Vibration tests of structures

Abstract

Dynamic tests of three structures are described. In two cases linear vibration theory is applied to explain the behavior of the structures. In the third case a new method of analyzing the vibration records is introduced to define nonlinear properties of the structure. Free and forced vibration tests were conducted on a reservoir outlet structure consisting of a reinforced concrete tower, 149 feet in height, with a steel truss bridge, 3139 feet long, connected to the tower near the top. Measurements revealed five natural frequencies and mode shapes, and indicated the extent and significance of foundation movements. A detailed theoretical analysis of linear vibrations of the structure is carried out to show good agreement with the observations and to illustrate a general technique for the dynamic analysis of framed structures. An earth dam 485 feet long by 60 feet in height by 450 feet thick at the base was subjected to a sinusoidal lateral exciting force at the top. Application of the theory of a truncated wedge vibrating in shear modes is made to determine an effective shear wave velocity in the earth fill and to estimate damping in the modes. A general procedure is presented for experimentally determining the restoring and dissipating functions in lumped mass. structures, linear or nonlinear. An experiment on a single degree of freedom laboratory structure with bolted joints is used to illustrate the method. The question of instrumentation suitable for structural dynamic work is considered and recommendations are made on the basis of tests and examination of many commercially available components.