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.

In most of the structural design codes, the foundation sites are classified through the mean stiffness of the soil profile, obtained on the basis of the mean shear wave velocity of the upper 30 m Vs30. Because it is an averaged index, the site classification does not directly take into account the properties of shallower soil layers, neither the fact that sites with different soil stiffness distribution are classified as the same site class. Moreover, the way in how this situation can affect the response of the structures is not clearly specified. In this work, the effect of the site stiffness distribution on the seismic performance and level of damage of structures is explored by means of numerical models. Under a direct approach, two-dimensional numerical models are developed on OpenSees, where the structure has a nonlinear elastoplastic constitutive law. The soil is represented by nonlinear elements sensitive to confinement pressure. The soil-foundation interface is modeled with contact elements allowing the sliding and rocking of the foundation. Different structures (with periods between 0.3 to 1.2 s) are subjected to dynamic analyses using seismic records of different frequency content and amplitudes. The results show significant differences on the structural response among sites of the same class, but with different stiffness distribution, particularly when the structure remains on the elastic range, suggesting that the classification of foundation sites through an average index can underestimate the effects of the dynamic coupling of the soil, the foundation and the structure.

La mayoría de normas de diseño estructural clasifican los sitios de fundación a través de la rigidez media que presenta el suelo, obtenida a partir de la velocidad promedio de ondas de corte en los primeros treinta metros Vs30. Al tratarse de un promedio, la clasificación no considera directamente las características de los estratos cercanos a la fundación, ni que sitios con distinta configuración de rigideces son clasificados como un mismo tipo. Por otra parte, tampoco hay indicaciones claras respecto de la forma en que esto podría afectar la respuesta estructural. Este trabajo busca establecer si la distribución de la rigidez del suelo de fundación afecta el desempeño sísmico de las estructuras mediante modelos numéricos en OpenSees. Bajo un enfoque de análisis directo bidimensional, la estructura es modelada con una constitutiva elastoplástica. El suelo se representa por elementos no lineales sensibles a la presión de confinamiento. La interfaz suelo-fundación se modela con elementos de contacto que permiten el deslizamiento y balanceo de la fundación. Se someten distintas estructuras (con períodos entre 0.3 a 1.2 s), a análisis dinámicos con registros sísmicos de diferentes contenidos de frecuencia y amplitudes. Los resultados muestran que existen diferencias significativas en la respuesta estructural entre sitios clasificados como un mismo tipo, pero con distinta configuración de rigideces, especialmente cuando la estructura se mantiene en el rango lineal, lo que sugiere que la clasificación sísmica de sitios de fundación en base a un indicador medio podría subestimar los efectos de acoplamiento dinámico del sistema suelo-fundación-estructura.