[Paul Copioli]

Trusses are very efficient means of transferring load. Weight efficient, that is. When it comes to pure buckling, total weight really doesn't matter. Critical load, the load at which buckling occurs, equals pi^2*E*Ia/L^2.

E is the Young's Modulus of the column (or column/core combination)
Ia is the area moment of inertia (mm^4 or in^4)
L is the length of the beam.

This equation is based on some assumptions, but the criteria for validity is, generally, that L/r must be grater than 100. r is the radius of gyration mentioned before. r^2 = Ia/Area.

Would a bridge with solid plate sides instead of trusses hold more load? Probably, depending on geometry conditions.

Would it weigh too much? Yep.

Would it be better or worse for dynamic conditions (think Tecumseh Narrows bridge)? Much worse. Mass is a major player for dynamic performance.