Mechanical Equipment area on roof assumed dead load.

[diegotorres2020]

I am working on a multi-family residential project were it is still too soon to determine the HVAC equipment, and I am trying to get calcs out for the dead load at the roof and start sizing some wood members. Most of my experience comes from the commercial side, were we also deal with this and we usually assume an equipment area and use 50psf dead load over the roof dead load itself to cover us for all possible equipment and begin sizing members. what would your recommendations be to make a safe assumption about the dead load from equipment? is 50psf overkilled for residential?

 

[jayrod12]

What type of multi-tenant residential? Townhouses? 3-4 storey? Apartment?

That will play into the answers you get.

Where I practice, an additional 50 psf wouldn't even account for the snow build-up around the units. Due to the level of build-up where I am, we just apply the 84 psf snow load everywhere (36 is the base value and 84 is the value directly adjacent an obstruction) and that usually provides enough structure around the area that I can deal with the unit weights once they come in. Generally the unit itself is around 40 psf, and the snow on top of the unit would be 36. Total, that's less than the 84 I've accounted for everywhere.

 

[diegotorres2020]

It' s a 5 story apartment structure, located out in Colorado. so we do have the same snow conditions as the ones you mentioned.

 

[r13]

Isn't the maximum load on the purposed equipment area equal to 50 (equip DL) + 36 (typical roof snow SN) = 86 psf(DL+SN)? Or 50+84=134 (not likely)?

 

[jayrod12]

So I'd be looking at it something like that. In the entire area that could be used for mechanical on the roof, I'd be considering designing everything for the maximum build-up value around the units.

Then, once the actual unit sizing and locations come in, you can go back and finesse the numbers. Usually my method results in some reductions in size, which are always more welcome than increases. And in the event some things increase, you can usually offset it with reductions elsewhere in the area so it comes out net neutral. No one likes seeing structure increasing in size late in the project. But they love when you can reduce it.

 

[diegotorres2020]

@Jayrod12 Thanks so much, I'll get that going on my end and see where it gets me. I agree, it's always best to reduce member size later down the road, than actually increase.

 

[jayrod12]

And if they complain about the member sizing after your preliminary design, tell them to get you the final mechanical unit layout and weights and you'll see where you can reduce things. Puts the ball back in their court and usually starts the client hounding the mechanical consultant so you don't have to.

 

[dik]

I usually treat mechanical equipment as dead load unless there is significant vibration.

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

-Dik

 

[jayrod12]

In my final calculations, I consider mechanical load dead as well.

I was just pointing out my own method for doing preliminary design of a rooftop mechanical area when the information isn't available yet. At that design stage, the key is just making sure you account for enough load. And always usually better to overestimate live loads for deflection calcs. With the only obvious difference being long term deflection and creep where overestimating the dead load my be more conservative.