# Retrofit Fatigue Cracked Diaphrag Cutouts Using Improved Geometry in Orthotropic Steel Decks
## 1 Introduction  
Orthotropic steel decks (OSDs) are widely used in cable supported bridges and girder bridges due to their light weight and the short onsite construction time [1; 2; 3]. However, several types of fatigue cracks occur at the edges of added holes and at welded joints in OSD bridges [4; 5; 6]. In particular, the types of cracks that originate from the edges of diaphragm cutouts at the connections of U-ribs to diaphragms account for 42.3% to 85.5% of the total number of fatigue cracks in OSDs [4; 5; 6; 7; 8; 9; 10]. The lackof timely and essential retrofits causes these cracks to elongate and propagate throughout the entire diaphragm, which decreases the serviceability of OSDs, and hence reduces the service life and safety of these bridges. Conventional repair and retrofit technology of fatigue damages in steel structures include the stop-hole method [11; 12; 13], weld repair method [14; 15; 16], reinforcement with steel/carbon fiber reinforced polymer(CFRP) plates method [17; 18; 19; 20; 21], concrete overlay methodology [22], etc. These methods are common in fatigue reinforcement engineering projects of OSDs. However, removing short cracks by enlarging and optimizing the geometry of the cutout is a fast and effective method of mitigating fatigue damage; such a process excises short cracks around diaphragm cutouts and simultaneously reduces stress concentrations [7; 23]. In addition, partial or complete removal of long fatigue cracks around diaphragm cutouts is often necessary before diaphragm rehabilitation by bolting or bonding with steel plates near the cutouts. Therefore, the geometry of diaphragm cutouts is very important for effective fatigue repair. The effectiveness of fatigue retrofits of diaphragm cutouts is often determined by the maximum concentration stress [23; 24; 25]. The cycling stresses and the magnitudes of stress amplitude lead to fatigue failure [26; 27; 28; 29], which is also reflected in the \"safe life\" design [30; 31; 32]. It has been incorporated in the AASHTO LRFD code (AASHTO 2017), Eurocode 3 (\"Design of steel structures\"), and other specifications, where the S-N curve and similar algorithm of diaphragm cutouts are given based on nominal stress or hot spot stress [33; 34]. Another local stress method, named the effective notch stress approach, is adopted in the recommendations of the international institute of welding (IIW) as an alternative method. To improve the fatigue life of diaphragm cutouts, several normative documents and experiences suggest some typical cutout geometries for newly built bridges. Six typical diaphragm cutouts are shown in Figure 1 that include the cutout in highway bridge (Type I) and cutout in railway bridge (Type II) in Eurocode 3, AASHTO cutout (Type III), Japanese standard cutout (Type IV), the cutouts used in the Williamsburg bridge in US and the Humen bridge in China (Type V), and the large radius cutout (Type VI). Comparative studies have been carried out for these typical cutout geometries. In these studies, the rib spacing is often set constant at 600 mm, which is the most commonly-used value. The values of the upper width of the U rib and the depth are often 300 and 280 mm, respectively. The deck plate and rib thicknesses were taken into consideration in some research, but the results showed that the above parameters have a slight influence. Several typical diaphragm cutouts have been studied to compare their stress state. They found that the stress concentration regions are located at the free edges of diaphragm cutouts, and the geometry and size of diaphragm cutouts are vital parameters affecting the maximum principal stress [35; 36]. [PERSON] et al. [37] investigated the effect of the diaphragm cutout height on the stress distribution and the max-principal stress of typical diaphragm cutouts.