With old steel structures – such as bridges from the 1960s, historic canopies or old factory halls – it is often not a matter of a design error or a sudden overload, but of the slow, inexorable wear and tear of the elements and prolonged use.
No protective coating (such as paint or zinc) lasts forever. After 20 to 30 years, coatings begin to flake or become porous due to UV radiation and weathering. Once the steel is exposed, rust has decades to eat into the material. In aging structures, this leads to loss of cross-section : the steel beams become physically thinner because the steel has rusted away, literally reducing their load-bearing capacity.
Decades of temperature fluctuations (expansion in a hot summer, contraction in a cold winter) and daily vibrations take their toll. Bolted connections can slowly loosen over the years. Moreover, in historic structures, you often see that after a century, the old rivets have become brittle and lose their clamping force or even break off spontaneously (the “heads” pop off).
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