LEED project - maximum limit allowed to simplified wall azimuth.

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Hello,

Does anyone know how to interpret "no more than 45 degrees" of exception 5.b - table G3.1 (90.1-2007/2010/2013) ? :

5. Building Envelope
All components of the building envelope in the proposed design shall be modeled as shown on architectural drawings or as built for existing building envelopes.

Exceptions:
b. Exterior surfaces whose azimuth orientation and tilt differ by no more than 45 degrees and are otherwise the same may be described as either a single surface or by using multipliers.

Because "no more than 45 degree" include 45 degree, does the exception still apply if the azimuth of two walls is exactly 45 degrees?
In other words, considering a LEED project, can a perfect octagonal building shape be modeled as a square building (as long as they have the same floor area and the 8 walls have a similar construction) ?

Thanks

Julien
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Julian -

The text is written to include 45 degrees. So if they are 45 degrees, then you can make it a square, although the exception was intended for curved surfaces.

Hope this helps.

Leonard Sciarra, AIA, ASHRAE, LEED ap+
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Put another way:

If you have a perfectly circular floorplate, ASHRAE wants you to model an octagon, with respect to a minimum number of facets.

This specific query reminds me: There was a pretty interesting thought-study presented at ASHRAE's energy conference in 2015 where someone tested this logic, by asking the question (paraphrasing) "how few facets can we get away with and still accurately represent a perfect circle?" I don't recall exactly the answer but the results surprised me with how far you could go in simplifying the shape & still maintain relative accuracy. I believe a 4 sided square (corrected geometrically for volume) in fact performed very closely to the baseline.

If you're sincerely & specifically interested in modeling an octagon as a square, I think you'll find that could actually be pretty accurate so long as you are correcting for conditioned volume / exposed area. I'd seek out that actual paper/presentation while you're at it ;-).

~Nick

[cid:image001.png at 01D256CE.3B872510]
Nick Caton, P.E., BEMP
Senior Energy Engineer
Energy and Sustainability Services
Schneider Electric

D 913.564.6361
M 785.410.3317
E nicholas.caton at schneider-electric.com
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[cid:image001.png at 01D189AB.58634A10]

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Thanks Leonard and Nick.

I will model a building with octagonal shape and one with square equivalent shape in the next day. I will let you know the results.
Nick, if you find the "circular to square" study, I will more than happy to read it!

Have a good day

Julien

De : Nicholas Caton [mailto:Nicholas.Caton at schneider-electric.com]
Envoy? : 15 d?cembre 2016 13:25
? : Leonard Sciarra ; Dutel, Julien (BEIE)
Cc : bldg-sim at lists.onebuilding.org
Objet : RE: [Bldg-sim] LEED project - maximum limit allowed to simplified wall azimuth.

Put another way:

If you have a perfectly circular floorplate, ASHRAE wants you to model an octagon, with respect to a minimum number of facets.

This specific query reminds me: There was a pretty interesting thought-study presented at ASHRAE's energy conference in 2015 where someone tested this logic, by asking the question (paraphrasing) "how few facets can we get away with and still accurately represent a perfect circle?" I don't recall exactly the answer but the results surprised me with how far you could go in simplifying the shape & still maintain relative accuracy. I believe a 4 sided square (corrected geometrically for volume) in fact performed very closely to the baseline.

If you're sincerely & specifically interested in modeling an octagon as a square, I think you'll find that could actually be pretty accurate so long as you are correcting for conditioned volume / exposed area. I'd seek out that actual paper/presentation while you're at it ;-).

~Nick

[cid:image001.png at 01D256DA.0A3E4CC0]
Nick Caton, P.E., BEMP
Senior Energy Engineer
Energy and Sustainability Services
Schneider Electric

D 913.564.6361
M 785.410.3317
E nicholas.caton at schneider-electric.com
F 913.564.6380

15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States

[cid:image002.png at 01D256DA.0A3E4CC0]

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Julien,

Also make sure that the surface areas of the walls and windows are exactly the same between the different models.

The only test should be of wall orientation, not of surface-to-volume ratios. I used to play these games, like taking a rectangular footprint and distribute the walls equally in the four cardinal directions in order to remove any directional bias.

Joe

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Nick,

I think the relative impact of the degree of simplification would probably depend on a number of factors. Three that immediately come to mind are:

? Calculation method for transmitted solar radiation - does the software incorporate natural incident angle dependent transmittance values that are a part of how glass behaves? I know there's a big difference between using the simplified method in eQUEST and actually using the detailed input (either the LBNL window way or just picking an actual glass from the library).

? Type of building - if it's a more skin dominant load, then you will see a greater impact from the simplification.

? Orientation (I'm guessing some orientations are more sensitive to this).

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Can does not mean you will have an accurate model. All too often the
questions are about what can or what must I do and not about what is best to
do.

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Andrew,
Not trying to be difficult here, but it might be better to substitute
"reasonable" or "representative" for "accurate". From a certain perspective
none of our energy models are accurate. Especially for Appendix G
comparisons of new building designs, there are so many variables which are
not known with high confidence!

What's the infiltration rate - really?
What is the occupancy or lighting or plug load diversity - really?
What is the part load performance - as installed?
etc.

I'm always glad to learn of research which quantifies the impact of topics
about which I have a gut check, but don't have time to actually
investigate. The paper Nick cited would be one of them. Those ASHRAE
people are also pretty good at doing their homework when writing standards
such as 90.1, too.

I ALWAYS attempt to input the best available information, using a good
understanding of building science, physics and heat transfer. Nonetheless,
I also realize that there are dozens of variables that will not match with
actual operation.

p.s., I used 16 facets for my last round tower just to be more confident :)
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Agree with everything you say below. Reasonably achievable is probably the best, lol. Just wanted to emphasize getting reasonably achievable is the goal not just getting done.

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We are on the same page.

A friend once said (probably quoting someone else), "Become good, then
better, then best. After that, set a new standard for 'best'."

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