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Civil Engineering/tuned mass damper and other dampers

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Question
sir,
thank u for answering my previous queries...pls enlighten me with the following queries..
1.can u tell me whether the base shear of my building gets decreased or  increased after the application of viscous dampers....because i have seen in one journal where it says the base shear has increased...if it indeed increases can u please explain the process how a viscous damper increases the base shear..because as per my knowledge a damper decreases majority of the responses be it base shear or displacements..
2.im also trying to apply tuned mass dampers in my model ...now i know that the displacements of the building will get decreased i have given the correct parameters and done the modelling of the damper properly in sap 2000...will the base shear get increased or decreased...
3.what is the difference between viscous dampers,viscoelastic dampers and friction dampers...do all these dampers help in reduction of base shear.....beacuse i have read that friction dampers reduce the base shear while viscous dampers increase the base shear....if so plzz explain the methodology behind the decreament or increament of each damper above specified....

Answer
Dear Shoaib

1- Normally the calculated base shear shall be reduced due to increase in damping. But when you are comparing the actual behavior of 2 alternatives in same structure under a specific time history then you may know that an increase in damping may shift the actual damped period of vibration and to be actuated more intensely. But for a design purpose I don't accept to have an increased design base shear.

2- In the case of tuned mass , you are reducing the effect of earthquake not by reducing the base shear but by stimulating higher modes of vibration to influence the effect of the first essential modes of vibration. Then it could increase or reduce the base shear.

3- Visco elastic dampers are like viscous one from calculation point of view. The design base shear will reduce in this kind of damping system. The friction is different. If you tune the friction dampers not to be activated under earthquake effects, then you don't have any energy dissipation and the base shear is the maximum. If your friction damper is designed to behave actively then you must have designed it for a less intense earthquake. Then it is like design of usual structures. You must use a R factor which may be assumed to be more than actual ductile structures R factor. Thus the base shear will be reduced by some extents using different higher R factor.

Regards
BEH

Civil Engineering

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Babak Esmailzadeh Hakimi

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