[Bldg-sim] Modelling simplification for structural concrete columns

[Hall, Brendan]

To the original post, I would use the line of the wall and ignore the columns. If you feel that there is a considerable shading effect, then use side fins or a building shade. Look at a section of wall the width of one column spacing as calculated the combined U value and thermal mass of the wall to see if the columns will have enough of an effect to make it worth modeling. I don't know how large the columns you are referring to are but it is likely that they would have a negligible effect on the model as a whole, especially when you consider the uncertainty of other values that are input. There is a line in Table G3.1.5 saying that other envelope assemblies that cover less than 5% of the total area need not be explicitly model and can be averaged into other wall assemblies.

As to Jeremiah's post,


  *   ASHRAE 90.1 does not require thermal bridging to be modeled in the baseline.
Thermal bridging should also be taken into account when calculating wall and floor U Values. External un-insulated assemblies must be explicitly modeled.

  *   ASHRAE 90.1 does not require internal mass to be modeled in the baseline.
I don't think any internal constructions are addressed in 90.1. Generally the effect of any internal walls will be negligible compared to the external walls unless your typical wall is heavy weight like a concrete block wall. Some schools are like this. Either way I would say internal mass would not need to be included in the baseline since the baseline is a metal framed building, which implies GWB internal walls.

  *   ASHRAE 90.1 does not require self shading to be modeled in the baseline.
  *   ASHRAE 90.1 requires a light weight wall assembly for the baseline.
The baseline should used a metal framed wall, which still has some mass effects that are required to be modeled. The way I understand it, a "light weight" wall refers to neglecting the mass effects of walls in your calculations.


Brendan Hall

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Jeremiah Crossett
Sent: Wednesday, March 13, 2013 12:36 AM
To: Patrick Bivona
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Modelling simplification for structural concrete columns

Group,
Please do correct me if I am wrong--->>

  *   ASHRAE 90.1 does not require thermal bridging to be modeled in the baseline.
  *   ASHRAE 90.1 does not require internal mass to be modeled in the baseline.
  *   ASHRAE 90.1 does not require self shading to be modeled in the baseline.
  *   ASHRAE 90.1 requires a light weight wall assembly for the baseline.
So the conclusion is that this does not need to be modeled in the baseline and the question becomes how to model this in the proposed building, or actually "what gets the project more LEED credits" If I where to do it I would: (in E+)

  *   Ignore the shading element of the concrete pillars and consider the walls that separate the zones the only thing that matters for shading.
  *   Dependant on the other wall construction-- if mass than add the thickness to the total wall area-- if steal framed, wood&other or metal than bridge wall with column and add internal mass to account for thermal mass.
My conclusion is your #1 should be fine, especially if your not attempting to get credit for the exposed thermal mass..

On Tue, Mar 12, 2013 at 8:36 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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Jeremiah D. Crossett  | Senior Analyst  |  Phase Change Energy Solutions
120 E. Pritchard St.  | Asheboro, NC 27203  | Mobile 503-688-8951
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