Why are steel bridges declared unsafe after a long use?

There are a number of reasons why steel bridges may be declared unsafe after a long use. These include:

* Corrosion: Steel is susceptible to corrosion, which can weaken the structure of the bridge over time. This is especially true in areas where the bridge is exposed to salt water or de-icing chemicals.

* Fatigue: The repeated loading and unloading of a bridge can cause metal fatigue, which can lead to cracks in the steel. This is especially a concern for bridges that carry heavy loads or are subjected to high levels of traffic.

* Damage: Bridges can also be damaged by accidents, such as collisions with ships or trucks. This damage can weaken the structure of the bridge and make it unsafe for use.

* Design flaws: Some bridges may have been designed with flaws that make them unsafe. These flaws may not be apparent at the time of construction, but they can become evident over time as the bridge ages.

In addition to these factors, the age of a bridge can also be a factor in its safety. Older bridges may not have been designed to meet the same safety standards as newer bridges. They may also be more susceptible to deterioration and damage.

For all of these reasons, it is important to regularly inspect steel bridges and assess their safety. Bridges that are found to be unsafe should be repaired or replaced as soon as possible.