ABSTRACT
Since the first orthotropic steel deck bridge was developed by German engineers in the 1930’s, orthotropic steel decks (OSD) have been utilized successfully for thousands of steel bridges. Wheel loads cause large local stress variations, stress reversals, and an increased number of stress cycles that must be considered in fatigue design of OSD. Many of the earlier orthotropic decks with closed ribs experienced fatigue cracking problems (Wolchuk, 1999).
For the rib-to-deck welded detail, the cracks may initiate and propagate in ways as shown in Figure 1. Cracks A initiates from the weld root and propagate through the deck plate, Cracks B initiates from the weld toe in the deck plate and propagate through the deck plate, Cracks C initiates from the weld root and propagate through the weld throat, and Cracks D initiates from the weld toe in the longitudinal rib and propagate through the rib wall (Xiao 2008). Typical cracks at rib-to-deck welded detail. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/fig271_1.tif"/>
Currently, taking local effects into account, some local approaches including hot spot stress approach and the notch stress approach were used for fatigue assessment of welded steel structures. The fracture mechanics based crack propagation approach provides the most versatile and accurate method for fatigue life assessment for welded details.
To investigate the SIF of 4 kinds of typical cracks on rib-to-deck welded detail, a partial OSD with 3 longitudinal ribs was created with 2D-plane elements using a general purpose FE software package ANSYS. Two kinds of wheel load case are investigated, which is acting on the deck plate at the rib and acting on the deck plate between the ribs.
The FEA result show that the stresses near the tip of crack is tension/shear mixed-mode. The SIF is replaced by an equivalent stress intensity factor (ESIF) K eq. () K e q = K I 2 + K I I 2 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315207681/cd556cd4-4dcf-4efe-8e29-56fc67b8bfbd/content/eq161.tif"/>
To improve the fatigue resistant of rib-to-deck welded detail, it is important to reduce the ESIF of crack A. The relationship between ESIF and some design parameters including deck plate thickness, initial crack size, rib height and rib wall thickness were investigated. ESIF is increasing with the increment of initial crack length. It is an effective method to improve the fatigue life of rib-to-deck welded detail, limiting initial crack length by reducing weld flaw. ESIF is decreasing with the increment of deck plate thickness. It means that fatigue life is increasing with the increasing of deck plate thickness.
In Eurocode 3, FAT 71 is proposed for rib-to-deck welded detail of OSD. The design stress is the nominal stress in the wall of rib. Using LEFM approach, the S–N curve of rib-to-deck welded detail was obtained. It was showed that most of fatigue life prognosis results greater than the S–N curve (detail category 71) in Eurocode 3. It is significant that defining reliable material constants for fatigue life evaluation of welded detail in LEFM approach.
