[Bldg-sim] Modelling simplification for structural concrete columns

[Joe Huang]

Patrick,

I frankly think that you're looking at the modeling of building geometry 
in simulation programs too literally.  Please keep in mind that in the 
end all
the simulation programs are just doing hundreds of one-dimensional heat 
flow calculations through thousands of time steps.  Wall thickness is needed
only to compute the heat flow characteristics (U-value, response factor, 
transfer function, etc.) but is not modeled explicitly in the 
simulation, i.e.,
walls have no thickness in the simulations.  Similarly, the space volume 
is used only to determine the amount of air participating in the space 
heat balance, so therefore you should always input the actual enclosed 
volume, even if it does jive with the dimensions of the enclosing 
surfaces.  If you're concerned about the shading effect of the columns, 
they can be model them as narrow vertical building shades with widths 
equal to the protruding part of the columns, but if the walls are 
opaque, the effect must be extremely small.

However, I think you're ignoring  what I think is the most significant 
heat transfer aspect of these columns, which is the two-dimensional 
conduction through them, because the columns act like stubby fins on the 
wall, particularly if they're solid concrete with no insulation.  That's 
where I would concentrate my energies if I wanted to get the modeling right.

To do that, you would really have to model the wall-column-wall assembly 
with a 2-D conduction program like THERM (available from LBNL), although 
that was written for modeling window frames and can only output a 
steady-state U-value.  I still have a 2-D conduction program written in 
Fortran called WALFERFN (sounds German, doesn't it ? but the acronym 
stands for WAll Finite Element Response Factor New :-) ) that I use to 
calculate response factors for composite walls, but that can only handle 
planar surfaces.

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 108D
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
www.whiteboxtechnologies.com
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"


On 3/13/2013 8:27 PM, Jeff Haberl wrote:
> FYI.
>
> ASHRAE RP1468, just finished, which provides BIM-to-thermal modeling 
> examples, contains example modeling DOE2 input files and BIM files for 
> concrete thermal bridging, and  other examples like curved windows, etc.
>
> The final report can be I obtained from Mike Vaughn at ASHRAE.
>
> Jeff
>
> Jeff S. Haberl, Ph.D., P.E., FASHRAE, FIBPSA
> Department of Architecture
> Texas A&M University
> College Station, Texas 77845-3581
> Office: 979-845-6507, Lab: 979-845-6065
> Fax: 970-862-2457, jhaberl at tamu.edu <mailto:jhaberl at tamu.edu>, 
> www.esl.tamu.edu <http://www.esl.tamu.edu>
>
> On Mar 12, 2013, at 10:36 PM, "Patrick Bivona" 
> <patrick.bivona at gmail.com <mailto:patrick.bivona at gmail.com>> wrote:
>
>> Hi everyone,
>>
>> I'm wondering how best to simplify a model when considering 
>> structural concrete columns.
>> My modelling is in the context of the LEED whole building energy 
>> modelling. So I'm concerned with accuracy as much as acceptability by 
>> the LEED reviewers. I've read the ASHRAE 90.1 user's manual but 
>> didn't find a satisfactory answer.
>>
>> Here's an example:
>>
>>         ----
>> wall   |    | wall
>> -------|  |------
>>        |    |
>>         ----
>>  concrete column
>>
>> I can see 3 options for simplifying, each with its own issues:
>>
>> 1. Make the external surface of the column at the same level of the 
>> surrounding walls and model the column surface as having its true 
>> thickness . The issue is that it alters the area of the space inside 
>> and doesn't account for the shading effect of the column.
>>
>> 2. Follow the internal boundary of the column, which introduces 3 
>> surfaces. Not sure what construction thickness to assign to these 
>> surfaces. This preserves the internal space area but alters the 
>> shading effect of the column.
>>
>> 3. Follow the external boundary of the column. This also introduces 3 
>> surfaces. Not sure what construction thickness to assign to these 
>> surfaces either. It preserves the shading effect of the column but 
>> alters the area of the internal space.
>>
>> I'm leaning towards option 1. What do you think? Are they other 
>> better options I overlooked?
>>
>> Of course, things get a bit more complicated when the walls either 
>> side are not aligned or in the same plan...
>>
>> Thanks!
>> Patrick
>>
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