A constrained optimal Rayleigh damping coefficients for structures with closely spaced natural frequencies in seismic analysis

A constrained optimization method is proposed to determine Rayleigh damping coefficients for the accurate analysis of complex structures. To this end, an objective function was defined to be a complete quadratic combination of the modal errors of a peak base reaction evaluated by response spectral analysis. An optimization constraint was enforced to make the damping ratio of a prominent contribution mode exact. Parametric studies were conducted to investigate the effects of the constraint, the cross term of modes, and weighting factors on the optimization objective. A two-story building and a real-world lattice structure were analyzed under six earthquake ground motions to understand the characteristics and demonstrate the accuracy and effectiveness of the proposed optimization method. Unlike the conventional Rayleigh damping, the optimization method provided an optimal load-dependent reference frequencies that account for varying frequency characteristics of earthquakes around the prominent contribution mode.

Cross-Correlation-Based Method vs. Classical Fft for Spectral Analysis of Impulse Response

The paper presents comparison of results of impulse response spectral analysis that has been obtained using a method based on cross-correlation with results obtained using classical FFT. The presented non-Fourier method is achieved by correlating the analyzed signal and reference single-harmonic signals and using Hilbert transform to obtain an envelope of cross-correlation. The envelope of crosscorrelation makes it possible to calculate appropriate indicator and make its plot in frequency domain as a spectrum. The spectrum obtained this way has its advantage over the FFT that the spectral resolution does not depend on duration of signal. At the same time, the spectral resolution can be much greater than spectral resolution resultant from FFT. Obtained results show that presented non-Fourier method gives frequency readout more accurate in comparison to FFT when the impulse response is a short-time signal e.g. few dozen of miliseconds lasting.

Rat hippocampal neurons in culture: properties of GABA-activated Cl- ion conductance

The actions of gamma-aminobutyric acid (GABA) on the membrane properties of rat hippocampal neurons maintained in dissociated cell culture were studied using intracellular recording techniques. All the neurons tested were responsive to GABA applied by pressure from micropipettes containing 10-20 microM GABA. The response consisted of a marked increase in conductance associated with a potential change. The inversion potential was sensitive to the Cl- ion gradient across the cells. It was about -60 mV when measured during recordings utilizing K acetate-filled microelectrodes, about -15 mV when measured during recordings with KCl-filled microelectrodes, and about +15 mV when measured with KCl electrodes in a medium containing low [Cl-]o. These results indicate that the membrane conductance evoked by GABA primarily involves Cl- ions. There were no apparent differences between the inversion potential of responses elicited at the level of the cell body and those evoked on processes. The two-electrode voltage-clamp technique was used to study the membrane mechanisms underlying these responses. GABA generated current responses that were associated with an increase in both conductance and membrane current variance. At a given potential both the conductance change and increase in variance were directly proportional to the amplitude of the current response. Spectral analysis of the membrane current variance evoked by GABA revealed that many of the computed spectra could be fitted by a single Lorentzian equation, suggesting that GABA activates two-state (open-closed) Cl- ion channels whose durations are exponentially distributed. The mean duration of these channel openings was estimated to be 22.9 +/- 2.1 ms, while the average conductance was estimated to be 19.8 +/- 2.7 pS in 13 cells. Large-amplitude GABA responses evoked at -70 mV frequently faded in amplitude, often by as much as 50%, with little or no change in the associated conductance. Since the membrane current variance decreased in direct relation to the fading current response, while estimates of channel lifetime did not change, the results suggest that the fading is due to a shift in the Cl- gradient. Responses to constant amounts of GABA evoked at different membrane potentials showed that the macroscopic conductance activated by GABA varied with membrane potential. Often 4-5 times more conductance was generated at depolarized (0 to +10 mV) relative to hyperpolarized potentials (-60 to -70 mV).(ABSTRACT TRUNCATED AT 400 WORDS)

Fast Floor Response Spectra Generation Technique

A consistent technique is derived for generating floor response spectra for equipment in nuclear reactor systems using response spectral analysis. The use of this technique eliminates the requirement for a time history analysis. The technique is based on the dynamic theory of coupling the supporting building with a very light spring-mass system representing the equipment. The response of the spring-mass system in the coupled systems is the floor response spectrum at the spring-mass system frequency. Resonant and off-resonant cases are derived separately. This technique is more efficient in the use of computer time than the conventional time history techniques. Moreover, a more realistic response spectrum is generated by this technique.