live load reduction two way slabs

huangyhg

live load reduction / two way slabs
i have a long span pt concrete office building that was being designed with a shallow beam and one way slab system.  45' span beams and 28' conventionally reinforced one way slabs.  we are using an office load of 100 psf and due the large tributary area we used live load reduction to the beams on this 100 psf live load.
we have been asked to provide bay studies for a two way flat slab (with drop panels) to be compared and priced.  obviously to compare apples to apples i would need to use live load reduction for this two way slab as well.  i have never considered reducing live loads to two way slabs, the bays typically are not large enough, however i suppose that it is possible.  does the code specifically restrict this?  does anybody regularly design two way slabs by reducing the live load?  
thanks for any input.

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i don't know the us codes well enough to help you (and am presuming you are us codes from your units), however i would wonder about your vibration criteria from your description.  have you reviewed the proposed structural framing for walking excitation?
also, on topic, i have applied live load reduction factors (llrf)to two way systems because llrf are based on the satistical chances that the floor is fully loaded.  you must still apply them rationally, and it is entirely possible that not all of your   
i think i'd be tempted to use a single bay size for the tributary area for two-way slabs.

typically i stay away from using live load reduction when designing the slab because it may restrict the future use of this floor.  
that isn't to say that it can't be used for two-way slabs.  in fact, two way systems have more redudancy "to share the peaks of imposed load between the various elements" than a one-way system so if anything, you should probably be less inclined to use live load reduction for the one-way spanning system, particularly with such a large live load (i think 100 psf converts to around 4.5kpa).
using a conventially reinforced flat slab system, you may be required to reduce your column grid spacing to 22' (6.5m) to be economical.
to me the code seems a bit arbitrary when it allows no reduction for a 101 psf and greater ll but allows reduction for 100 psf and less.
in the end you use your judgement.
thanks for the input
i have a copy of asce 7 in front of me now.  it does define live load reduction for two ways slabs.  the kll value for two ways slabs is 1, and the value for interior beams is 2. so by code i can reduce the live laod, but it will be be reduced as much for the two way slab as it will for the slab and beam option.
mijowe,
make sure you let the formwork designer know that the slab can only carry a reduced load. you cannot use live load reduction for formwork design so they will have to work with the lesser capacity for the design of the backptopping.
if you work it out, you will probably find it is better to design for the full load than have more expensive formwork.
rapt, that's very interesting.  is that just your opinion or have you performed actual cost comparisons of structure vs. formwork with regard to live load reduction?
i used to work for a large structural firm that designed a lot of pt office buildings.  their standard was to always use live load reduction and additional backprops as required.  so i would be interested to see a cost study on this.
taro,
i have yet to meet a builder who, when given the option of adding 10% more reinforcement to a slab or backpropping 6 floors instead of 3, has gone for the extra backpropping. both for the cost of the backpropping and the interference with and delay of followup trades.
we also find these days that most clients with office buildings want the flexibility to place light weight compactus anywhere on the floor. this is not possible if live load reduction is allowed for in the design. for ease of letting of the building space, the more flexibility the better and that is where the money is. 10% more slab reinforcement is nothing compared to flexiblity of rental space.
we would only ever use live load reductions for columns and transfer memebers, never for floors.
thinking of llrf for columns, i have heard that a number of codes prohibit this, but do not know any specifically.  anyone aware of a code which does not permit live load reduction factors on columns?  in my opinion it would not make sense, as while some floors may experience full live loads at some point through their lives, rarely if ever would a column see all of the supported floors loaded at full live load.
it would seem to me that the reverse would make more sense, and coincidentally align with rapt's preference for "rentability".
any thoughts?
cheers,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
i am just finishing the design of a large two way flat slab, and i did not use any ibc live load reduction.  but i did use the aci live load reduction.  the aci allows the use of 3/4 ll when patterning live loads (alternate spans, adjacent spans).  you still use full ll when loading all spans.
daveatkins