###### Abstract  
Wind loading on a transmission tower structure is jointly influenced by the wind field, structural parameters, and the geo-spatial configuration of the transmission line. Considering the multi-parametric effect, this paper aims at developing a limit capacity model for transmission towers under strong winds. To this end, the limit capacity of the tower is expressed via two equivalent means: one is the limit wind speed as a function of the wind angle of attack and the span of transmission line; the other is a limit  
of the tower structure are necessary to promote the structural design, optimization, and safety assessment [8; 9; 10]. The tower structure fails under strong winds mainly because the wind loading effect on the structure exceeds its resistant capacity. Most failure cases show the buckling damages on the main force-bearing members of the tower body [11; 12; 13]. One approach to explore the wind-resistant capacity of the tower is the progressive failure simulation in which the process starting from plastic hinges and ending with buckling failure is considered [13; 14]. The generated capacity curve relating the displacement at the top of the tower to the shear force at the bottom of the tower is used to describe the mechanical performance of the tower. Based on the capacity curve, the elastic/linear behavior and elastoplastic/nonlinear behavior of the tower structure are captured. The nonlinear stage is not expected in a normal working state, so the transition point between the linear and nonlinear stages is usually taken as the damage threshold of the tower structure. Wind-resistant capacity analysis on the tower structure is essential to assess the fragility/reliability of transmission line towers under winds. [PERSON] et al. [15] discussed the effect of the tower-wire interaction and wind fluctuation on the limit capacity of the tower and then analyzed the fragility of transmission towers subjected to winds. [PERSON] et al. [16], considering the combined effect of the wind and rain, generated fragility curves for transmission towers with the stochasticity of loads involved. [PERSON] and [PERSON] [17; 18] also simulated a series of fragility curves for transmission towers by taking into account the uncertainty of structures and the stochasticity of winds, respectively. These fragility simulations aim to the two-dimensional fragility curves that relate the failure probability of the tower to the magnitude of wind. However, the wind-induced fragility of transmission towers in a line is a multi-parametric issue [19] with the combined effect of the wind field, tower-wire structure, and geo-distribution of transmission line involved. Therefore, the study of wind-resistant capacity modeling of the transmission tower is necessary for improving and subtilizing the fragility assessment and analysis on transmission line towers. This paper is dedicated to establishing a limit capacity model for the tower, which can be further applied to the multi-parametric fragility assessment of the transmission line/network under winds. Four parts are involved in the following. First, two limit capacity surfaces described by three external parameters (i.e., the wind speed, wind angle of attack, and the horizontal pan of line) and three wind load components (i.e., the transverse and longitudinal wind loads acting directly on the tower, and the wind load transferred from the transmission wires) are proposed respectively. The two capacity surfaces are approximated by quasi-static gradual-loading nonlinear simulations on the finite element model of the tower, and the mechanical behavior and failure of the tower structure under different wind loadings are demonstrated and discussed. Second, limit state functions are yielded with the capacity function term expressed by the two presented capacity surfaces. Considering the existence of structural uncertainties, the capacity function based on the capacity surface is apparently implicit and high-dimensional.