Analytical approach for improving damage equivalence factors

The fatigue design of bridges under variable amplitude traffic loads is not straightforward. To simplify the task, the current Eurocode provisions apply fatigue load model 3 and damage equivalence factors (λ). However, the fatigue load model and the damage equivalence factors have several shortcomings: (1) the critical span length is defined for limited bridge influence lines, (2) the damage equivalence factors neglect simultaneous presence of several heavy vehicles on bridges, and (3) the safety margin of damage equivalence factors is not uniform for all bridge influence lines. The current study identifies the effective parameters in the damage equivalence factors through a step-by-step analytical approach. Based on the analytical studies, new propositions for two main parameters of the fatigue load model and fatigue equivalent length are made. These modifications not only improve the accuracy of damage equivalence factors (both λ and λmax) but also extend the application to any bridge influence line. In addition, the proposed modifications are generalized by introducing a partial damage equivalence factor (λ5) which takes into account the effect of repetition in influence lines. The proposed parameters are justified with traffic simulations for various bridge cases.