**2. Test Overview**

#### *2.1. Overview of the Original Bridge Project*

The Tian'e Longtan Grand Bridge is an upper-bearing, steel tube-reinforced concrete rigid-frame arch bridge, with a main span of 600 m, a rise of 125 m, and a rise-to-span ratio of 1/4.8 [20], as shown in Figure 1. The main arch curve is in the form of a catenary, with an arch axis coefficient of m = 1.9. The arch ring adopts a concrete box section with constant width and variable height, with a box width of 6.5 m, a box height of 12 m at the arch foot, and a box height of 8 m at the arch crown. The transverse center distance of the arch rib is 16.5 m, and the thickness of the arch box web gradually changes from 0.45 m at the arch crown to 0.95 m at the arch foot. The top slab thickness is 0.65 m, and the bottom slab thickness gradually changes from 0.65 m at the arch crown to 1.3 m at the arch foot. The rigid steel skeleton primarily uses Q420 steel, while the internal concrete employs C80 self-compacting micro-expansion concrete, and the external concrete utilizes C60 concrete. Notably, the external concrete is cast using a segmented ring method, dividing the external concrete horizontally into three rings, including the bottom slab, the web, and the top slab, and adopting a symmetrical casting approach with eight working faces longitudinally.

**Figure 1.** Elevation view of Tian'e Longtan Grand Bridge (unit: m).

#### *2.2. 1:10 Scale Model Test of the Main Arch Ring*

A 1:10 scale model experiment was conducted on the main arch ring of the original bridge, with stress equivalence as the design principle [21,22]. To minimize the influence of the scale effect on the structural response [23], the overall geometric parameters of the model bridge, such as span and rise, were strictly converted according to the scale ratio. Local designs adhered to the principle of achieving rigidity similarity and construction requirements in order to replicate the actual project conditions to the greatest extent and to strive for consistency of stress conditions between the model bridge and the original bridge. Additionally, by conducting nearly one hundred sets of experiments on the concrete mix ratio, we ensured that the concrete material properties of the model bridge closely resembled those of the original bridge [24], thereby reducing the potential impact of the scale effect on the material side.

It is important to note that the scale effect primarily affects discontinuous regions where beam theories [25], such as the Euler–Bernoulli beam theory, are not applicable. However, in the context of the present experiment, the rigidity of the skeleton structure remains continuous throughout the investigation process. When integrated with the results from Section 4.3, it becomes evident that the scale effect has not significantly impacted the precision of our experimental outcomes.

#### 2.2.1. Scaled Model Test Scheme

The model test was carried out in the State Key Laboratory of Mountain Bridges and Tunnel Engineering at Chongqing Jiaotong University, with a main span of 60 m, as shown in Figure 2. This span exceeds the largest scaled test ever conducted for similar bridge types [14,26], making it the largest scaled model test for an arch bridge in the world to date.

**Figure 2.** Picture of model test. (**a**) Figure of the arch bridge model test under viewpoint 1; (**b**) Figure of the arch bridge model test under viewpoint 2.

This structure's arch axis adopts a catenary, with an arch axis coefficient (m) of 1.9, a calculated rise of 12.5 m, and a span-to-rise ratio of 1/4.8. The arch rib adopts a concrete box-type section with uniform width and variable height, as shown in Figure 3. The box is 0.65 m wide, the box height at the foot of the arch is 1.23 m, and the box height at the apex of the arch is 0.83 m. The thickness of the box web plate gradually varies from 55 mm at the apex to 95 mm at the foot of the arch. The top plate is 65 mm thick, while the bottom plate thickness gradually varies from 65 mm at the apex to 130 mm at the foot of the arch. To enhance the local stability of the main arch ring, transverse partitions with a thickness of 50 mm are arranged every 2 m along the length of the bridge.

The model test's main arch ring mainly includes three parts: a rigid steel framework, C80 internal concrete in the pipe, and C60 external concrete. The rigid steel framework was assembled using the large-section lifting method, with a total weight of approximately 5.0 tons. The internal concrete was pumped from both sides of the arch base section symmetrically toward the arch crown using the pumping and jacking method, with a total volume of approximately 1.5 m3.

The casting of the external concrete is consistent with the original bridge, dividing the external concrete into three rings in the transverse direction: the bottom plate, the web, and the top plate. In the longitudinal direction, each ring is divided into eight working faces, with each working face being divided into six segments for casting. The model test's segmented positions and the casting length of each segment are consistent with the original bridge, which has been scaled proportionally, and the age difference between the concrete segments is basically the same as the original bridge.

The main test content of this experiment included the strain, displacement, environmental temperature fields, and structural temperature field throughout the construction process [27]. The strain testing primarily involved the establishment of test cross-sections at 16 longitudinal points of the bridge (a total of 17 test sections are shown in Figure 4a), with fiber-optic strain sensors being arranged at each cross-section on the main steel tube, bottom plate, web, and top plate, as shown in Figure 4a. The displacement testing was mainly conducted at 8 longitudinal points of the bridge (a total of 7 test sections are shown in Figure 4b), with four measurement points set near the upper and lower chord steel tubes at each cross-section. The related testing was completed using wire-drawing displacement sensors, as shown in Figure 4b. The environmental temperature field and structural temperature field were monitored using PT100 sensors. Given that the temperature distribution of the lab environment and the structure was relatively uniform, an environmental temperature sensor and a structural temperature sensor were placed along the bridge longitudinally at intervals of 10 m.

**Figure 3.** Cross-section of the main arch ring of the model bridge (unit: mm). (**a**) Dimensional drawing of mid-span section; (**b**) Dimensional drawing of arch foot section; (**c**) Dimensional drawing of the cross-section of the mid-span diaphragm.

2.2.2. Model Test Construction Stages

By referring to the original bridge's construction process, the construction of the main arch ring is divided into 20 stages, with the specific content of each stage shown in Table 1.

**Table 1.** Division of the construction stages of the main arch ring.


**Figure 4.** Schematic diagrams of test cross-sections and test methods. (**a**) Cross-section of strain test and sensor display diagram; (**b**) Displacement testing cross-section and display diagram of testing equipment.
