[Bldg-sim] Modelling simplification for structural concrete columns

[Michael tillou]

Brendan,

I expanded on some of the points you made below.

Mike

Sent from my iPad

On Mar 13, 2013, at 8:49 AM, "Hall, Brendan" <BHall at karpinskieng.com> wrote:

> 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.

Using the ASHRAE 90.1 Appendix A assembly u-value tables is the easiest way to factor in thermal bridging effects when creating external wall constructions because they are already factored in.   If you're in CA, the same Title 24 tables have very different results so just be aware their isn't agreement on the magnitude of those effects.
> 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.

Internal surface properties including massing and furniture massing can have a big impact on how a model performs.   Some modeling software does a good job of defaulting these inputs but other tools don't do such a good job.  The result of not accounting for these effects is that a very high fraction of solar and thermal load inputs are assumed to directly impact the space air temperature each time step rather than being absorbed and retransmitted in a future time step.  This becomes most obvious if you build a simple single zone model and turn off the HVAC system after a certain period of time and look at the space temperature response over a multi day period.  Without calibration of interior massing you can get very large (>50 degree) diurnal space temperature swings that would never be possible outside of "Model World". 

From an ASHRAE 90.1 modeling perspective unless a specific baseline condition is specified the baseline inputs should be the same as in the proposed design.  The proper way to deal with internal mass is to model it the same in both models.  If treating it as an efficiency measure it should be run as an exception calc since no explicit baseline condition is specified.
> 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.
>  
I disagree with your understanding of light weight.  You should always take into account mass effects of walls in a model. Light weight just means thermal transmission of loads happens over a shorter time period (1-3 hrs) compared to heavier weight constructions that pass a given thermal load over a longer time period (6-8 hrs).  Some modeling software will show you a plot of how the different construction assemblies you create will transmit thermal loads on an hourly basis which is useful for communicating these differences.


>  
> 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> 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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>  
> --
> Jeremiah D. Crossett  | Senior Analyst  |  Phase Change Energy Solutions
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