pillar
楹 centers on a structural pillar or column, typically made of wood, that supports a building.
楹 combines 木 (tree/wood) with 盈, which likely contributes the sound and may relate to fullness or filling, suggesting a solid wooden pillar. The exact historical development is uncertain.
The left side is wood 木, and the right side 盈 looks like a vessel 皿 being filled. Imagine a sturdy wooden pillar holding up a full vessel, supporting the structure.
For エイ, picture a wooden pillar shaped like the letter A: A -> エイ, and it stands tall supporting the roof.