building project of nanjing south station, covering an area of 380,000㎡, with steel structure of 110000 tons, is the largest unit steel structure project in china. it includes three parts: composite frame, grid frame roof system and steel canopy. component of unit work weighs great. the components include several structures such as pipe truss, steel net frame, profile steel truss, steel pipe column, welding boxy, h type steel. those components are made of high strength (q390gjc) mid-thick steel plate. of those, 2/3 plates are thicker than 60mm, the thickest one is 180mm, with complicated welding process and undergone strict process control.