An engineering challenge on an active fault
In the rugged terrain of northern Shan State, the Kunlong Bridge is an important transportation link, crossing the powerful Salween River. The modern bridge, a 286-meter-long box girder structure, was completed in 2023 to replace an older, deteriorating suspension bridge built in 1965. This newer bridge, with a load capacity of 75 tonnes, facilitates trade along the Hsenwi-Kunlong-Chinshwehaw road, an important route to China. The bridge's location, however, places it directly over the Nantinghe Fault, a seismically active feature, presenting a significant engineering problem.
Myanmar is a region of high seismic activity, primarily due to the collision between the Indo-Australian and Eurasian tectonic plates. This collision zone is dominated by the Sagaing Fault, a major right-lateral strike-slip fault that runs for over 1,200 kilometers through the country. The Nantinghe Fault is part of this larger, active system. Probabilistic Seismic Hazard Analysis (PSHA) is a method used by engineers and seismologists to estimate the likelihood of certain levels of ground shaking at a given location. For the Kunlong Bridge, PSHA indicates a 10% probability of a magnitude 7 earthquake occurring within 50 years, a standard benchmark used in infrastructure planning. Such an event could generate ground shaking severe enough to exceed the bridge's structural capacity.
Assessing the structural risk
The vulnerability of a bridge during an earthquake depends on its design, materials, and the specific characteristics of the ground motion it experiences. Engineers assess these risks by developing "fragility curves," which are probabilistic models that show the likelihood of a structure reaching or exceeding a certain damage state for a given level of ground shaking. This analysis considers potential failure points, such as the bridge's piers, bearings, and overall structural integrity.
The design of infrastructure in earthquake-prone regions requires specialized engineering to ensure resilience. For a structure like the Kunlong Bridge, seismic analysis would involve calculating the expected peak ground acceleration (PGA)—a measure of the intensity of ground shaking. Deterministic and probabilistic seismic hazard analyses for Myanmar show that areas along the Sagaing Fault and associated structures can be subjected to significant PGA levels. While the new Kunlong Bridge was built with modern engineering standards, its location on an active fault necessitates an understanding of its potential performance during a major seismic event predicted by hazard assessments. The structure's ability to withstand the expected ground motion from a magnitude 7 earthquake is a point of scientific and engineering interest.