Larger bridges, such as the Golden Gate, would more likely be planned for a 100-year lifespan at the time of construction. But many smaller bridges are expected to last only 20 years or less before they must be replaced.
The life expectancy of a bridge is generally calculated by dividing its length by its average daily traffic (ADT). A longer span with higher traffic counts will have lower maintenance costs over time. Older bridges tend to need their surfaces repaired more frequently because they're made of materials such as wood or concrete that are damaged by age and use. Newer bridges are usually built with stronger materials that don't require replacement as often.
Most major bridges in the United States were built between the 1920s and the 1960s. At that time, engineers began using statistical methods to estimate how long bridges should last so they could plan maintenance schedules accordingly. They found that steel bridges tended to last longer than those made out of other materials, such as wood or concrete. The typical lifetime was thought to be between 30 and 50 years.
Since then, many bridges have been updated or replaced with newer models. For example, the new Bay Bridge is expected to last about two centuries.
But some bridges fail to meet the expectations set by historians and engineers.
Bridges are now constructed to last 75 to 100 years, and adding new monitoring systems and expecting them to be resilient and functional for such a long length of time has never been done before. But the world's oldest standing bridge is still serving its community more than 1150 days after it was built.
Old bridges were not meant to be permanent. They were built as needed, usually over rivers or other bodies of water where they could be replaced if they were found to be inadequate after testing. Many old bridges have since been replaced because of changes in traffic patterns or improvements made possible by modern technology.
But some older bridges are still in use today even though they are well beyond their design life. These bridges were often built without any consideration of future maintenance costs, so they suffer from poor planning and lack of funding. Without these upgrades, many bridges would need to be replaced sooner rather than later.
Modern bridges are expected to last much longer than those built just a few decades ago. As engineers become better able to predict how structures will perform over time, they can make updates or modifications to keep bridges in good condition for longer. For example, engineers may choose to add additional support beams or change the shape of a bridge's cross-section to reduce stress on specific areas.
Concrete and steel bridges are projected to endure 75 years or more, but timber bridges (which engineers are less experienced with) are predicted to survive just 20–30 years. However, new bridges are usually built using modern materials so their lifespan will be much longer.
The average lifespan of a wooden bridge is around 40 years. The main factor in determining how long it will last is the quality of construction. A well-built structure that receives adequate maintenance will last for many decades; a poor structure that is heavily used will need replacement sooner. Other factors such as the type of wood used, exposure to the weather, etc. may also affect the life span of the bridge.
Most older bridges were constructed of timber because they are cheaper than concrete or steel. These days most new bridges are made from concrete because it is stronger and requires less maintenance. However, some countries with hot climates and dry conditions like Mexico and Australia use timber due to its drought-resistant properties.
During World War II, wood was the primary material used for bridges because there were shortages of other materials. Today, steel is generally used instead. Old wooden bridges tend to deteriorate quickly when exposed to water because wood absorbs moisture which causes the wood fibers to expand and contract, leading to cracking and deterioration of the surface.
In the United States, the average bridge is 43 years old. Because the majority of the country's bridges were intended for a lifespan of 50 years, a rising number of bridges will require extensive renovation or retirement in the near future. The most common type of bridge that reaches 40 years of age is the steel truss, followed by pre-stressed concrete and then composite (metal decking with cross-members of fiber optic cable). On average, new composite bridges last about 60 years before they must be replaced.
The life expectancy of a bridge is based on its use and exposure to environmental conditions. Older, heavily used bridges tend to deteriorate more quickly than newer ones. Unpredictable events such as heavy traffic loads, inclement weather, and vandalism can also cause damage to a bridge that will eventually need to be repaired or replaced.
Most countries have laws governing the construction and use of bridges. These laws include requirements for maintenance, insurance, and qualification of personnel who work on bridges. They may also limit the weight that can be carried on a particular kind of bridge or require warning signals to indicate when a bridge needs to be repainted.
Bridges play an important role in our daily lives, whether we are aware of it or not.