Fatigue life assessment of roadway bridges, PhD thesis defence

Fatigue is an important consideration in the design of bridges, especially those made of steel. Cycles resulting from the passage of a truck over a bridge depend essentially on bridge type, detail location, span length and vehicles axles’ configuration. To address the complex limit state of fatigue, damage equivalence factors (lamda-factors) are given in the SIA codes and the Eurocodes. However, the lamda-factors given in the codes are not valid for all bridge influence lines. Another issue in the application of damage equivalence factors is that the fatigue equivalent length is defined for limited cases in the codes. Moreover, the lamda-factors are determined with very simplified load models of real traffic that entirely neglect simultaneous presence of multiple heavy vehicles.
The main objective of this research is to study the relationship linking influence line and damage, to find the effect of main parameters on damage equivalence factors for roadway bridges with actual loads and improve them. As far as accurate traffic simulations need actual traffic data, it is also necessary to study real traffic flow. The first two chapters introduce the thesis objectives and give a literature review in the various directly related research fields. In the next chapter, a series of simplified traffic models (the same models applied in the codes) are used in order to evaluate the accuracy of the damage equivalence factors as defined in the code for different bridge types. Initial traffic simulations are also performed to compare the output with the damage equivalence factors given in the codes.
Then, the author profoundly investigates the relationship between bridge influence lines, loads geometry and fatigue damage sum. Some modifications for improving the damage equivalence factors, which are developed through a step-by-step analytical approach, are proposed. The proposed modifications aim not only to improve the accuracy of the damage equivalence factors for continuous traffic flow, but also enhance their applications to various bridge types and traffic conditions. A partial factor, lamda5, dedicated for influence lines that have repetition is also defined. In addition, the lamda4-factor for double lane traffic is updated in order to take into account the effect of crossing.
Before final traffic simulations, the most important parameters needed to express realistic traffic model and their range of variation are determined by analyzing the available traffic data from measurement stations in Switzerland. Finally, the accuracy of the proposed modifications for damage equivalence factors are assessed by comparing them with the final simulation results obtained for different traffic conditions. It shows that the proposed fatigue load model and fatigue equivalent length can properly represent damage equivalence factors for any influence line. In addition, for several double lane traffic conditions, the crossing ratios are determined.

KEYWORDS: bridge; fatigue design; damage equivalence factor; fatigue load model; influence line; traffic model; Monte-Carlo simulation; heavy vehicle; traffic flow; crossing effect.

Thesis Directors: Prof Alain Nussbaumer, Prof Jean-Paul Lebet

The presentation will take place on Friday, April 12, 2013 at 17:00 in GC C3 30 at EPFL