# Buckling-Restrained Bracing System with Ultra-High-Performance Fiber Concrete
## 3 Finite Element Analysis Results for UHPFRC and Conventional Concrete Filler  
As mentioned previously, cyclic analysis was conducted to investigate the behavior of the BRB model under applied amplitude. There was a small gap between the core and the filler material to provide sufficient space for the core to buckle. This localized buckling phenomenon facilitates the dissipation of energy during compression loading. The observed deformation pattern is a result of core local buckling. As BRB undergoes cycles of tension and compression, the locations of these local buckling occurrences may change or shift within the core element. This dynamic behavior contributes to the overall energy absorption and dissipation capability of the BRB system. Consequently, it effectively mitigates the detrimental effects of cyclic loading and enhances the structural performance of the system under seismic conditions. Based on the numerical results, the maximum  
Figure 16: BRB boundary condition. Figure 15: BRB interaction contacts (**a**) core and concrete, (**b**) concrete and restrainer. deformation in the BRB was observed due to buckling occurring in the middle of the plastic core, as shown in Figure 17.  
Figure 17: BRB steel core, **(a)** BRB (G40) displacement core, **(b)** BRB (UHPFRC) displacement core, **(c)** BRB (G40) strain core, **(d)** BRB (UHPFRC) strain core, **(e)** BRB (G40) stress core, **(f)** BRB (UHPFRC) stress core. The maximum deformation of the core in the BRB model filled with ultra-high-performance fiber reinforced concrete (UHPFRC) is lower than that of the G40 model. This is attributed to the higher strength properties of UHPFRC, which enhance the confinement of the core and consequently reduce the local buckling. The reduced curvature observed in the shape of core buckling in UHPFRC BRB does not indicate imperfect performance; instead, it demonstrates the effectiveness of UHPFRC in restraining the core against significant buckling. Similarly, core strain follows the deformation pattern and exhibits lower values in BRB with UHPFRC filler. The increased strength of the UHPFRC material also enhances the resistance force of the BRB against compressive loads, leading to an increase in the maximum stress experienced by the core (Table 6). As it can be seen in Figure 18, core local buckling and its deformation have a direct effect on concrete filler. Consequently, UHPFRC exhibits less deformation compared to conventional concrete due to its advanced material properties, allowing the steel core to buckle without causing significant damage to concrete. However, the high brittleness of conventional concrete results in inadequate restraint for the steel core, especially under higher applied loads, which can lead to concrete damage and global buckling of the BRB. Furthermore, it is worth mentioning that the strain in conventional concrete is greater than that of UHPFRC due to increased pressure from the core element on the concrete surface, whereas stress in UHPFRC is higher due to its superior resistance to applied loads (Table 7). \begin{table}
\begin{tabular}{l l l l} \hline
**Steel Core** & **Displacement (mm)** & **Strain** & **Stress (MPa)** \\ \hline BRB (G40) & 37.8 & 0.0023 & 200 \\ \hline BRB (UHPFRC) & 22.2 & 0.0017 & 348 \\ \hline \end{tabular}
\end{table} 
Table 6: UHPFRC and G40 BRB steel core results. \begin{table}
\begin{tabular}{l l l l} \hline
**Steel Core** & **Displacement (mm)** & **Strain** & **Stress (MPa)** \\ \hline BRB (G40) & 0.84 & 0.0081 & 345 \\ \hline BRB (UHPFRC) & 0.51 & 0.0044 & 475 \\ \hline \end{tabular}
\end{table} 
Table 7: UHPFRC and G40 BRB concrete results. Figure 18: _Cont._  
Concrete filler is restrained by an additional layer of buckling protection known as the steel tube restrainer. Any deformation or buckling that occurs in the concrete filler also affects the steel tube restrainer. As depicted in Figure 19, the effective confinement of the concrete results in negligible load transfer to the restrainers in both models.