Impact Load of RTUs

[a2fan]

I am designing a roof top structure to support mechanical equipment. Some questions I ran into:

Impact Loads per ASCE 7-10, 4.6.3 - For the purpose of design, the weight of machinery and moving loads shall be increased as follows to allow for impact: (1) light machinery, shaft- or motor-driven, 20 percent; and (2) reciprocating machinery or power-driven units, 50 percent.
-For instance, the RTU weighs 10000lb and it is motor driven.
--Will the addition be to live load or dead load? I believe LL as it is discussed under the Live Load chapter.
--Is the LL=1.2*10000 or 0.2*10000? I believe LL=1.2*10000lb
--What if there are vibration isolators installed; is there any impact load then? can the impact load be reduced?
--Also, are vibrations from a roof top mechanical unit considered under impact loads?

Thanks

AK

 

[ChorasDen]

I don't have an answer for you, but I'm curious if anyone else does, so I'll sub to this thread.

 

[WesternJeb]

I talked to a unit mfr. about this once and was told that their "maximum loads" provided on their cutsheets included this weight / maximum load from machinery.

I have also been taught to typically apply RTUs as a dead load.. They can't be readily moved and their weights are very well known and not subject to much fluctuation, so the load factors associated with increasing dead loads are more representative for the RTU's.

 

[canwesteng]

I'd generally only bump up the weight of the motor and fan 20%. And treat the whole thing after bumping load up as a dead load.

 

[ChorasDen]

I'd generally only bump up the weight of the motor and fan 20%. And treat the whole thing after bumping load up as a dead load.

This is how I design framing supporting mechanicals and RTU's at roof level as well, however, I'm not sure if that is necessarily the correct interpretation now after reading 4.6.3. The part most interesting to me is that this is the live load chapter of ASCE.

 

[phamENG]

I've always interpreted it to be the weight of the machine is the dead load, and 20% of the machine's weight is the live load. So D=1.0*weight, L=0.2*weight.