Cut Solid by Curves

Question

Back into elementary difficulties: 
 Why won't this work? - pic - Cut (Split, retain both) all the roof Solids (Free Forms) by the green Curve. Either refuses, or removes the cut-off part of ea Solid. 
 dgn accompanying.

T_Wollet · Answer

fostertom, 
 You need to have a closed shape when using Cut Solid By Curves or Construct Opening. 
 If all you are trying to do is to trim the elements back, I might suggest that you just use Stretch. The reason for this is that if you are using the Cut Solid By Curves or Construct Opening tools, the Forms will each get a Feature Cut stored on the element (you can see these by using Building Element Info on the form and switching to the Feature tab). This is one additional layer of information that is stored on the element, which can increase file size and reduce performance as more and more feature cuts are stored on elements. What I mean by this is that the form without the feature cut is basically just storing the length, width, thickness, position, rotation, Family/Part, and calculated quantities. If you add a feature cut the geometry used for the feature cut is now stored on the element, plus the calculated quantities of the feature cut and the net quantities of the form subtracting the feature cut. 
 It might not be noticeable in a file this size, with just the features being discussed in this question, but if it gets compounded over the life of the project you might see a performance difference. As an example, In the file you posted, I deleted everything except the roof elements and the shape to cut the roof forms. Then I compressed the file and it was 344 KB. Stretching the forms instead of cutting the forms and then compressing again did not have any impact on the file size, still 344 KB. I reset the file, cut all the forms using Cut Solid by Curves, and compressed. The file size is now 442 KB. 
 I am saying don't use Features, as they are very useful. I am just suggesting that you avoid unnecessary features if you are doing manipulations that do not require them. 
 I hope this helps, 
 Travis

T_Wollet · Answer

fostertom, 
 My apologies, I misinterpreted what you were trying to achieve. Based on what I now understand to be your goal, I would suggest that using Linear Forms, rather than Free Forms, would make this easier to accomplish. If you had used Linear Forms, you could use the Modify Form tool to Extend a Linear Form. Setting the Method to Add Distance and toggling ON the options for 'Add Form' and 'Distance'. Setting the distance to a positive value would leave the existing form as it is and create another form at the selected end that is equal to the specified length. If you use a negative value for distance it will leave the end of the Linear Form where it is, but split it the specified distance from the selected end. 
 In the file you posted, if you had used Linear Forms you would follow these steps: 
 Select Modify Form > Extend a Linear Form 
 Dialog Settings: 
 Method = Add Distance 
 Distance = -215mm 
 Add Form = ON 
 In the case of the file as you have it, using Free Forms, I believe that you would have to use the Solid Modeling manipulation boolean tools, which would convert the Free Forms to MicroStation Solids. This is typically not desirable in BIM workflows, but depending on what kind of reporting you would need to perform downstream, this may be acceptable. Another option would be to select all of the elements that you want trimmed back, Copy (from the north edge of the roof) & paste (at the south edge of the roof) so that you have two free forms for each element that you wanted to split. Then use the stretch tool to stretch both the south edges of the original forms and the north edges of the copies forms. Use the stretch tool again to align the south edge of the copied forms with the desired roof edge. 
 Travis