Suspension Bridges: What They Are and How They're Built
When you think of a suspension bridge you likely picture a small overpass hanging precariously over a vast canyon, allowing travelers and tourists to cross from one side to the other. If you've ever walked across a suspension bridge, you know how much it can shake and sway under your weight. This feature is loved by some more brave-hearted tourists but is also the reason many people are scared of crossing them.
While this may be our personal relationship with what we often think of as suspension bridges, they belong to a much larger category than we give them credit for. Did you know, for example, that the Golden Gate Bridge, which is perhaps the most famous bridge in the world, is a suspension bridge? So are Tower Bridge in London, Brooklyn Bridge in New York, and many more of the world's most iconic bridges. Yet we're never scared of driving and walking across these ones, despite them being engineered in similar ways as many of the suspension bridges you might be afraid to cross on a hike.
What is a Suspension Bridge?
A suspension bridge is a specific type of bridge where the platform, usually a road, is hung below small vertical cables called suspenders. In turn, these suspenders are connected to much larger cables that are secured to towers on either side of the deck, which are anchored to the ground.
The description above matches a suspension bridge that functions as a road or crossway for vehicles, like those famous bridges we talked about, but there are much simpler designs made for pedestrians, nature enthusiasts, and tourists in mountainous terrains such as Golden, British Columbia and Canon City, Colorado.
How Suspension Bridges Work
Suspension bridges made for pedestrians and hikers do not have massive towers like the well-known bridges made for vehicles we discussed before. These structures that are popular with tourists are usually narrow paths made of wood, with suspenders connecting the deck to the cables that are doing double duty as railings for those crossing the bridge.
Those cables are successively attached to 3-4 foot-tall 'anchors' on either side. Known for their shakiness and seeming instability, simple suspension bridges are loved by many tourists, but cannot carry near the load of their more complex counterparts.
Speaking of, how does a bridge built with so little support underneath, if any, hold such mind-boggling weights as the 4,000 lbs per foot that the Golden Gate Bridge can support? This is what makes these majestic structures such an impressive modern-day engineering feat.
Suspension bridges are built so that the compression forces from the weight applied to the deck are transferred through the suspenders to the large cables running between the towers. The force then follows the cables through the towers and is passed into the ground.
Each part of the bridge plays a significant role. Without strong anchors, the weight of the cables would drag the towers inwards, making them collapse down onto the bridge. The towers are necessary to pass the weight through the cables and on into the ground. The cables and suspenders must be capable of holding the entire burden being applied to the deck or one may snap and the bridge would crash down to the water or valley below.
Advantages and Disadvantages of Suspension Bridges
You may be asking yourself, what's the point of a suspension bridge? Are there no other bridge types that would serve the same purpose? There are many pros and cons to suspension bridges, but let's focus our attention on the top three in each category:
Advantages
1. Suspension bridges are relatively cheap.
The first advantage to these structures is that they're economical to build. Not needing as many supports as other types of bridges, suspension bridges require a minimal amount of supplies to get the job done.
2. They can cover long spans.
Another major advantage of this type of bridge is that it can easily span long distances, even up to 7,000 feet (2,133 meters). This benefit comes from the design of a suspension bridge. Whether covering 70 feet, 700 feet, or 7,000 feet, the support mechanisms can remain unchanged without compromising the effectiveness of the bridge.
One excellent example of this is the Akashi-Kaikyo Bridge in Japan, which has a central span between two towers of 2,000 meters. The longest suspension bridge in the world is the 1915 Çanakkale Bridge in Turkey. Opened in 2022 after five years of construction, this is the second tallest bridge in Turkey, and including the approaches and the main span reaches a total length of 3,563 meters.
3. Construction requires little access from below the deck.
Finally, a major benefit to suspension bridges is that after the installation of the first temporary cables, there is very little access required from below to build the structure. This is an advantage for a few reasons: primarily, since most suspension bridges are built over water, it allows the waterway to remain open during construction. Especially important in port cities, building this type of bridge won't overly interfere with water traffic coming in and out of the port.
Disadvantages
1. Suspension bridges can't hold as much weight as other bridges.
The design of the suspension bridge, while great for covering long distances, has a limit to the amount of weight it is able to withstand. With so little support from underneath the deck, as heavy objects cross the overpass, the weight from the object can never exceed the weight limit of any one cable, or else the entire structure would be at risk. For this reason, if a bridge is required to withstand very heavy loads such as rail traffic, it is best to use another design.
2. Very little room for error.
As mentioned above, the weight from traffic on the bridge can never exceed the weight limit of a single cable. In 2011, the risks of such a small margin for error were shown in the collapse of the Kutai Kertanegara Bridge in Indonesia. This catastrophe was caused by flaws in engineering which allowed stresses on the bridge to break just one support cable, leading to one of the worst bridge collapses of the 21st century. At least 20 people were killed and many more were injured as the 710-meter-long bridge fell to the river below.
3. They can require significant foundation work.
If construction is taking place in an area that has a lot of soft ground, then a suspension bridge may require significant time and effort to be put into building the foundations and anchors for the structure. The anchors are vitally important to the success of the bridge, and they would need to be heavily reinforced to prevent them from sinking further into the ground over time.
When Was The First Suspension Bridge Built?
The first modern suspension bridge was built between 1819-1826, connecting Wales and Anglesey in Great Britain. Designed by Scottish civil engineer and architect Thomas Telford, the Menai Bridge stands approximately 100 feet above the water, is supported by sixteen huge chains, and spans 579 feet.
Although Telford is credited with designing the first modern suspension bridge, he was about 400 years late to the party. The first-ever suspension bridges are credited to the Inca civilization and date back to the 1400s. The bridges were constructed using grass that was woven into bundles, and each cable was replaced yearly by local villagers. As you might imagine, repairing these bridges was very dangerous. With no real equipment, these villagers had to be out on the overpass as they replaced the cables, and these "repairmen" often faced death.
How Are Suspension Bridges Built?
Construction on suspension bridges most commonly starts on either end with the anchors. In the case of the Golden Gate Bridge, giant concrete blocks each weighing 60,000 tons were sunk into the ground to act as anchors for the future cables of the bridge.
After the anchors come the towers. One way for workers to build these massive structures over the water running underneath is using caissons. A caisson is a large watertight chamber that is sunk to the bottom of a body of water to act as a foundation for construction projects. While building the Brooklyn Bridge, caissons were lowered and then filled with concrete as bases for the towers that reached 278 feet above the surface of the water. Temporary piers can also be built to provide easier access to the towers for the construction workers.
Towers are most often built from either steel or reinforced concrete, and both of these materials have been used regularly for many of the world's most well-renowned suspension bridges. Going back to Golden Gate as an example, the famous towers were made out of pure steel. After the underwater foundation was built, giant concrete blocks were added to what would be the bottom of the tower. These were to be used both as a foundation and as protection for the tower in case of collision from shipping boats.
The actual construction of the towers requires the lifting of sections of steel or concrete up and into place. In the case of Tower Bridge, four concrete pieces were lifted atop each other and laid onto massive granite slabs. The slabs were covered in specially prepared canvas to facilitate the concrete being set onto it, and the towers were keyed to their foundations using very large bolts built into the piers.
Now for the cables. Cables for suspension bridges are hardly ever individual wires that span the length of the bridge. Instead, thousands of wires are twisted together to create the finished product. An example of this is the 27,000 individual pieces that were twisted together as part of the 80,000 miles of wire that make up the cables of the Golden Gate Bridge. That's enough to wrap around the earth 3.5 times!
To put the cables in place, ships are often used to spread them across the waterway, where cranes can then lift the wires and attach them to the tops of the towers. After the initial attachment, crews then must work to wrap more wire around these main cables, strengthening them until they are capable of supporting the weight necessary to the bridge.
From these main cables, the suspenders are then dropped down to hold up the deck. Crews will attach trusses and beams to the suspenders which create the foundation of whichever type of overpass will end up being built, most often a roadway. Concrete is then poured onto the foundation to build the path across the bridge.
Final Thoughts
Suspension bridges are some of the most majestic, iconic, and awe-inspiring structures we have in the world today. A true feat of engineering, these beautiful bridges have become famous in China, Japan, Canada, the UK, and America, among many other countries, both for their ability to transport vehicles and for making great experiences within nature, despite how scary they may be.
So what should we feel when we see them? What can we take from the designs of a few of the greatest engineers of our time? The answer is simple: inspiration. The inspiration that we can build things previously unheard of. The inspiration that we can span gaps that were once seen as too vast to cross.
Suspension bridges show us that we can do what seems impossible. So be creative, problem-solve, and accept new challenges, because maybe, just maybe, that impossible project you have has been possible all along.
