三、主肢截面形式的选择。桅杆主肢一般有格构式、半箱式、箱式等几种形式。由于箱式结构的桅杆操作使用不方便,在大、中型桅杆中一般不采用,运用较广泛的是前面两种。吊装能力在100～150吨左右的中型起重桅杆一般采用格构式,截面为四主肢正方形分布,以满足安装互换的要求。主肢较多采用等边角 Third, the main section of the choice of cross-section form. Mast main limbs are generally lattice structure, half-box, box and other forms. As the mast box-type structure is not easy to use, in large and medium-sized masts are generally not used, the more widely used are the first two. Lifting capacity of about 100 to 150 tons of medium-sized lifting masts generally lattice structure, the cross section of the four main limbs square distribution to meet the installation and exchange requirements. Main limbs use more isometric corners