Small oscillations of truss bridge with two-section deck

 

The simplest possible truss bridge that can still be called a bridge. Two deck sections, three movable joints (points) in a plane – i.e., 6 degrees of freedom and thus also 6 modes.

The slowest mode No. 6 represents transverse oscillation of the bridge, the second slowest mode No. 5 is longitudinal oscillation.

The constant ω0 is given by the mass of the rods and their stiffness, ω0 = ω0(m, k). Its specific value is not important.

 

Mode No. 1, ω = 1.82 · ω0

Mode No. 2, ω = 1.61 · ω0

Mode No. 3, ω = 1.51 · ω0

Mode No. 4, ω = 1.18 · ω0

Mode No. 5, ω = 0.58 · ω0

Mode No. 5 corresponds to longitudinal oscillations of the bridge.

Mode No. 6, ω = 0.49 · ω0

Mode No. 6 corresponds to transverse oscillations of the bridge.