[Bldg-sim] Modelling simplification for structural concrete columns

Joe Huang yjhuang at whiteboxtechnologies.com
Thu Mar 14 01:00:07 PDT 2013


Sorry, caught a couple of typos, meant to write in the second paragraph, 
"...even if it doesn't jive.." and "they can be modeled..."

Joe

On 3/14/2013 12:35 AM, Joe Huang wrote:
> 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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>>
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