hollow core under uplift 9wind loads0

huangyhg

hollow core under uplift (wind loads)
good morning,
i'm looking at a concrete pump station in south florida. the wind pressures are reaching 83 psf (gross uplift) on the roof. i plan on using a precast concrete hollow core deck that will span 28 feet.
anyone have a feel for when it's a good idea to just use a heavy deck, rather than top slab resteel? an 8" hollow core slab will span 28 feet, but only weights 62 psf. we are the design engineers for the building, not the actual deck. i just want to make sure we show something practical.
hollow core slabs are usually not symmetric in the vertical location of the prestress cables, the vertical load capability is different in the downward vs upwards loading.
in using a topping, the additional concrete increases the strength for gravity loads (but only increases the weight for resisting uplift).  the difference between the weight and the uplift is all the slab must resist in the uplift wind loading, (assuming the uplift is greater than the constructed weight), and the inverted strength of the hollowcore is usually able to resist this small force.  if the 62 psf weight is subtracted form the 83 psf uplift, then the net uplift is 21 psf. a 2" topping makes this net uplift go to nothing which eliminates the analysis of connections at the edges and load path to the footing.
can you use an angle to secure the hc with bolts between the planks?
dik
are you allowed to just take 83-62 = 21?
don't you have to use 0.6d + w?
why dont you put l bars from the wall into the cores. most manufacturers will allow for the top of 1 or 2 cores to be removed at the ends for grout placement.
the following website is australian, but it gives great fixing details.
if you need any of these details to make it work then tell the hollowcore manufacturer that is required. it is the responsibility of the primary engineer to ensure that the interfaces are taken care of.
you should also specify to the hollowcore designer that their slabs be designed to resist the required uplift loads.
as they are symetric, there is no reason why this cant be done.
if you go along the route of adding extra concrete to the top, re  
quote:
the difference between the weight and the uplift is all the slab must resist in the uplift wind loading
i would agree with jike - you must use 0.6d + w as the controlling combo for this uplift.
i would also add that 83 psf for component wind seems pretty high.  for 120 mph wind, exp c, i get c&c roof uplift in the mid to upper 30's.  just checking....
jae - south florida, if you're in the high velocity hurricane zone, is 146 mph exp c.  south florida has it's own section of each fbc chapter, termed "high velocity hurricane zone".  with that velocity and exposure you can get over 130 psf for your c&c negative roof pressures in some cases.  even 130mph can get you over 100 psf.
i agree also about using 0.6d+w, not just d+w.
thanks for all the great information.  our south florida client factors up the 3 second wind gust.  we are designing for roughly 158 mph due to client provisions.
my big concern is about the amount of reinforcement that is required in the top of the deck to resist the suction.  we can either use more steel in the deck, or a heavy deck.  i'm just not sure what is cheaper.
the topping mentioned brings up a good point.  at 83 psf uplift, can the cold joint between the deck and the topping hold?
as i understand the code, the 0.6d is 60% of the likely minimum dead load at the time of the wind event.  typically i add in some dl for mechanical and electrical.  this is conservative for gravity design, but would not be conservative for wind uplift design.  i typically have two dead load numbers based on upward or downward loads.   
l shaped bars in the topping and at the grout keys between the hollow core slabs and into the supporting walls allow you to include wall dead weight and doweled footing in the d total. the cold joint between the top of slab and topping is keyed into the edge grooves and if nominally clean is in the order of vacuum strength for 3000 psi f'c (14.7 psi).