Date of Award

Winter 2019

Project Type


Program or Major

Civil Engineering

Degree Name

Doctor of Philosophy

First Advisor

Erin Santini-Bell

Second Advisor

Raymond Cook

Third Advisor

John Leander


Fatigue damage in welded structural steel components has a complex presentation, which is influenced by the geometric configuration of the component and load path in a structural system. The classic fatigue assessment methods, using nominal stresses and S-N curves, may not capture nor predict the complicated performance of the component with respect to fatigue. Recent novel complex steel structural connections that experience multi-axial behavior or do not fit any conventional fatigue categories are not explicitly addressed in the existing fatigue design codes. An ideal fatigue estimation for the complex structural components is dependent on a thorough understanding of structural performance of the component within the global structural system and application of an appropriate fatigue assessment method.

This dissertation presents a fatigue assessment protocol for complex structural components

of steel bridges, using numerical methods and field-collected structural response data. Multiple

fatigue assessment methods are implemented, including the nominal stress method, hotspot stress

method, and linear elastic fracture mechanics method to estimate fatigue performance of a complex welded structural component. Accordingly, for each method, a set of computationally efficient finite element models of a large-scale bridge are created. Each model corresponds to the requirements of a specific fatigue assessment method and provides the required stress responses, under simulated dynamic traffic loads.

A major contribution of this research is the development of a novel a multi-scale modeling method to accommodate multiple dimensions of elements and multiple axes loading configurations. The multi-scale models are created for a case study, the Memorial Bridge in Portsmouth, NH, which is a vertical lift steel truss bridge and includes a novel gusset-less connection. The gusset-less connection includes a complex web geometry and curved fillet welds connecting the web to the flange. The bridge is also equipped with a long-term structural health monitoring program, with arrays of installed sensors. Field data are collected from the sensors to report the health status of the bridge. Additionally, field-collected data are utilized to validate the finite element models created for this study.

Due to the limited sensor location available, finite element models are used to predict structural responses that will supplement the field-collected data to appropriately provide stress- concentrated responses at the welded components of the bridge. The multi-scale model results illustrate that the geometric shape of the weld impacts the variability of the generated hotspot stresses along the weld toe. The changes in the stress state and estimated fatigue life are investigated during the crack propagation procedure, using a multi-scale model with a simulated crack.