风荷载作用下林木的力学响应

Functions of forest in maintaining a stable eco-system and environment protection have been incrementally paid attention. And due to the wind disasters, a large number of financial lost had happened in forest annually, which follows the enhancement of the cost of forest management. Strong wind nurtured by the gradually changing climate leads to more timber lost by years. So, knowing the mechanism of how the tree is damaged by strong wind well is the key to handle this problem. Our project aims to study the tree’s mechanical characteristics (which basically are shelter forest species such as Pinus thunbergii, Robinia pseudoacacia and Populus tomentosa) by the experiments of applying simulated and real wind loads on a given tree and analyzing the digital recorded data from the sensors placed on varied height of the tree, which monitoring the stress, strain, displacement and torsion separately and simultaneously. The experiment of simulated wind loads was to study the simulative wind’s temporal and spatial distribution by applying the average and fluctuant load on several positions of a tree. Basing the real and simulated wind load experimental study, then combining the theoretical analyze of structure vibration under dynamic load, we can reach a model which shows the reaction of a tree to the wind loads. A 3-D model will be made to fit for the finite element method study then to value the probability of wind damage happens in a forest. With the help of this model, more reasonable forest management methods will be taken and the financial lost of shelter forests could be cut down.

森林在维系生态系统及保护生态环境中日益被人们认识和重视。大风作用造成林木破坏，给林场造成了巨额的经济损失，并极大提高了森林管理和经营的成本。随着气候的不断变化，强风造成的损失还在逐年增加。应对风害，需要了解大风破坏森林、毁坏林木的机理。本项目以北方沿海防护林主要树种黑松（Pinus thunbergii）、刺槐（Robinia pseudoacacia）和毛白杨（Populus tomentosa）为对象，进行实际风荷载和模拟风荷载作用下林木的力学响应特性试验：沿林木高度方向多点布置测定应力、变形等的传感器和加载装置，并与风荷载同步采集，实时反映林木的力学响应特性。在试验研究的基础上，结合结构振动分析理论，建立风荷载作用下林木力学响应模型。进行基于3D建模的有限元数值计算，预测大风作用下防护林林木发生破坏的风险，为防护林管理者采取措施保护林木、减少损失提供依据。

本课题设计了不同树种（刺槐、毛白杨和黑松）林木物理力学特性、微观结构和化学成分测定试验以及林场条件下的风速、风向和位移测量试验，成功试制了一套能够准确测量沿海防护林在水平和竖直方向上风速风向分布情况的试验系统。该系统包括2个竖直方向风速风向观测塔、10 组水平方向风速风向观测架、风速风向传感和采集系统、传感器数据采集仪移动供电系统、数据采集和分析软件。基于上述内容研究了水平和竖直两个方向上林带对林场内风速、风向的影响以及不同林木特性对风速、风向和位移的影响，得到了水平和竖直方向上的风速变化特征和风场内风速变化的描述曲线，分析了林带宽度、林木特性、林场风环境等因素对风速变化趋势的影响。同时研究了不同树种的树枝、树干和树根物理力学性质对比，探究了不同树种林木的主要化学组成、微观结构和力学特性（抗拉、抗压、抗剪、抗弯和弹性模量等）的内在关系以及力学性质差异机理。现场试验研究与基于双向流固耦合的理论数值模拟分析了不同造林方式（株距、行距、林带宽度、林带间距）以及林龄、树种对风的阻滞作用，并现场测定和数值分析了不同树种、树龄林木在不同风速风荷载作用下林木的受力、变形和位移等力学特性。基于数值模拟和理论分析建立了不同风速作用下风荷载对林木影响的风场模型，并用国家气象局大数据进行了模拟对比分析，为深入研究林木抗风特性和准确模拟风荷载作用下林木力学特性提供试验成果和理论基础。.在该课题研究过程中设计测定林场风速、风向和林木位移试验装置一套，并申请国家发明专利1项，编写风速测量分析软件程序2套。在该课题资助下，项目组共发表学术论文7篇，其中SCI、EI刊源各2篇，项目负责人在国际会议中做学术报告1次。取得的研究成果为系统评估风和林木耦合作用特征提供了一个研究平台，并将为防护林管理者在保护林木不受大风破坏、降低风灾造成的防护林损失等方面提供了理论支持和科学依据。