The　magnetorheological　(MR)　fluid　damper-based　semiactive　control　systems　have　received　considerable　attention　for　protecting　structures　against　natural　hazards　such　as　strong　earthquakes　and　high　winds.　In　this　paper,　a　novel　modal　controller　using　wavelet　packet　transform　(WPT)　is　proposed　for　the　vibration　control　of　distributed　structures.　In　the　proposed　control　system,　the　WPT　method　is　utilized　to　decompose　the　acceleration　measurement　and　select　the　modes　containing　most　of　the　WPT　energy　component　as　the　dominant　modes.　Then,　a　modal　controller　is　designed　to　control　the　dominant　modes　and　the　optimal　active　control　force　is　solved.　Finally,　Clipped-optimal　control　law　is　adopted　to　determine　the　voltage　applied　to　each　MR　damper.　A　Kalman-filter　observer,　which　estimates　the　full　controlled　modal　states　from　local　accelerometer　feedbacks,　is　designed　for　rendering　the　controller　to　be　more　applicable　to　distributed　structures　with　a　large　number　of　degrees　of　freedom.　A　numerical　example　of　a　stadium　roof　structure　installed　with　MRF-04K　damper　is　presented.　The　effectiveness　of　the　controller　is　evaluated　under　both　Tianjin　and　El　Centro　earthquake　excitations.　The　superior　performance　and　adaptability　of　the　controller　for　versatile　loading　conditions　are　demonstrated　through　the　comparison　with　traditional　truncated　modal　controller.　©　2011　Tianjin　University　and　Springer-Verlag　Berlin　Heidelberg.