Dynamic investigation of an ancient masonry bell tower with operational modal analysis: a non-destructive experimental technique to obtain the dynamic characteristics of a structure

This paper shows the results of an experimental analysis on the bell tower of “Chiesa della Maddalena” (Mola di Bari, Italy), to better understand the structural behavior of slender masonry structures. The research aims to calibrate a numerical model by means of the Operational Modal Analysis (OMA) method. In this way realistic conclusions about the dynamic behavior of the structure are obtained. The choice of using an OMA derives from the necessity to know the modal parameters of a structure with a non-destructive testing, especially in case of cultural-historical value structures. Therefore by means of an easy and accurate process, it is possible to acquire in-situ environmental vibrations. The data collected are very important to estimate the mode shapes, the natural frequencies and the damping ratios of the structure. To analyze the data obtained from the monitoring, the Peak Picking method has been applied to the Fast Fourier Transforms (FFT) of the signals in order to identify the values of the effective natural frequencies and damping factors of the structure. The main frequencies and the damping ratios have been determined from measurements at some relevant locations. The responses have been then extrapolated and extended to the entire tower through a 3-D Finite Element Model. In this way, knowing the modes of vibration, it has been possible to understand the overall dynamic behavior of the structure.

Analysis of a metallic pedestrian bridge under dynamic human loads in pre and post reinforcement phases

The purpose of this work is to study the dynamic behavior of a pedestrian bridge in Alicante, Spain. It is a very slender footbridge with vertical and horizontal vibration problems during the passage of pedestrians. Accelerations have been recorded by accelerometers installed at various locations of the bridge. Two scenarios, in free vibration (after the passage of a certain number of pedestrians on the bridge) and forced vibration produced by a fixed number of pedestrians walking on the bridge at a certain speed and frequency. In each test, the effect on the comfort of the pedestrians, the natural frequencies of vibration, the mode shapes and damping factors have been estimated. It has been found that the acceleration levels are much higher than the allowable by the Spanish standards and this should be considered in the restoration of the footbridge.

Sistemas de protección individual anticaídas sometidos a impacto. Simulaciones numéricas

Los sistemas anticaída se establecen en UNE-EN-363 para evitar o retener caídas de personas sin que las fuerzas de retención causen daños relevantes. Constituyen el último recurso para prevención de caídas tras considerar otras medidas. Este artículo analiza las variables que afectan al comportamiento del sistema: altura de caída, longitud de cuerda de retención, rigidez y amortiguamiento del material de retención, y peso del operario. A tal fin, el fenómeno ha sido simulado con modelos dinámicos de elementos finitos mediante una cuerda elástica y un lastre rígido. Ello permite obtener valores verosímiles de las máximas fuerzas sobre el accidentado y el sistema durante el impacto. Los resultados demuestran el papel fundamental de la proporción entre altura de caída y longitud de cuerda (factor de caída), más determinante que la propia altura de caída. Las elevadas fuerzas máximas obtenidas indican la necesidad de incluir en el modelo absorbedores de energía elastoplásticos, en futuras investigaciones, con el fin de disminuir el impacto a valores razonables.