Rule of Thumb for Roof Top Unit Weights based on Heating/Cooling Tonnage? 2

[FoxSE14]

If a mechanical engineer says "oh, and we need your help to support three 10 ton RTUs", what number does your brain start to think of in regards to self weight of the RTU? (On a current project the mechanical equipment schedule shows that a 10 ton roof top unit has an operating weight of 1300 lbs).

This would be nice to know in the future when in those field conversations with mechanical engineers. I thought I came across a rule of thumb that: X-tons heating/cooling equates to a range of Y to Z lbs of operating weight. Does anyone know of any general rules of thumb on this?

Thanks

 

[jayrod12]

In my experience there's no such rule. I've had RTUs that the cut sheet during design was say 2000lbs, then during tender there's a request for equal that ended up weighing twice that. They can make parts out of aluminum instead of steel and save a ton of weight but at a significant cost, and vice versa.

 

[mtu1972]

Years ago, at second job with design-build industrial contractor, this was the prank sprung on new structural engineers. Well into the design phase, the ME group would tell the structural engineer that they had many 10 ton units on the roof. I had 2 years experience working with Architects, so I did not fall for it.

I did see all of the new grads, hired after me, return to their desks panic stricken as to how they could support that much weight on their roofs. The mechanical designers thought it was funny.

gjc

 

[FoxSE14]

Allow me to elaborate a bit: I've done quite a few structural retrofits for RTU changes on existing buildings. At a previous firm, someone had taken the time to compile a list of RTUs. This list reported the heating/cooling tonnages for each unit and their associated operating weight. They then came up with a 'very ballpark' number for the Approximate Operating Weight per Ton Heating/Cooling. This is what I am after. Unfortunately, I do not recall what that number was.

Jayro12 - point well taken regarding the aluminum vs. steel swapping to save weight.

 

[JLNJ]

I just ask for the plan dimensions. Rooftop air handling units are usually in the 50 to 60 psf range (but more for a chiller).

One thing to look for is the snow drift when the unit gets to be over 15' wide.

 

[WARose]

Looking back at some equipment cut sheets on some old jobs.....a sample of TRs vs. weights for some units:

3 ton- 600 lbs
5 ton- 700 lbs (skid size: 80"x45")
6 ton- 700 lbs
7.5 ton- 960 lbs
15 ton- 2300 lbs (skid size: 92"x 125")

On the ones where I give the skid size, if you work that out.....it comes to about 26-29 lbs/ft².

I don't know if all that gives you any rule of thumb. JLNJ's number (above) looks pretty good to me.

By the way, these units include the compressors which (I think) makes them heavier than chill water units (for big buildings).

 

[JohnnnyBoy]

I think its also worth noting that depending on the spec sheet it can be equivalent static load or just self-weight. If it is self-weight only a dynamic load factor needs to be used for a compressor type unit, which I believe is 1.25 but I don't have the building code in front of me.

 

[dik]

The bigger issue is the size and the potential for being an obstruction for snow load accumulation.

Dik

 

[FoxSE14]

Thanks everyone.

And, a quick aside: yes, in these conversations with Mech Engs, I always try to educate them on the 15-foot threshold for snow loads. They're not used to taking suggestions from SEs, however, when I inform them that a unit slightly bigger than 15' may result in an additional $5K, $10K, $30K, cost to the Building Owner to reinforce the structure because of drift requirements, some of them appear to take note.