Showing papers on "Modal testing published in 1976"

Modal Analysis for Building-Soil Interaction

Abstract: An approximate method of modal analysis is presented for the earthquake response of linear building-foundation systems. Modal damping is obtained as a formal extension of the classical method of modal analysis and is shown to be equivalent to the energy ratio criterion proposed by Jacobsen. By making use of the modal shapes of the fixed-base superstructure, modal damping in the system is expressed as a weighted sum of the critical damping ratios of the superstructure plus an additional term representing the energy dissipated by the soil. To estimate the accuracy of the suggested method, both analytical and numerical studies are performed. Several examples are included to illustrate the effect of hysteretic soil damping on the system response.

Damping Synthesis from Substructure Tests

Abstract: A new method is proposed for synthesizing structural damping from substructure test data. It utilizes the off-diagonal coupling terms in the substructure modal damping matrices as well as the diagonal terms which correspond to uncoupled modal damping. The coupling terms are evaluated from complex resonant response measured at each substructure mode. Both analytical and experimental verification of the method have been made. Accuracy requirements on experimental data and data reduction have been established.

Modal structure inferred from static far-field noise directivity

Abstract: Turbofan noise directivity calculated for two directivity models was compared with experimental, blade passing frequency data from two fans at 60 and 90 percent speeds. Experimental data indicated similar directivity patterns which were well represented by a single average data curve. Calculated points using the equal amplitude model showed over-prediction near the fan axis and near the 90 degree position. Calculated points using the equal power model showed a very good match with the average data lending support to theory of equipartition of modal power from a random source such as the interaction of the rotor with inlet flow distortion. The equal modal power model also gave good agreement with individual data points.

Experimental techniques for acoustic modal analysis of cavities

Abstract: This paper describes two experimental techniques which were developed to determine acoustic resonances and mode shapes in cavities. Application to an automobile passenger compartment is discussed as an example of the use of each technique.

Modal analysis of coupled motion of horizontally excited embedded footings

Abstract: A modal analysis formulation of the equations of motion of horizontally excited embedded footings is presented. Numerical solutions obtained with these equations are compared with the results obtained from direct solutions. It is shown that the two methods of solution are in good agreement for frequencies near to the first resonance.

Identification of natural frequencies and modal damping ratios of aerospace structures from response data

Abstract: An analysis of one and multidegree of freedom systems with classical damping is presented. Definition and minimization of error functions for each system are discussed. Systems with classical and nonclassical normal modes are studied, and results for first order perturbation are given. An alternative method of matching power spectral densities is provided, and numerical results are reviewed.