NATURAL FREQUENCY OF BLOCK FOUNDATION& UNDER FREE AND FORCED VIBRATIONS

Abstract

Natural frequency of block foundations under conditions of free and forced vibrations has been studied. A relation has been obtained for determining the natural frequency at any force level from a knowledge of the natural frequency of free vibra-tions, dynamic force and static weight. The relation has been established both for horizontal and vertical modes of vibra-tion. Two model footing resting on the surface of medium sandy soil were tested. The natural frequency of free vibra-tions was obtained by hitting the block with a hammer and the natural frequency of forced vibrations from resonance tests. Field test data reported by D.C. Gupta and B M. Basavana was also analysed to check the validity of the frequency relation obtained. Validity of the frequency relation was further checked by exciting the blocks to resonance at different force levels with SRTEE oscillator which was also calibrated experi-mentally. The frequency relation connecting the natural frequency of free vibrations, the natural frequency of forced vibrations, the static weight and dynamic force can be of utility in comput-ing soil constants for the design of machine foundations from field tests on a more rational basis. The problem is simplified to determination of natural frequency of free vibrations only from which the resonant frequency at any force level can be computed. -2- The values of Co, the coefficient of elastic uniform compression of soil and CT, the coefficient of elastic uniform shear of soil have been determined both from free and forced vibration tests. The damping factors were also computed from both the free and forced vibration tests. Limitations of the testing programme have been given and suggestions for further research have been made