Retrieval of forest canopy height in a mountainous region with ICESat-2 ATLAS

Forest canopy height is one of the important forest parameters for accurately assessing forest biomass or carbon sequestration. ICESat-2 ATLAS provides the potential for retrieval of forest canopy height at global or regional scale, but the current canopy height product (ATL08) has coarse resolution and high uncertainty compared to airborne LiDAR-derived canopy height (hereafter ALCH) in mountainous regions, and is not ready for such applications as biomass modeling at finer scale. The objective of this research was to explore the approach to accurately retrieve canopy height from ATLAS data by incorporating an airborne-derived digital terrain model(DTM) and a data-filtering strategy. By linking ATLAS ATL03 with ATL08 products, the geospatial locations,types, and (absolute) heights of photons were obtained, and canopy heights at different lengths (from 20 to 200 m at 20-m intervals) of segments along a track were computed with the aid of airborne LiDAR DTM. Based on the relationship between the numbers of canopy photons within the segments and accuracy of ATLAS mean canopy height compared to ALCH, a filtering method for excluding a certain portion of unreliable segments was proposed.This method was further applied to different ATLAS ground tracks for retrieval of canopy heights and the results were evaluated using corresponding ALCH. The results show that the incorporation of high-precision DTM and ATLAS products can considerably improve the retrieval accuracy of forest canopy height in mountainous regions.Using the proposed filtering approach, the correlation coefficients (r) between ATLAS canopy height and corresponding ALCH were 0.61–0.91, 0.65–0.92, 0.68–0.94 for segment lengths of 20, 60, and 100 m, respectively;RMSE were 1.90–4.35, 1.55–3.63, and 1.34–3.23 m for the same segment lengths. The results indicate the necessity of using high-precision DTM and using the proposed filtering method to retrieve accurate canopy height from ICESat-2 ATLAS in mountainous regions with dense forest cover and complex terrain conditions.

冷弯加劲高强度钢梁受力性能及经济性分析

随着炼钢技术的进步,高强钢材在建筑领域逐渐得到应用。相比于普通钢材,高强度钢材具有节约钢材、减小截面尺寸、减轻结构重量、提高抗震性能等优点。对于宽厚比较大的板件,在压力作用下易发生局部屈曲从而降低了构件的承载能力。而设置加劲肋改变了原板件的受力性能和屈曲行为,可以提高被加劲板件的综合刚度,且冷弯加劲具有制作方便、加工成本低的优势。为了探明采用冷弯半圆形加劲的高强度钢梁的受力性能及经济性,采用ABAQUS有限元模型,并与两端夹支、跨中单点加载的H形钢梁试验结果进行对比,验证有限元模型的准确性后,建立翼缘和腹板加劲的H形和箱形截面钢梁受弯分析模型。分别选择4种截面尺寸的H形截面钢梁考察翼缘加劲对钢梁强度和稳定承载力的影响大小,对H形、上翼缘加劲、上下翼缘均加劲的三种截面类型进行数值模拟,对比不同截面形式钢梁受弯强度承载力,通过改变上下翼缘厚度,得到与Q355级H形截面钢梁极限承载力相同时翼缘的厚度,得出不同尺寸情况下相应的用钢量减少比例。分别对H形和箱形腹板设置加劲肋的钢梁截面进行尺寸优化,在保持截面用钢量不变的情况下增加翼缘厚度,对比不同优化截面尺寸时Q355级腹板加劲截面相比于不加劲H形截面钢梁的受弯强度承载力;通过优化的截面尺寸,得到相同承载力时加劲高强Q690钢梁相比于Q355钢梁用钢量的节约程度。研究表明:H形钢梁上翼缘朝外加劲与朝内加劲相比具有更高的受弯强度承载力,翼缘加劲会显著降低整体失稳承载力;相同截面面积的翼缘加劲Q690钢梁比Q355钢梁受弯强度承载力提高1倍以上,相同强度承载力的翼缘加劲,Q690钢梁比Q355钢梁用钢量节约50%左右;腹板减薄厚度且设置一个加劲肋,保证用钢量不变的情况下增加翼缘厚度,钢梁受弯强度承载力更高;H形Q690钢梁采用优化截面时在保证承载力与Q355钢梁相等的情况下,节省用钢量40%左右;箱形Q690钢梁采用优化截面时在保证承载力与Q355钢梁相等的条件下,节省用钢量30%左右。