The amplitude dependent dynamic characteristics of an existing building before and after seismic retrofit

Amplitude dependency of the dynamic characteristics of an existing 9-story RC building with uni-axial eccentricity is addressed in the paper, based on the various kind of observations and vibration tests such as earthquake observation, micro tremor observation and forced vibration tests using various exiting forces like excitor machine, man induced load and the air gun type impactor, obtained these 35 years after its completion. The building was damaged during the 1978 Miyagi-ken Oki earthquake. During this earthquake, the maximum acceleration recorded at 9th floor exceeded 1G, 1040 cm/s2, and the deflection angle of the building was of the order of 10-2 radian. The seismic retrofit work was performed from year 2000 to 2001, and it was the first large scaled reinforcement work. The vibration tests were performed before and after the retrofit work. After the retrofit work, the earthquake observation records were obtained for several earthquakes with different amplitude level. The dynamic characteristics of the building are comparatively discussed for three terms in the lifetime of the building, namely, Term-1, comprising the years after completion until before the Miyagi-ken Oki earthquake, this was the most severe earthquake for the building; Term-2, comprising the years after the Miyagi-ken Oki earthquake until before the retrofit work; and Term-3, after the retrofit work