Suspension bridges are a type of structure where the deck is hung below a series of suspension cables that are on vertical suspenders. The first modern examples of this design began to appear in the early 19th century. Simple bridges without the suspenders have been in place in the mountainous regions of the world for many centuries.
Suspension bridges have cables that suspend between its towers to help carry the weight of the deck with its vertical suspenders. This design allows for the deck to arc upward for additional clearance or to sit level. Many of them built without falsework.
For the suspension design to work, there must be cables anchored to each end of the structure. Any load applied to the bridge transforms into tension that the main cables must support. That’s why they continue beyond the pillars to the supports at the deck, and then continue into the ground anchors to provide enough support.
There are several unique advantages and disadvantages of suspension bridges to consider when communities start looking at the need for a new span.
List of the Advantages of Suspension Bridges
1. Suspension bridges are relatively affordable to construct.
Suspension bridges are relatively economical to build in the sense that only a minimum amount of supplies is needed to create the structure. You only need the anchors to support the construction, cables that support the deck, and then the pathway for pedestrians or vehicles.
This structure can prove to be a comprehensive solution for communities looking to build a functional bridge without plenty of funds, primarily because wood and rope are just as useful as steel or iron for its construction. Despite the lack of funds, a suspension bridge can still look visually appealing and thoroughly operative from a practical viewpoint.
2. Bridges using this technique can cover long spans.
China has built half of the 10 longest suspension bridges currently operational in the world today. The current record holder is the Akashi-Kaikyo Bridge in Japan for a suspension standpoint, with a central span that measures almost 2,000 meters. The six lanes of combined traffic that it supports offer a combined length of almost 4,000 meters. It took ten years to complete the construction, supporting Honshu and Shikoku by highway and rail. It can withstand an 8.5-magnitude earthquake or wind speeds of up to 286 kilometers per hour.
The Xihoumen Bridge connects Cezi Island with Jintang Island in China, providing a main span of 1,650 meters. The four expressway lanes measure a total of 5,300 meters, built from 2004 to 2009 as part of a long-term infrastructure improvement plan. This design is effective for long spans because the support mechanisms remain the same.
3. It is easier to maintain suspension bridges when compared to other styles.
Once the construction of the suspension bridge is complete, there are relatively few materials required to maintain its upkeep. Most communities can get by with routine checks of the hardware and structural components, a layer of paint every so often, and some deck repair. There are fewer needs for major overhauls because there aren’t as many moving components with this design. That means the ongoing expenses for the span are less, so you can save money establishing the bridge and while taking care of it.
4. Suspension bridges provide a landmark for many communities.
The undulating shapes of the standard suspension bridge make it an aesthetically pleasing experience. The linear features and curved shapes help communities to design a landmark that can draw tourism dollars to the region. Iconic bridges like the Golden Gate, Tower, or Brooklyn all bring people to the structure because they want to see it. Suspension bridges already provide a familiar shape and user confidence with their design, encouraging all of them to become famous in their own way.
5. This bridge design requires very little access from the bottom of the deck.
Except for the installation of the first temporary cables during the construction of the bridge, there is very little access required from below to finalize this structure. That makes it possible to leave a waterway open while the bridge is under construction since almost all of the work occurs on top of the bridge. This advantage also makes the cost of labor less than it is for other designs, allowing for a lower overall installation cost.
Even when ongoing maintenance becomes necessary with a suspension bridge, this advantage still applies. Unless there are pipes, cables, or additional items outfitted to the span for some reason, there is no need to service it from below.
6. There is more flexibility with the configuration of the suspension bridge.
When installing a new suspension bridge over a span of any size, the deck can be built so that it can have sections of it replaced. This advantage allows a community to expand the section to widen the lanes of traffic or add pedestrian-specific areas for travel. You can also make adjustments to the amount of weight that the bridge can support over time. That means the structure can evolve with the area so that it can continue to offer a useful deck for whatever traffic needs to cross.
7. It takes less time to build suspension bridges when compared to other designs.
You don’t need as many anchorages with a suspension bridge when comparing it to non-suspension options. There are fewer anchors necessary as part of the overall design, which means you don’t need as many cables to support the deck. If a community decides to go with a cable-stayed design, then a single tower may be enough to complete the entire foundational process. This advantage makes it possible to reduce the amount of time it takes to take the bridge from concept to reality.
8. There are several variations available for suspension bridges to follow.
When building a suspension bridge, there are different deck structure types that engineers can use to complete the structure. Most of them use an open truss structure after discovering the issues with plate girders after the collapse of the Tacoma Narrows Bridge in 1940. That deck option twisted and vibrated violently at wind speeds of only 40 miles per hour. There are sharp entry edges and sloping under-girders that are also useful in some designs.
Engineers can opt for suspender cable terminations. These are created by inserting a suspender wire rope into the narrow end of a cone-shaped cavity orientated into a line where strain on the bridge exists. There are different cable types too, such as chain, linked bars, or multiple strands of wire. The latter adds strength and improves the reliability of the span since a flawed strand won’t have the same impact as a broken link.
9. Underspanned suspension bridges are a possibility.
When engineers decide that an underspanned suspension bridge is useful for an installation point, then the primary cables hang entirely below the deck of the structure. They are still anchored into the ground in a manner similar to the conventional design. Only a few bridges of this style have ever been built since the deck is much less stable when suspended below the cables. The Micklewood Bridge is one of the best-known examples of this design, and it may have been the first of its type. It was 103 feet in length, had heavy cross braces, and the main cables were chains.
The only remaining underspanned bridge in the United States currently in use is the Kellams Bridge. It spans the Delaware River between Hankins, NY and Stalker, Pennsylvania. It recently underwent a $4.5 million renovation project.
List of the Disadvantages of Suspension Bridges
1. Suspension bridges can struggle to support focused heavy weights.
The goal of a suspension bridge is to continually transfer the tension and weight of traffic as it moves along the span. All of the cables work together to make this happen, but there is an upper weight tolerance that one must consider. If the focused weight of a heavy object crossing the span were to be greater than the weight limit of a single cable, then the entire structure would be at risk of collapse. That’s why you do not usually see this design in place when significant rail traffic needs support.
2. There is less flexibility with a suspension bridge.
All bridges have a risk of failure when extreme stresses impact the structure. Suspension bridges tend to struggle with this issue more readily because the deck “floats” with the use of the anchors and cables. Since each point has limitations that must be taken under consideration, a combination of vertical pressures and side wind speeds can lead to a failure of the span. This disadvantage causes them to not remain sturdy under high wind conditions unless reinforcements are built into the structure.
3. High winds can cause a suspension bridge to start vibrating.
When there is a stiff breeze that comes across the face of a rigid suspension bridge, then the forces acting on the structure can cause the deck to begin vibrating. Most of the spans in use today require aerodynamic profiling or added stiffness to its design so that this outcome does not occur. This disadvantage can be cumbersome in some situations since the added weight or extra components may not be fully supported by the installation point.
4. Some access below the deck may be necessary during construction.
Suspension bridges try to limit the amount of time that workers spend underneath the deck because access points can be challenging. This issue cannot be entirely removed from the construction plan, even with cable-stayed bridges, because of the need to support the overall structure. That means a backup plan is necessary for waterways so that traffic can get through accordingly. Most of the work involves lifting the deck units and the initial cables, but there can be other requirements to manage as well.
5. Suspension bridges have a lower deck stiffness compared to other designs.
The typical suspension bridge design offers a relatively low deck stiffness compared to other spans that don’t use this engineering concept. That makes it virtually impossible for the span to carry heavy rail traffic when highly concentrated live loads occur frequently. Reinforced designs can minimize this disadvantage to a certain extent, but the design will rarely have a chance to compete with non-suspension options. That’s the reason why you usually see the suspension designed used for vehicular traffic, cycling, or pedestrian needs and not on railroad crossings.
6. Some suspension bridges require extensive foundation work.
If the suspension bridge needs to be built in an area that has a lot of soft ground, then engineers must spend a considerable amount of time working on the foundation of the structure. Extensive reinforcement of the tower anchors is necessary because the weight of the bridge would force it to begin sinking into the ground over time. Every heavy load that the span would support would force it deeper into the earth. Even when this work takes place, there is no guarantee that the result will be successful.
7. The failure of one cable can be enough to cause the entire bridge to collapse.
The Kutai Kartanegara Bridge in Indonesia suffered a catastrophic collapse in 2011. There were 20 people killed and 40 injured, along with 19 people listed as being missing. The deck of the structure was completely destroyed. Two bridge pillars were standing at the time of the collapse. The reason for the incident was human error. The structure collapsed when workers were repairing a cable.
Only one support cable snapped to cause the suspension bridge to suffer a catastrophic failure. That’s why continuous maintenance and inspection must occur, along with the support of experienced engineers and laborers who can support the health of the span. Almost every recorded collapse of a suspension bridge has been due to overloading in some way.
8. There are cheaper bridge options to build.
Suspension bridges are one of the more affordable span options that communities can build to support transportation networks. It is also more expensive to install them when compared to some of the other options. When a suspension bridge fails for some reason, it is usually replaced with an arch bridge because of the effectiveness of the design and its overall tensile strength when combined with the final cost in comparison.
Humans have constructed suspension bridges since the beginnings of our history. The earliest versions of this technology were a simple rope span that was slung from one end of a canyon to the other. Then the deck was either at the same level or slightly below the ropes to facilitate pedestrian movement.
Thangtong Gyalop began using iron chains to build simple suspension bridges in eastern Bhutan during the 15th century. This effort was so strong that the final one, located in Duksum on the route to Trashiyangtse, didn’t wash away until 2004.
When looking at the advantages and disadvantages of suspension bridges, it is clear to see that there are some weak points in the design to consider. When engineers can create an effective design that counters these problems, then the potential of this span is pretty amazing. If that outcome is not possible, then there is a significant risk of collapse that must be taken under consideration.
Keith Miller has over 25 years experience as a CEO and serial entrepreneur. As an entreprenuer, he has founded several multi-million dollar companies. As a writer, Keith's work has been mentioned in CIO Magazine, Workable, BizTech, and The Charlotte Observer. If you have any questions about the content of this blog post, then please send our content editing team a message here.