# Retrofit Fatigue Cracked Diaphrag Cutouts Using Improved Geometry in Orthotropic Steel Decks
## 2 Outline of Retrofitted Bridge  
### Fatigue Cracks around Diaphragm Cutouts  
A self-anchored suspension bridge opened to traffic in December 2006, having an overall length of 680 m and a main span of 350 m. Steel box girders with OSDs were used in this suspension bridge to reduce weight and enhance stability. Figure 2 shows a cross section of OSD box girder with five traffic lanes. The spacing between the transverse diaphragms are 3.0 m, and the thickness of the diaphragms is 10 mm, except for the diaphragms at the hangers is 12.0 mm.  
Figure 1: Representative diaphragm cutouts including (**a**) Type I: highway cutout in Eurocode 3, (**b**) Type II: railway cutout in Eurocode 3, (**c**) Type III: AASHTO cutout, (**d**) Type IV: Japanese standard cutout, (**e**) Type V: cutout used in Williamsburg bridge, (**f**) Type VI: large radius cutout.  
The U-ribs run through each diaphragm, and cutouts are made at the cross position on the diaphragm. The radius of the diaphragm cutout is 10.0 mm; the arc segment connects with the U-rib. A conventional structure and size was used at the connection of each U-rib to the diaphragm, such that the spacing between the U-ribs is 600 mm, the width of the upper U-ribs is 300 mm, the width of the lower bottom plate is 170 mm, the height of U-ribs is 280 mm, the thickness of the U-ribs is 8 mm, and the radius of the internal arc between the web and the lower bottom plate is 5 t, where t is the thickness of U-rib. The details of U-ribs and cutout on diaphragm are shown in Figure 3a.  
The detection of the cracks was performed on the OSD at anywhere of the whole bridge by manual inspection, especially focus on the potential cracking position, such as diaphragm cutouts, connection between U-rib and top plate, and so on. In 2015, 141 fatigue cracks were detected around the diaphragm cutouts in this bridge by manual inspection. Among them, there are 20 weld fatigue cracks originating from the connections of diaphragm and U-ribs, and another 121 parent material fatigue cracks originate from the free edges of the diaphragm cutouts. The number of fatigue cracks in free edges account for 85.8% of the total number, and is much more than other locations, and hence need effective rehabilitation. These cracks originated at the intersections of the arc segments (\(r=10\) mm) and the straight segments of the diaphragm cutouts, as shown in Figure 3b. The length, inclination and position of such cracks are shown in Figure 3a, and the corresponding data are listed in Table 1.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Crack Length/mm** & **0–25** & **25–50** & **50–100** & **100–250** \\ \hline number & 52 & 38 & 26 & 25 \\ percent & 36.8\% & 26.9\% & 18.4\% & 17.7\% \\
**Inclination Angle** & **0\({}^{\text{\text{\textminus}}}\)–30\({}^{\text{\textminus}}\)** & **30\({}^{\text{\textminus}}\)–45\({}^{\text{\textminus}}\)** & **45\({}^{\text{\textminus}}\)–60\({}^{\text{\textminus}}\)** & **60\({}^{\text{\textminus}}\)–90\({}^{\text{\textminus}}\)** \\ number & 61 & 51 & 29 & 0 \\ percent & 43.3\% & 36.2\% & 20.5\% & 0\% \\ \hline \hline \end{tabular}
\end{table}
Table 1: Fatigue crack characteristics and quantity in cutouts.  
Figure 3: Structural details and fatigue cracks in the rehabilitation region: (**a**) The drawing: (**b**) the photo.  
Figure 2: Cross-section of the steel box girder of a suspension bridge.