Mode 4. Period = 0.32 sec Mode 5. Period = 0.29 sec
"More complex structures may be analysed by considering not only the fundamental modes, but also the higher natural modes of vibration, which are a characteristic of the stiffness and mass distribution of the structure. Some of these natural mode shapes, which are independent of the forcing vibration, are shown above for a typical ten storey building. Many computer programs exist to peform this calculation."
Edmund Booth, from 'Analysis for earthquake effects' in 'Concrete structures in earthquake regions'