Gust Effect Factor for Wind Load Estimation of Tree Supporting Systems

Abstract

Strong winds have caused an increasing amount of damage to fruit trees, such as uprooting and fruit drop, and various prop systems or support systems have been introduced to prevent this wind damage. When a prop system is designed against strong winds, it is essential to calculate the wind load acting on each tree in order to accurately assess the wind resistance of the prop system. In particular, the fluctuating effect is significant and cannot be ignored when the natural frequency of a tree is relatively small. In this study, vibration tests were performed to measure the natural frequencies and damping ratios of trees in order to evaluate the wind load acting on the trees. Both ambient and free vibration tests were performed, and the dynamic properties were identified and compared. It was found that the average natural frequency of trees was approximately 1.0 Hz, and therefore the dynamic effect against fluctuating wind load needs to be considered. The analysis results of the gust effect factor indicated that the wind load may be underestimated or overestimated considerably if the flexible nature of trees is neglected, or if the exact values of natural frequency and damping ratio are not available.

Keywords

• wind load
• tree supporting system
• gust effect factor
• ambient vibration test
• system identification