ASHRAE Journal - July 2022 - 32

TECHNICAL FEATURE
TABLE 1 Thermal properties of testing facility.
WALLS
ROOF
BUILDING
TYPE
Heavy
Light
R-VALUE
(h·ft2·°F/Btu)
6.24
19.86
THERMAL MASS
(Btu/°F·ft2)
27.93
2.13
R-VALUE
(h·ft2·°F/Btu)
16.13
11.96
geometry of the space will become
undefined, and loads will no longer
be able to be calculated on this space
(no " garbage out " ).
Therefore, if a spatial element
should have a 10 ft × 10 ft (3 m × 3 m)
exterior wall, but only has a 6 ft ×
10 ft (2 m × 3 m) wall due to an architectural
modeling mistake, loads
will not be able to be calculated on
that spatial element.
These mistakes do not happen
frequently because architects use a
spatial element for purposes such as
building area programming, door
schedules and name/number tagging.
Therefore, the architect typically
curates the spatial elements.
The equivalent spatial element for
MEP design has MEP data attached
to it, rather than architectural data.
The MEP design spatial element
isn't just good for determining what
is bounding them; they are also
good at determining what is inside
them. This is helpful when it comes
to calculating internal loads. They
keep track of all of the electrical elements,
such as light fixtures, that
are contained inside the space. It
can read the electrical data from
those elements and store it in the
spatial element. This allows the user
to have the same power that is on the
electrical engineer's panel schedule
automatically show up in load
calculations.
This also works with electrical
receptacles; however, mechanical
engineers typically account for plug
32
ASHRAE JOURNAL ashrae.o rg
THERMAL MASS
(Btu/°F·ft2)
4.61
1.27
FLOOR
R-VALUE
(h·ft2·°F/Btu)
4.16
2.92
THERMAL MASS
(Btu/°F·ft2)
3.66
2.66
load on an equipment level. They
may prefer to override this receptacle
power, but it is still a good quality
check to be able to automatically
compare the plug load assumed for
the cooling load and the plug load
that the electrical engineer assumed
side-by-side.
Program Overview
There is not a " person " element
in BIM software, so typically occupant
load is derived from its space
type occupant density parameter.
Furniture, however, is often
included in the BIM model, so it
is easy to automatically count the
number of seats in the spatial element
and compare that to the occupant
density and select the most
appropriate value.
Once all of this data is captured, it
can be used in two ways. Most BIM
software includes a built-in load
calculation engine, but if a thirdparty
engine is preferred, a Green
Building XML (gbXML) file can be
exported. gbXML is an extensible
markup language (XML) database
that is structured to an industry
standard so different software platforms
can read and understand the
data.2 Both methods will be validated
in the following sections.
Existing Validation Literature
A seminar entitled " The Current
State of the Art for an Automated
Pathway from BIM Building Models
Direct to HVAC Heating/Cooling
J U LY 2022
Load Calculations " at the ASHRAE
2017 Winter Conference compared
four different methods for calculating
loads on the old ASHRAE headquarters
located on Tullie Circle
in Atlanta.3 The first method was
to use spreadsheets based on the
radiant time series method (RSTM).
The three other methods used BIM
data. While the calculation results
generally agreed with one another
on a building level, they did not
compare the results with a true
measured building load. By validating
BIM load calculation procedures
against actual loads experimentally
measured on a real-life structure,
the reader can be more confident in
using BIM for loads.
Daniel E. Fisher, Ph.D. and Jeffrey
D. Spitler, Ph.D., at Oklahoma State
University pioneered load calculation
validation with their seminal
research project ASHRAE RP-1117
entitled " Experimental Validation
of Heat Balance/RTS Cooling Load
Calculation. " 4
This research consisted of building
two different 12 ft × 12 ft (3.7 m ×
3.7 m) test facilities in Stillwater,
Okla. (latitude = 36° 1', longitude =
-97° 1'). One test facility was built
using heavy construction materials,
and the other was built using light
construction materials. The thermal
properties of the constructions can
be viewed in Table 1. Both facilities
were fitted with uncoated 1/8 in.
(3 mm) thick single-pane windows
(normal SHGC = 0.86, U-Value =
1.04 Btu/h·ft2·°F [5.9 W/m2·K]) that
accounted for 50% of the area of the
south and west wall.
No internal loads were added to
the test facility because the heat
gain procedure for internal loads
is uncontroversial. Once they are
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