<p>We have implemented the project which connects the radiate railway lines existing individually with new line “Tohoku-Jukan line”. This line is now under construction just above the in-service Shinkansen line, because it is very difficult to obtain the space for new line in urban area of Japan. The construction is realized by applying the viaduct structure with double-deck system, which consists of the existing viaduct for Shinkansen and new deck and new pier for the new line added on the steel rigid-frame pier of the existing viaduct.</p><p>For this renewed structure, we had studied on the seismic performance by carrying out the analysis and the experiment. The study gave us the result that the base of the piers had not enough capacity against the large-scale earthquake, which we have experienced up to now in Japan. From this result, we also developed the reinforcement method, which was ascertained by horizontal cyclic loading test, for the base from the inside of the hollow steel pier, and conducted it. The reason why the reinforcement had to be conducted from the inside of pier was that the space underneath the viaduct is normally used for various building and it is impossible to construct something from the outside of the pier.</p>