Saw some seasoned guys do this on the jobsite today:
They are running a rack of conduit on strut. After they filled the strut with all the conduit that could fit, the super tells them he forgot a pipe; need to add one. So they strapped a short section of strut to the bottom of the just-installed conduit (full rack); then with all-thread extending from this new short section of strut up above the full rack, they added another short section of strut, upon which they were able to install the "forgotten" conduit.
To summarize: the new pipe is supported by strut which is supported solely by pipe (which is itself supported by strut which is supported by purlings).
If you can visualize this, please tell me: doesn't this violate some code about not using wiring methods to support other wiring methods?
