Steel truss bridges have ‘secret’ defences against collapse

Steel truss bridges have been the backbone of transport networks since the late 19th century. They are built from interconnected steel bars and they can span long distances and carry heavy loads, rendering them ideal for railways and highways. The Pamban, Howrah, and Saraighat bridges in India are some famous examples.

Many of these bridges remain in use today and often carry traffic far more than they were designed for. They’re also exposed to more intense natural hazards like floods and storms, faster rates of material corrosion due to environmental change, and the simple wear of a century of service.

When one part of a truss bridge fails, the entire structure can collapse suddenly and disastrously. Such collapses incur human tragedies as well as economic shocks, since closing a busy bridge can cost crores of rupees a day. Engineers understand the primary resistance of these bridges well: the way intact parts carry normal traffic loads. But they have been less clear why some bridges survive after one component breaks while others collapse quickly.

A study in Nature on September 3, by researchers from Spain, has revealed why.

The team built a scaled-down steel truss bridge in the laboratory based on a common railway design called a Pratt truss. Then they simulated damage by cutting through specific components, such as chords and beams, to mimic sudden failure. In each scenario, sensors recorded how the structure responded. The team also created advanced computer models that reproduced both the intact and damaged states, allowing them to simulate more than 200 different damage scenarios.

The experiments revealed six fundamental secondary resistance mechanisms that activated when a main component failed: panel distortions, torsion of the whole structure, hinged rotations, out-of-plane bending, simple bridging by nearby members, and uniaxial bending. Like a spider web adapting to the loss of a thread, each of these mechanisms rerouted loads through alternative paths, preventing immediate collapse. Which mechanism dominated depended on which part failed. For example, losing a diagonal mainly triggered panel distortions while losing a chord involved global torsion and rotation.

Even when damaged, the bridge specimen was surprisingly robust. It could withstand loads up to 3x higher than standard operating levels before collapsing. The failures propagated differently depending on the role of the original component. For instance, members that sustained compression, like upper chords, led to brittle failures while tension-bearing members like lower chords led to more gradual and ductile failures. In all cases, however, the bridge only collapsed following a cascade of buckling failures spreading through the structure.

These insights open new doors for engineering practice. Just as understanding secondary mechanisms reshaped building design worldwide, the same knowledge can be used to guide safer engineering. For new bridges, engineers can refine designs to bolster secondary resistance mechanisms. In existing structures, inspections and retrofits can focus on critical areas that help activate these ‘secret’ defences. The study also provides a roadmap to make bridges more resilient to accidents, nature disasters, and the test of time.