underdeck

plural underdecks

1. The part of a bridge that lies beneath the deck, or surface.

   1 more example

   During my nearly 20 years as an engineer with the New York City Department of Transportation, I witnessed numerous bridge failures, including the collapse of the West Side Highway in 1973, the fatal snap of a Brooklyn Bridge cable in 1981, and the 1989 fall of a concrete slab from the underdeck of Franklin D. Roosevelt Drive, which crushed a Brooklyn dentist in his car.

   2007 August 13, Samuel I. Schwartz, “Catch Me, I’m Falling”, in New York Times