薄壁断面的几何特征

代码说明：

  计算桥梁箱形截面的特征，包括面积，形心距，形心轴到底面的距离，对X轴的惯性矩 ，对Y轴的惯性矩，自由扭转惯性矩，约束扭转惯性矩(calculating box-shaped bridge characteristics, including size, shape from the heart, heart-shaped shaft from the surface in the end, the moment of inertia of the X-axis, Y-axis of the moment of inertia, the Liberal reverse moment of inertia, the moment of inertia 450052)

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