Advanced Building Simulation

36 de Wit
ASHRAE (1997), ISSO (1994), and the Polytechnic Almanac (1995). For a few
parameters a range was assumed for lack of data.
Apart from the uncertainty ranges for the individual material properties, estimates
must be made of the statistical dependencies between these properties. If two properties
are dependent, that is, have a strong positive correlation, then high values for one
property tend to coincide with high values for the other. If the two properties are independent,
however, the value of one property does not change the expectations with
respect to the value of the other. In this crude uncertainty analysis we will only distinguish
two levels of dependency: completely (positively) correlated or uncorrelated.
To estimate the correlations between the properties of different components and
materials, each property x has been considered as the output of the hierarchical
model:
x�� x��x 1��x 2��x 3
(2.2)
where � x is the general mean over the whole population; �x 1, the variation between
types, which satisfy the description in the design specifications; �x 2, the variation
between production batches within a type; and �x 3, the variation between individual
components within a batch.
It has been assumed that the variation in the material and component properties
predominantly arises from the first variation component �x 1. Hence, complete correlation
has been considered between properties of the same name, if they belong
to components and materials of the same name. Dependencies between different
properties or between unlike components or materials have not been considered.
UNCERTAINTY IN WIND PRESSURE COEFFICIENTS
In our case, the ventilation flows through the building are mainly driven by the local
(wind) pressures at the locations of the windows in the façades. These pressures
depend on the wind velocity upstream of the building, the position on the building
envelope, the building geometry, the wind angle with respect to the orientation of the
building, the geometry of the direct environment of the building and the shape of the
wind profile. In the simulation, only the wind velocity and wind angle are explicitly
taken into account, the effect of all other factors is captured in a single coefficient,
the wind pressure coefficient. In fact, this coefficient can be considered as a massively
simplified model of the airflow around the building and its environment. It is clear
that not specification uncertainty, but modeling uncertainty will be dominant for this
coefficient.
Several tools have been developed to assist the assessment of mean wind pressure
coefficients on the basis of existing experimental data from prior wind tunnel studies
and full-scale measurements. The tools from Allen (1984), Grosso (1992),
Grosso et al. (1995), Knoll et al. (1995), and Knoll and Phaff (1996) have been applied
to the current case to assess the required wind pressure difference coefficients. 2
The results are shown in Figure 2.5. A more detailed analysis of the wind pressure
difference coefficients can be found in Section 2.4.
As a start, we will use the intermodel scatter in Figure 2.5 as an estimate for the uncertainty
in the wind pressure difference coefficients in this crude uncertainty analysis.