Z.Q. Chen

Turbulence Integral Scale Corrections to Experimental Results of Aeroelastic Models with Large Geometric Scales: Application to Gust Loading Factor of a Transmission Line Tower

Full aeroelastic model tests are often carried out to investigate the buffeting responses and the corresponding gust loading factor (GLF) of flexible structures. The geometric model scale for complicated structures may sometimes be excessively large in comparison with the ratio between turbulence integral scales measured in wind tunnel and in real atmospheric environments. This inconsistency in scale modeling leads to severe distortions of turbulence integral scale similarity whose influence manifests itself by modifications in resonant component of buffeting response and in resulting GLF as well, and numerical correction is needed when these testing results are converted to full-scale. This paper presents a correction procedure in terms of correction factors for experimental GLF obtained from full aeroelastic model tests to account for the inconsistency in scale modeling. The correction is simply made on analyzing the prototype in two air flows with different turbulence integral scales, namely the nominal value in the real atmosphere environment and the full-scale value correspondent to aeroelastic model test. The correction procedure is applied to the full aeroelastic model experiments of a transmission line tower with an unusually large geometric scale of 1/40 in order to examine the effect of turbulence integral scale on GLF. An appreciable effect is observed. It is found that the GLF obtained from wind tunnel experiments tends to be conservative for most structures and becomes unsafe for structures with extremely low modal frequencies when disregarding the correction. Correction factors are then derived to accommodate the inconsistency in modeling turbulence integral lengths. As a first generalization, a set of more general correction factors defined in terms of modal damping ratio and frequency ratio, the ratio between modal frequency and the dominant frequency in wind power spectra, are further developed through parametric analysis. The correction factors decrease with the augment of modal damping ratio and frequency ratio.

Wind loads and effects on rigid frame bridges with twin-legged high piers at erection stages:

This article presents a procedure for analyzing wind effects on the rigid frame bridges with twin-legged high piers during erection stages, taking into account all wind loading components both on the beam and on the piers. These wind loading components include the mean wind load and the load induced by the three turbulence wind components and by the wake excitation. The buffeting forces induced by turbulence wind are formulated considering the modification due to aerodynamic admittance functions. The buffeting responses are analyzed based on the coherence of buffeting forces and using finite element method in conjunction with random vibration theory in the frequency domain. The peak dynamic response is obtained by combining the various response components through gust response factor approach. The procedure is applied to Xiaoguan Bridge under different erection stages using the analytic aerodynamic parameters fitted from computational fluid dynamics. The numerical results indicate that the obtained peak structural responses are more conservative and accurate when considering the effect of each loading component on the beam and on the piers, and the roles of different loading components are different with regard to bridge configurations. Aerodynamic admittance functions are source of the important part of the error margin of the analytical prediction method for buffeting responses of bridges, and buffeting responses based on wind velocity coherence will underestimate the results.

Modal parameter identification of a long-span footbridge by forced vibration experiments

Performing forced vibration tests on full-scale structures is the most reliable way of determining the relevant modal parameters in structural dynamics, such as modal frequencies, mode shapes, modal damping, and modal masses. This study describes the modal identification of a double-level curved cable-stayed bridge with separate deck systems for pedestrians and vehicles via forced vibration tests. The steady-state structural responses to sinusoidal excitations produced by an electrodynamic shaker are recorded under varying excitation frequencies, and the frequency response functions are established. The measured frequency response functions are curve fitted to estimate the modal parameters. The numerical simulation of frequency response function–based modal parameter identification of an elastically multi-supported continuous beam structure is carried out, and the emphasis has been placed on the evaluation of the effect of an additional shaker mass, excitation frequency step and range, multi-mode vibration, and noise on identification results. Finally, the modal parameters for the first lateral mode of a double-level curved cable-stayed bridge are identified by forced vibration experiments, and the results are compared with those from ambient vibration tests and free vibration tests. The effect of the unmeasured wind excitation on identification is discussed. It is shown that the effect of ambient vibration is minor for wind velocity of 3–5u2009m/s. The damping ratios identified by forced and free vibration tests are comparable, while those from ambient vibration are subject to large variations. The modal mass obtained from forced vibration tests is in good agreement with finite element prediction, which provides design basis for mass-type dampers.