How to build bridges that are tsunami-resilient

By Andrew Wright

The nuclear plant at Fukushima was the most famous casualty of the 2011 Tōhoku tsunami, but it was far from the only infrastructure affected in Japan. Hundreds of bridges were swept away, often hampering the relief effort – the island of Miyatojima, for example, where over 70 percent of homes were destroyed, was cut off from the mainland when its bridge collapsed.

Bridges built after Japan introduced new design codes in the 1990s – in response to the 1978 Miyagi earthquake – generally survived the 2011 earthquake itself. But these bridges had not been built to cope equally with tsunamis, say academics Kazuhiko Kawashima and Hiroshi Matsuzaki: “No single word about tsunami was included in the design codes”.

Indeed, a report by professors Akira Hosada and Kyuichi Maruyama suggests that the retrofitting of some older bridges to improve their resilience to earthquakes could have unexpectedly increased their vulnerability to the tsunami: additional devices to prevent girders from falling might have restricted their horizontal movement, transferring more of the tsunami’s force onto the reinforced concrete pillars and causing them to fail.

Weak points

Bridges that failed in the tsunami did so for two main reasons, say Kawashima and Matsuzaki: some failed when the horizontal force of the tsunami dragged them over; others when the tsunami’s vertical force lifted up spans of the bridge and floated them off.

The experience in Japan – after bridges were also destroyed in Indonesia in the 2004 tsunami – was a “clear warning for scientists to pay more attention and investigate, assess and mitigate the effect of tsunami loads on various structures including bridges”. This was the conclusion drawn by Iman Mazinani, Zubaidah Binti Ismail and Ahmad Mustafa Hashim in the Journal of Earthquake and Tsunami.

Building strength

Japan’s experience provided some positive examples, too. Kawashima and Matsuzaki point to bridges that survived the tsunami, despite having been completely submerged.

There are some general principles of building for tsunami resilience. Rollo Reid, Director of Reid Steel, observes that after the 2004 tsunami “well-built buildings survived in the middle of areas that were otherwise completely devastated”.

What makes for a well-built building when a tsunami hits? According to Reid, helpful factors include being built diagonally to the shoreline, so waves hit pointed corners first; and the steel beams of the frame being encased in a concrete slab, rather than planted on concrete pads – tsunamis can scour around such pads, whereas the weight of the water pushing a slab downwards can actually help to keep a building upright.

New findings

On bridges specifically, the science is advancing. A December 2018 paper in the Journal of Marine Science and Engineering calls for a “paradigm shift” in thinking about tsunami risk. Pedro Lomonaco and Solomon Yim from Oregon State University, and Dennis Istrati and Ian Buckle at the University of Nevada, make the case for including “not just the estimation of total tsunami load on a bridge but also the distribution of this load to individual structural components”.

The researchers constructed a 1:5 scale model of an open-girder bridge in Nevada, then shipped it to Oregon for testing in a wave flume over 100 metres long. These experiments allowed them to understand more about the horizontal and vertical forces that bridges have to withstand.

Their research sets out a “new physics-based methodology… which can be used by practicing engineers for the tsunami design of bridge connections, steel bearings and columns” – and which should help to keep future bridges standing when tsunamis hit.