# Seismic Retrofit of Steel Truss Bridge Using Buckling Restrained Damper
## 1 Introduction  
Earthquakes are the biggest threat to bridge structures, and design codes have been revised after each natural disaster. For example, in Japan, the design specifications for highway bridges was revised several times right after Hyogoken-Nanbu (1995) and Tohoku (2011) incidents which include the capacity loss of bridges [1,2]. Currently, many existing bridges which were designed by old design codes exist. The seismic performance of those bridges needs to be assessed by the new seismic design code criteria. If the seismic performance is not satisfactory, existing bridges should be retrofitted [1]. To that end, many dampers are being developed such as fluid viscous dampers, solid viscoelastic dampers, friction dampers, added damping and stiffness dampers (ADAS), and buckling restrained braces (BRBs). In these, the design process of BRBs is the most familiar to design engineers due to its material behavior [3]. It is also important to note that BRBs have been proved to be superior in durability and long-term reliability. BRBs have only been focused on seismic-resistant building designs, and have been gradually developed for bridge application since first being reported in 1988. BRBs are new for bridge structures [4], and their application in the domain is discussed in the literature [5,6]. [PERSON] et al. [6] conducted an analytical study on a steel arch truss bridge in which diagonal members and lateral brace members were replaced by BRBs. Based on the result, the authors highlighted that the employed approach could be an efficient solution not only for earthquake-resistant new designs but also for retrofitting existing steel truss bridges. As can be seen in previous studies, BRBs are usually used in such a way that they replace members of truss bridges. Generally, buckling restrained braces (BRBs) have been considered as part of a bracing system, rather than as part of a damping system in seismic-resistant designs [3]. Therefore, we propose the idea of using BRB as an additional individual member or an external damper, not as the structural component or bracings. In this case, the device may be called a buckling restrained damper (BRD). The damper 'BRD' and the bracing 'BRB' are identical in terms of behavior and the design. They belong to the metallic damper that works based on the inelastic behavior of steel, and typically consist of a steel core and buckling restrainer case (i.e., concrete filled tube; Figure 1). During seismic load, the steel core or the yielding member of the damper undergo plastic deformation and dissipate the seismic energy by hysteretic behavior [3; 4; 5; 6; 7; 8; 9; 10; 11]. In order to enhance the seismic behavior of bridges using BRD, design parameters such as the length, the cross-sectional area, the location, and the inclination of BRD shall be taken into account. Regarding the length of BRDs, some studies are available in the literature, for example, [7; 8; 9; 10; 11]. [PERSON] et al. [10] have conducted analytical and experimental studies on the entire model of the BRD with a yielding part and a rigid part. The ratio between the length of the yielding core and the total length (L\({}_{\text{c}}\)/L\({}_{\text{total}}\)) of the BRD was taken to be 0.2 to 0.4, and the axial strain was shown to be higher than 3%. For common BRBs, the length ratio typically varies from 0.6 to 0.8, and the axial strain is 1% to 2%. Therefore, they investigated the buckling load of the BRD in two cases: (i) the yielding core was placed in the middle of the BRD, and (ii) the yielding core was placed at the one end of the BRD. The authors also compared the energy dissipated by the BRD with a reduced length of the yielding part and the energy dissipated by a conventional BRD. The study showed that higher buckling capacity is achieved when the short yielding core is placed at the one end of the BRD; therefore, the BRD with the short core has a higher energy absorption capacity of 1.3 times than that of the conventional BRD.