Why Are Bridges Built So High off the Ground?

High Suspension Bridge

We’ve all marveled at the iconic Golden Gate Bridge and its towering piers, either in real life or through a screen. But have you ever wondered why exactly those piers are that tall? And why are bridges built so high off the ground?

Bridges are built so high off the ground to allow water traffic to pass underneath safely. They are built high to reduce the tensile forces acting upon the bridge wires. A greater angle between the ground and cables reduces horizontal tension as the forces get verticalized with higher towers.

While suspension bridges might not seem too complex at first glance, there’s a lot of clever engineering that goes into optimizing their design. Read on to find out more about them!

How Tall Should Bridge Towers Be?

The height of the bridge towers isn’t just a design choice, as a lot of engineering thought goes into calculating the right height for a bridge and its support piers. Among other factors, the optimal height also depends on whether it’s a cable-stayed bridge or suspension bridge.

Bridge towers should be at least as tall as 20% of the bridge span, or the distance between both towers. For suspension bridges, the tower height should at least be 10% of the bridge span. Suspension bridges are preferred over cable-stayed ones in most scenarios, as no one goes underwater.

Suspension bridges are an economical solution for creating pathways over rivers with sufficient clearance for water traffic passing under them. They are neither too costly to build nor require massive piers to support the deck. And no one needs to go underwater to build them.

Besides managing the bridge’s weight distribution, there are certain height requirements that a bridge needs to meet to make sure there’s sufficient space for waterway traffic passage underneath it.

State-imposed regulations ensure a certain degree of vertical clearance under bridges built over waterways. The higher the bridge deck gets, the taller its towers need to support its weight, which brings us to the next question.

How Tall Do Bridge Decks Need To Be?

While bridges are constructed to facilitate traffic and allow it to pass over rivers, they shouldn’t get in the way of waterways and ships. For that, they need to be built high enough that ships and water traffic of all sizes can safely pass through.

Bridge decks need to be about 200’ (61 m), while vertical clearance for water traffic is 155’ (47 m) at mid-span. Different states have varying regulations on the minimum height of bridges. This height is considered safe enough to allow sufficient clearance for water traffic to pass through.

Keep in mind that sailors and captains should always be careful while passing under bridges.

Sometimes, the water level can be abnormally high, which makes the passage riskier during high tides. The bridge deck can also droop slightly lower at the boundaries of the navigation channel, so due diligence is required before approaching the bridge.

Bridges with relatively lower vertical clearance require careful observation for safe passage, especially if the vessel is tall enough to be close to the bridge’s clearance height. Even if the ship is slightly shorter than the bridge’s listed clearance, factors like high tides, wires hanging off, or even the wave wakes from passing by water traffic can cause collisions.

The captain can also request an opening when it’s necessary for safe passage.

However, in most cases, that can be avoided with sufficient knowledge about the vessel’s height and lowering the ship’s outriggers or antennas. Note that unnecessarily requesting a bridge opening can result in significant fines.

How Do Suspension Bridges Work?

Crossing waterways without a boat requires a platform built over it, and suspension bridges offer the most popular solution. Besides looking majestic, they play a critical role in allowing traffic to pass over rivers. But, how exactly do they work?

Suspension bridges consist of two tall towers with cables attached to them, which carry the weight of a suspended bridge deck. Since these cables are connected to piers, the deck weight gets transmitted to the ground. Many vertical suspender wires connect the deck to the main supporting lines.

The deck’s weight gets transmitted by the mainline to the towers, putting an inward force on each of them through the attached cables.

But the mainline support cable stretches past the two towers and anchors to the ground at both ends. This way, the inward force from the weight of the bridge deck gets canceled out by the extended cables secured in solid rock or concrete blocks on both sides of the bridge.

The height of the towers also plays a vital role in managing the inward pull and tensile forces. The higher the angle between the cables and the ground, the more verticality it adds to the structure.

With taller towers, the direction of the cable’s pull becomes more vertical.

On the other hand, shorter towers result in more horizontal tension and hence more inward pull. Shorter piers also increase tensile forces on both sides, which require stronger, heavier, and costlier suspension wires.

Thanks to this balance of outward and inward forces, these towers can be pretty thin and still support a full-size suspension bridge.

The anchors on each side of the bridge are strong enough to pull the towers with a force that’s equal to the deck’s weight. As a result, all weight gets centered on the piers that transmit it to the ground.

Modern Suspension Bridges Are More Reliable

Suspension bridges go way back, but they weren’t always as reliable and capable as the ones we have today. Some design flaws included chain cables and unstable bridge decks, which could be safety hazards under more weight or unpredictable environmental forces.

The thin bridge decks were unstable and unreliable under strong natural forces and bad weather. Even heavy winds could shake up the entire structure.

If the platform isn’t strong enough, no degree of clever engineering on the support cables or tower height could prevent the bridge from falling apart. Modern suspension bridges have sturdy bridge decks that don’t sway with heavy winds.

The chain cables in earlier bridge designs also weren’t ideal for supporting their weight as the entire system could collapse if one link broke lose. Modern designs replaced them with sturdy high-tensile strength steel cable bundles.

That way, even if the bundle fails partially, the bridge doesn’t collapse. Thanks to the advancements in civil engineering, we’ve optimized the safety, practicality, and beauty of suspension bridges.

The world’s longest traffic suspension bridge is the Akashi Kaikyo Bridge in Japan, which is 3,911 m (12,831 ft) long and 282.8 m high (928 ft).

Conclusion

Bridges and piers are tall due to various reasons. These include optimal weight distribution along the support lines of the bridge and meeting vertical clearance requirements. These requirements ensure that the water traffic passing under the bridge gets enough space to get through safely despite the size of the vessel.

Suspension bridges and cable-stayed bridges also droop lower in a parabola shape near the two ends, so the bridge deck’s height needs to account for that to ensure sufficient vertical clearance across the entire bridge span.

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