A cable-stayed bridge offers a design that is similar to a suspended bridge It will have towers that help to support the structure, while the deck is held in place by cables. The difference in the design is that the cables hold the deck by connecting it directly to the support pillars instead of using suspending wires or cables to stabilize the span.
This bridge type is useful for numerous traffic options, including automobiles, trucks, bicycles, and pedestrians. In some situations, a cable-stayed bridge is suitable for light rail as well. Engineers use this option when a span must be longer than what a cantilever bridge can support because of its weight, yet it is also short enough so that a suspension bridge is not the most practical option.
The first cable-stayed bridge in history is credited to Fausto Veranzio, who published his idea about this design in his work entitled Machinae Novae. The first bridges that were actually built using this method bean to appear in the 19th century. Many of the first suspension bridges would use elements of this design option as well. That includes several famous designs, including Brooklyn Bridge, Victoria Bridge, and Albert Bridge.
There are several advantages and disadvantages of a cable-stayed bridge to consider, so these are the key points that you will want to review.
List of the Advantages of Cable Stayed Bridges
1. Cable-stayed bridges take less time to complete than other options.
One of the most significant advantages to consider when evaluating a cable-stayed bridge is the amount of time required to complete the construction. This option does not require the same levels of anchoring that you will find in alternative designs. There are also fewer cables required to help support the deck because of how it ties to the support pillars or towers.
2. The strength of a cable-stayed bridge is unquestionable.
Because the cable-stayed design is similar to a suspension bridge, it is only natural to compare the two option. In most situations, the former will offer more strength to span a gap than the latter. The cable-stayed bridge can handle more pressure on a consistent basis compared to the suspension design, allowing the deck to have more resilience against wear and tear because there is greater rigidity in its construction.
There is also an element of resilience against natural pressures which may impact the bridge in negative ways over time. It withstands the shaking mechanisms of an earthquake better than most other bridge types. You can potentially place it in locations where a cross-wind might make other designs unsuitable for the span. It will even maintain its shape better while supporting the heavy loads.
3. It can be significantly cheaper to build a cable-stayed bridge.
Because there are fewer labor elements to consider with this design, the installation costs can be significantly less because there are fewer manhours involved. Most designs are roughly 30% cheaper to construct when comparing the cost to other design options that are available today to cross that span. This advantage is one of the primary reasons why this type of bridge is the most common type that you will see when traveling on roadways around the world. The cost factor is so cheap that some communities have found that a new bridge using this design is less expensive than trying to maintain an older design indefinitely.
4. Cable-stayed bridges can be constructed to almost any length.
Although the span length of a cable-stayed bridge is restricted because of its design, what is unique about this option is that engineers can connect different spans together with the support pillars or towers to create a bridge of almost indefinite length. The Jiaxing-Shaoxing Sea Bridge is one such example of this advantage at work, offering consistent support for a span that is over 6.2 miles in length.
Sometimes called the Jiashao Bridge, this span allows drivers to cross Hangzhou Bay without difficulty as it can accommodate up to eight lanes of traffic at once. Drivers can travel at speeds above 60 miles per hour safely while using the structure. Local laws prevent vehicles with a max speed of 45mph from using the span. Construction was completed on July 6, 2013, with traffic using it about two weeks later.
5. There are multiple design options from which to choose with a cable-stayed bridge.
Engineers have several different options that they can use when designing a cable-stayed bridge to cross a span. The side-spar design tends to be the most common as it offers only one tower, requiring supports that are on just one side of the structure. Some locations may require a cantilever-spar design, which provides a single spar that is found on one side of the bridge. You can also use cradle systems, multiple span options (like the Jiashao Bridge), or extra-dosed options to create the needed supports for a consistent deck that can support the expected weight that will be placed on the structure one day.
6. The design of the cable-stayed bridge supports itself.
The cables that are used to create consistency and stability for this bridge design provide the structure with the temporary and permanent supports it requires simultaneously. Whenever more weight is added to one specific section of the bridge, then the cables will help to displace the extra pressure throughout the remainder of the structure to prevent one section from receiving the brunt of the stress. These cables will also maintain the stability of the structure as it distributes the unexpected pressure, allowing for safe usage in almost any situation.
7. Cable-stayed bridges offer the possibility of a symmetrical design.
Although a suspension bridge and a cable-stayed bridge look very similar in their final design, the one significant advantage that you will find with the latter option is that the symmetry one can build into the span can help it to provide more stability and strength. When the spans on either side of the pillar or tower are of the same length, then the horizontal forces help to balance out the effects of each other. That means there are fewer requirements for large ground anchors to ensure the structure can remain supportive as traffic passes along the deck.
8. Designers can use four different classes of rigging to create results.
When the decision is made to install a cable-stayed bridge to cover a span that is usually 3,000 feet or less in length, then there are four different types of rigging for the cables from which to choose. Each offers unique benefits that can lead to a better user experience for the local community.
• The mono design for a cable-stayed bridge uses a single cable from its towers to provide support. This option is rarely seen unless the span being crossed is relatively small.
• The parallel design, sometimes referred to as a harp option, offers cables that are virtually parallel to each other so that the height of their attachment is proportion to their distance from the tower and their deck mounting.
• The fan design requires that the cables all connect to or pass over the top of the towners. This option is preferred when access is necessary to the cables while maximum supports are needed to create a stable deck. Engineers can modify this option for specific environmental requirements too.
• The star design spaces the cables apart on the tower, connecting to one point or closely-spaced points on the deck instead of being spread out across the entire span.
9. Cable-stayed bridges can also use four arrangements for their support columns.
One of the most common design options for a cable-stayed bridge is called the “single arrangement.” This option uses on column for cable support, usually through a projection in the center of the deck. It can also be placed along one of the sides of the structure. If a double arrangement is used, then pairs of columns are placed on both sides of the deck. The portal arrangement adds a third member that connects the tops of the two columns to create a visual effect that is reminiscent to a door-like shape, offering additional strength for traverse loads.
The final option is called the “A-shaped arrangement,” which achieves the same goal as the portal design by angling the two columns toward each other so that they meet at the time. Depending on the exact structure of the bridge, designers can have the columns be vertacle, curved, or angled relative to the bridge deck.
List of the Disadvantages of Cable Stayed Bridges
1. Cable-stayed bridges do have a maximum length to consider.
The introduction of computer-aided design for cable-stayed bridges has helped architects and engineers make the maximum range of a span longer now than ever before, but this option still has limits. Most of these bridges will cover a span that is between 100 to 1,100 meters in length. That is why they are an exceptionally attractive option for pedestrian bridges or places where unusual loading configurations might be present.
The main body of the Jiashao Bridge in China is measured at 2,680 meters, which makes it the most significant span using this design option when multiple connections are in place to create the final crossing. When looking at a single span option for a bridge, the longest in the world today is the Russky Bridge in Vladivostok Russia, which offers total coverage of 1,104 meters.
2. This design option can become unstable in specific environments.
Although a cable-stayed bridge can help to provide a consistently supportive deck when there are crosswinds present over a span, this option does not work well when the speed of that wind remains consistently high. This disadvantage occurs because of the rigidity that the cables provide for the overall structure. In regular situations, this would contribute to a higher level of durability. Under the pressure of a high-speed crosswind, the deck would start rocking. Over time, this issue begins to loosen the support cables, making it possible for the structure to eventually fail over time.
We saw this disadvantage occur when a cable-stayed bridge collapsed in Genoa, Italy, on August 14, 2018. This bridge was built in 1967 and made largely of concrete, which is typical for the design. When it collapsed, the failure claimed 43 lives as motorists found themselves plunging into the depths below.
3. Cable-stayed bridges can be challenging to inspect and repair.
The design of most cable-stayed bridges will place the bundle areas for the support structures in regions where a physical inspection becomes very challenging. When you add in the reduction of anchors for the support structure, the routine maintenance for this design option can be intensive. Although communities might be able to save upwards of 30% on the installation costs for this option, the increase in labor costs for ongoing maintenance will eventually eat into those savings.
When you start talking about a bridge that must last 50-100 years, then a community will eventually pay more for this structure than they would with other designs despite the lower initial capital costs.
4. It is a design that can sometimes be susceptible to rust or corrosion.
Most of the cable-stayed bridges that are built today use a combination of concrete and steel to create a rigid, supportive structure. Unless there are protections in place that maintain the quality of the metals used for the span, the support cables can be highly susceptible to corrosion and rust. Even if the materials do not show signs of this issue, the higher levels of fatigue that even a minor problem might create could have a devastating effect on the health of the bridge.
That is why you will see coastal regions painting their bridges with a water-resistant material to prevent the effects of corrosion due to the salt in the air. This issue is why you do not typically see cable-stayed bridges of a significant length anywhere in North America. The Baluarte Bridge in Mexico, at 520 meters in the longest on the continent. It is also the highest bridge, standing over 1,320 feet over the river below. In just 6 years of use, the total maintenance cost for the structure are over $132 million.
5. The strength advantages typically apply to short spans.
The cable-stayed bridge design fell out of favor in the early 20th century because it simply lacked the strength to support traffic. Although it was still used for short-to-medium spans, suspension bridges and other design options grew in use because they offered more durability despite the higher costs of installation. It was only when funds became scarce during the massive rebuilding efforts of the 1940s and 1950s that this option made a dramatic comeback. Even with computer-aided designs extending the reach of this design, it is still far behind the length of spans that other bridge options could provide for a community.
A Final Thought on the Advantages and Disadvantages of Cable-Stayed Bridges
When the populations of the planet began to recover after the devastating effects of World War II, the cable-stayed bridge design began to take on a higher priority. This structure could be built quickly and cheaply to create the necessary infrastructure to support the rebuilding effort. Although the idea was far from new, it was an option that seemed to check all of the necessary boxes.
These bridges are aesthetically pleasing, incredibly strong, and exceptionally durable. Many of the first designs that were built in the late 19th century are still standing today, with most still used for their initial purpose. Computer-aided designs are now letting architects and engineers maximize the potential of this option.
The advantages and disadvantages of cable-stayed bridges must involve more than consistency and cost. We must consider the needs of the actual span, the amount of traffic the bridge must support, and what potential disasters could impact the structure one day. Although this option may not be suitable in every situation, the actual benefits it provides almost always outweigh the potential disadvantages, which is why it is such a popular bridge design.
Natalie Regoli is a child of God, devoted wife, and mother of two boys. She has a Masters Degree in Law from The University of Texas. Natalie has been published in several national journals and has been practicing law for 18 years.