Acceleration amplitude

Murphy, J. R. and L, J. O Brien, 1977, The Correlation of Peak Ground Acceleration Amplitude with Seismic Intensity and Other Physical Parameters, Bull. Seismol. Soc. Am. 62 p 877. 

During SG (evaporators) overhaul in pursuance with the results of vibration investigations the pipes were plugged that were operated at maximum vibration acceleration amplitudes <3g. 

The model was subjected to a total of eight earthquakes during two flights. The earthquakes were pseudo-harmonic wavelets except for the last earthquake fired during the first flight that was a sine sweep. This signal is actually a sine wave of decreasing acceleration amplitude and frequency. The main characteristics of the input motions are tabulated in Table 22.3 both in model and prototype scale (bracketed values), while the time histories are depicted in Fig. 22.5. 

This type of input acceleration was imposed by sinusoidal excitation consisting of 15 steady cycles. To smoothen out the transition between transient and steady-state response, the excitation comprises of a 5-cycle ramp up to full test level at the beginning of the excitation, and a 5-cycle ramp down to zero at the end. With reference to frequency and acceleration level, a set of five frequencies (4, 7,13, 25 and 43 Hz) was used at low acceleration amplitude of 0.05g, to study the dynamic response of the system. The excitation frequency of 7 Hz was then selected for a series of harmonic excitations with increasing amplitude, until failure. The conditions of the excitation are considered to be essentially pseudostatic, as the above frequency is much smaller than the resonant frequencies of the system, with respect to both the free field and the soil-wall system. 

Accelerometers are used to characterize the vibrational levels associated tvith low-g experiment platforms. One useful anatytical tool is to convert acceleration measurements via Fourier transformation into acceleration amplitude vs. frequency diarts. Certain features tend to appear consistent in these measurements. Examples include vibrational modes associated with the flexing of spacecraft structural components, equipment operations, compressors, tons, etc. 

Especially in an environment with rotating masses, sinusoidal vibrations, often with a constant frequency, are common. A sinusoidal excitation can be characterized by its frequency and its acceleration amplitude. An important test of sinusoidal vibration is the sine sweep test. In a given frequency band, the frequency changes at a given rate and the specimen is monitored to... 

Resolution It is the minimum acceleration amplitude that it may measure. In early accelerometer digital recorders, it was limited by the digitizer (A/D converter) resolution. In modem instmments, the digitizer is usually 24 bit, and the resolution is related to the self-noise level of both the accelerometer and the digitizer. So it is usually given as self-noise level, or it may be obtained from the dynamic range. Good-quality accelerometers have noise levels under 1 pm/s rms (root mean square). 

Dynamic Range The relation (expressed in decibels, dB) between the maximum acceleration amplitude (FS) and the resolution. A ratio between two amplitudes ai and a.2 expressed in dB is 20 log(ai/a2) for energy or power ratios, a factor 10 instead of 20 is used. The best accelerometers now have a dynamic range up to more than 150 dB, which means a ratio... 

Seismic Network and Data Quaiity, Fig. 3 Frequency-amplitude plot for octave-wide band passes of ground-motion acceleration. The blue polygon represents the sensitivity limits of an STS-2 broadband seismometer, where the frequency limit at 50 Hz is given by the Nyquist frequency for a sample rate of 100 Hz. The lower acceleration amplitude limit in the band 0.001-50 Hz is the minimum sensitivity of the sensor,... 

Local site conditions not only influence the peak acceleration amplitudes but can also strongly affect the frequency content of surface motions. Fig. 9 shows the effects of local soil conditions on the shape of the normalized response spectra computed from ground motions recorded on different sites for periods above 0.5 s, spectral accelerations for soil sites are higher than those for rock sites. The figure clearly indicates that deep and soft soil deposits enhance the transmission of low frequencies (high periods). The results also show that the use of a single response spectrum shape for aU site conditions is not appropriate. This evidence has been incorporated in a large number of seismic codes worldwide which propose the use of different spectral shapes for different subsoil conditions. 

Figure 2.2 Patterns in vertically vibrated granular materials, (a) Overhead view of a square pattern produced by vibrating 60,000 0.55 mm lead spheres in a 55 mm cell at / = 22 Hz and maximum acceleration amplitude of 3.000 times gravity. The pattern is illuminated with low angle lighting to accentuate the peaks and valleys, (b) Oblique view of the square pattern from (a). The pattern is //2 subharmonic, repeating every other drive cycle. On the other cycle, the peaks become valleys and the valleys become peaks, (c) Collection of patterns at various driving frequencies and amplitudes, (top row) //2 Stripes, //2 Hexagons //4 Stripes, (bottom row) //4 Hexagons, //4 Phase bubbles, //4 Phase domain pattern. (Adapted from Bizon, C. et al., Phys. Rev. Lett., 80, 57, 1998 Moon, S.J. et al., Phys. Rev. E, 65, 011301, 2002.)...

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