generator on one way slab

huangyhg

generator on one way slab
i have an existing one way slab concrete with 7x23" joists at 2'-3 o.c. with 3" slab.  the client wants to install 2500 lb generator.  i have done the analysis and it looks like the joist will be able to handle the load (not even close).  now, how would you mount this generator?  i dont know how many bolts i need yet because i have not talked to the manuf.  but i just want to know general idea what you usualy do on this situation.  would it be ok to put mount something on the 3" slab?  or do i need to tell them to only put the bolts on the top of the joist? (2'-3" apart).  since the generator is about 105" long, that means i can put 3 bolts each side.   i am just worried that the skid plate already mounting holes so that we cant dictate where to put the generator.   
not almost anymore!  
have you increased the load to allow for dynamic effects?
have you checked punching shear under the support points?
if the slab can take the load then i would tend to bolt through the slab, with a backing plate and then chemical anchor around the bolt in the slab. this gives good uplift resistance for any shaking effect.
what you can do if you have extra capacity is to use a 4 inch or thicker housekeeping pad of concrete, reinforced to help bridge across the joists and avoid localized overstresses on the thin 3" slab.  
then you can bolt through the pad, between the joists, and through the 3" slab to attach the unit.
be sure to include in your analysis some live load around the unit in addition to its weight.
oh, i wouldn't bolt down into the joists themselves for fear of damaging the top joist reinforcing bars.
i would suggest that you mount the generator on vibration isolators (springs) to keep the floor from vibrating. i recommend that you contact the following vibration isolator manufacturers:
the vmc group
mason industries
kinetics
you need to ask the manufacturer to submit the layout of the skid including dimension of the mounting bolts. you also need them to supply loading information on both static and dynamic. you are likely to face some problem in mounting if the slab is 3" thick only, even it is composite (with steel deck), since the reinforcing would be very minimal.
jae, good idea!!!  how would you pursue this?  how do i add 4" of concrete on the top of already poured concrete?  how do i create bond to the slab?  your wise words are always appreciated.
not almost anymore!  
the generator has a built in vibration damper.  i think jae's idea will solve the problem.   
not almost anymore!  
we don't normally try to bond a housekeeping pad to the underlying concrete - (the unit might need to move or be replaced someday).  if there is vibration, your through-bolts should be adequate to prevent walk-off.
kslee1000's suggestion to get the loading/skid info is good.
for further vibration control, an elastomeric mat between slab and housekeeping pad could also be considered.
you could mount the unit on a steel curb that spans over the joists.  through bolt the curb to the slab to hold it in place laterally (or use simpson titen hd botls), the curb would carry the vertical load over the slab and not load it vertically.  then the top flange of the curb can be drilled in the specific locations required by the skid plate to mount the unit.
jae, so just put the concrete form on the top of slab and pour it?  the normal force from the equipment will create enough friction force to avoid sliding?  is there someting i need to know when i specify this housekeeping pad?  the only think i can think of right now is to make sure i have enough edge distance for the mechanical anchor.  how about reinforcing wise?  just do minimum reinforcement for thermal?  sorry, i am a little green.
not almost anymore!