Considering dynamic soil structure interaction (SSI) effects on seismic isolation retrofit efficiency and the importance of natural frequency ratio

In analyzing seismic behaviour of highway bridges constructed on soft soil deposits, it is important to take account of soil-structure interaction effects. In this paper, seismic response of a bridge pier-foundation is investigated based on earthquake acceleration records measured simultaneously on the pier crest and on the ground surface near that bridge. Four motions were used in the analysis, i.e., two were induced by two earthquakes with magnitudes of 7.5 and 6.6, respectively; and two by their aftershocks. In the former two earthquakes, the maximum accelerations were 186 and 438 gals on the ground surface, and 310 and 230 gals on the pier top, respectively. Analyses of frequency characteristics of the motions showed that the predominant frequencies of pier-foundation were always approximately identical with the fundamental natural frequency of the subsoil. Analyses of micro-tremors measured at the sites revealed that the natural frequency of the pier-foundation system is higher than the fundamental natural frequency of the subsoil. Analytical models were formulated to calculate the seismic response of the pier-foundation assuming the subsoil and pier-foundation to be a shear column model with an equivalent linear shear modulus and an elastically supported beam on the subsoil, respectively. Bedrock motions were computed from the measured ground surface motions and then applied to the bedrock of the analytical model. The seismic responses of pier-foundation were thus calculated and compared with the measured records giving a good agreement.

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Experimental Evaluation of the Natural Frequency of an Offshore Wind Turbine’s Scaled Model

Volume 9: Offshore Geotechnics; Torgeir Moan Honoring Symposium ◽

10.1115/omae2017-61423 ◽

2017 ◽

Author(s):

Laura Kerner ◽

Selim Benfeddoul ◽

Jean-Claude Dupla ◽

Gwendal Cumunel ◽

Jean Canou ◽

...

Keyword(s):

Wind Turbine ◽

Natural Frequency ◽

Soil Structure ◽

Offshore Wind ◽

Resonance Phenomenon ◽

Soil Structure Interaction ◽

Offshore Wind Turbine ◽

Scaled Model ◽

Structure Interaction ◽

Precise Evaluation

Offshore Wind Turbines (OWT) are slender structures with sensitive dynamics, strongly influenced by the soil-structure interaction. The structure is subjected to cyclic and dynamic loads with frequencies close to the first natural frequency of the offshore wind turbine. To avoid any resonance phenomenon, a precise evaluation of the initial first natural frequency of the wind turbine is essential. The present work deals with the evaluation of the natural frequency of an OWT’s scaled model with monopile foundation. The main factor influencing the natural frequency is the soil-structure interaction which needs to be assessed precisely. To do so, a simple method presented by [Adhikari and Bhattacharya, 2012] assimilates the offshore wind turbine as an Euler-Bernoulli beam on a flexible foundation with lateral and rotational springs. The key factor in the evaluation of the natural frequency is the value of the stiffness of these springs. In this way, this paper presents a method combining experimental measurements and a finite element model on Abaqus which allows a precise evaluation of the stiffness of the springs. The proposed method is compared to the existing methods used to evaluate the soil’s stiffness (such as [Eurocode 8, 2003]). The suggested method gives a fine evaluation of the response of the structure with a mean deviation below 1%, compared to the average errors obtained for the previous methods ranging from 6.6 to 17.4%.

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