Civil Engineering Reference
In-Depth Information
Chapter 3
Analysis and Mitigation of Vibration of Steel Footbridge
with Excessive Amplitudes
S. Posp´ˇil, S. Hra
ˇ
ov, S. Urushadze, and D. Jermoljev
Abstract
The article analyses the dynamics response of a footbridge crossing the Morava River between Slovak and
Austrian border. After the construction, the bridge with very flexible construction showed excessive acceleration amplitudes
in both vertical and horizontal direction when excited by usual pedestrians. Main footbridge part of 180 m length across
waterway is a three span suspended steel structure. The longest middle span of 120 m length is hanged by four prestressed
tendons over pylons, which are the part of prestressed lateral portals. Footbridge deck is made using triangular lattice girder
with the top orthotropic plate. The bridge has a vertically curved shape. The adverse vibration has been successfully
suppressed by multiple tuned dampers with the adjustable friction elements installed beneath the bridge deck. The
experimental modal analysis comprising both forced vibration as well as ambient vibration from wind was carried out
before and after the installation of vibration absorbers. The results of experimental tests were confronted with the outputs of
numerical analysis.
Keywords
Steel footbridge • Human induced vibration • Pedestrian comfort criteria • Tuned mass dampers
3.1
Introduction
Long-span footbridges rank among engineering structures with a high social significance. Their architecture design must in
general satisfy matters such as attractive look, reliability and low weight. These requirements determine the bridge to
be sensitive to the wind effects and the assessment against the wind becomes crucial. According to the character and the
intensity of excitations, the adequate treatment to reduce or avoid the oscillations should be carried out also in cases when
the vibration amplitudes could lead to the pedestrian discomfort or panic. In the terminal years of the last and the initial years
of this century the number of footbridges has increased fast, as they provide shortcuts enabling pedestrians and cyclists to
reach easily their destinations. While bridge structures are designed to withstand permanent load increased by a dynamic
coefficient and not assessed with reference to user's comfort, the footbridge structure must resist both static and particularly
dynamic loads. Moreover, its dynamic response must satisfy also the requirements of pedestrians' comfort usually expressed
by response acceleration. From 1970 to the present the Central laboratory of the Institute of Theoretical and Applied
Mechanics of the Academy of Sciences has tested some 15 footbridges of various type, either in situ or on their dynamic/
aeroelastic models. The tests verified in particular the magnitude of dynamic response; the model tests provided also
information on wind effect.
Regulations and standards, e.g. [
1
-
4
], recommend the avoidance of such structures, the vertical natural frequencies of
which vary from between 1.6 and 2.4 to between 3.5 and 4.5 Hz (this second range is connected with the danger of second
harmonic response excitation). However, these requirements cannot be fulfilled in all cases. The hatched zone in Fig.
3.1
is a
zone through which the curves pass expressing the probable upper and lower limits of the vertical footbridge natural
