In the newest release of Revit Architecture 2010, certain long-overdue gbXML related enhancements have proved that good things come to those who wait. The revamped interface goes beyond a cosmetic change and introduces functionality that will definitely help building performance modeling enthusiasts.
By the same token, the new releases of Green Building Studio and Ecotect 2010 offer additional functionality via gbXML exchange that streamlines the geometry manipulation and translation process among several BIM compliant platforms.
Let’s start by reviewing the new interface, with settings that are now better exposed to the designer that would like to have a more comprehensive understanding of a proposed design’s implications on overall performance.
At the project level choices are given to determine a facility’s operational mode by selecting its category from the Building Type list which adheres to gbXML schema 0.37 and Building Type spaces as outlined in ASHRAE 90.1 2007. Another important aspect of the new interface is the ability to determine the level of detail, if you will, at which the geometry will be translated into the gbXML file.
It is important to mention that from a designer’s perspective an attempt to analyze a model at the end of the SD phase, or even later, might yield frustrating moments as one is attempts to fix inevitable “leaks” that are caused by both compounded user errors and Revit’s inability to interpret complex (rich in detail) design intent via gbXML. During this translation process Revit can ignore all of the leftover spaces that did not get a room assignment, such as wall cavities, and override them by setting the appropriate value for Sliver Space Tolerance. What is unique in the new interface is the preview of the exported model with its corresponding rooms/spaces and surfaces, in a similar fashion as with the IES export utility. The two available tabs within the gbXML export dialog box, “General” and “Details” allow for two levels of model information access as well as for some post processing of information that is submitted for the future analysis. The “Details” tab is particularly valuable as it allows for a more comprehensive look into the analytical model by allowing a user to either highlight or isolate the individual rooms as shown in pic.4 or to highlight and isolate the individual analytical surfaces for the purpose of verifying potential model drawbacks.
Switching to the Analytical Surfaces preview allows for an even greater level of detail in previewing translated geometry and in a slightly different fashion from the VRML preview where all of the surfaces are color coded, the orientation of the surface is contained in the translated name of the surface and the function of the surface is color coded. Nevertheless, beyond that almost aesthetic discrepancy, the functionality of this surface preview surpasses any of the currently available ways to validate translated geometry.
Now this is all great, but by the same token this exposes some not so great things about Revit’s modeling engine and about the ever growing lack of self imposed modeling discipline when creating content in BIM applications. As most of today’s software compensates for “sloppy” modeling, this compensation factor cannot cover up for inefficiencies within the analytical model. This leads us to another good reason for the “Less is more” approach to energy modeling. In pic.8 it can be seen how the surface tessellation that is conducted by Revit has a relatively hard time interpreting the optimal face layout for the given representation and instead of having only 4 properly oriented faces, the software interprets this wall as a combination of more than 4 faces, which translates into more analytical surfaces that are being pushed for the analysis. More surfaces do not necessarily mean a better model, but on the contrary it can produce results that are less accurate and harder to interpret.
This newly added export functionality is a significant step toward the desired interoperability between Autodesk’s BIM supporting platforms and a range of energy modeling tools that are either attributed to the same software vendor or are available as free of charge, third party Building Performance Analysis applications. One of the most obvious applications of gbXML exchange within the Autodesk family of products is model information exchange between Revit and Ecotect bundled with Green Building Studio. This somewhat convoluted way of cross application geometry and limited object information exchange will be the focus of several upcoming BIMology posts.