I am trying to understand how the Weight values get assigned to structural members in the Structural Quantity Spreadsheet, in particular Weight (Vol * Density) as well as Volume which I assume is used to calculate the Weight. I thought perhaps the "Weight" and "Area" values from the shape catalog were being used but it appears that it is using something different.
The Weight (Vol * Density) cell in the Structural Quantity Spreadsheet is populated based on the value of “Volume (Gross Modeled)” or “Volume (Net Modeled)” from the Structural Quantities property panel, which will match if there are no features. This value is multiplied by the shape's Density value to produce the resulting Weight (volume X density) value. The reason these property values are used is because they represent the actual modeled values of the Structural members in the DGN file. And, these volume values are the result of “Area | Cross-Sectional Modeled” X member length.
The values for the other quantity properties such as Volume (Gross), Volume (Gross) x Density, Volume (Net) x Density and Area | Cross-sectional are different and described as "ideal" or manufactured version. The help topic "Structural Quantities Property Details" describes all of the available properties:
Also, note that the values are rounded to four decimal places within the OpenBuildings Designer Properties dialog and two or three places in the spreadsheet.