HVAC equipment roof loads

[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

 

[quark]

You did the correct calculation but, designing your entire slab for the load of a particular equipment which occupies only portion of the slab may cost you a fortune.

Suppose, your total slab area is 400 sq.ft out of which only 40 sq.ft is used to place some equipment, you are unnecessarily designing 360 sq.ft area for more than 600%. But when your slab is designed for lower loads and when you have to place higher loads (if this is indicated during desing stages) the structural engineer will do a trick or two to span the load for wider areas to have the strength with in the limit.

Having said all these, you will get exact idea if you drop your question at related forum507. This is an extremely good and dynamic forum.