Is there a specific word for these diagonal bars? I doubt engineers would call them diagonal bars since they're widely used in engineering, radio towers and bridges off my head use them.
This is called cross bracing--diagonal supports used to reinforce the structure of a building or bridge or similar.
Individually, the bars might be called beams or girders.
The overall structure might be called a space frame, although that term is more often used for structures that cover, roof, or enclose an area, rather than support a tower. Still the bascic structure of triangular bracing is similar.
In Structural Engineering, this kind of building group is called Trusses. Trusses are a Structure that is made from 3 Sides, as the triangle is considered the Strongest Shape to build with. That said, they use it to create Cross-Bracing made up of Members as they are called; Beams or Girders (Horizontal Members), Columns or Posts (Vertical Members) and Panels (Wall/Cover Members).
A good truss reference: https://www.dictionary.com/browse/truss