prop, shore
橕 centers on supporting something from below or from the side: a prop or a shore that holds something up.
橕 combines 木 (wood) with 牚, which likely contributes the sound and the idea of propping or supporting. The exact historical development of 牚 is uncertain.
The wood radical 木 on the left suggests a wooden beam. The right side 牚 looks like a sturdy structure with a cover and a base. Picture a wooden prop holding up a heavy load.
For トウ, imagine a wooden prop holding up a tall tower: tower -> トウ, and the wood keeps it from falling.