Uncertainties in the modal estimation from output-only data in civil structures

Cualquier estructura vibra según unas frecuencias propias definidas por sus parámetros modales (frecuencias naturales, amortiguamientos y formas modales). A través de las mediciones de la vibración en puntos clave de la estructura, los parámetros modales pueden ser estimados. En estructuras civiles, es difícil excitar una estructura de manera controlada, por lo tanto, las técnicas que implican la estimación de los parámetros modales sólo registrando su respuesta son de vital importancia para este tipo de estructuras. Esta técnica se conoce como Análisis Modal Operacional (OMA). La técnica del OMA no necesita excitar artificialmente la estructura, atendiendo únicamente a su comportamiento en servicio. La motivación para llevar a cabo pruebas de OMA surge en el campo de la Ingeniería Civil, debido a que excitar artificialmente con éxito grandes estructuras no sólo resulta difícil y costoso, sino que puede incluso dañarse la estructura. Su importancia reside en que el comportamiento global de una estructura está directamente relacionado con sus parámetros modales, y cualquier variación de rigidez, masa o condiciones de apoyo, aunque sean locales, quedan reflejadas en los parámetros modales. Por lo tanto, esta identificación puede integrarse en un sistema de vigilancia de la integridad estructural. La principal dificultad para el uso de los parámetros modales estimados mediante OMA son las incertidumbres asociadas a este proceso de estimación. Existen incertidumbres en el valor de los parámetros modales asociadas al proceso de cálculo (internos) y también asociadas a la influencia de los factores ambientales (externas), como es la temperatura. Este Trabajo Fin de Máster analiza estas dos fuentes de incertidumbre. Es decir, en primer lugar, para una estructura de laboratorio, se estudian y cuantifican las incertidumbres asociadas al programa de OMA utilizado. En segundo lugar, para una estructura en servicio (una pasarela de banda tesa), se estudian tanto el efecto del programa OMA como la influencia del factor ambiental en la estimación de los parámetros modales. Más concretamente, se ha propuesto un método para hacer un seguimiento de las frecuencias naturales de un mismo modo. Este método incluye un modelo de regresión lineal múltiple que permite eliminar la influencia de estos agentes externos. A structure vibrates according to some of its vibration modes, defined by their modal parameters (natural frequencies, damping ratios and modal shapes). Through the measurements of the vibration at key points of the structure, the modal parameters can be estimated. In civil engineering structures, it is difficult to excite structures in a controlled manner, thus, techniques involving output-only modal estimation are of vital importance for these structure. This techniques are known as Operational Modal Analysis (OMA). The OMA technique does not need to excite artificially the structure, this considers its behavior in service only. The motivation for carrying out OMA tests arises in the area of Civil Engineering, because successfully artificially excite large structures is difficult and expensive. It also may even damage the structure. The main goal is that the global behavior of a structure is directly related to their modal parameters, and any variation of stiffness, mass or support conditions, although it is local, is also reflected in the modal parameters. Therefore, this identification may be within a Structural Health Monitoring system. The main difficulty for using the modal parameters estimated by an OMA is the uncertainties associated to this estimation process. Thus, there are uncertainties in the value of the modal parameters associated to the computing process (internal) and the influence of environmental factors (external), such as the temperature. This Master’s Thesis analyzes these two sources of uncertainties. That is, firstly, for a lab structure, the uncertainties associated to the OMA program used are studied and quantified. Secondly, for an in-service structure (a stress-ribbon footbridge), both the effect of the OMA program and the influence of environmental factor on the modal parameters estimation are studied. More concretely, a method to track natural frequencies of the same mode has been proposed. This method includes a multiple linear regression model that allows to remove the influence of these external agents.

"Creeping" civil war in the North Caucasus. OSW Commentary No. 50, 2011-03-15

Over the last year, the situation in Russia’s North Caucasus has become further destabilised. Attacks and armed clashes happen daily, and destabilisation is spreading to an increasingly large area. The extent of violence in the region is so great that it can already be stated that a de facto civil war is taking place, the warring parties being the Islamic armed underground movement which operates under the banner of the so-called Emirate of the North Caucasus, and the secular governments of the individual republics, who are supported by local and federal branches of the Russian Federation’s Interior Ministry and Federal Security Service. Moscow has no idea how to successfully tackle the Caucasus rebellion. Force has proved to be costly and unproductive, while the attempts made since early 2010 to integrate the region with the rest of Russia by implementing development programmes have not brought the desired results, because of widespread corruption and faint interest from businessmen who are afraid to invest in such an unsafe region. A growing problem for Moscow, particularly for the prestige of the state, is attacks by militants on areas near Sochi, where the 2014 Winter Olympics is to take place. It must be assumed that over the next 3 years before the Olympics, Moscow’s priority in the region will be to ensure the safety of Olympic preparations, and then the games themselves. It cannot be ruled out that the North Caucasus Federal District with its ‘troubled republics’ will be surrounded by a kind of cordon sanitaire (Sochi is situated in the neighbouring Southern Federal District). This could in turn strengthen these republics’ isolation, maintain the state of permanent instability, and postpone the prospects of solving the region’s acute economic and social problems.

Discrimination against minorities and women in pensions and health, life, and disability insurance.

Includes bibliographical references.

Forecasts of passenger traffic of the domestic trunk air carriers, domestic operations, scheduled service, 1965-1975 /

"September 1965."

Civil affairs handbook.

Consists of separtate handbooks for 21 countries, issued in sections.

Study of Civil War sites in the Shenandoah Valley of Virginia : pursuant to Public Law 101-628.

"September 1992."

The essays; or, Counsels civil and moral. First published in 1597, and as he left them newly written.

Mode of access: Internet.

Civil rights project : report.

Separate reports for U.S., each state and D.C., each divided into several parts usually titled: Provisions of the Constitution ... relation to the infringement of civil rights, Elections, Education, Housing, Employment, Admin. of Justice.

Civil engineering bibliography on the design and construction of nuclear facilities /

"DTIE Issuance Date: December 1962."

The photographic history of the Civil War : thousands of scenes photographed 1861-65, with text by many special authorities /

Includes index.

Civil commitment and social policy : an evaluation of the Massachusetts Mental Health Reform Act of 1970 /

Mode of access: Internet.

Procedure for condition survey of civil airports /

Mode of access: Internet.

Radiological protection and decontamination of civil aircraft.

Mode of access: Internet.

The warden service.

Mode of access: Internet.

Executive summary of systems analysis to develop future civil aircraft noise reduction alternatives /

Mode of access: Internet.