1. Introduction

2. Materials and Methods

2.1. Bridge Testing

Bridge monitoring can be divided into two parts: inspection-based monitoring and instrument-based monitoring [1]. Diagnostic load testing proves to be a suitable method for assessing the condition of a bridge. This inexpensive and rapid technique demonstrates the bridge's response to an applied load, with measurements including displacements, strain measurements, and the determination of bridge dynamic parameters. The test results obtained allow for a more accurate diagnosis of the actual bridge condition [2].

Static and dynamic load tests were conducted to evaluate the bridge's capacity to carry the design load and to calibrate the finite element model. In the static loading tests, vertical displacements and strains of the structural elements were measured [3]. The static load test was divided into three types of loading conditions, each assessing the stress and deflection of different main parts of the bridge. For the dynamic load test, lateral and vertical dynamic displacements of the piers and center span sections were measured as trains passed over the bridge at various speeds. The entire bridge's natural frequency, vibration mode, and damping ratio were also measured [4]. Both static and dynamic loads were applied using two 2M62 heavy locomotives with a total load of 2328 kN. Five dynamic tests were performed at speeds ranging from 20 to 100 km/h. The results of the static tests provided the maximum displacements of the bridge girders, while the dynamic tests yielded information on vibration characteristics, particularly modal parameters such as vibration mode shapes, frequencies, damping ratios, and dynamic amplification factors [5].

Railway bridge girders were designed with camber to accommodate vertical deflections that may occur during the construction or operation of trains. The camber of the girders was determined based on the dead load of the main girder, following applicable regulations [6]. External prestressing is considered one of the most effective reinforcements for increasing the service life of a structure. Since the 1950s, this reinforcement technique has been successfully applied to bridge structures in many countries and has proven to be an efficient solution for various types of bridges. Prestressing reinforcement is defined as a system in which tendons are placed outside the cross-section, and the prestressing force is transmitted to the girder through end anchors, deflectors, or saddles [7]. Utilizing an experimentally validated model, numerical simulations were performed to evaluate the effect of the ratio of deviator spacing to span length (Sd/L). The internal forces decrease linearly with increasing Sd/L. An increase in Sd/L results in a decrease in the ultimate stress of the tendon [8].

The bridge is currently located in Garut City, West Java Province, Indonesia. Figure 1 displays a map of the Cibatu-Garut railway line. Figure 2a is an aerial view of the bridge, while Figure 2b provides a detailed look at the steel frame structure. The bridge has a total length of 41.055 meters, as depicted in Figure 3.

Table 1. Bridge Data.

Table 1. Bridge Data.

Bridge Type	Girder and Truss Bridges
Located	Pasirjengkol – Wanaraja
Bridge Length	41.055 m
Bridge Width	1.30 m; 3.60 m; 1.30 m
Width Between Rails	1.20 m
Number of Spans	3 spans
Span Configuration	9.485 m; 21.935 m; 9.635 m
Bottom Structure Type	Stone Masonry

The static and dynamic tests of the bridge were conducted by applying the load of the CC206 locomotive, which has a total weight of 90 tons and six axles, each with an axle load of 15 tons. The specifications of the CC206 locomotive are provided in Table 2. The bridge test scheme, performed on three bridge spans, is illustrated in Figure 4.

The dimensions of the BH 25 bridge structure are presented in Table 3. The main structure of the bridge is constructed using steel, ironwood material is employed for the rail bearings, and the lower structure is made of masonry, as indicated in Table 4.

3. Result

3.1. Bridge Modeling

According to Minister of Transportation Regulation No. 60 of 2012, the actual structural deflection value for steel truss railway bridges should not exceed the allowable deflection of L/1000, and for girder bridges, it should not exceed L/800. The deflections resulting from the combination of service loads at 1/2L of the center of the bridge span are illustrated in Figure 10.

The deflection of the bridge resulting from the modeling due to the service load at 1/2L of span-1 structure is -3.27 mm, which is less than the allowable deflection of L/800 (9485/800 = -11.85 mm). For span-2 structure, the deflection is -21.88 mm, which is less than the allowable deflection of L/1000 (21935/1000 = -21.93 mm). Similarly, for span-3 structure, the deflection is -3.28 mm, which is less than the allowable deflection of L/800 (9635/800 = -12.04 mm). It can be observed that the actual deflection in the structure does not exceed the allowable deflection, and the bridge structure meets the requirements of the stiffness parameter against deflection.

The purpose of the capacity ratio (R) check is to determine whether the structural members meet the safe or unsafe criteria based on the comparison between the ultimate internal force resulting from the maximum load combination and the factored nominal strength of each member. A structure is considered to meet the strength requirements if the value of R is less than 1 or at least equal to 1, while if R is greater than 1, then the structure does not meet the strength requirements. The capacity ratio generated by the maximum load combination on the bridge structural elements is presented in Table 5.

Figure 12. Capacity Ratio of Bridge Structure Elements.

Figure 12. Capacity Ratio of Bridge Structure Elements.

3.4. Bridge Strengthening by External Prestressing Method

On the BH 25 bridge, temporary reinforcement was implemented by installing 2 supports on span-2 for the safety of the bridge, as there were indications of a -8.0 mm decrease in the bridge structure, which should have had a camber height of +300 mm. One of the efforts to enhance the serviceability and capacity of the bridge is to apply bridge reinforcement using the external prestressing method. The principle of applying external prestressing to a steel frame is to provide a force that reduces the tensile stress in the bridge truss by using cables or steel bars placed on the outside of the bridge structure.

In structural modeling, restoring the target camber of +300 mm to the entire structural element leads to overstressing, making it an impractical option. Therefore, in modeling the structure to achieve the opposite deflection (camber), it is adjusted to the actual deflection results of the structure due to the combination of service loads acting on the bridge, with a prestressing force of 180 tons, considering an initial loss of 10% [10]. The required prestressing force is then calculated as 10% x 180 tons = 198 tons. The tendon area is determined using the following equation.

$$\mathrm{A}=\frac{P}{\sigma }=\frac{\mathrm{P}}{\mathrm{0,70}\mathrm{x}{f}_{pu}}=\frac{\mathrm{1941716,7}\mathrm{N}}{\mathrm{0,70}\mathrm{x}1860\mathrm{N}/{\mathrm{mm}}^2}=\mathrm{1491,334}{\mathrm{m}\mathrm{m}}^2$$

Table 7. Modeling Criteria for Bridge Structure Reinforcement.

Table 7. Modeling Criteria for Bridge Structure Reinforcement.

Tendon Diameter	43.575 mm
Tendon Area	1491.334 mm2
Strand Type	7 wire strand
Dimensions of Deviator Profile	H 200.200.8.12
Prestressing Force	198 tons

Table 8. Deflection Comparison of Bridge Structures.

Table 8. Deflection Comparison of Bridge Structures.

Description	Load Combination	Deflection Structure (mm)
		1/2L
Field Testing	1,0 D + 1,0 SD + 1,0 LL	-11.30
External Prestressing Reinforcement	1,0 D + 1,0 SD + 1,0 PR	11.82
	1,0 D + 1,0 SD + 1,0 LL + 1,0 PR	-3.38

The initial stress $\sigma =\mathrm{0,70}x{f}_{pu}$ (SNI 2847-2019)

The external prestressing method applied to span-2 of the bridge can reduce the deflection of the existing bridge structure at 1/2L by 70.08%, equivalent to 7.92 mm, under the influence of the service load applied to the bridge.

The capacity ratio values for several structural elements of the span-2 bridge do not meet the safety criteria, as determined by comparing the ultimate internal force resulting from a combination of maximum loads with the factored nominal strength of each element. Consequently, to enhance the structural members of span-2 of the bridge, 10 to 15 mm plates are added to the flange of the structural members. The capacity ratio resulting from the combination of maximum loads on the bridge members, to which plates were added, is presented in Table 9.

Figure 18. Deflection of Bridge Structure at 1/2L after External Prestressing Due to Load Combination of 1,0 D + 1,0 SD + 1,0 PR.

Figure 18. Deflection of Bridge Structure at 1/2L after External Prestressing Due to Load Combination of 1,0 D + 1,0 SD + 1,0 PR.

Figure 19. Deflection of Bridge Structure at 1/2L after External Prestressing Due to Load Combination of 1,0 D + 1,0 SD + 1,0 LL + 1,0 PR.

Figure 19. Deflection of Bridge Structure at 1/2L after External Prestressing Due to Load Combination of 1,0 D + 1,0 SD + 1,0 LL + 1,0 PR.

Figure 20. Dominant Frequency of Vertical (z) Mode 45 Bridge Structure at 17.07 Hz after External Prestressing and Plate Reinforcement.

Figure 20. Dominant Frequency of Vertical (z) Mode 45 Bridge Structure at 17.07 Hz after External Prestressing and Plate Reinforcement.

Figure 21. Capacity Ratio of Bridge Elements after External Prestressing and Plate Reinforcement.

Figure 21. Capacity Ratio of Bridge Elements after External Prestressing and Plate Reinforcement.

4. Discussions

4.2. Bending of Steel Elements

One of the failure modes in steel structures is buckling. In general, buckling is induced by axial forces or forces acting on the principal axis of the structural section. Buckling is a phenomenon in which a structure is unable to maintain its original shape, leading to a change in shape to establish a new equilibrium. Local buckling is observed in the lower flange area of the longitudinal girder structure of span-1 in the bridge. This is attributed to tensile stress surpassing the capacity of the section to bear tensile stress. Additionally, the slender/thin shape of the steel section makes it more susceptible to buckling failure. Slenderness is defined as the ratio of length to thickness, and the longitudinal section of span-1 falls into the category of non-compact slenderness. Buckling does not necessarily result in the complete failure of the structure, but it can diminish the structural performance and lead to more significant damage.

Figure 22. Local Buckling in the Flange Section.

Figure 22. Local Buckling in the Flange Section.

5. Conclusion

References

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