Units for building component types

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1

Hi

I am trying to understand the logic behind the units for building component types in RTLCA. I’ve tried rewatching all the tutorial videos but they do not touch this subject.

I just created a new floor from scratch of 134m2. The floor should include concrete and a reinforcing mesh. I manually added to new materials:

  • 134m2 concrete
  • 134m2 Reinforcing steel

I then found materials from the library that match what I needed.

For the concrete, the material in the library is declared in m3. Does that mean that I have to enter m3/m2 (meaning the thickness of the slab), or am I asked to enter the m3? Entering the thickness would definitely make most sense.

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The same is the case for the reinforcing mesh. I’m I being prompted to enter kg og kg/m2. If the answer is kg, what influence does the m2 I’ve already made have on the results?
The construction engineers usually just declares kg/m2 to us, so that would be the most logic unit in this case.

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2

Hi Mark

The EPD’s decide which units you can use.

In this specific case I would recomend to build your own construction with a declared unit of m2 with the exact concrete and how many m3 per m2 and amount of steel in kg.
Otherwise you have to manually enter the correct amount of m3 or kg in the whole project.

Reach out again if I can be more specific.

Best regards,
Mads

3

I understand that the EPD decides the unit, but I still don’t get what to input in the example above as there are two units in one material - respectively m2 and m3.

In LCAbyg, the structure is quite clear. You declare the unit for the construction and then the unit of the materials is defined by both the unit of the construction and the unit of the EPD.

In the example below, the unit of the construction is m2. The unit of the EPD for concrete is m3 and for the reinforcing mesh it is kg, so the units for the construction becomes:

  • Concrete: m3/m2 (which corresponds to the thickness of the material)
  • Reinforcing mesh: kg/m2

The hieraki is quite clear in LCAbyg, while having two units for one material in RTLCA still confusses me. But it might just be a matter of me still having a “LCAbyg-mindset”, so to say :wink:

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Could you maybe answer, what I should enter in the to examples from the first message in this tread, if I want a 120mm concrete slab with 3,6 kg/m2 reinforcing mesh?

4

For homogenous material layers, I would do exactly what you’re doing in LCAByg… You’ve set the construction unit in RTLCA to m2… Material unit should correlate to this unit, hence if the EPD is in m3 you need to use that unit for material quantity, just like you do in LCAByg by using m3/m2…

For inhomogenous material layers, like the one you have set up with concrete and reinforcement mesh you have to create a construction with a inhomogenous layer… here you can specify construction unit as in LCAByg and then every material unit corresponding to that construction unit. i.e m3/m2 or kg/m2… as illustrated below… hopefully…

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above is the ‘Manage Units’ view for the construction I was creating below;

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6

Thanks for chipping in Mirnes.

So if I understand correctly, I should enter 0,12 in the marked field below if I want to have a 134 m2 with a thickness of 0,12m, right?

From looking at the material, I would have thought that I would have to calculate the volume manually (134 m2 * 0,12m = 16,08 m3) to get the propper thickness.

Of cause you are right though, that it would make more sense in this case to treat an inhomogeneous layer with both steal and concrete in this case. I actually didn’t think of that, but the question above is still relevant.

7

No! - not 0,12m… In this case where you have manually created a construction and not imported from Revit, RTLCA does not know the mass of the concrete slab, you have only supplied info on how large it is in m2… You’ll have to manually enter the mass here (i.e. 16.03 m3)… either that or create a construction, like the one I suggested above that tells RTLCA how much m3 pr. m2 you have. With that construction created you are then able to apply it on your concrete slab object as it contains square meter data.

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8

Okay, I can now see the difference between imported and manually created constructions. The imported constructions already have area and volume reported through Revit, whereas this isn’t the case for the manually created constructions.

That being said, it would still be very useful to have the thickness informed for materials that area “cubic” like gypsum, insulation, etc. Thickness is a normal parameter in Revit, so that should be possible, right? That way, I know that an insulation batt of 308,12m2 (see the example below) and 7,09m3 is actually 43mm thick. This is very important information when QC’ing the imported data.

So in my minds eye, there should be an extra column between Preferred and Unit, that says Area, Length, Count, Volume and (as a new feature) Thickness.

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Of cause, you won’t be able to report a thickness for non-rectangular objects like steel beams etc., like it isn’t possible to add the length of this insulation batt in this example.

Inhomogeneous layers
The other option you mention is to use constructions instead of materials from the library, as the sublayers have the unit m3/m2 or kg/m2. That’s more intuitive in my head.
However, when creating a custom construction, you are asked to specify the thickness of the layer. So if I have a project with a timber frame, like the one a have modelled below, in different thickness - lets say a thickness of 70mm, 100mm and 250mm respectively - then I wouldn’t be able to use this material, which is modelled with a thickness of 1000mm. I would need to model three timber frames as separate custom materials with the correct thicknesses, right?

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9

Update
I talked to Mads from the RTLCA team the other day and got the answer to this question. Thought I would share it here, in case someone else has the same question.

When working with constructions, where the inhomogeneous layer is squared, like the example below, the thickness of the layer doesn’t matter. What matters is the percentages of the two layers.

However, if I want to model a non-square inhomogeneous layer, such as a C-profile steel stud, the thickness is important as the percentage of steel varies according to the thickness of the wall. So in this case, one would need to make a custom construction for each wall thickness.

Hope that explanation makes sense :sweat_smile:

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10

Now, I have another question related to the lacking transparency of the units in RTLCA :thinking:

I’m working on a LCA in RTLCA on a project, were we’ve already done the calculation in LCAbyg, to compare the two tools. In the process, I noticed that the GWP wasn’t the same for a the ground floor. Diving further in, I could se that the difference was in the emission from the gravel.

LCAbyg: 686,2 kg CO2e
RTLCA: 148,3 kg CO2e

So the GWP from this material is 4,6 times larger in LCAbyg than RTLCA. I can see that the emissions of the EPD’s are identical, so I’ve concluded that the problem is the units.

In LCAbyg, the unit is kg/m2, as the declared unit for the material Grus 2-32 mm is kg. That’s fairly straight forward. So in LCAbyg, I’ve entered 430 kg/m2, which equals to 71.951,2 kg gravel.

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In RTLCA, I am note sure what unit is used for the layer of gravel. The gravel geometry is imported from Revit and has the dimensions:

Area: 167,33 m2
Volume: 8,37 m3

Hence, the thickness is ~ 50mm.

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When I map the material Grus 2-32 mm (having the declared unit kg), I don’t get an extra dimension in my building component type called “kg”. So the amount of gravel included in this layer is apparently a black box to me.

So how many kg gravel do I have in this case? :thinking:
Or is there another way of comparing the results and the amounts in LCAbyg vs RTLCA?

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Hi’ @MarkLabrosse

We are aware of this.
If you map an EPD or a generic dataset which has kg as Declared Unit, then we should add kg as the preferred unit and make the conversion for you. Correct?

/Martin

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Yes, exactly that would be great :+1:

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13

Got it!
It already calculates the conversion and uses that in the results, but it is not showing the result as it should in kg.

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Yes, I realise that now. Using the conversion ratio from the EPD, I’ve concluded that the 430 kg/m2 gravel I had entered in LCAbyg corresponds to approx. 230 mm of gravel, where it should have been 50 mm. So in the end, the difference between the results was due to me having input a too large quantity in LCAbyg.
Just wanted to clarify, so that people don’t think that RTLCA was the problem here :wink:

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That being said, showing the total weight in RTLCA is still good to have to be able to QA/QC the inputtet quantities :+1:

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15

It would be great if constructions could be constructed :smile: from real components like c-studs.
As a user I want to avoid as much of this in between calculation - to avoid mistakes.
So if I could select studs shape (c-shape and so on…) and tell c/c distance, fill in the rest with insulation and let RTLCA calculate the ratio, that would be great.
One for the “Features and Ideas”.

16

Wow Mark, I am impressed. Loving the curiosity, with subject and personal insights! #fan

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