Vibration observations, Sather Tower, University of California, Berkeley, California

Deflection curves for the first three modes of transverse vibrations induced by external forced motion of Sather Tower are presented for both the E-W and N-S directions. Also presented are data on two modes of torsional vibrations.

In the fundamental transverse modes, substantial correlation is obtained with the work of previous observers, including deflections greater in proportion than height, as found by Carder. Also, striking similarity exists between observed modes in the two directions, a condition reflecting the known symmetry of the structure. This fact was used for substantiation of the trends established from the data when precise interpretation of tower motion was found difficult. It is noted that nodal points occur in the two higher modes between the eighth and ninth levels in both directions. This would be expected, since an abrupt change in section occurs between these floors; however, a computation for the deflection of a simple cantilever beam of length equal to the tower height (from base of tower footing,—18ft. Elevation) and concentrated load applied at the height of the shaking machine (seventh floor) shows less than 6 per cent deviation of thirteenth-floor deflection from the N-S fundamental deflection curve presented. Finally, knowledge of the vibrational characteristics of Sather Tower, and similar structures, has been extended through the determination of the second and third transverse modes and torsional modes of vibration.