ABSTRACT
The 2016 Kumamoto Earthquake of April 2016 caused severe damage to expressways in Kumamoto Prefecture. The damage included the collapse of a rocking pier bridge that crossed over an expressway. A rocking pier bridge consists of multiple columns with hinge structures at the top and bottom ends of the columns. A rocking pier is unusual in not being free-standing nor resisting horizontal inertial forces acting on the superstructure. The bridge that fell was equipped with structures for confining the displacement of the superstructure in the direction perpendicular to the bridge axis so as to conform to seismic design standards. However, the great seismic force (1,399.4 gal) generated by the first shock on April 14 destroyed the confining structures, and the displacement of the superstructure went uncontrolled. In addition to the loss of confining structures, the aftershock, which occurred about 28 hours after the first seismic motion and generated a great seismic force (1,156.9 gal), brought about excessive horizontal displacement of the superstructure. Under this excessive displacement, the midpoint support lost vertical bearing capacity. From this, the bridge underwent failure. It is possible for the rocking pier bridges in Japan to become unstable from displacement and to collapse in an earthquake of similar strength to that of the 2016 Kumamoto Earthquake; therefore, it is necessary to secure safety of the expressways by urgently improving the seismic performance of rocking pier bridges. This report introduces a design for the seismic retrofit of bridges with rocking piers and construction measures against the toppling of such bridges that were implemented by the Hokkaido office of NEXCO East Japan.
