Sunday, September 23, 2007

Revit – a "Shade" too green - Revealed !

Well the shades are still there but thanks to Revit’s management and development team, the explanation for their occurrence was given promptly. In this instance Revit’s team (I just do not like to call them the factory as in my view programming is anything but a metaphor for Modern Times), has proven once again that they pay the attention to their product and the people using it. So “Shades”, here we go. From now on you can think of them as a “negligible” artifact that is the result of overlapping polygonal geometry when host and hosted elements interact.

(image courtesy of AutoDesk)

Think of this “negligible” factor within gbXML file as your comfort level threshold when it comes to the accuracy of GBS simulation results. As illustrated these shade surfaces are formed from the centerline of Revit’s wall to the exterior surfaces of either walls or roof eaves.
Their depth depends on the construction type and the amount of overhang, meaning the thicker the exterior wall the deeper the shade surface, and the wider the overhang the larger area of that roof's surface will be designated as the Shade surface.

(image courtesy of AutoDesk)

Therefore the difference in the amount of applicable shading will be different with respect to the construction method as a 12” wide wall over 10 feet will produce an additional 5 SF of shading, and a 24” thick wall over the same distance will generate 10 SF of shading surfaces. For now, if you seek for a no questions asked simulation model, go ahead and get rid of all of the Shade surfaces that are not functional from the designer's perspective by manually deleting them from the gbXML file.The good news is that Revit's team is working hard to streamline the gbXML translation tool even more by increasing its accuracy and flexibility.
It is also important to emphasize the relationship between the Room object and its upper boundaries.
This subject is covered in
Revit’s MEP white paper and it deals with a limited range of roof forms and the required relationship with Room objects.
To make this long story short, one should ensure that the Room object’s upper boundary extends above the highest vertex of the roof. The analytical volume that is reported to gbXML file is not the one that reflects Room object's volume but the one that is encapsulated by the boundary surfaces of the adjacent building elements.I would encourage everyone to open their gbXML file and examine its content from time to time and verify that all of the model elements are correctly exported from Revit’s model. If reading XML is not one’s cup of tea, the GBS client has a convenient tool that translates the gbXML file into a VRML model and indicates the potential conflicting analytical surfaces. This model can be easily shared online and in order to visualize it the user should install a VRML viewer such as Cortona by Parallel Graphics.

In the next post I will illustrate some basic building forms and the best strategies for schematic design zoning in order to achieve quick and usable comparative results from GBS simulation.

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