understructure

noun

understructure

noun

Etymology From under- + structure.

1

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

Holonym: infrastructure (facilities sense)
Near-synonyms: infrastructure (foundations sense), underpinnings

1 more example

Four small suspension bridges, complete with towers and cables like those on the span across the East River, are now encased inside the Capitol’s subbasement understructure.2008 January 22, Kirk Johnson, “Strength Renewed, a Grand Old Building Is Back”, in New York Times:

Entry derived from the Wiktionary, under licence CC BY-SA 4.0 — list of authors.