HVAC equipment dead load

[CoreTech01]

Hello all,

Will looking at a set of prints, architectural, they called for a roof dead load of the HVAC equipment at 4 PSF.
Please correct me if I am wrong here but Pounds Per Square foot is calculated by weight / area in feet. So if the rooftop unit had a weight of 1000 lb and had a dimension of 8’x 5’ the PSF would be 25 PSF.

My question is “Did I calculate something wrong here?” By my calculations I would only be able to install something the size of a residential condenser on this roof.

This is an 8,000 square foot store that’s parts of a strip mall. What am I missing here?
There are currently no mechanical drawings.

Thanks

 

[JStephen]

This could be a uniform load over the whole roof for such equipment (it may be anticipating piping, duct work, and that kind of thing rather than a single big unit). There could also be a concentrated load allowance somewhere in there for the load case you describe. Might be worth a call to the engineer in question if it's not clear.

 

[JAE]

That's correct - the 4 psf is an overall weight that primarily affects the larger members that support large areas of roof.

For specific units that weigh higher, there should be some indication on the plans as to where these units can be located - usually with special, stronger framing to support them. If you don't see anything indicated for the heavier units (like what JStephen suggests) then call the engineer.

 

[csd72]

Best to use the rule of thumb, that anything heavier than a person needs to be specifically agreed with the structural engineer.

 

[Gilbertl]

For a typical retail or commercial bldg design, the assumed 4psf for MEP collateral loads is typical or std. procedure for design of the roof framing. It is because this load (4psf) is blanketed across the entire roof structure that it is actually smaller than the 25psf load that you came up with in your example.

When significantly larger unit weights come into play, such as those frequently encountered with industrial bldg. or industrial platform or mezzanine designs, then the method of collateral load determination must be looked at more closely, and will lean toward the more intuitive method that you used initially (i.e. 25psf).