Assessment of dynamic responses and acceptance criteria for steel warren truss footbridges in Hong Kong with respect to pedestrian-induced vibrations

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Assessment of dynamic responses and acceptance criteria for steel warren truss footbridges in Hong Kong with respect to pedestrian-induced vibrations

 

Author: Li, Wai-man
Title: Assessment of dynamic responses and acceptance criteria for steel warren truss footbridges in Hong Kong with respect to pedestrian-induced vibrations
Degree: M.Sc.
Year: 2000
Subject: Footbridges -- China -- Hong Kong -- Design and construction
Bridges, Iron and steel -- China -- Hong Kong -- Design and construction
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Dept. of Civil and Structural Engineering
Pages: ii, 86, [34] leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1523532
URI: http://theses.lib.polyu.edu.hk/handle/200/5248
Abstract: The structural form of steel warren truss has been frequently adopted for footbridges in Hong Kong, particularly for long span ones. Steel Warren truss footbridges having a span length of over 30m and possibly up to 50m across major road junctions or railway reserves are not uncommon. These footbridges have been made more and more slender in appearance, and they are more susceptible to vibration problem arising from their dynamic responses under pedestrian movements. Footbridges in Hong Kong are designed to BS5400 together with some amendments stipulated in the structures Design Manual. Accordingly, the first flexural frequency of footbridge superstructures should preferably be not less than 5Hz to avoid unpleasant vibrations felt by pedestrian. In case the first flexural frequency (f) is less than 5Hz, the maximum vertical acceleration excited by pedestrian motions should not exceed 0.5 √fm/s2. This paper presents a review of the structural configurations of some typical steel Warren truss footbridges built in Hong Kong. All of them employed steel structural hollow sections as the principle truss members. The results from the review were then used to prepare several representative structural configuration models of warren truss footbridges. Corresponding plane frame models and 3-dimensional frame models were constructed, and normal modes dynamic analysis was carried out using a general purpose structural analysis program. A parametric study with respect to the effects on the natural frequency due to variations in span length, truss height, deck width, member sizes and panel length between inclined bracings. It was found that changing the panel length had little effect on the flexural frequency. It was also found that treating a Warren truss as an equivalent beam based on composite moment of inertia of the top and bottom chords would overestimate the first flexural frequency: a difference of exceeding 25% when the span to height ratio is 8 or less. Charts for estimating the first flexural frequency and maximum vertical acceleration due to a single pedestrian load for various truss configurations were prepared. These charts may be used as a quick reference for preliminary design of warren truss footbridge with respect to compliance with the SDM criteria. It was found that the allowable maximum acceleration criteria due to a single pedestrian load would likely be exceeded only when the Warren truss footbridge was of very light weight construction such as steel plate deck and steel sheet roof cover. It was also found that the calculated maximum vertical acceleration under a single pedestrian load remains almost constant despite a substantial increase of truss height. Increasing the truss height merely raises the first flexural frequency. However, as human are more tolerable to high frequency vibration, increasing the truss height is still an effective way to reduce discomfort due to bridge vibration. The allowable maximum acceleration criteria due to a continuous stream of pedestrians moving along a steel Warren truss footbridge was checked. It was found that the calculated maximum acceleration was well below the limiting value of 0.5√f. Therefore, it is concluded that there is no need to check this criteria of continuous pedestrian load for commonly encountered steel Warren truss footbridges in Hong Kong.

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