Final answer:
Soldiers are ordered to walk out of step across a bridge to prevent forced oscillations and resonance. This unsynchronized walking pattern disrupts the rhythmic pressure that can induce large amplitude oscillations and threaten the structural integrity of the bridge.
Step-by-step explanation:
The question, 'Why are soldiers in general ordered to "route step" (walk out of step) across a bridge?' pertains to the concept of forced oscillations and resonance in physics. The reason behind this order is to prevent the possibility of resonance. When soldiers march in step, their synchronized footsteps can create rhythmic pressure on the bridge structure. If the frequency of this pressure coincides with the bridge's natural frequency, it can induce resonance, potentially leading to large amplitude oscillations which can cause structural failure.
To avoid this, soldiers are instructed to walk in an unsynchronized fashion—route step—which varies the forces applied to the bridge and prevents any potential for resonance that could compromise the structural integrity of the bridge.