Roof Top HVAC Equipment 1

[paco131]

I am unclear on a note on structural drawings. I have to place two HVAC units on the roof and the drawings state the following- The roof framing design includes one or sum of roof supported mechanical unit(s) weighing no more than 1,500 lbs. supported by at least two joists per bay. (adjacent bay loading pattern is not considered) Heavier unit(s) require additional structural analysis. What does the adjacent bay loading pattern statement mean (looking for a simple opinion). I take it the unit has to be supported by the joists correct? See pic attached, but what is the meaning of the adjacent bay loading statement?

Thank you for your opinion!

 

[Ron]

To me it means that loading one bay has no influence or carry-over to another bay. Based on this, you could do the same loading in each bay of the building.

 

[msquared48]

Well, the joists are supported by beams that are supported by columns. The column to column spacing is considered a bay. I would assume the beams have been checked for one unit up to 1500 pounds, but not two units.

Mike McCann, PE, SE (WA)

 

[paco131]

Thank you Ron. Now if the HVAC unit curb is not large enough to span the distance between joists (5 foot) I wonder if the equipment curb can just be placed on the metal deck. It states that the girder panel point load includes 1,500 roof top unit load...

 

[Ron]

I would not place a prefabricated curb on an unsupported metal deck. This will affect the roof flashing and contribute to leaks. While it might be ok structurally, it is not a good practice. Further, there is always some vibration of HVAC equipment when running, so mounting on an unsupported deck will increase vibration in the deck and contribute to interior noise and roofing issues.

If the curb won't span the distance, provide supplementary structural reinforcement under the metal deck to transfer the load to the joists.

 

[paco131]

Ron- thank you, great input and is much appreciated!

 

[jimstructures]

I would take a more limiting interpretation. I would say the statement means that no consideration for adjacent loading has been checked. The designer has looked at one unit in one bay and nothing else. The OP does not say whether the roof structurals are simple span (i.e. Bar Joists or Beams) or continuous span (i.e. cold-formed "Z" purlins). The designer must say whether it is allowable to have Two RTU's less than 1/2 bay length apart on either side of a beam so the beam sees the entire load from more than one Unit or some fraction there of. I just feel more needs to be specified or a clearer statement of what is allowed.

Jim

 

[Ron]

I've read the OP several times and, Mike and Jim, you might be right! I took it to mean that any bay could receive the loading, which I think is correct, but all bays receiving the loading at the same time might not be covered as you've noted. That; however, only requires analysis of the beam and perhaps the columns, not the joists. Beams could take 2 loadings, columns up to 4. Loads are relatively low, so hopefully, there's enough capacity to handle.

 

[paco131]

Sounds good thank you. The landlord has a Structural Engineer so I will have to cover the exact placement with notes and have the Structural Engineer review before any penetrations are cut on the roof. Thank you so much for your advice.

 

[jimstructures]

Ron,
I agree that the note writer meant any bay could take one RTU, but the note writer must specify how close together the RTU's can be located. We often say each bay of a craneway can support One (1) design crane load but we also add that the cranes must be installed with a stand-off or a distance limiter must be installed that prevents the individual crane loads from being closer together than one bay lengths distance. In theses cases we say the craneway must be simple span crane beams not continuous 2 bay or more crane beams.

Jim

 

[hokie66]

If this is a newly constructed building, ask the design engineer for clarification. If it is an old building for which you have the structural drawings, you still may be able to ask the engineering firm which designed it. If the firm no longer exists, I see your problem, but as the landlord has a Structural Engineer, he can make the call.

 

[Triangled]

Gotta luv construction speak: by others where occurs typ unto. This note is poorly written precisely because it allows multiple interpretations and ultimately each interpreter can justify their different interpretation while the poor building is built incorrectly.

I agree that you definitely need to go to the engineer of record for clarification in writing!

I think, but, hey, it's just my interpretation, and I am likely as wrong as anyone else,
I think a pair of joists was designed to support 1500 lbs between them, whether that 1500 lbs is composed of a single 1500 unit or perhaps (3) 500 lbs units, but a total of 1500 lbs max.
I think by "bay" he means joist bay and not girder or column bay... (but, hey there's definitely problems with my interpretation too), and that the note is saying an additional unit cannot be placed in the adjacent joist bay such that the middle joist of the two joist bays is overloaded.

 

[MikeHalloran]

Here in SoFla, I'd recommend some additional structure to support a sturdy cage over each of the a/c units, to deter the copper thieves.

One employer of mine lost 22 units before finding a cage that was sturdy enough.



Mike Halloran
Pembroke Pines, FL, USA

 

[Ron]

Hey MikeH....tell him to move the business to Broward. Won't stop the thieves but at least they'll be driving BMW's and it will look nicer on the security video.

 

[paco131]

Nice- you wouldn't happen to know if it was a pre-manufactured cage? We have some areas here in San Antonio which could use some sturdy caging as well.

 

[MikeHalloran]

I don't know if our cages were special or standard, but there are a bunch of offerings now:

https://search.yahoo.com/yhs/search...ft+cage&ei=UTF-8&hspart=mozilla&hsimp=yhs-004

Mike Halloran
Pembroke Pines, FL, USA

 

[msquared48]

Need to put the copper in A/C cage traps and catch the b%_€¥¥¥%!

Mike McCann, PE, SE (WA)

 

[MikeHalloran]

We seriously considered switching to an acutely poisonous refrigerant, but that meant our crew would need to suit up and clean up the mess before repairing and recharging.



Mike Halloran
Pembroke Pines, FL, USA