Calculated and Observed Vibration Amplitudes

Comparisons between the observed and calculated (Sung solution) maximum displacement amplitudes suggest that for sandy and silty soils the stress distribution beneath the footing changes from parabolic at small mass ratios to the rigid-base type of stress distribution at higher mass ratios. For clay soils the theoretical displacement amplitudes for a rigid base stress distribution were in approximate agreement with observations. Based upon the Sung and Hsieh solutions for displacement amplitude theoretical expressions have been developed for acceleration amplitude. Observed amplitudes were found to be of the same order of magnitude as theoretical predictions. For small mass ratios and in situations in which the operating frequency of a machine is greater than the resonant frequency, a theoretical analysis indicated that excessive vibration amplitudes may develop at the operating frequency even though amplitudes at the resonant frequency are not excessive.