contractor calls about construction loading

huangyhg

contractor calls about construction loading
"we just want to verify we can use a scissor lift on your slab"
how do you usually handle it when contractors call to make sure they can use a lift / forklift / other machine on your slab, either on grade or suspended?
we have basically said yes in an extremely roundabout, no liability way, assuming it seems reasonable, but i'm curious as to how others handle the calls.
check out our whitepaper library.
i've had this happen before.  my first reaction is to state that the floors were not designed with wheel loads in mind, but that if they provide me with spec sheets on the lift, including wheel load and wheel footprint data, i could check it...for a fee.  usually the fee is simply a couple of hours of work, but i insist that if i'm giving them an answer, and being held liable, i want to get paid for it.
in most floors, the live load value is at least 40 psf if not higher, and with a parking garage live load requirement of 50 psf, this usually doesn't pose a concern relative to overall capacity.  but does present a wheel load issue that should be check in terms of slab capacity and punching shear.
thanks jae.  i always check the slab shear capacity, including punching shear, before giving them the roundabout answer.
this brings up another question though.  when designing floor slabs, why aren't they designed for the point wheel loads from scissor lifts?  i don't think i've seen a commercial construction job that didn't use a scissor lift on each floor.
means and methods are contractor's responsibility and they should retain an engineer to check this sort of thing.  jae is right about your right to a fee if you end up with the task.  personally i'd charge more than a few hours as it takes longer to obtain all the information you need than to actually do the work.
when checking construction loads such as scissor lifts, the point loads are usually not in addition to the full design live loads.  i check the worst case point load in the midspan of each element without the full uniform live load (use 10 psf? or whatever you feel comfortable with)
how far below is it shored?
or is it even shored?  
whyun, i knew someone would call me on my "couple of hours".
you are right that there's more time involved usually.  also, we as engineers should get paid for the value of the services we provide, not the time it takes to perform them.
it seems to me, that the prudent thing to do, if you know a wheel load will be applied, is to specify maximum wheel load in the design.
there seems to be an assumption that the contractor that calls you is shirking his duty and the ones that don't are hiring their own consultants to re-engineer your building for that specific load.  you have to consider the distinct possibility that the one calling you is the conscientious one and that no wheel-load design gets done in the other cases.  of course, if they've built enough slabs of similar thickness with similar equipment on top, they probably have a fair idea that it'll work without actually designing anything.
i don't have my ibc code handy but i thought there is a minimum concentrated load specified for commercial buildings. isn't it 1500#?
the contractor has the option of using the eor or someone else to calculate those capacities but should expect to have to pay for that additional service.
i agree that jae's proposal is an excellent way to get the answer, but some owners may have a problem with "their" engineer being paid by both the owner and the contractor. makes the engineer appear to have a conflict of interest. i realize that this is really a "technicality", but there could be contract language that requires the contractor to retain his own, third party engineer to determine the answer.
scissor lifts are typically small and hold around 3 or 4 people max with little space to move around.  they are common in construction sites even on concrete filled decks.
fork lifts to transport construction material may impose more critical loading.
it is up to the contractor to decide whether they want to pay a nominal design fee or take a risk.
per 2000 ibc, concentrated design live load for office floors is 2000#.  heavy manufacturing is 3000# and vehicular driveways subject to trucking is 8000#.