What does "understructure" mean?

The supporting structure underneath something, such as a vehicle or bridge.

What is a synonym for "understructure"?

Common synonyms include: base, base, basement, basis, bearing wall, bed.

noun

noun ·Rare ·Advanced level

Noun

1
The supporting structure underneath something, such as a vehicle or bridge.
"Holonym: infrastructure (facilities sense)"
2
lowest support of a structure wordnet

"Holonym: infrastructure (facilities sense)"

From under- + structure.