[Bldg-sim] Modelling simplification for structural concrete columns

[Patrick Bivona]

Joe, Edwin,

I take your point that I'm probably too literal in my attempt at modelling
the geometry of this wall.

Thanks for the collegial suggestion about THERM. It seems really good and
I'm pretty sure it's going to be useful to me further down the line.

The building I'm modelling is a tower with a square floor section of
1400m2. There are only a few uninsulated columns but they are relatively
big (1.2m wide, 2m deep) and spaced by 8m or 18m. It doesn't seem
appropriate to me to give an overall U-value to the assembly, spreading the
effect of the columns over such "long distances". I'd rather model the
columns as surfaces separate from the  surrounding walls, especially as
they are glazed curtain walls.

Patrick

---------- Forwarded message ----------
> From: Joe Huang <yjhuang at whiteboxtechnologies.com>
> To: bldg-sim at lists.onebuilding.org
> Cc:
> Date: Thu, 14 Mar 2013 01:00:07 -0700
> Subject: Re: [Bldg-sim] Modelling simplification for structural concrete
> columns
>  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"
>
>
>
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