Assessment Of Wind-induced Fatigue Damage Of Metal Roof Components

The metal roof system is widely used in large-span roof structures with its light weight,large span and simple construction.The existing researches mostly concentrate on the equivalent static wind load of the main structure and the extreme value of wind loads.However,the systematic research on the fatigue damage and fatigue failure of the metal roof components is seldom.The research shows that the accumulation of fatigue damage of metal roof components under wind loads and the strength and stiffness degradation caused by that are the main reasons of the structure failure.Based on this,three aspects of work have been carried out.Firstly,the fatigue wind speed range of the metal roof components is determined based on the measured data of wind speed,which provides the basis for the estimation of fatigue damage or life.Secondly,the improved frequency domain fatigue assessment method is proposed to make it better suited for the estimation of fatigue damage caused by non-Gaussian stress and multi-axial stress.Thirdly,a new method is presented to consider the uncertainty of structural parameters based on uncertainty theory.This paper focuses on wind-induced fatigue damage of metal roof components,and the main work and innovations are as follows.A method for calculating wind induced fatigue wind speed interval of metal roof system is proposed.(1)Anew method is presented to calculate the wind speed range of wind-induced fatigue of metal roof components.According to the measured data of wind speed and pressure coefficients derived from the existing wind tunnel test,a method for calculating the wind speed range of wind-induced fatigue of metal roof components is proposed,which provides the basis for estimating fatigue damage,crack initiation life and crack propagation life and the influence of the parameters of S-N curve on fatigue life of the hot spots of the structure.In the presented method,the mean wind speed is defined by the non-stationary model,and the power density method is used to calculate the probability density function of the non-stationary mean wind speed.Based on those,the upper bound of fatigue wind speed range is determined.The dynamic response of metal roof components subjected to wind loads is obtained by numerical simulation.Then the lower bound of fatigue wind speed range is determined by considering the stress threshold.The results show that the fatigue wind speed range of the typical metal roof components in Beijing is recommended as 1 m/s to 10 m/s.(2)An analytic formula is proposed to calculate the fatigue damage of non-Gaussian uniaxial and multi-axial stress.According to the characteristics of the wind-induced dynamic response of the metal roof components,the analytic formulas for calculating the expected fatigue damage rate and variance of the structure subjected to non-Gaussian uniaxial and multi-axial stress are proposed,which makes up the shortcomings of the traditional frequency domain fatigue analysis method in considering non-Gaussian and multi-axial characteristics of the stress.Based on the Tovo-Benasciutti method and non-Gaussion transform process,the limitations of Bendat formula that can only be used to assess the Gaussian narrow-band stress are improved,which makes it suitable for calculating the variance of fatigue damage subjected to uniaxial non-Gaussian broadband stress.Meanwhile,based on the equivalent Lemaitre stress,the analytic formulas of the expected fatigue damage rate and variance are proposed,which considers non-Gaussian and multi-axial characteristics of the stress.Finally,take fatigue damage estimation of the metal roof components subjected to wind loads as an example,and the expected fatigue damage rate and variance by the proposed method are compared with the results of time domain method,which verifies the accuracy and applicability of proposed method.(3)A new method is presented to assess the fatigue damage of uncertainty structure.Based on the improved interval analysis and spectral approach of fatigue damage estimation,two new approaches,the full combination method and simplified method,are presented to evaluate fatigue damage of structures with uncertain parameters under stationary Gaussian loads.In the presented methods,uncertain parameters of structures are defined by interval parameter models.Then approximate expressions of the spectral moments and spectral bandwidth parameters are derived by the improved interval analysis and interval Taylor expansion.The monotonic property of the expected fatigue damage rate with respect to uncertain parameters is proved through the numerical approach.On the basis of that,the proposed full combination method can accurately estimate intervals of the expected fatigue damage rate of structures with uncertain parameters by wholly combining the bounds of interval spectral moments.The simplified method approximately calculates intervals of the expected fatigue damage rate through explicit expression by properly combining the bounds of interval spectral moments and bandwidth parameters.Finally,numerical examples are used to illustrate the accuracy and applicability of the proposed methods.