01 Accident Overview The Rainbow Bridge is located on Shunping Road in the southeastern part of Shunyi District, Beijing, spanning the Chaobai River and connecting Shunyi with Pinggu. It is a landmark building and an important transportation hub of Shunyi. Figure 2 Schematic diagram of the accident bridge location The Rainbow Bridge is the only half-through arch bridge in Beijing, with a total length of 640.22 m. The approach bridges at both ends consist of 13 spans of 35 m prestressed concrete simply supported T-beams. The main bridge is a half-through flying-swallow-type concrete-filled steel tubular tied-arch bridge with flexible tie rods and rigid arches, which balances the horizontal thrust of the large-span central arch by tensioning the tie rods and using the weight of the flying-swallow-type side spans at both ends. The main bridge has a total length of 180 m, including a central span of 108 m and side spans of 36 m on each side. According to public information, the arch ribs on both sides of the main bridge are provided with a total of 18 suspender rods, 9 per arch rib, each made of 127φ7 parallel high-strength steel wires; the tie rods use 27×7φ5 high-strength low-relaxation steel strands, 12 bundles in total, 6 at each arch rib. The structural schematic diagram of the Rainbow Bridge is shown below. For convenience of description, the main arch rings are designated A and B, and the cross beams and corresponding suspender rods are numbered 1# to 9#. Figure 3 Schematic diagram of the bridge structure The Rainbow Bridge was built in October 1999 and had been in service for nearly 26 years at the time of the accident. According to public information, the major defects and maintenance history of the bridge are as follows: 2006: Bridge inspection found relatively serious defects, including vertical cracks in the main bridge piers, longitudinal cracking of the deck pavement, cracking of the main bridge hollow slabs, and corrosion of suspender rods. To address these problems, measures such as steel structure anti-corrosion treatment, crack reinforcement and replacement of suspender and tie rods were taken to control general defects, and a health monitoring system was installed to control structural defects; 2015: As traffic volume on Shunping Road gradually increased and heavy vehicles became more numerous, defects in the approach bridges became prominent, with potholes, cracks and ruts appearing, and serious local damage to sidewalks and railings. In July 2015, the Rainbow Bridge was closed for major repairs for one month, including replacement of deck pavement and sealing of cracks; 2020: During routine inspection of the Rainbow Bridge, the Beijing Shunyi Highway Sub-bureau found abnormal expansion joints on the west approach bridge, overall westward sliding of the girders, reduced spacing of expansion joints at the abutment, and the girder ends pressing against the back wall. After demonstration, it was confirmed that no structural defects had occurred in the main bridge body, and without interrupting traffic or damaging the bridge, 160 ultra-thin large-tonnage jacks of 200 tons were used for jacking reset and lateral deviation correction—the first application of synchronous jacking technology for a large bridge in Beijing. 02 Accident Site Analysis According to reports, at about 4:40 on April 23, a fire broke out in the landscape cables under the Rainbow Bridge. At 4:59, nearby residents called the police, and relevant departments arrived at the scene immediately to organize firefighting, traffic control and other response work. During the handling of the cable fire, the bridge structure gradually collapsed under the influence of the fire, until the open flames were extinguished at 11:09 a.m. The structural remains are shown in the figure below. It can be seen that the suspender rods of arch ring A of the Rainbow Bridge are all intact, while the suspender rods of arch ring B are disconnected from the main girder at all locations except beam 1#. One end of the cross beam corresponding to each suspender rod fell into the Chaobai River, and obvious fire marks remain on the surface of the main arch ring. According to publicly available online videos, the collapse of the Rainbow Bridge went through multiple stages: Stage 1: The suspender rods at cross beams 5# and 6# fractured, and the main girder collapsed; Stage 2: The fire spread, the suspender rods at cross beam 7# fractured, and the main girder collapsed; Stage 3: The fire spread further, and the suspender rods at cross beams 2#, 3#, 4# and 9# fractured and collapsed; Figure 5 Schematic diagram of the Rainbow Bridge collapse process Existing research shows that the tensile strength and elastic modulus of parallel steel wire bundles decrease significantly under fire, while thermal expansion and high-temperature creep also occur, greatly reducing structural load-bearing capacity, as shown in the figure below. Figure 6 Stress-strain curves of parallel steel wires [1] Combined with the fire alarm at the Rainbow Bridge before the collapse, it is inferred that the cable fire at arch ring B caused a reduction in the load-bearing capacity of the tie rods and multiple longitudinal suspender rod steel bundles, thereby triggering the structural collapse. 03 Development of Structural Types of Half-Through Arch Bridges Half-through suspended concrete-filled steel tubular arch bridges can be mainly divided into the following three categories according to the structural characteristics of the deck system, as shown in Figure 7. (1) Integral deck system. This type of deck system has good robustness. Although the structure is complex and construction is difficult, its application has shown an increasing trend in recent years. (2) Deck system without longitudinal girders. Early half-through concrete-filled steel tubular arch bridges mainly used deck systems without longitudinal girders, in which suspender rods connect to cross beams, the deck slab is supported on the cross beams, and no stiffening longitudinal girders are provided. (3) Deck system with longitudinal girders. At present, deck systems with longitudinal girders are mostly used, which have relatively high redundancy.
According to the Opinions of the Ministry of Transport on Further Improving the Safety and Durability of Highway Bridges (Jiao Gong Lu Fa [2020] No. 127), the structural redundancy of suspender arch bridges without stiffening longitudinal girders is obviously insufficient. Meanwhile, the Technical Code for Concrete-Filled Steel Tubular Arch Bridges (GB 50923-2013) explicitly stipulates that "the suspended deck system of half-through arch bridges shall adopt an integral structure; suspended deck systems mainly relying on cross beams for load bearing must be provided with stiffening longitudinal girders and shall have the capacity to prevent girder falling after the failure of suspender cables corresponding to both ends of one cross beam."
The Rainbow Bridge was built in the 1990s and belongs to the deck system without longitudinal girders,which was relatively common at the time. Due to the lack of longitudinal girders with relatively strong fire resistance to share the deck load, the load is transmitted directly to the arch ring through the suspender rods. On the one hand, the fatigue resistance of the suspender rods is reduced; on the other hand, when the suspender rods encounter sudden events such as fire, the resulting temperature rise reduces the strength of the parallel steel wires, making collapse failure highly likely.
04 Bridge Fire Cases
(1) October 26, 2024, Shenzhen-Zhongshan Link: a car caught fire while driving.
Figure 8 Schematic diagram of the bridge fire site (Shenzhen-Zhongshan Bridge) (2) April 2, 2025, Binjiang Bridge in Jiangmen, Guangdong: sparks dropped during construction ignited the anti-collision gelatin on Pier No. 2, causing a fire. (3) November 5, 2023, Hutuo River Bridge in Shijiazhuang, Hebei: a short circuit at the wire joint of the lighting circuit of stay cable No. 2 caused a fire that burned and quenched the cable. Figure 10 Schematic diagram of the fire site (Hutuo River Bridge, Shijiazhuang, Hebei) 05 CableFire Protection
For bridge structures, the load-bearing components above the deck include main cables, stay cables, suspender rods, etc. Once damaged, they will cause huge losses and are difficult to repair, and their strength is very sensitive to temperature. They should be the focus of bridge fire protection. The PE sleeves of traditional suspender rods are also prone to burning at high temperatures and can hardly achieve fire protection.
Cable fire protection has received increasing attention in recent years. Taking typical bridges such as the Shenzhen-Zhongshan Bridge and the Xihoumen Bridge as examples ofcablefire protection, a common practice is to wrap the cable body with fireproof materials to ensure that the main cable steel wires do not lose strength for a short time under continuous fire, such as new fireproof and heat-insulating protection systems like composite aerogel fireproof materials, meeting the requirement that under a 1000°C fire environment (tanker truck fire) lasting 60 min, the surface temperature of the cable steel wire bundle does not exceed 300°C.
Analysis by Safety Science and Emergency Management Research:The direct cause of the collapse of the Shunyi Chaobai River Bridge was the fire in the cables laid in the trench, which led to the fracture of the bridge tie rods. Preliminary analysis suggests the specific causes may be the following three points:
1. Cable fire generates high temperature
Abnormal ignition of the lines in the externally mounted cable trench on the bridge produced high temperatures that acted directly on the concrete-filled steel tubular tie rods. The strength of steel begins to decrease significantly above 400°C; when the temperature reaches 600°C, the strength of steel loses about 50%, causing the tie rods to be unable to bear the original load.
It is understood that the bridge is a half-through tied-arch bridge, and the tie rods are the core load-bearing components that maintain the horizontal thrust of the arch ribs. After the tie rods fractured, the tensile force originally borne by the tie rods was instantly transferred to the suspender rods, causing overload fracture of the suspender rods and ultimately leading to instability and collapse of the main span structure of the bridge.
3. Hidden hazards of externally mounted cables
The notice specifically mentioned that "the whole city will carry out an investigation of externally mounted bridge cables," implying that the cable laying may have had design defects: the cable trench was too close to the tie rod structure, or no effective fireproof isolation layer was provided, so that the fire directly affected the main structure.
At present, the accident investigation team has intervened and is conducting tests on the cause of the cable fire (such as short circuit, overload, aging, etc.) and the high-temperature resistance of the tie rod materials. This incident has also exposed the hidden hazards of the safety linkage between bridge ancillary facilities and the main structure, and bridges across the country may subsequently strengthen fire protection standards for externally mounted facilities.
Special ThanksSenior engineer Mr. Li, a bridge technical expert, participated in the guidance. The above work was completed collaboratively by graduate students Zhang Mingjian, Zhang Xiaoxin, Shou Zhekun, Zheng Qian, Yuan Zishun and Yue Yuhan of Zhejiang University of Technology, and the text was edited by graduate student Zhu Yuting. This work is merely practice material for learning accident analysis; corrections and criticisms are welcome if there are any inaccuracies. Note: Some videos and images are from public online reports; if there is any infringement, please contact us for timely deletion.
References:
[1] Feng Jinpeng, Li Jinglun, Gao Kang, et al. Study on the fire resistance of stay cables of in-service cable-stayed bridges under vehicle-induced fire [J]. China Journal of Highway and Transport, 2024, 37(09): 34-45.
[2] Chen Baochun, Wei Jiangang, Zhou Jun, et al. Current application status and prospects of concrete-filled steel tubular arch bridges in China [J]. China Civil Engineering Journal, 2017, 50(06): 50-61.
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