## 1 Introduction  
Wind loading is a major loading to high-rise structures, which not only causes vibration but may bring about fatigue problems as well, for the fluctuating wind as a random loading can be regarded as a sort of cycle loading for high-rise structures and may cause fatigue crack initiation in components where large stress concentration exists and finally leads to fatigue failure. Several cases of wind-induced fatigue failure of steel mast structures have been found in history [1]. As to high-rise steel frame structures whose beam-to-column connections are mostly welded and local stress concentration inevitably exists, fatigue cracks are prone to initiate near welded joints in beam-to-column connections, which certainly brings potential danger of fatigue failure.  
At present, fatigue life assessments in the field of civil engineering are mainly based on nominal stress or hot spot stress theories. For example, [PERSON] [2; 3; 4] selected several types of mast structures, including telegraph poles and lamp-posts, and, by considering the changing wind speed and wind direction simultaneously, performed fatigue assessment of along-wind and crosswind response effect on structures from the frequency domain and time domain, respectively. [PERSON] [5; 6] presented a practical and efficient approach for calculating wind-induced fatigue of tubular structures and studied the effects of the wind direction and wind grid size on the high cycle fatigue of the structure. However, the fatigue assessment of welded joints based on nominal stress and hot spot stress methods cannot accurately consider the effect of the notch effect, which is a strong stress concentration near notch roots or notch toes. As a result, many new methods and concepts based on local stress have been introduced into practical engineering structures. [PERSON] [7] used the theory of critical distances (TCD) to perform fatigue analysis of a riveted railway bridge by a global-local FE (Finite element) model. [PERSON] [8] applied notch stress concept for several engineering structures including MAG-welded offshore K-nodes, sandwich panels for ship decks, spot-welded automotive doors and MAG-welded automotive trailing links.  
However, the drawbacks of the methods stated above are very clear. In nominal stress, joints are classified into various joint types and each type owns a design S-N curve, which inevitably confuse engineers when selecting the accurate curve, while in hot spot stress and local stress, the results are largely dependent on the meshing style, which will bring high requirement of meshing density and thus exhaust massive computing resources and time, especially in large scale complex structures. Therefore, the desire to establish a mesh-insensitive method capable of unifying various curves into one single curve has received considerable attention. The equivalent structural stress method is the ideal combination of these two advantages.  
In this paper, the equivalent structural stress method is introduced into the fatigue life assessment of a typical high-rise steel braced frame structure. A multi-scale FE model is established and the corresponding wind simulation is performed. The fatigue life of the welded beam-to-column connection is assessed based on equivalent structural stress method, hot spot method and nominal stress method and comparisons are made.