I am preparing my quantities for a project pursuing this credit and have a few questions about how much “massaging” the USGBC would like us to with our data when it doesn’t quite fit the options in the Eco-Calculator. For example:
The slab on grade in the Eco Calculator assumes a 4” thickness. If we have a 5” slab should we scale our square footage by 1.25 (5/4) to get the same volume of concrete, or would the USGBC prefer that we input the actual square footage and then note this in the discussion phase?
A similar scenario exists for the cast-in-place concrete wall. A thickness is not specified, but the impact data numbers are roughly twice those of the 4”, slab so it appears that that the program assumes that the wall is 8” thick. Our project has a number of 18” and 20” walls. Should we scale the square footage to account for differences in wall thickness?
Thanks,
Dirk
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Whit Faulconer
Director – LEED Technical DevelopmentUSGBC
51 thumbs up
September 19, 2010 - 10:42 pm
Dirk, Thanks for your question. I have been in contact with Athena Institute for clarifications and/or advice for how to best address the issues you raise.
My first inclination is to say that you should keep on rolling making choices that favor your project, the path of least resistance. It is just these types of conflicts that we want to learn from. As long as you are documenting your choices and explaining your rationale, it helps us learn how to better design the requirements and submittals for real world application.
I do expect an answer from Athena early this week, so let's see what they come back with as the best solution technically. I expect they will have a simple recommendation that should keep you moving forward. -whit
Whit Faulconer
Director – LEED Technical DevelopmentUSGBC
51 thumbs up
September 22, 2010 - 11:25 am
Below is the response from The Athena Institute.
All assemblies in EcoCalculator (EC) are presented holistically, that is to say that the results reflect the impact measures of the entire assemblies, and you can't separate the individual materials form each other and scale them separately. So increasing your square footage by 25% will give you the effects of a 5" slab on grade because EcoCalculator models a 4" slab. But that means you'll get the effects of 25% more concrete as well as 25% more welded wire mesh, since those are the two materials in the slab assembly.
The question is, would you actually increase the amount of welded wire mesh if you poured a 5" slab? There is no flexibility in EC, to increase one without the other. So it's not a perfect solution, but I would say in the overall results, if you accurately account for the extra volume of concrete and get "penalized" with the extra wire mesh, the result is closer to the "truth" than if you left it as a 4" slab. The same applies to the cast in place wall. EC models an 8" wall with #5 rebar. Increasing your square footage 2.5 times will account correctly for the volume of concrete but will also increase the amount of rebar. I'm not a structural engineer, but I'm guessing you would add more rebar for a 20" wall, but I don't know if it would be 2.5 times.
Dirk Kestner
PrincipalWalter P Moore
26 thumbs up
September 23, 2010 - 6:24 pm
Whit,
Thanks for the timely response. Athena's response seems to make sense, but as you mentioned I will document all assumptions and explain my rationale. Hopefully in the final version there will be a bit more flexibility with the data the user can input.
Dirk