understructure

under- +‎ structure

understructure (plural understructures)

1. The supporting structure underneath something, such as a vehicle or bridge.
   • 2008 January 22, Kirk Johnson, “Strength Renewed, a Grand Old Building Is Back”, in New York Times‎:

     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.

• unrestructured