Advanced Building Simulation

194 Augenbroe
vendors. The description of an actual building is a set of data items organized according
to the Building Model schema. The actual data exchange is realized by shipping the
data items in a format defined in the STEP standard, specified in ISO-STEP, part 21.
This is often referred to as the STEP physical file format. Anybody familiar with
XML (XML 2003) will recognize the similarity. The building representation could
also be developed as an XML schema whereas actual data then would be shipped as
an XML document. The application interfaces could be defined as XML schema
mappings. The advancements in Enterprise Application Integration (EAI) have been
largely driven by the increasing number of XML tools for this type of data transactions,
and all breakthroughs in B2B applications are largely based on this technology.
Although the data complexity encountered in EAI is an order of magnitude less than
that is the case with multiactor integration around product models, the adoption of
XML tools in PDT is growing. For the time being though, the EXPRESS language
remains superior to the XML conceptual modeling tools, but the field is catching up
rapidly (Lubell 2002). Current developments typically try to take the best of both
worlds by doing the conceptual modeling work in EXPRESS. The resulting schema is
translated into an XML schema, which then enables data instances to be sent as XML
documents instead of STEP physical files. In the short run, it seems unlikely that XML
technology will displace current STEP-based PDT as the latter has an advantage in
operating on the scale and complexity encountered in product modeling.
The IAI has concentrated on developing its IFC as a robust, expendable, and implementable
structure. Its stability and completeness has reached the stage where it can
be tested in real-life settings (Bazjanac 2003), albeit in “preconditioned” scenarios.
The current IFC version is a large model that has reached maturity and relative “completeness”
in some domains, but it remains underdeveloped in others. A growing
number of studies are testing the IFC, ascertaining its ability to provide the data that
is needed by building simulation applications, for example, the data structures for an
energy load analysis for building envelopes. An example of such a study is reported
in (van Treeck et al. 2003).
Table 8.1 shows a typical outcome of a property matching study, in this case done
as part of a graduate course assignment at Georgia Tech (Thitisawat 2003). In this case
it concerns a comparison between properties needed for energy analysis applications.
It was found that coverage of the IFC is very complete for most standard simulations.
In this case the material properties are taken from three IFC Material Property
resources: IfcGeneralMaterialProperties and IfcHygroscopicMaterialProperties. In addition,
the IFC offers a container class for user-defined properties IfcExtended
MaterialProperties. This provides a mechanism to assign properties that have not (yet)
been defined in the IFC specification.
The IFC model is available from the IAI and an increasing number of tool vendors
have started to offer IFC interfaces (IAI 2002). Eventually, the IFC may attempt to
become a STEP standard, “freezing” a version of the IFC in the form of a building
industry standard.
8.2.2 The management of the data exchange
Figure 8.3 shows the realization of data exchange through transport of files between
applications. Each application is expected to accept all the instances of the model and