cautious of situations like the one explained above where you really need them. @Rana, why did you say that using stiffness modifiers for slab would result in higher sway? Isnt the lateral distribution (for this case) on rigid diaphragm assumption would result into same story force no matter how thick or thin the slab is, which will determine the story sway(considering frame stiffness as constant)..
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  2. update: 19.02.2014 Read about equilibrium nd compatibility torsion. You have to determine which case you have, if it is equilibrium torsion you cannot releaee torsion so your modifier will be 1.0 like in a cantilever slab or hollow core slab supporting beams because if you will release torsion it will redistribute to slab and as in equilibrium torsion there is no redistribution possible so you cant release torsion. But in case of compatibility torsion you can release torsion and it will redistribute. Si you can make its modifier equal to 0.35 but uf you make it 0.01 make sure there is no excess cracking in slab and you design slab for increased moments...thats why code does not explicitly tells u abt torsion modifier because it depends on case to case. Value of equilibrium torsion is not affected by the modifier so always apply 0.1 or 0.35 or whatever for torsion modifier whatever the case is.
    1 point
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