Abstract
The influence of the frecuency content of seismic excitations on the behavior of an optimal tuned mass damper (TMD) is studied in the context of a system with explicit consideration of soil-structure interaction. A stochastic analysis is made in the time domain for two random processes, one considering a broad bandwidth process (BBP) and other considering a narrow bandwidth process (NBP). A structure built over three different types of soil (soft, medium and hard) is considered. For the optimization of the TMD, the minimization of the ratio between the standard deviation of the displacement of the main structure with TMD with respect to a structure without TMD, is used as the target function. It is found that for seismic excitations with high frecuency content, the ratio of the TMD frequencies compared to the fixed base frequency of the structure approaches to 1 as the soil becomes more rigid. It is also observed that the TMD become tuned with the flexible base frequency for all soil types, producing perfect tuning for small mass ratios and detuning gradually for higher mass ratios. On the other hand, the TMD optimal damping ratio increases as the TMD mass ratio is higher, independently of the soil type. The TMD is more efficient for higher values of the TMD mass ratios, especially on soft soil. In structures built over flexible base, that are subjected to low frequency content excitations, the optimal TMD is tuned with the flexible base, independently of the type of soil and the fixed base period of the main structure. The TMD optimal damping is not sensitive to the flexible period for small mass ratios, and reaches its minimum value when it matches with the predominant period of the seismic event. On the other hand, the TMD reaches its maximum efficiency when it is tuned with the flexible period of the soil-structure system, and coincides with the predominant period of the seismic exitation and is higher on soft soil. A deterministic analysis is made using two seismic records, an artificial earthquake compatible with the Chilean code NCh2745 characterized by high frequencies content and other similar to the event in 1985 in Mexico, characterized by low frequencies content. It is seen that the optimal TMD is efficient controlling the response of the structure in all types of soil analyzed.
Highlights
One of the main objectives of structural design is to provide safety and serviceability to the users
The third row shows the tuned mass dampers (TMD) optimal damping factor optt. The behavior of these optimal parameters is analyzed for a range of mass ratios between 0.01 and 0.1, for all three soil types: soft soil, medium soil and hard soil, and for three fixed base periods of the structure (Ts ), i.e. 0.5, 1 and 1.5 seconds
The reduction becomes less sensitive to the soil type. l) The TMD produces a greater reduction in soft soil for the entire range of fixed base periods analyzed. m) There is an aspect ratio value where the TMD efficiency changes with regard to the soil type, independent of the soil type and the fixed base period of the structure
Summary
One of the main objectives of structural design is to provide safety and serviceability to the users. This is done, fundamentally, by controlling the response of structures subjected to dynamic excitations. One of the structural control systems used currently, are tuned mass dampers (TMD). The first studies on the behavior of TMD were made by Den Hartog in 1947, who determined the TMD design parameters to control a one degree of freedom system subjected to harmonic excitation (Den Hartog, 1947). Research focused on the optimization of the design parameters and evaluation of the TMD performance in reducing vibrations of structures subjected to different types of dynamic excitations.
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