Introduction

Building Information Models are usually stored with closed proprietary file formats. This leads to the necessity to export them to other formats or even having to redo the whole model in another application.

Additionally, the original files usually pack excessive information, when compared with the requirements for 3D Computational Fluid Dynamic simulations. However, there is already a common platform for all these software, devised to exchange information between different applications. It is called gbXML, which stands for Green Building XML Schema.

The Open Green Building XML Schema, Inc., is a non-profit organization dedicated to developing and maintaining this schema, in order to give the industry an open and simple file format, for exchanging information between building information models (BIM and CAD) with software for analysing energy efficiency of buildings based on the 3D digital models.

Today this is a defacto industry standard, adopted by Autodesk, Graphisoft, Bentley, Carrier and many others.

gbXML is in a textual, human readable, organized XML format, providing rich model information for energy analysis, including:

• complete and simplified building geometry;

• indicates opening, wall characteristics, as well as shades;

• building geographic location and orientation;

• energy analysis information, before and after simulations.

The gbXML.org website has a list of applications that currently use gbXML (here), further evidence to the fact that this is now a widely accepted format among the BIM/CAD and energy efficiency building model analysers.

Our approach to the problem has been via Computational Fluid Dynamics, where the current set of tools available in dedicated Building Analysis Environments has a somewhat limited feature set. Moreover, the absence of a reliable bridge between the BIM world and the CFD world meant the existence of an additional obstacle to the work of CFD students, professionals and enthusiasts with an interest in the exciting world of aiding the development of energy efficient buildings.

Also, the absence of a simple way to take BIM data into more established formats, is keeping other forms of 3D simulations on the side, e.g.: simulation and development of domotic robots for cleaning and managing houses, warehouses, malls and other big and small building constructions, adapted/optimized to specific buildings and areas.

gbXML2STL comes to fill in a gap between the world of building model designing and the world of simulation, by providing direct 3D geometric representations of the building models in STL files.

The main difference between this exporter and other STL exporters, is that the other ones are mostly used for rapid prototyping, and do not care about the discrimination of openings and rooms.

There are 3 export modes available through the command line:

full building geometry into a single object, with openings defined as individual objects, all into a single ASCII STL file: gbXML2STL building.xml building3D
full building geometry, but with rooms saved into separate STL files, with openings saved as individual solid objects along with their respective room; here export of room connections is optional: gbXML2STL -s -c building.xml building3D
export only one room with openings saved as individual solid objects along with it: gbXML2STL -r sp-1-room -c building.xml building3D

For all of these modes, one can choose to export internal, external or both types of openings (optionally including air openings), into individual STL files, for those CFD applications that are unable to:

• handle more than one solid object per file;

• create meshes inside connected, but closed, volumes.

Also, to provide easy script-ability for this process, the gbXML2STL Exporter is provided as a command line executable with console output. With that in mind, it has an option to output a list of room names, with respective openings names, the latter discriminated by type.

gbXML2STL 0.65 uses the following packages:

• MinGW gcc 3.4.2

• Qt 4.5.1 (under the GNU Lesser General Public License)

• VTK 5.4.0 (with a BSD license)

• OpenCV 1.0, October 18, 2006 (with the Intel License Agreement For Open Source Computer Vision Library)

Capabilities:

• Nearly full export capability of internal geometries of buildings defined in gbXML file format.

• Export external surfaces, ex/including shades as walls/openings.

• Curved walls are properly processed. There should be only minor bugs (see limitations).

• Wall thickness is taken into account when the gbXML model provides them geometrically.

• All measures defined in the XML Schema for gbXML, are converted to International System units.

• Export to ASCII STL.

Limitations:

• It's currently specialized for exporting internal geometries.

• External geometries are exported, but there are still some limitations.

• There is a limit of 50cm thick walls.

• When exporting, make sure there aren't curved walls (with openings), that have arcs with angles larger than 100 degrees. If so, split those walls, in order to avoid export errors when exporting from the BIM tool to gbXML. In the extreme situation, you might need to split the wall between every opening.