When soldiers march over bridges, they are routinely instructed not to do so in step. This practice, which might seem odd at first glance, is rooted in both historical events and physics. The reason lies in the prevention of what engineers call "resonant vibrational frequency." Simply put, if the rhythm of the soldiers' synchronized steps matches the natural oscillation rate of the bridge, it can cause increasingly amplified vibrations.
This concept is similar to pushing a child on a swing; if you push at regular intervals consistent with the swing's natural rhythm, the swing goes higher and higher. When applied to a bridge, these vibrations could potentially become powerful enough to cause structural damage, or in extreme cases, lead to the collapse of the bridge. The most famous example that illustrates this phenomenon is the Broughton Suspension Bridge near Manchester, England, which collapsed in 1831 reportedly due to vibrations caused by a battalion of soldiers marching in step across it.
Following incidents like these, military forces worldwide have adopted the practice of "breaking step" on bridges. This simply means soldiers walk out of their usual synchronized rhythm to avoid matching the bridge's frequency. This dispersion of forces ensures that no synchronous pressure points are hit continuously, essentially scattering the impact of the troop's feet over different points in the bridge's structure at any given time.
Catastrophic situations aside, the physics behind this practice is vital for preserving the long-term integrity and safety of bridges. Engineers design bridges with various load limits and resonance frequencies in mind, but an unanticipated force like a marching platoon can offset these calculations. Therefore, the practice of breaking step is an additional precaution to safely distribute weight and impact.
In contemporary settings, while many modern bridges are designed with materials and engineering techniques to better withstand such forces, the practice of breaking step remains a prudent safeguard. It underscores an awareness of how seemingly benign activities can interact with engineering structures in significant ways and showcases a fascinating intersection of human activity and scientific principles.
