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Bridges are commonly seen as static constructions. In reality, they behave more like dynamic, living entities. They are continually changing in order to function, adapting to varying loads, weather patterns, and other sorts of stress. Bridges, like people, must "react" to very stressful events such as accidents, explosions, fires, earthquakes, and hurricanes in order to survive in some situations.

This article will look at how different types of bridges are designed to withstand stress. We'll also look at some of the most typical forces that cause bridges to fail. These pressures can have a significant impact on how bridges age, deteriorate, and potentially fail.

Understanding them can help architects design long-lasting structures and inspectors and maintenance people extend the life of existing structures.

The gravity dilemma

Gravity is the most powerful force impacting bridges, continually pulling at them and attempting to drag them down to earth. Gravity isn't so important when it comes to buildings, especially massive ones like skyscrapers, because the ground beneath them is always pressing back.

When it comes to bridges, however, this is not the case. Their decking stretches across open space. "Space" offers no resistance to gravity. Longer span bridges are more vulnerable to gravity than shorter span bridges. Similarly, heavier constructions are more likely to be destroyed by gravity than lighter structures.

Bridge failures are relatively uncommon. So, what is it that prevents them from collapsing due to the force of gravity?

The solution is the same regardless of the type of structure:

1. Tension (a force that stretches and pulls outward) is carefully balanced with compression (a force that pushes or squeezes inside).

2. The load (the total weight of the bridge structure) is channeled onto the abutments (the supports at either end of the bridge) and piers (the supports that run under the bridge throughout its length).

3. These forces are distributed differently on different types of bridges:

Beam Bridge

A beam bridge has its deck (beam) in tension and compression. (The beam can be squeezed and stretched depending on conditions.) The abutments are in compression, which means they are always being squeezed.

Arch Bridge

An arch bridge supports loads by distributing compression across and down the arch. The structure is always pushing in on itself.

Suspension Bridge

The towers (piers) of a suspension bridge are in compression and the deck hangs from cables that are in tension. The deck itself is in both tension and compression.

Cable-stayed Bridge

A cable-stayed bridge is similar to a suspension bridge. However, the deck hangs directly from the piers on cables. The piers are in compression and the cables are in tension. The deck experiences both forces.

Truss Bridge

A truss bridge is a variation of a beam structure with enhanced reinforcements. The deck is in tension. The trusses handle both tension and comprehension, with the diagonal ones in tension and the vertical ones in compression.

Cantilever Bridge

A cantilever bridge is one of the simpler forms to understand. Basically, it addresses the forces of tension (pulling) above the bridge deck and those of compression (pushing) below.

Stressors beyond gravity

The fact that compression and tension on a bridge are continually fluctuating due to pressures such as:

Changing loads

Bridges would be simple to construct if the stresses on them were constant. The forces acting on them would not change. In reality, loads can fluctuate substantially and dynamically throughout the day and over time.

Bridges transport anything from trains, autos, and trucks to water pipes and other utility equipment. The amount of traffic and utility flow fluctuates throughout the day, generating considerable fluctuations in the live load, which can cause tensile and compressive forces to increase and decrease across the structure.

• Example: When a railroad travels over a bridge, the structure bends and flexes, then returns to its original relaxed state once the train passes by.

Environmental forces

Bridges constantly react to Mother Nature. Environmental sources of stress include:

• Tides, waves, and water back-ups. Water is one of the most powerful forces on earth. Engineers often insert openings into bridge abutments to allow water to flow through rather than push against them.

• Winds. Large gusts of wind can cause bridges to sway and twist. Modern ones are lighter and more aerodynamic, allowing wind to pass through them, which prevents them from moving.

• Earthquakes. Seismic forces cause bridge sections to shake and crash into each other, which can make them crumble. Designers include dampers to absorb vibrations and bumpers to keep sections from banging into each other on bridges in active earthquake zones.

• Hurricanes and other major storms can have devastating effects on exposed areas of bridges. Construction teams often install protective equipment around vulnerable sections, such as utility infrastructure.

• Ice, cold, and blizzards. Cold weather and freezing conditions cause contraction on certain bridge elements. Thawing can have the opposite effect. The impacts of expansion and contraction have been exacerbated in today’s more extreme climate conditions. Engineers account for this by incorporating more responsive and flexible components into bridges constructed in cold places.

Accidents and other unexpected events

Accidents in traffic and construction, boats hitting abutments, and explosions can all cause substantial bridge stress and, in some cases, failure. To decrease the impact of extreme occurrences on the balance of forces impacting a bridge, builders can use strong, fire-retardant materials and isolating devices.

Summary

Some of the forces described above may cause instantaneous catastrophic damage or failure of bridges. These pressures also erode bridges over time, causing long-term harm. Bridges, like biological beings, have ways of signaling when they are strained. Inspectors, supervisors, and engineers must be on the lookout for these indicators. It can assist them in keeping current structures safe while also providing the information required to develop even more durable and responsive structures in the future.