This paper describes a study on improving redundancy in a 240 m span steel Pratt truss bridge. Redundancy was evaluated from two perspectives. The first perspective was how many members would induce bridge collapse owing to their own damage, and the second perspective was how many members would be damaged owing to another member's damage. Members corresponding to these two perspectives were widely present throughout the subject truss bridge. As a measure to improve redundancy of the subject truss bridge, the method of installing alternative load paths by retrofitting additional braces was studied. Three proposals with different geometry of the additional braces were compared, and "X bracing", which equipped X-shaped braces by adding braces in the opposite direction to the existing diagonal member, demonstrated excellent performance. In addition, the effect of the method retrofitting the cable along the member was also studied. Next, the placement patterns of "X bracing" on 18 truss panels of the subject truss bridge were examined using an optimization technique. It was clarified that examination of the placement for the additional braces was important for the efficient improvement of redundancy, and that the optimum placement of X bracing was in the alternating areas. Finally, the buckling gusset plate of the I-35 W bridge was also in the alternating area, suggesting that reinforcement of the alternating area might be important for redundancy of truss bridges.
