How to consider Mech Unit weights on Roof (DL/LL)?

[Light1997]

On a Metal Deck roof top, I have Mech Units weighing 20000lbs, I have the cut sheet so I know the area as well. If I'm getting 80psf load for that area, do I consider it as Dead Load or Live Load (NOT Roof Live) on the roof? I think DL makes more sense but can anyone share their experience?

 

[bones206]

I consider that dead load since it is permanently installed to the structure.

 

[ProgrammingPE]

Mechanical units are dead load. They are "fixed service equipment".

In determining dead loads for purposes of design, the weight of fixed service equipment, such as plumbing stacks and risers, electrical feeders, and heating, ventilating, and air conditioning systems shall be included.

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[phamENG]

The weight of the unit itself is a dead load, but there is a live load component to consider for operation.

4.6.3 Machinery. 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. All percentages shall be increased where specified by the manufacturer.

On the surface this sounds like it should be DL=1.2*weight, but I disagree. This is the live load chapter. DL=1.0*weight; LL=0.2*weight.

 

[dik]

Thanks pham... didn't know that.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Light1997]

Wow! Thanks ProgrammingPE and pham for this info.

 

[straub46]

I agree with DL. I generally don't include the DL of the units in the uplift load cases though, in case new/future units are moved or are lighter.

Go Bucks!

 

[phamENG]

No problem. The commentary is largely silent on this section. To expand on it a little more: I've taken it to be a general consideration of impact from a seized bearing or similar mechanical failure that redirects an otherwise balanced and contained kinetic energy to a large applied force on the supports AND a general allowance for a machine being slightly out of balance or other minor and usually unexpected vibrations. This is NOT a replacement for serious vibration considerations if so mandated by a machine supplier.

I've heard some engineers talk about using the "shipping weight" as their DL and the difference between shipping and operating weights as a live load, but I disagree on that one. I think the operating weight should be the basis of the dead load since it's unlikely to go long periods without coolant or other fluids.

 

[dik]

I've used the dead load for decades, but have increased them, arbitrarily by 10% to 25% depending on how much coffee I've had that day... generally 10%... it's nice to have a prescribed solution. Again, thanks.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Agent666]

If it's small stuff which could easily be moved or replaced over it's lifetime, I usually just cover it with a general live load allowance that allows for maintenance type loading and the plant element itself.

For heavier stuff that is more permanent, I consider as dead load.

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[dik]

Biggest problem in these environs is the snow accumulation that sometimes accompanies the RTUs... in particular if you're adding an RTU to an existing roof. I really dislike doing whole roofs for a 'built up' future load...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik