Abstract: Steel structure is widely used in industrial and civil buildings for its lightweight, beautiful, environmental friendly, easy to be transformed and other characteristics. Considering the time pressure and economic rationality of engineering projects, steel structure is generally strengthened under load conditions, which is quite different from the traditional design. There will be hidden dangers if traditional calculation method is used in the strengthening design. The calculation formulas and regulations in two industrial standards concerning steel structure strengthening are compared, the differences between them and their differences from traditional calculation methods are introduced, and an example of the strengthening design of steel structure under load conditions is given

Key words: strengthening, steel structure, stress ratio, load