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Damping ratio is an important component in the dynamic analysis and plays a key role in design of supertall buildings. It is necessary to have reliable field measurement records for estimation of structural dynamic characteristics. The main objective of this paper is to estimate eigen mode damping ratio of TV towers as a kind of supertall buildings with the height of more than 250 m. In this regard, field measurements were performed on Milad TV tower, located in Tehran, which has a height of 435 m by ambient vibration monitoring. Damping ratio of this tower is determined using random decrement technique and compared with available data of a few other TV towers.For proposing eigen mode damping ratio of TV towers, it is shown that Rayleigh damping coefficients could not be calibrated properly for tall slender TV towers. A damping estimation chart of TV tower that is a function of natural frequency and structural aspect ratio is proposed, and the chart validity has been checked with field measurement results of other TV towers. The chart shows that structural damping for wind analysis proposed in design codes and standards is overestimated for tall slender TV towers. Copyright a difference = ((measured À calculated)/measured). 872M. M. AMIRI AND M. YAHYAI

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Tuned mass dampers (TMDs) and active tuned mass dampers (ATMDs) are control devices added in tall buildings to suppress the response of buildings in irregular external excitations such as earthquakes and wind. The performance of both passive and active damper systems can be assessed by parametric studies. This paper presents a brief description of the theory of passive and active tuned mass dampers. In this work, the effect of TMDs has been studied by changing the frequency ratio of the damper to the first mode of the structure. It was observed that the extra damping provided by TMDs, called effective damping, can primarily be obtained from design charts developed for this purpose. A 25-storey building is designed and analysed using these charts as an example. The behaviour of a structure with an active tuned mass damper is investigated. The related parameters and equations of the active mass damper for the building are solved and its behaviour arising from external loads is studied. The conventional studies, which are based on white-noise excitations, usually overestimate the performance of TMDs and ATMDs. Hence, in order to study the behaviour of a building with a TMD and an ATMD in real excitations, El-Centro and Tabas earthquakes are selected. The results show a significant increase in the effect of TMD performance in controlling the structural displacements by using additional control devices.