A suspension bridge in its simplest definition is where the road hangs from cables. The roadways are suspended over a span of distance between two towers, through cables, chains or ropes. A classic example of a suspension bridge is San Francisco’s Golden Gate Bridge. The entire structure is composed of several parts.
- The towers support most of the weight of the bridge. Compression pushes down the deck and then travels up to the cables, chains, or ropes that connect the towers and then dissipate.
- Anchorages are solid rocks or massive concrete blocks where the cables run horizontally.
- The supporting cables receive the tension forces from the bridge, pass it to the anchorages and into the ground.
- The deck truss is the supporting system beneath the bridge. It stiffens the deck and lessens sway and ripple.
The simplest suspension bridge was made of ropes slung across a chasm. The first version of early suspension bridges that used iron chains were built by Thangtong Gyalpo – a saint and bridge builder of Tibet, while the first structure that resembled the modern suspension bridge was attributed to Fausto Veranzio, a Venetian polymath.
Despite being the design used for some of the world’s iconic bridges, a suspension bridge has its share of pros and cons.
Pros of a Suspension Bridge
1. It can span over long distances
Suspension bridges can span anywhere between 2,000 to 13,000 feet, which is farther than any type of bridge can accommodate. This is why it is often the design of choice when connecting very distant locations. But it can be underspanned when needed, similar to what was used in building the James Smith’s Micklewood Bridge and the Pont des Bergues. The top 3 suspension bridges with the longest spans are the Akashi Kaikyō Bridge in Japan that spans over 1,991 m, the Xihoumen Bridge in China at 1,650 m, and the Great Belt Bridge in Denmark at 1,624 m. The top longest suspension bridge has held its title since 1998. Are you wondering which rank is the Golden Gate Bridge of San Francisco? It is at #12 at just 1,280 m.
2. It is inexpensive to build
Suspension bridges can be inexpensive in the sense that only a minimal amount of materials are needed to create it. You only need the anchorages, cables and roadways. This can prove a perfect solution for communities looking to build a functional bridge without plenty of funds, especially because wood and rope can be used for its construction. Despite the lack of funds, a suspension bridge can still look visually appealing and fully functional from a practical standpoint.
3. It is easy to maintain
Once the construction is completed, suspension bridges do not require plenty of materials to maintain and ensure its integrity and longevity. As long as the structure is regularly checked, it can last a very long time, without requiring a major overhaul. This cuts down ongoing expenses for bridge maintenance that a community has to shoulder. Along with the need for a minimal amount of construction materials, a suspension bridge is inexpensive overall.
4. It is incredibly versatile
As long as there is a place where a support tower or anchorage can be installed, a suspension bridge can be built practically anywhere. There is no need for inflow restrictors to be placed underneath either. Compared with other bridge types, suspension bridges may also withstand earthquakes. Strong and high winds, however, may be a different story.
5. It is aesthetically pleasing
As opposed to beam bridges and truss bridges, a suspension bridge is visually appealing because of its undulating shapes. It has linear and curved features that make up a beautiful structure. How the cables are suspended also enable a versatile suspension bridge construction.
Cons of Suspension Bridges
1. It is vulnerable to the wind
Although these are fitted with a truss system beneath the roadway to keep the bridge from bending and twisting, a suspension bridge is designed to be flexible enough to move with the wind, but not break entirely. So when the wind is strong or high, the bridge may incur damage. If too much weight is put on support cables, they can break when strong winds blow. This is why engineers must account for these elements when building the support bridge and ensuring that the road is stable.
2. It has load limitations
Cables would be incapable of holding up heavy roadways, which is why a suspension bridge has to be build light enough for the cable to support. This means it can’t accommodate heavy loads of traffic, and it can only allow a certain amount of weight to pass through. Only certain types of traffic can cross a suspension bridge, and some communities even resorted to limiting access to a certain amount of the population at a given time.
3. It costs time and money
It is confusing, yes, considering that suspension bridges are generally inexpensive. But it is costly in the sense that it takes a lot of time to build and for the entire duration of the construction, access under the bridge are strictly limited, especially through the waterways. This means ships carrying loads or supplies would have to divert to a different course. The loss of income or business over an extended period of time can have a huge impact on the local economy. This makes suspension bridges costly in a way. If a community heavily relies on boat traffic, the construction of a suspension bridge can inconvenience a lot of citizens.
4. It has limited applications
A suspension bridge maybe versatile construction-wise, but it can only be used in specific situations. Because it needs to be flexible enough to keep the wind from destroying the roadways. It can’t be stiff and strong enough to support rail traffic. So it can only be used by general traffic but not rails, especially because it can’t support heavy freight that rails often carry.
Moreover, suspension bridges require anchorages, massive concrete blocks or rocks that call for equally huge land area. Without the right site for the anchorage to be installed, construction of a suspension bridge would not be possible.