Read modelling best practice

At the outset, agree with the design team the level of detail going into the BIM model to allow for successful creation of the gbXML/IFC. (Please refer to file type notes below for further information^{File Type Notes}

^{GBXML – This is Green Building Schema. For more information, please visit:}^{http://www.gbxml.org/}

^{IFC – Industry Foundation Class. For more information, please visit:}

^{http://www.buildingsmart-tech.org/specifications/ifc-overview}

^{VEXML – IES standard of XML schema}

). At each stage check that the rooms/spaces/zones placed are fully bound and are flood filling the room as anticipated and no other areas of the model. (Please refer to ’Check Volumes’ section for more information)

Also check that the height of the volumes placed are taking into consideration the full volume as required and that ceiling voids have been considered. (See sections ‘Check Volumes’ & ‘Utilise floor levels/stories’) It is key to thermal modelling to make sure that there are no air gaps within the full boundary of your building model.

It is best practice when creating a BIM model for export via gbXML/IFC to build up the geometry in a particular order.

· Start with the external shell.

· Add in the floor Levels

· Then add in core areas

· Finally add in all other internal partitions.

When developing the model always make sure to have only a single external boundary wall type, a single internal type partition if possible and a single floor slab type. Avoid modelling two walls directly adjacent to each other as this can cause issues with the gbXML/IFC file you want to export. Walls can still be tagged to apply different thermal values once in the Virtual Environment (VE). Please refer to section ‘Consider model element properties’ for more information.

If you plan iterative work flows (e.g. import design changes into existing Virtual Environment models) or plan to export VE loads calculation back to gbXML files, it is crucial that you use unique room names. Revit and ArchiCAD generate fresh unique room ids each time files are exported, so to allow the energy model to be easily updated from the main BIM model each time, unique room names and numbers are required.

Vendor-specific recommendations

Graphisoft ArchiCAD – gbXML: External Wall modelling

When exporting a gbXML file type from ArchiCAD it is best practice to split the external walls per floor as these elements have a direct correlation with the stories’ function within ArchiCAD.

Graphisoft ArchiCAD IFC / Revit IFC & gbXML: External Wall modelling

When exporting these file types the external walls can be modelled as normal with a single external wall element covering multiple floors.

In summary the key points of modelling are as follows:

· Make sure a rooms/spaces/zones are placed throughout the entire model. Please refer to ‘Check Volumes’ section.

· Make sure that there are no air gaps within the full external boundary of your building model and that all volumes are accounted for including ceiling/roof voids & risers. Please refer to ‘Check Volumes’ & ‘Check ceiling Voids’ section.

· Use levels/stories to tie volumes to. Please refer to ‘Utilise floor levels / stories’ section.

· With Revit in particular make sure only main wall/floor/roof elements have room bounding ticked on. Please refer to Consider model element properties’ section.

· Keep models simple i.e. low levels of detail