SMACNA Duct Weight Formula Issues

[casquatch]

Hello. I'm updating our take-off schedule for duct weight calculation but have run into a series of snags (I think they are all related).

In our schedule, we list out both round and rectangular ducts. But since rectangular and round ducts don't use the same sizing parameters, I have added all of them in for visibility. So we have Width, Height, and Diameter.

From there I apply a gauge size to them with a calculated value. This is handled by two different formulas currently, because I was not able to get anything to combine and work properly for both WxH & Diameter in the same formula. I tried, but it would either process the rectangular or the round ducts, NOT both (just depended on which was first in the IF statements). So here I am with two different calculated columns, one for each.

The formulas for the gauge sizes are (we only go down to 24 and up to 18, nothing should be 26 or 16 GA):

Gauge Size Rectangular = 
if(or(Width > 85", Height > 85"), 18, if(and(or(Width > 54", Height > 54"), or(Width < 85", Height < 85")), 20, if(and(or(Width > 30", Height > 30"), or(Width < 54", Height < 54")), 22, if(and(or(Width > 0", Height > 0"), or(Width < 30", Height < 30")), 24,0))))

Gauge Size Round = 
if(Diameter > 84", 18, if(and(Diameter > 54" , Diameter < 85"), 20, if(and(Diameter > 30" , Diameter < 55"), 22, if(and(Diameter > 0" , Diameter < 31"), 24, 0))))

Now we get to the pounds per square inch, where things fall apart (at this point). I'd like to combine both the round and rectangular here and apply the proper weight value based on the gauge of the ducts. I'm basing this off of the SMACNA rules (basically the same as this chart for galvanized). Formula is:

Pounds per sq ft = 
if(Gauge Size Rectangular = 24, 1.156 psf, if(Gauge Size Round = 24, 1.156 psf, if(Gauge Size Rectangular = 22, 1.406 psf, if(Gauge Size Round = 22, 1.406 psf, if(Gauge Size Rectangular = 20, 1.656 psf, if(Gauge Size Round = 20, 1.656 psf, if(Gauge Size Rectangular = 18, 2.156 psf, if(Gauge Size Round = 18, 2.156 psf, 0 psf))))))))

NOTE, I have tried this formula MANY different ways, here are a couple...

I've reversed the order of sizes:

Pounds per sq ft = 
if(Gauge Size Round = 18, 2.156 psf, if(Gauge Size Rectangular = 18, 2.156 psf, if(Gauge Size Round = 20, 1.656 psf, if(Gauge Size Rectangular = 20, 1.656 psf, if(Gauge Size Round = 22, 1.406 psf, if(Gauge Size Rectangular = 22, 1.406 psf, if(Gauge Size Round = 24, 1.156 psf, if(Gauge Size Rectangular = 24, 1.156 psf, 0))))))))

I've setup OR statements for both round and rectangular at the same time (and reversed the orders as well):

Pounds per sq ft = 
if(or(Gauge Size Rectangular = 24, Gauge Size Round = 24), 1.156 psf, if(or(Gauge Size Rectangular = 22, Gauge Size Round = 22), 1.406 psf, if(or(Gauge Size Rectangular = 20, Gauge Size Round = 20), 1.656 psf, if(or(Gauge Size Rectangular = 18, Gauge Size Round = 18), 2.156 psf, 100 psf))))

But nothing seems to want to work. All that happens is that whatever is first in the pounds per sq ft formula that is what happens and then the rest just result in a null value. Not a 0, just nothing. So I can either get a 1.156 psf applied, or the other end of the spectrum with a 2.156. And it only applies to the proper GA ducts, then it stops.

I believe this has everything to do with the two different shapes and trying to pull those together in the end, but I have no real way of knowing. The odd thing is is that the PSF formula isn't using those dimensions at all, it's using the Gauge Size results, which are both Numbers (I've tried integers too, doesn't matter). I mean going all the way back, the gauge sizes are using the duct dimensions, so it could be the problem. 

Should I just give up and make two different schedules? One for round and one for rectangular. Or is there something I'm missing? Anyone else run into this, thoughts/questions?

[HVAC-Novice]

What is the objective? We can give better answers when we know what it is you want to accomplish. 

If it is for structural reasons, then you are ignoring the re-enforcements, seams, hangers and insulation.

[casquatch]

The objective is to have a single working schedule for both round and rectangular duct that will give us an end total weight. The weight is based on the pounds per square foot multiplied by the total area of that duct. This is just for duct and just for quick basic sheet metal estimation.

The issue is that I can't seem to get both round and rectangular duct to give me a proper PSF value...in the same schedule that is.

I'm thinking I may just have to split them up. I just went through that process and they work fine individually, just not combined. But would be ideal to have them both together.

[HVAC-Novice]

You seem to be determine the gauge based on duct dimension. But the gauge actually can be different based on pressure class, and type of reinforcement and spacing of the latter. For example a 10" duct will have different gauge size depending on if it is 1" or 10" pressure. It also will differ based on if the reinforcements are spaced every foot, or every 3 ft and so on. Then there are stainless steel, and aluminum ducts that change everything. 

As a designer I determine the size, and specify to be built based on SMACNA standards and what pressure class. The contractor then can decide to use thin gauge, and many reinforcements, or thicker gauge, and fewer reinforcements. I don't have knowledge which is more economical, that is up to contractor to chose from the options SMACNA allows. 

I realize this doesn't answer your actual question, just an observation. I guess what I meant by objective, is what will you do with the weight? Are you a designer, or a contractor? What is that schedule supposed to accomplish? 

I'm not sure it would help, but a key-schedule with the area weight based on size may help. But I don't know off-hand how to use that in an automated way. Then you can just edit the key schedule as needed. Maybe a parameter could be added to ducts to add a thickness. If you add a thickness parameter to the schedule you could calculate volumes and weight. Again, I'm not sure it is possible, but if it was, it would be much less complex. 

[casquatch · Accepted solution]

We are using the calcs for the design side. Which is why we don't really need a ton of accuracy. Sounds like most of our work is lower pressure as well, so there is even less of a need for exact values.

For now I've just split up the schedules into round and rectangular. Not great, but it works. My other option was to just have multiple columns for the items that won't calculate together. Then I'd at least be able to have them in one schedule, just not combined together in a single column...too bad you can't use the combine columns tool with a calculated value, that would at least make it appear to be correct and in one column.