Overview
Sustainable design and Building Information Modeling (BIM) are the hottest topics in the AEC industry today. They are simultaneously reshaping not just how we design, but our entire approach to it. As traditional workflows are reshaped, the software tools we use rapidly evolve (and increase in number) to meet new demands in the design process.

Sustainable design changes how we work in two ways: First, the design and validation of energy reduction measures necessitates more frequent and complex analysis such as energy modeling, daylighting simulation, and life cycle cost analysis. Second, a successful sustainable design requires an integrated multi-discipline approach. These two events require more tools and an increased need for those tools to communicate and share data effectively. 

Discussions about BIM usually involve diagrams, islands of software tools overlaid with crisscrossing arrows depicting the complex relationships and fluidity of information in between. The tools we use are important, but their interoperability, those arrows connecting the dots, become equally significant.

Green Building XML (gbXML) is one of the unsung heroes of this industry transition. As an open XML schema, it is free for anyone to use and any software manufacturer to incorporate into their products. The schema attempts to create a universal language for all things “green building” to facilitate interoperability between an ever-expanding number of software tools for the built environment.

In its simplest form gbXML acts as a conduit between two software tools. Typically it is used to export the building geometry of a 3D architectural model and then can be imported into various analytical tools. This potentially saves a great deal of tedious recreation of building geometry. Once the analytical model and calculations are completed, some results can be exported back to the architectural model.

A Little History and the Current State of gbXML
gbXML was created by Green Building Studio (GBS) in 1999 with funding from the California Energy Commission and Pacific Gas and Electric, and it was released soon thereafter. It quickly became the standard schema for building information exchange and was adopted by major BIM authoring and analysis software vendors. In 2008, GBS was acquired by Autodesk, and gbXML was spun-off into its own non-profit called the Open Green Building XML Schema, Inc or gbXML.org. This non-profit now includes a board of directors consisting of 11 software companies, all of whom are stakeholders in gbXML. Also, gbXML.org holds quarterly board of director meetings, and promotes the schema through live webinars, quarterly e-newsletters, an active website and forum, and much more.

Some Technical Details About gbXML
The gbXML schema is a tree-like hierarchy of data placeholders. At nearly 400 elements and attributes, there are more placeholders than are probably ever used. In order to successfully transfer an element, the source software needs to write/export it and the destination software needs to read/import it. Some examples of the type of data the can be incorporated into a gbXML file include information related to building geometry (planar and/or rectangular geometry), envelope properties such as wall u-values and window emissivity values, building and equipment schedules, weather data, shading properties, point-by-point lighting data, hydronic loop equipment properties, air loop equipment properties, surface adjacency info, zone and HVAC system-related data, and much more.

Incorporating all of this data in the schema allows it to export to software tools that perform a wide variety of analysis including:

1. Whole building energy use & costs
2. Water use & costs (indoor, outdoor, and catchment)
3. Carbon emissions
4. Heating and cooling load analysis
5. LCA
6. Renewable energy
7. HVAC equipment sizing
8. Lighting analysis
9. CFD analysis
10. Fire analysis
11. Solar/shading analysis
12. Energy code compliance
13. Thermal comfort
    

What Software Tools Currently Integrates With gbXML
Currently, gbXML is integrated into 45+ software tools world-wide, some of which are:

1. Autodesk

• AutoCAD Architecture
  
• AutoCAD MEP
  
• Revit Architecture
  
• Revit MEP
  
• Green Building Studio
  
• Autodesk Ecotect Analysis

2. Bentley

• Architecture
• Building Mechanical
• Hevacomp

3. Cadsoft

4. Envisoneer

5. Google SketchUp

Plug-in (Greenspace Live)

6.     Graphisoft

Plug-in (Encina)

7.     Onuma BIMStorm

8.     Carmel Software

9.     DIALux

10.  EDSL - TAS

11.  IES VE

12.  Carrier – HAP

13.  CADLine - Cymap

14.  Elite Software

15.  EnergySoft – EnergyPro

16.  blueCAPE – CFD

17.  DesignBuilder

18.  HVAC Solution

19.  Greenspace Live

gModeller

20.  Solar Computer

21.  EnergySoft

22.  EnergyPro

gbXML Workflow Examples
An example of a workflow involving gbXML involves Autodesk’s new Conceptual Energy Analysis tool. This software tool is available as a plug-in to Autodesk Revit, and it allows architects, designers and engineers to compare energy consumption and costs of multiple design alternatives right within the Revit environment. Results are presented in a highly visual and graphical format, that enable further refinement of the designs to achieve optimal levels of efficiency. For each analysis, Revit Conceptual Energy Analysis results can be exported via gbXML to EnergyPlus and DOE-2.2 (eQuest) for more detailed analyses.

Another example of a workflow involving gbXML is from GreenspaceLive. Based in the UK, they have recently released a software tool called gModeller. It is an energy analysis plug-in for Google SketchUp that is based on gbXML for designing resource efficient buildings. gModeller enables the building designer to decorate 3D SketchUp building models with gbXML surfaces and openings, to identify and label gbXML spaces, and to import and export gbXML models to EnergyPlus.

Other common examples of workflows involve software tools such as  Autodesk Revit MEP to Trane TRACE and back to Revit.

A Short Case Study from DLR Group
A firm wide retreat in July 2010 brought together integrated teams of DLR Group, an architectural and engineering firm located in ?????. For two days, architects and engineers from offices across the country formed six teams to focus on incorporating the metrics of energy performance into project designs during the earliest phases of the design process. The teams worked rapidly to propose potential design alternates and modeled those options in Autodesk Revit and SketchUp while working simultaneously in IES-VE, Green Building Studio, and Ecotect Analysis energy modeling software. The energy impact and performance of climate, site orientation, daylighting and systems were tested and ideas and potential solutions to building systems revised on the fly.

The results were illuminating: In a mere eight hours over two days, teams produced excellent architectural solutions, architectural and engineering systems that met the energy metrics of Architecture 2030, and produced design communication report-outs that visually communicated the combined energy and aesthetic improvements for their projects. All of this was made possible through the gbXML format and integrating energy and BIM models in a rapid, iterative fashion.

Some Pitfalls
Working directly with gbXML is not without pitfalls. Sometimes users need to modify a gbXML file that has been exported from Revit. This modification means there is a violation of the “parametric” paradigm of BIM. As analysis models proceed in one direction, the architectural model is, more than likely, proceeding in another. While methodical routines to cleanup, repair, or modify gbXML are certainly convenient, they can become more work than they are worth if they must be repeated at each iteration of the design. Relying on gbXML to manipulate a model’s output is no replacement for a good self-contained model which exports cleanly.

Conclusion
As the use of gbXML becomes more widespread, tools which utilize the schema must continually improve their use of it. The analytical model wants accurate yet simplified geometry. The reduction of a complex architectural model to what is essentially just text in gbXML format is not a straightforward task. As building models continue to gain in complexity so too must gbXML’s ability to accurately capture the intent of our designs. Currently, gbXML.org is accepting comments from interested stakeholders on schema updates. As gbXML’s utility is increased, supporting documentation needs to be provided as well. Investigating how gbXML works and the breadth of the schema has proven extremely useful for creating analytical models from architectural models. As models for design and analysis continue to gain in complexity and the number of software tools used grows, gbXML offers a powerful means of interoperability and access to BIM data.

About the Author
Stephen Roth is Principal at Carmel Software, a company that develops mobile and web-based applications for the AEC (architectural, engineering, and construction industries). Stephen sold his prior company to Autodesk in 2008 and worked there for 1-1/2 years to help integrate the software into Autodesk’s 3D modeling tool, Autodesk Revit. Stephen is also President of gbXML.org, a non-profit that houses the Green Building XML schema that allows disparate software tools such as 3D BIM and building analysis tools to share information with one another. Stephen is active in various technical committees at ASHRAE and the USGBC. He is also a Professional Engineer and a LEED AP.