## 2 Composite Truss Bridge Test Design
### Selection of Members and Nodes  
In the rectangular concrete-filled steel tube composite truss bridge, the structure's bending moment and shear force were converted into axial forces in the truss members. Tension and compression members are the most fundamental force-bearing units in composite truss bridges. The possible cross-sectional forms for tension and compression members include rectangular steel tubes, rectangular concrete-filled steel tubes, and I-beams. The main truss chord utilized a PBL-stiffened rectangular concrete-filled steel tube section, with the steel grade specified as Q345, with an elastic modulus of 2.06 \(\times\) 10\({}^{5}\) MPa, a yield strength  
Figure 4: Cross-section layout of the 110 m through-type simple-span truss bridge (unit: mm). Figure 3: Elevation layout of the 110 m through-type simple-span truss bridge (unit: mm). of 345 MPa, and a density of 7850 kg/m3. The infill concrete was ultra-high-performance concrete (RPC120), with an elastic modulus of approximately 30-50 Gpa, a yield strength of about 120 MPa, and a density ranging of 2000-2500 kg/m3. Due to the high strength-to-weight ratio of UHPC compared to steel, this design not only enhanced the load-bearing capacity of compression members and nodes, but also changed the force transmission and failure modes of the nodes. This can further improve the load-bearing capacity of the components while effectively reducing the structural weight. Additionally, the composite section ensured efficient load transfer without transverse diaphragms and stiffening plates, thus saving steel material. However, as the lower chord member functioned as a tension and bending component, the low tensile strength of the concrete limited the ability of the steel tube and concrete to work together to carry loads, making it challenging to improve the tension and bending load-carrying capacity. To address this issue, the proposed design incorporated prestressed ultra-high-performance concrete-filled steel tubes for the lower chord member. By applying prestress to the concrete within the tube, pre-compressive stresses were introduced, which helps delay concrete cracking and enhances the interaction between the steel tube and the concrete. This approach effectively resolves the tension and bending load-carrying capacity issues in composite structures, improving the overall performance and durability. This design was based on the original steel truss bridge, and the size of each component of the truss bridge was appropriately adjusted through trial calculations. To ensure structural safety, economic feasibility, and standardization, the design employed uniform thickness plates, where possible, an approach which facilitates standardized design and manufacturing and improves construction efficiency. The chord sections had an outer width of 1000 mm. The upper chord had an outer height of 800 mm, while the lower chord had an outer height of 1000 mm. The standard plate thickness was 20 mm, with the web plates of the chords thickened to 36 mm at the connections with the F1 web members. Additionally, the lower chord concrete contained four to six bundles of 192\(\lx@math@degree\)s15.2 prestressing tendons. The web members were rectangular steel tubes with dimensions of 1200 mm \(\times\) 1000 mm and a plate thickness of 36 mm (F1-type web members) or dimensions of 1150 mm \(\times\) 1000 mm with a plate thickness of 20 mm (F2-type web members). The node design featured integral node plates, with members and node plates welded together in the factory. The nodes were then assembled externally, simplifying manufacturing and installation. PBL-reinforced rectangular concrete-filled steel tube nodes were used. These nodes featured perforated steel plates within the main tubes to enhance interface bond strength and prevent debonding. The concrete tongue within the PBL holes increased the load transfer path and shortened the transfer length, effectively improving the load-bearing performance of the nodes.