Fundamental Frequencies of Vibration of Footbridges in Portugal: FromIn SituMeasurements to Numerical Modelling

Abstract

Since 1995, we have been measuring thein situdynamic characteristics of different types of footbridges built in Portugal (essentially steel and precast reinforced concrete decks) with single spans running from 11 to 110 m long, using expedite exciting and measuring techniques. A database has been created, containing not only the fundamental dynamic characteristics of those structures (transversal, longitudinal, and vertical frequencies) but also their most important geometric and mechanical properties. This database, with 79 structures organized into 5 main typologies, allows the setting of correlations of fundamental frequencies as a negative power function of span lengthsL (L-0.6toL-1.4). For 63 footbridges of more simple geometry, it was possible to obtain these correlations by typology. A few illustrative cases representing the most common typologies show that linear numerical models can reproduce thein situmeasurements with great accuracy, not only matching the frequencies of vibration but also the amplitudes of motion caused by several pedestrian load patterns.

Estimation of the dynamic characteristics of base-isolated single-span RC footbridges in Portugal based on in situ measurements

“…Data treatment consisted in analyzing peak acceleration values, predominant frequencies of vibrations in the three orthogonal directions through FFT techniques, and damping from amplitude decay in a few cases. A cross examination of results, together with the interpretation of modal shapes for this simple geometrical layout and prior knowledge of previous work [1], led to the identification of ''first'' modal frequencies in the three directions of space (T transversal; L longitudinal; V vertical), which lay in the range of influence of pedestrian loading (1.5-3.5 Hz). For all analyzed structures, damping is quite small with values varying in the interval 1 [ f [ 0.5 %, even for large amplitude motions.…”

“…15, 16 and 17 and Table 10, where the frequencies of first mode in the V, T and L directions are plotted against the largest span length. Based on our previous work [1] we used a power function (f = a 9 L -a ) to fit the data. Figure 15 presents the results in the V-direction and it can be seen that the new data increase the correlation coefficient from 0.793 to 0.805.…”

“…In a recent paper, Oliveira [1] presented a database with information on 79 footbridges representative of the existences in Portugal, containing the geometric and mechanical properties as well as the fundamental dynamic characteristics of those structures, which are the most important transversal, longitudinal and vertical frequencies, damping and amplification factors due to pedestrian crossing. Further studies have extended this database to 100 footbridges.…”

Effect of Vinyl Flooring on the Modal Properties of a Steel Footbridge

“…The first method may be applied successfully during the design phase of the footbridge [5] or after its construction when the structural modifications are compatible with the aesthetic and functionality of the footbridge [6]. The second method is usually applied to existing structures or it can be also applied during the design phase if the slenderness of the structure makes impossible to control its response via the modification of either its stiffness or its mass [7].…”

Control de vibraciones en estructuras flexibles mediante amortiguadores de masa pasivos, adaptativos, semiactivos y activos

“…Tabla 3.1 Niveles de confort en pasarelas peatonales Tabla 4.15 Resultados de las pruebas de campo realizadas en el vano 2 de la PMC con distintas estrategias de control. (Casado, et al, 2013 (Borges, et al, 1969) Cuando alguno de los modos de las estructuras están dentro del espectro de frecuencias de las cargas dinámicas (Figure 1.1 Respecto a los parámetros modales de las pasarelas peatonales, analizando los valores de frecuencia del primer modo de flexión, estos suelen estar entre 1 y 5 Hz como puede verse en las gráficas que Zoltowsiki (Zoltowski, et al, 2006) y Oliveira (Oliveira, 2014) obtuvieron de sus estudios (Figura 1.2). Un dato interesante que puede obtenerse de la primera de las gráficas, es que el material usado en la construcción de las estructuras no tiene mucha influencia en el valor de la frecuencia del modo principal.…”