Seismic retrofit for the truss bridges of the Seto-Ohashi Bridges

In original seismic design of the Seto-Ohashi Bridges, an inland near-field earthquake was not considered. And there is concern that large-scale earthquakes would occur in the near future. Therefore seismic retrofit for truss bridges of the Seto-Ohashi Bridges has started. In the seismic retrofit design for the truss bridges, it was decided that seismic response of overall bridge was reduced firstly and the truss members were strengthened by additional stiffeners secondly. The stiffeners were attached to the truss members by bolting, because it was difficult to ensure quality of field welding. As inner surface of the closed box-section truss members has no coating, drilling bolt holes in the members was thought to increase the probability of corrosion of the members. Therefore it was decided to apply airtightness measures for the inside of the closed box-section members after their drying.