I think you probably mean "bridge span" rather than "spam bridge." A bridge span is the part of the bridge between its supports.
Main span means the longest part of the bridge.
span bridge
A bridge's span is the length that crosses the gap.
The span is the distance between supports. The sum of the span lengths is the bridge length.
its the span of a bridge for which the cost of building both substructure and superstructure is same....
A span of gap is something in a gas leak or under a bridge that smokes.
the span
Akashi-Kaikyo Bridge, Japan is the longest single span bridge which is 1,991 meters long.
Tsurumi Tsubasa Bridge has a center span of 890m making it the largest bridge in the world!
That is the correct spelling of "span" (a distance, or to bridge).
A truss is a diagonal brace which provides structural support for a bridge span by extending between a vertical member and the bridge span.