What is the significance of S-N curves in fatigue analysis of steel bridges?

S-N curves are significant in fatigue analysis of steel bridges because they provide a graphical representation of the relationship between the stress range (S) experienced by a steel component and the number of cycles (N) it can withstand before fatigue failure occurs. The S-N curve allows engineers to estimate the fatigue life of steel components under cyclic loading conditions. By knowing the stress range that a component will experience due to traffic and other loads, and by referring to the appropriate S-N curve for the specific type of steel and connection detail, engineers can estimate the number of load cycles the component can endure before a fatigue crack is likely to initiate. This information is crucial for assessing the fatigue resistance of steel bridge structures and for designing details that minimize stress concentrations and extend fatigue life. Different S-N curves exist for different types of steel and weld details, reflecting their varying resistance to fatigue cracking.