tree trunk, support, prop
榦 centers on a sturdy central pillar: a tree trunk that supports the rest of the tree, or any prop or support that holds something up.
榦 combines 木 (tree) with a phonetic component that historically relates to a pole or stem, giving the idea of a tree trunk or support. The exact historical development of the right side is uncertain.
The left side is 木 (tree). The right side looks like a sturdy pole with a crossbeam on top. Picture a strong tree trunk used as a prop to support a heavy load.
For カン, imagine a tree trunk used as a can support: a large can resting on the sturdy trunk. Can -> カン.