Donald E. Hudson

Bio: Donald E. Hudson is an academic researcher. The author has an hindex of 1, co-authored 1 publications receiving 24 citations.

Synchronized vibration generators for dynamic tests of full-scale structures

Abstract: The general considerations behind the design of a rotating weight sinusoidal vibration generator for dynamic studies of full-scale structures have been given in a previous report which pointed out the advantages of a system of multiple synchronized machines that would permit a distribution of exciting forces throughout a structure so as to most efficiently excite various modes of vibration. Under the sponsorship of the California State Division of Architecture a set of four synchronized vibration generators has been completed and tested, and the purpose of the present report is to summarize the design information on this new system, and to give detailed operating instructions for its use.

Extracting the Building Response Using Seismic Interferometry: Theory and Application to the Millikan Library in Pasadena, California

Abstract: The motion of a building depends on the excitation, the coupling of the building to the ground, and the mechanical properties of the building. We separate the building response from the excitation and the ground coupling by deconvolving the motion recorded at different levels in the building and apply this to recordings of the motion in the Robert A. Millikan Library in Pasadena, California. This de- convolution allows for the separation of instrinsic attenuation and radiation damping. The waveforms obtained from deconvolution with the motion in the top floor show a superposition of one upgoing and one downgoing wave. The waveforms obtained by deconvolution with the motion in the basement can be formulated either as a sum of upgoing and downgoing waves, or as a sum over normal modes. Because these deconvolved waves for late time have a monochromatic character, they are most easily analyzed with normal-mode theory. For this building we estimate a shear velocity c 322 m/sec and a quality factor Q 20. These values explain both the propagating waves and the normal modes.

The Observed Wander of the Natural Frequencies in a Structure

Abstract: The Southern California Seismic Network (scsn) has recently installed seismic stations in two buildings on the Caltech campus (Millikan Library and the Broad Center). Continuous real-time accelerometer data from these structures are now freely available to the community. This dataset provides a new opportunity to observe, and better understand, the variances in the primary dynamic property of a building system, its natural frequencies. Historical data (triggered strong-motion records, ambient and forced vibration tests) from the well-studied Millikan Library show dramatic decreases in natural frequencies, attributed mainly to moderately large local earthquakes. The current forced vibration east–west fundamental frequency is 22% lower than that originally measured in 1968. Analysis of the new continuous data stream allows the examination of other previously unrecognized sources of measurable change in the fundamental frequencies, such as weather (wind, rain, and temperature), as well as nonlinear building vibrations from small local and moderate regional earthquakes. Understanding these nonlinear shifts is one of the long-term goals of real-time building instrumentation and is critical if these systems are to be used as a postearthquake damage assessment tool.

Comparisons between ambient and forced vibration experiments

Abstract: The ambient and forced vibration techniques for testing full-scale structures are critically compared. Both methods, based on small level excitation, may be used to determine many mode shapes and frequencies of vibration and the corresponding damping values, with adequate accuracy for most purposes. The two techniques give mutually consistent results. The mode amplitudes determined by ambient and forced vibration tests show systematic departure for high modes and near the top levels of buildings tested. This phenomenon is attributed to the participation of all mode shapes and is a consequence of excitation by a concentrated force near the top of a building and at a frequency differing by only a few per cent from a natural frequency of vibrations. A new way of showing the effect of unwanted modes on the response near resonance of the mode being sought is developed. It is particularly useful for the analysis of steady, forced vibration tests of structures using eccentric mass vibration generators.

Interaction of a shear wall with the soil for incident plane SH waves: Elliptical rigid foundation

Abstract: The closed-form solution of the dynamic interaction of an elastic shear wall and the elastic homogeneous half-space, previously known only for the rigid foundation with circular cross section, has been generalized to apply for the foundation with elliptical cross section. It is shown that the interaction equation depends on the incidence angle of plane SH waves and that this dependence gradually disappears as the elliptical cross section approaches the circular one. The effectiveness with which the rigid foundation can scatter the incident energy has been found to increase with the depth of the foundation.