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Putting dampers on

Letters

The article on tuned mass dampers applied to the new Oakland Bay Bridge (NCE 14 October) assumes that the tuned mass system (TMS) could provide an answer to a potential cost overrun in the current bridge design because it could accept a lighter design under earthquake loading.

First of all a difference should be made between a passive TMS and an active TMS, the former being more robust and cheaper and the latter more versatile.

Having had an interest in the application of passive TMS to offshore structures the following results were obtained:

lThe system is very suitable for mechanical equipment running at a single frequency lTall buildings under wind loading may show a coupling between the wind excitation and its natural frequency and hence a passive TMS can be effective lUnder random wave loading (as compared with single frequency loading) the application of a passive TMS on a dynamically sensitive structure is not effective lUnder impulse loading the passive TMS is not effective at all, at least not for maximum amplitude displacements of the structure.

For these reasons it can be argued that the ability of the passive TMS to reduce maximum amplitudes displacements under earthquake loading of tall land-based structures should be questioned.

The alternative is an active TMS but it requires a highly advanced system to cope with the impulse type accelerations under a designed earthquake.

Pim Visser (M), Zomervlinderberm 66, 3994 WR Houten, the Netherlands

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