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    by Trevor Clayton

    Some of you may not be aware that Autodesk Sub-Assembly creator came with AutoCAD Civil 3D and a very useful tool that once you get you head around it, it will definitely help you overcome some corridor designs varying design, to help you the guys at Autodesk have written a blog that takes you through a specific sub assembly creation worth a look, I have quoted it below but is available at



    SAC - Subassembly with varying lane width without targets !

    This post is based on an earlier post made by Bart De Maesschalck from DEME on the Civil 3D Benelux user group page.

    The challenge is to create a subassembly with the ability to apply a varying width without the need for a target alignment or polyline/feature line.

    In this example the transition will be linear... a curved transition can be considered for another post.

    The solution lies in the possibilities that SAC (Subassembly Composer) offers. I'm sure there are many ways to get this done, but this is how I did it…

    STEP 1: Define the variables you need

    • Base width (Width before the transition, also used in subassembly mode)
    • Extended width (Width after the transition)
    • Transition length (Length of the transition zone)
    • Transition start station (station to start the transition)


    STEP 2: Implement this is SAC

    • Create the input variables we have defined in STEP 1.
    • Define an internal variables to be able to calculate the real width during the transition (ActualWidth)
    • Define the flows and decisions that need to be taken.
      For every assembly insertion of the corridor we need to define if the current station is before, in or after the transition zone.

    Calculate the actual width during transition

    STEP 3: Test in AutoCAD Civil 3D

    The Subassembly can be downloaded here: Download LaneWithTransition



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