## 5 Reinforcement of Steel Beams (Plates) Using Fe-SMA
### Practical Reinforcement of Steel Bridge  
Under long-term service conditions, fatigue cracks are easily generated at the diaphragm cutouts of orthotropic steel deck bridges subjected to vehicle-induced vibration and cyclical wheel load, and the traditional crack-stop method is prone to secondary cracking due to the inaccurate position of crack tips and stress concentration at the hole edge, as shown in Figure 11.  
The Sutong Highway Bridge, located in Jiangsu Province of China, is a double-tower cable-stayed steel box girder bridge, which was the largest cable-stayed bridge in the world at the constructed time, as shown in Figure 12. The authors' research group of Tongji University proposed and adopted the novel method of Fe-SMA plates covering crack-stop holes to reinforce the fatigue cracks of diaphragm cutouts in the Sutong bridge, as shown in Figure 13.  
Figure 11: Fatigue cracking of the diaphragm cutouts in orthotropic steel bridge decks.  
Figure 12: Sutong Highway Bridge: (**a**) integrated graph, (**b**) tower and cable.  
Figure 10: Friction clamp system and reinforcement details [103].  
In consideration of the internal space and geometry of the steel bridges, the dimension of Fe-SMA plates selected in this practical application is 75 mm \(\times\) 400 mm to effectively bond the Fe-SMA plates and the diaphragms, as shown in Figure 14.  
To obtain the mechanical properties of China-made Fe-SMA developed by the authors, the static tensile test and activation-recovery test were measured. The engineering stress-strain curves of Fe-SMA plates are shown in Figure 15a. The elastic modulus of the Fe-SMA plates is 172 GPa, the yield strength is 494 MPa, the ultimate strength is 960 MPa, and the ultimate strain is 38.31%. As shown in Figure 15b, the recovery stresses of Fe-SMA obtained using the pretension strain of 4% and activation temperatures of 150 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, 250 \({}^{\circ}\)C, and 300 \({}^{\circ}\)C are 192.3 MPa, 226.4 MPa, 270.8 MPa, and 294.1 MPa, respectively.  
The repair process is illustrated in Figure 16: (a) prestretch the Fe-SMA plates with the size of 75 mm \(\times\) 400 mm; (b) locate the position of the crack tips with the magnifying glass and lamp; (c) polish the diaphragm and clean it with alcohol to remove the oxide layer; (d) bond the Fe-SMA plates and maintain them for five days under pressure; (e) activate the Fe-SMA plates using a hot-air gun; (f) clean the diaphragm surface to complete reinforcement. According to the monitoring results, effective compressive stresses are introduced at the edge of crack-stop holes after activating Fe-SMA, which can significantly alleviate the stress concentration and avoid secondary cracking. Long-term monitoring on the Sutong bridge has proved this novel method can effectively strengthen the fatigue cracks of diaphragm cutouts and no secondary cracking has been observed.  
Figure 14: Schematic of the repair scheme for the diaphragm: (**a**) geometric dimensions, (**b**) reinforcement presentation.  
Figure 13: Crack repair of Sutong bridge: (**a**) job site, (**b**) group member in construction.