VIBRATION ABSORBERS FOR SYSTEMS SUBJECTED TO EARTHQUAKE TYPE EXCITATIONS

Abstract

There can be various methods of safeguarding a structure against excessive dynam:.c forces. One of them is to design the system as such for higher forces. Another method is to decrease the forces by providing the structure with some mechanism. There may be several methods of having such mechanism in the parent structure. One of them is by mounting vibration absorbers on parent system. This method has so far been used for systems wh:ch are subjected to sinusoidal excitations. The vibration absorbers may be provided to reduce the vibration amplitude at any one particular frequency and hence they are generally used at resonance condition. In this thesis, the efficacy of the use of vibration absorbers for earthquake tyoe excitations has been studied analytically. Since an earthquake has several frequency components, a greater number of vibration absor bers having different dynamic properties may have to be provided. The analytical and experimental study was also carried out for the use of absorbers for rnulti-frequency sinusoidal excitations* The parent systems considered in the analysis for earthquake excitations in most of the cases were single degree of freedom systems having linear spring force and viscous type of damping. For few cases investigated, the Ill parent system had two degrees of freedom system. Effect of the use of large variety of vibration absorbers, invol ving different types of damping and restoring force charac teristics on the response of parent system, has been studied. With single degree of freedom parent system, the combinations tried were, single and multiple absorbers with linear and non-linear springs and viscous and coulomb type of dampings. Besides these, the effect of mounting parent system on absorber was investigated. Furthermore, multiple absorbers having"linear spring and viscous damoing were tried with two degrees of freedom parent system. In addition, effect of changing the stiffness of single and two degrees of freedom parent systems was also studied. In the analysis of all above cases, the parameters of vibration absorbers like period, damping, mass and number were extensively varied to find the best combination. The parent systems were subjected to three actually recorded earthquake motions. Extensive use of digital computer was made in analytical study. The analytical study reveals that, a group of vibration absorbers could be used for earthquake type excitations. Maximum reduction obtained in the response of parent system among the cases investigated is about forty percent. As far as absorbers are concerned the •maximum force on them is very small. Analysis was also carried out for a single IV degree of freedom parent system when subjected to sinu soidal excitations. Multiple absorbers having linear spring force and viscous type of damping were tried for multifrequency sinusoidal excitations. The study indicates that an assemblage of absorbers is widely effective in reducing the forces on parent system. The effect of increasing or decreasing the stiffness of parent system on its dynamic response was studied analytically. Study was ^lso carried out when parent system is mounted on absorbers. It was found th=tt the response of parent system can also be decreased by judiciously changing its stiffness or by mounting it on absorber. Experimental investigations were carried out to study the effect of absorbers on the behaviour of a single degree of freedom model. A group of absorbers with different dynamic characteristics were tried along with the model. Sinusoidal excitations of varying frequency were chosen for the test. Experimentally obtained results were compared with the analytically calculated ones. Experimental study was also carried out on a concrete frame model to find the usefulness of absorbers. The frame was subjected to sinusoidal excitations and motion due to impact. A grouo of absorbers made of steel leaf sorings were used in this case.