## 12 Seismic Performance of Steel-Concrete Composite Bridges  
### Seismic Performance of Steel-Concrete Composite Bridges: State-of-the-Art  
Composite bridges, which integrate steel and concrete elements, have gained popularity in modern infrastructure due to their effectiveness and adaptability. These bridges capitalize on the advantages of both materials, with steel providing tensile strength and concrete offering compressive resistance. When exposed to seismic loads, composite bridges display intricate behavior that necessitates thorough analysis and design considerations. The interaction between steel and concrete components under dynamic loading can result in distinct stress distributions and energy dissipation mechanisms. Factors, such as shear connectors, deck-girder interactions, and material nonlinearities, significantly influence the overall seismic performance of these bridges. Understanding the behavior of composite structures during earthquake conditions is crucial for developing resilient design approaches and ensuring the safety of bridge infrastructure in seismically active areas. Ongoing research in this field aims to enhance analytical models, refine experimental techniques, and improve design guidelines to optimize the seismic performance of composite bridges. [PERSON] et al. (2020) [174] introduced a composite rigid-frame bridge that combines a steel-concrete composite box girder and concrete-filled double-skin steel tube piers with rigid connecting joints, showing improved static and dynamic performance over traditional bridges. Shake table tests of a scale model were conducted to assess seismic behavior and damage patterns. The results identified damage mainly at the upper and lower ends of the composite piers, with enhanced responses under near-fault ground motions. Numerical modeling confirmed the bridge's superior seismic performance, particularly in near-fault earthquakes. Furthermore, [PERSON] et al. (2018) [175] examined short-to-medium-span composite I-girder bridges, which are gaining popularity due to their reduced construction time and costs, as well as their suitability for seismic areas. The cyclic behavior of pier-to-deck joints using concrete crossbeams was analyzed and experimental tests showed good seismic performance of the joints, with different typologies tested. [PERSON] et al. (2020) [176] examined the behavior of dumbbell steel tube-confined reinforced concrete piers. These composite piers, which are shown in Figure 11, enhance strength and ductility compared to traditional reinforced concrete piers. Five composite piers and one reference dumbbell-reinforced concrete (RC) pier under pseudo-static loading revealed improved performance due to the confinement and enhancement effect of the steel tube. Seismic performance slightly improved with higher axial load ratios and wider webs. A simplified model accurately predicted lateral resistance, showing promising results for improving the performance of RC piers. [PERSON] et al. (2022) [177] introduced an enhanced type of concrete-encased column connections for circular CFT piers. They found that the new design eliminated the need for baseplates and anchor bolts. Experimental testing on four concrete-encased column connection specimens showed good seismic performance, with flexural failure and high-energy dissipation capacity. Calculations for strength degradation and load transfer mechanisms matched the test results well. Furthermore, [PERSON] et al. (2023) [178] examined the behavior of posttensioned precast segmental bridge piers under three-directional movements. They used shaking table tests to study an innovative posttensioned precast segmental pier under tri-directional ground motions and their results showed that adding more prestressing tendons or energy-dissipating bars reduced displacement responses. Piers with energy-dissipating bars resisted twisting better than those without.