How is the volume of steel structure plant work calculated?

How is the quantity of steel structure plant calculated? (I) Steel roof frame and steel net frame (1) Calculate the weight of steel according to the size shown in the design drawings without deducting the weight of eyelets, cut edges and cut limbs, and the weight of welding rods, rivets, bolts, etc. shall not be increased separately. (2) Irregular or polygonal steel plate, calculated by its external regular rectangular area