Bottom 木 - pictograph of a tree. As the indexing radical it identifies 梁 as something made of wood: a beam, ridge-pole, or wooden bridge. The whole character reads as 'a cut piece of wood laid over water', the literal etymology of a beam or footbridge.
Top-left three-drops water - the side-form of 水. Original 梁 meant a wooden beam laid across water to make a footbridge, so the water radical preserves the bridge-over-river image even though 木 below is the indexing radical.
phonetic
刅chuāng
cut; wound
Top-right 刅 - knife with a marker dot, supplying the ancient sound that drifted to liáng. Combined with 氵 it pictures cutting timber for a span. The same phonetic survives in 创 (create) and 疮 (sore), all once sharing a chuāng/liáng-family reading.
