Last modified: 2016-06-16
Abstract
Strong winds have caused increasing wind damages for fruit trees such as uprooting and fruit drop in orchards worldwide. In order to prevent these wind damages, the various prop systems or support systems have been introduced for fruit trees. When a prop system is designed against strong winds, it is essential to calculate the wind load acting on each tree for accurate evaluation of wind resistance of prop system.
It is often to treat the applied wind load acting on a tree as a static load and to use beam theory to determine the maximum bending moment at the base of the tree. However, the response of a tree is frequency dependent and is affected mostly by wind gusts at frequencies close to its resonant frequency. In this situation, the dynamic effects are likely to increase the bending of stems and hence the maximum bending moment at the base of the tree. These dynamic effects are likely significant and cannot be ignored when the natural sway frequency of a tree is relatively small, that is, the tree is flexible.
There are two approaches to quantifying the response of a tree to a given fluctuating wind load. First, the wind load and tree response spectra are experimentally measured and a transfer function from the wind load to tree response is developed. Alternatively, if the information on the dynamic properties such as natural sway frequency and damping ratio of trees are available, then it is possible to characterize their response to any fluctuating wind load by employing a wind engineering theory. In many design codes or standards, this dynamic effect is considered adopting the gust effect factor and empirical formulae for the factor are given as functions of the natural sway frequency and damping ratio. The threshold natural sway frequency in most design codes that the dynamic effect against fluctuating wind load needs to be considered carefully is 1.0 Hz.
This paper presents the system identification method to measure the natural sway frequencies and damping ratios of fruit trees for the evaluation of the wind load acting on the trees. Both the ambient vibration test and free vibration test are performed and the identified dynamic properties are compared. It is found the average natural frequency of fruit trees is less than 1.0 Hz, and thereby the dynamic effect against fluctuating wind load needs cannot be ignored. Further, it is found that the damping ratios of fruit trees are quite larger than those of civil and building structures due to the soil-structure interaction. Therefore, a special care is required when the prop systems for fruit trees are designed against strong winds.