A STUDY OF RESONANT FREQUENCY OF MACHINE FOUNDATIONS SUBJECTED TO HORIZONTAL. IMBALANCE FORCES

Abstract

In the present investigation the behaviour of block foundations subjected to horizontal force and moment has been discussed. Such a machine-foundation-soil system forms a two degrees of freedom system, having two natural frequencies. Due to limitations of equipment, only the lower natural frequency has been studied. Experimental investigations carried out so far by vari-ous investigators have revealed that the natural frequency of such a machine-foundation-soil system depends upon a. number of factors* The variables taken up for the present investigation are the base area height of the block and the dynamic load imparted to the system. Results of eight model footings subjected to translatory and rocking vibrations have been analysed. All the footings were resting on the surface of a medium sandy soil. The effect of contact area of the footing on the coefA.- eF tient of elasti4 uniform shear of soil, c, has also been discussed. It has been shown that ct is independent of the ratio of dynamic load to the static weight of the structural system. An attempt has been made to calculate the 'damping of the soil in the sliding mode of vibration. The damping factor is seen to increase with the area of contact of the footing with sJil, although its effect on natural frequency is negligible. The amplitudes of vibration have been coLputed on either side of the resonant frequency and compared with the experimentally obtained response curve. It is found that on the lower side of the resonant frequency, theoretical values lie above the experimental values. On the other side of the curve, theoretical values lie below the experimentally obtained values. The limitations of the testing programme have been given and suggestions for further research have been made