[Bldg-sim] Modelling simplification for structural concrete columns

[Graham Carter & Megan Lyall]

Joe & Mike's advice to use THERM is something I would support to convert a two dimensional heat transfer problem into one dimensional properties which most energy simulations are based on.  We have good success with this method when looking at edge beams and band beams that in the past have not been continuously insulated and where the extended surface area is significantly different from the projected surface area.

I assume your column has insulation somewhere?  While in annual energy terms the effect of insulating would be call if not a negative effect due to reduced free cooling, the effects in winter could be quite problematic.  The column surface temperatures will be very cool in winter after a weekend and I doubt anyone could sit near them comfortably unless insulated or compensated in some way (temperature, radiant heating etc).

I have had a few projects over the year where band or edge beams between carparks and office spaces have not been continuously insulated.  We have taken IR gun surface temperature measurements, modelled in THERM, taken THERM results to develop IES inputs and simulated remedial measures successfully and quite accurately relative to what the simulation would suggest.

Not knowing your climate zone, you can run different scenarios (boundary conditions and materials) in THERM to get a feel as to whether you have a radiant temperature asymmetry issue in winter.

So comfort not energy will (or should) govern the need to insulate...

Regards,
Graham


On 15/03/2013, at 5:53 PM, Patrick Bivona wrote:

> 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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