Japan’s Earthquake-Resistant Bridge Design Draws Praise After Kumamoto Quake

According to Reuters, on the afternoon of Tuesday, July 28, a powerful magnitude 7.1 earthquake struck Japan’s Kumamoto Prefecture on the island of Kyushu producing a maximum seismic intensity of 7 on Japan’s scale. After the earthquake, aerial footage of an elevated expressway circulated widely online, alarming many viewers who initially thought the bridge was on the verge of collapse.

The footage showed a section of the Kyushu Expressway in Yatsushiro City with clear displacement along the bridge deck. One side of an expansion joint appeared higher than the other, while the bridge’s main girder had also sunk.

However, many Japanese social media users responded by praising the structure, saying the visible damage demonstrated that Japan’s earthquake-resistant design had worked as intended.

Some users said that although the bridge appeared to have failed, the damage was not the result of a design flaw. Rather, the bridge had been designed to sustain damage in a controlled manner.

According to the Public Works Research Institute, Japan’s bridge engineering standards do not require every structure to remain completely undamaged during an exceptionally powerful earthquake. Instead, bridges are designed with sufficient strength, ductility and energy-dissipation capacity to withstand severe shaking without critical structural failure. Under Japan’s revised seismic-design approach, important bridges must perform with limited damage during extreme ground motions, while standard bridges must at minimum avoid critical failure.

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This means that some controlled deformation, displacement or localized component damage may occur during a major earthquake. Bearings, piers or other structural elements may absorb seismic forces and sustain damage, but the bridge system must retain enough capacity to prevent a catastrophic collapse. Japan’s specifications emphasize ductility and the transfer and dissipation of seismic energy throughout the bridge rather than relying solely on an extremely rigid structure.

One safeguard is the bridge unseating-prevention system. These devices are designed to reduce the danger that a bridge deck, or superstructure, will separate from its supporting piers or abutments after bearings fail or the deck experiences substantial lateral displacement. Japan’s post-1995 design revisions specifically required greater strength, ductility and energy-dissipation capacity for unseating-prevention devices.

The approach was shaped in large part by the destruction caused by the 1995 Great Hanshin-Awaji Earthquake. According to Hanshin Expressway, 17 bridge piers collapsed along a 635-metre section of the Route 3 Kobe Line in Kobe’s Fukae district. Across that route, 637 of 1,175 bridge piers and 551 of 1,304 bridge spans sustained damage.

Following the disaster, Japan substantially revised its Specifications for Highway Bridges in 1996. The changes strengthened requirements for lateral capacity and ductility, introduced stronger earthquake motions into bridge design, expanded the use of energy-dissipating bearings and required improvements to unseating-prevention systems. The revised standards recognized that a bridge might sustain damage during an extreme earthquake, while requiring it to avoid critical or catastrophic failure.

Another magnitude 5.8 earthquake strikes Kumamoto

At approximately 10:19 p.m. local time on July 29, a magnitude 5.8 earthquake struck the Amakusa and Ashikita region of Kumamoto Prefecture, producing a maximum seismic intensity of lower 5, Weathernews reported.

The Japan Meteorological Agency said the earthquake’s epicentre was in the Amakusa and Ashikita region and that its focal depth was extremely shallow. Based on the earthquake’s characteristics, officials said there was no risk of a tsunami and residents did not need to be concerned about a tsunami threat. The agency nevertheless issued an emergency earthquake warning and urged the public to take precautions.

The agency said it was the third-largest earthquake in the series of tremors that had affected Kumamoto since July 28. So far, five earthquakes in the sequence have registered a maximum seismic intensity of lower 5 or higher.