Sunday, September 16, 2012

“Core Only” Walls in Revit


Before the advent of CAD, on construction drawings walls were represented by two lines - which everyone understood to represent the wall’s entire thickness. The actual wall assembly was defined by details. The construction layers and finish materials of walls were always shown in larger scale views.

Then as now the standard for dimensions was to reference the face or edge or the core construction, because it must be placed first in the field.  General notes disclaimed “All dimensions are to face of structure unless noted otherwise”. When more specific direction was required, the letters ‘FOS’ or ‘FOW’ were placed next to the dimension line to denote the reference.

At one-eighth inch scale the size of a six inch wall is 1/16”, which is about as fine as can be drafted with pencil on vellum. In manual drafting, at smaller scales the same wall would be drawn thicker, by necessity. The level of detail was variable, depending on the scale of the drawing.

With CAD came the ability to add more detail and precision to drawings. Unfortunately, CAD did not have the human ability to automatically adapt drawing detail level to drawing scale, and management of this information by drawing layers (over hundreds of individual files) although possible, was eventually abandoned because it was impossible to maintain.

So it was all or nothing, and the practice of representing only the structural core of walls, and excluding finishes, evolved into the defacto standard for CAD.

The transition from manual production to CAD did not really resolve its problems; it simply exchanged old limitations for new ones. This compromise is one example of how the limitations of CAD were first accepted and are still perceived as obstacles to overcome in the transition from CAD to BIM.


Among the unintended side effects of this practice, with CAD room and building areas are often calculated by tracing a polyline around the perimeter of a space, not at its finish but at the wall core. Cabinetry and casework is often depicted as if the wall finish does not exist. These errors are not significant, but should hardly be defended when a better solution is presented.

Those familiar with its early history know that Revit was designed specifically to resolve the problems inherent in CAD. Revit allows dimensions to be referenced to structural elements, while also displaying the full thickness of the wall, finish to finish. Revit  also allows walls to be depicted with only two lines (coarse view) with unlimited options for color, fill pattern, and lines weight.

In Revit room areas are automatically calculated, with boundaries established at the user's preference. 

In addition, Revit area plans automatically apply rule-based algorithms for its calculations.


My advice to Revit users is to resist all temptation to use Revit to emulate the limitations found in CAD. Allow what is clearly obsolete to stay in the past where in belongs.

1 comment:

  1. Not a lot of people talking about this core vs finish idea. I am responsible for the BIM transition of an old firm where half the people want autocad old style drafting and half want revit. Since I'm setting up standards in Revit I'm really trying to figure out a system that works from plan to elevation to interior elev. to wall and building section. At this point I think we need to model the interior and exterior finishes- but Im not sure if they should be part of the wall or a separate, thin wall applied and locked to the structural wall... If you have any more thoughts on the matter I'd love to hear em! Thanks,

    Tyler

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