During the course of designing a duct system (these techniques can also be used with pipework systems) it may be required to represent a depression in the system (to avoid other ductwork/pipework) or other change in duct sizes.
In this case the center section of this run needs to be depressed by 100mm.
The depression and the sections either side should be drawn as 3 separate sections. The sections on each side of the depression are fixed accordingly.
Now the section that contains the depression should be fixed accordingly.
And finally we can see the depression here through the annotation.
As above, the centre section of this short run will be fixed to emulate a 100mm depression in duct sections. This is slightly more involved than the straight run as duct (and pipe) sections drawn around a bend usually heal into the attached sections, making them all one piece. Under these circumstances, providing different fixes to the same duct section would result in the whole of the section being fixed to the tightest constraints. To overcome this you need to consider using stub sections to allow the fixes in a way that would have the smallest effect on the system.The centre section is deleted:
And the 'stub' sections are added. For clarity, this is using a large grid. You may wish to make these stubs as small as possible. NOTE: Section numbering has been turned on here, so you can see how the stub sections become part of their adjacent section.
Finally, add back in the center to be depressed section and fix it accordingly. Show here with altered colours for clarity.