AN INVESTIGATION OF DISTORTION-INDUCED FATIGUE CRACKS IN STEEL HIGHWAY BRIDGES: FROM CRACK DETECTION TO FATIGUE RETROFIT

Abstract

Distortion-induced fatigue (DIF) cracking is the primary maintenance and structural safety concern in steel bridges built before the 1980s in the United States, requiring frequent inspections and costly repairs. Human visual inspections to characterize fatigue cracks have many drawbacks, including inconsistencies in identification, significant time and monetary costs, and safety risks posed to the lives of both inspectors and the traveling public due to lane closures. On the other hand, many retrofits aimed at mitigating the effects of DIF and stopping fatigue crack growth have been analyzed both experimentally and computationally; however, most of them require removal of the concrete deck, which disrupts the traveling public.In this dissertation, a holistic study is presented aimed at advancing the state-of-the-art in advancing detection, characterization, and repair of distortion-induced fatigue in steel bridge structures. A digital image correlation (DIC) based methodology is proposed for detecting and monitoring fatigue cracks in steel bridges. Also, a numerical study of three types of finite element (FE) analysis computational measures (Hot Spot Stress (HSS) analysis, J-integrals, and Stress Intensity Factors (K)) was performed to evaluate their predictive capabilities for characterizing DIF cracking propensity. Moreover, an analytical study was conducted to assess the effectiveness of CFRP applied over cracked steel plates, and to determine the influence of various modeling parameters on the analytical solution. Finally, a novel Carbon Fiber Reinforced Polymer (CFRP)-Steel retrofit is proposed to repair complex patterns of DIF cracks in steel bridge web gaps without the need for bolting to the flange or concrete deck removal. The DIC-based methodology was developed from in-plane compact fracture specimens then evaluated through a half-scale steel girder to cross-frame connection experimental test. The laboratory test results verified that the proposed DIC methodology could robustly quantify the fatigue crack length. The FE results of the three computational measures (HSS, K, and J) showed good alignment with experimental observations, and they were capable of predicting and characterizing DIF cracking propensity. The FE results of strengthening cracked steel plates with CFRP showed effectiveness in reducing maximum principal stress and stress intensity factor at the crack tips and in reducing crack propagation rate. The experimental results of the novel CFRP-Steel retrofit showed that the retrofit was effective in preventing DIF crack initiation and propagation.