crosspiece
棧 centers on a horizontal wooden piece used in construction: a crosspiece, plank, or beam that bridges or supports.
棧 combines 木 (tree, wood) with 戔, which likely contributes the sound and the idea of something narrow or layered, together suggesting a wooden plank or crosspiece.
The left side 木 is wood, and the right side 戔 looks like stacked narrow pieces. Picture a wooden crosspiece made from layered planks.
For サン, imagine a wooden crosspiece baking under the sun: sun -> サン, and the sun warms the wooden plank.