| Welded beam-to-column connections of steel high-rise structures may suffer fatigue damage due to local stress concentration under wind load.Therefore,it is urgent to develop a systematic fatigue life assessment method for such structures,which can provide reference to the design of structures.In this thesis,the combination research method of experiment,simulation and theory is adopted.First,the fatigue tests are carried out on the standard specimens of welded joints.Afterwards the fatigue tests are carried out on the local specimens of the beam-to-column connections.Based on this,the pressure-measuring wind tunnel test of a steel high-rise structure is carried out.A structural fatigue analysis method named “unified multi-scale fatigue analysis method” is proposed and,combined with multi-scale finite element modeling,different fatigue life assessment methods based on nominal stress,structural stress,local stress and fracture mechanics are used to predict the fatigue crack initiation life and the fatigue crack growth life of local welds in steel high-rise structures where stress concentration exists.The main research work and achievements of this thesis are as follows:1.Axial fatigue test study on standard specimens of welded joints.The axial high cycle fatigue tests and low cycle fatigue tests are carried out on different types of welded joint standard specimens.The post-process method of test results is improved and a linear slip model considering the slip effect between the clamps of the test machine and the end of specimens is proposed so that the precision of test's post-processed result is increased;The fatigue damage characteristics of the specimens are studied and typical damage mode and failure rule of each type of specimens are proposed.Based on finite element modeling,the fatigue life assessment is carried out by different fatigue assessment methods and the results are compared with the test results so that the precision of each method is discussed.The fatigue design curves of welded joints are improved and expanded.A unified design curve suitable for both high-cycle fatigue and lowcycle fatigue assessment of welded joints is established and provides reference to the design of welded components.The results show that although there is a little discrepancy between the fatigue life assessment results of welded joint specimens and the real test results,they agree with each other well globally.The slip effect between the clamp of the test machine and the ends of specimens can be considered by a linear slip model so that the precision of post-processed results is increased;The established unified design curves can be applied to both the fatigue assessment and design of welded components under high cycle fatigue and low cycle fatigue ranges and achieve better results.2.Fatigue test study on local specimens of welded beam-to-column connections in steel highrise structures.The current local specimens of welded beam-to-column connections in steel high-rise structures are improved and the local specimens with the clamp ends are designed,so that the specimens can be tested more conveniently;The axial cyclic fatigue tests of the local specimens are conducted and the fatigue property is analyzed and the damage characteristic of the specimens is summarized.Based on the finite element modeling,the newly-developed unified design curves suitable to high cycle fatigue and low cycle fatigue ranges are adopted and different fatigue assessment methods are used to predict the fatigue life of the specimens and the accuracy of life prediction methods in the local beam-to-column connection is discussed by the comparison with the experimental results.The applicability of the unified design curves to the beam-to-column connections is verified.The results show that under the action of cyclic load,the local weld location of welded beam-to-column connections in steel high-rise structures has good energy dissipation capacity,but the fatigue strength and fatigue resistance are relatively poor;Each fatigue life assessment method is able to achieve a good prediction effect in local weld location of welded beamto-column connections in steel high-rise structures;The unified design curve can avoid the elasticplastic finite element analysis and can be well applied to the fatigue life assessment and design of local weld location of welded beam-to-column connections in steel high-rise structures.3.Unified multi-scale fatigue analysis method for steel high-rise structures and verification of multi-scale modeling.The pressure-measuring wind tunnel test based on a rigid model is carried out using the scale model of a steel high-rise structure.The wind pressure distribution property on the structural surface is discussed by the index of mean wind pressure coefficient and the fluctuating wind pressure coefficient.The results are compared with the corresponding reference values in Chinese and Japanese Specifications and the results obtained by computational fluid dynamics simulation technology.Based on this,considering the action of the normal wind and the typhoon,the wind load time series on the structural surfaces are simulated;The unified multi-scale fatigue analysis method is proposed,which can easily determine the location of the most dangerous connection in the whole structure and based on that can predict the fatigue life of the local location of the most dangerous weld more accurately.The multi-scale finite element analysis model composed of the global scale model,the connection scale model and the weld scale model is established and the fatigue life assessment of the welded joints is carried out.On the base,the rationality of multi-scale modeling methods is verified,and the selection of key parameters during the modeling is proposed.The results show that the shape coefficient and the mean wind pressure coefficient obtained by the wind tunnel test are slightly larger than the relevant reference values specified by the Specifications and slightly smaller than the simulated results based on the computational fluid dynamics and overall variation trend is in good agreement with each other,so the wind tunnel test results are considered to be credible;The multi-scale fatigue analysis method can easily determine the most dangerous weld location of the whole structure and calculate its fatigue life more accurately.For the steel braced high-rise frame structures,the lateral weld toe of the weld connecting the upper beam flange and the the column in the beam-to-column connection in the lateral side on the first floor is easy to suffer fatigue damage;The multi-scale finite element model established by the constraint equation method or the sub-model method can achieve good accuracy in fatigue analysis;The sub-model method has higher precision than the constraint equation method,but is more susceptible to the size of the local mesoscale model.4.Study on fatigue crack initiation life assessment of wind-induced fatigue of beam-to-column connections in steel high-rise structures.The equivalent structural stress method,the theory of critical distances and the notch stress method are extended and improved to suit the case of civil engineering structures and stochastic wind loading.The high-cycle fatigue crack initiation life prediction of the weld location in the dangerous welded beam-to-column connections is carried out under the action of normal wind,and the sensitivity of the calculation results to the fineness of meshes and the convergence of the calculation results are further studied.The existing problems of each method are discussed by the comparison of results obtained by them.On the base of it,the low-cycle fatigue life assessment of welded beam-to-column connections in steel high-rise structures under the action of normal typhoon,strong typhoon and super typhoon is conducted,which provides reference to the related design of welded beam-to-column connections.The results show that the equivalent structural stress method,the notch stress method and the theory of critical distances can obtain precise calculation results in wind-induced fatigue life assessment of welded connections in steel high-rise structures after the improvement and extension;When there is large stress gradient change near the weld,the hot spot stress method or the equivalent structural stress method may underestimate the fatigue damage.The theory of critical distances makes the application more flexible due to the averaging of stress value in the region.When the local stress distribution in the depth direction of welds is very uneven,results of the notch stress method may deviate from the actual value due to the use of sub-model technology;Under the action of extreme strong wind,the local weld toe of the welded connections in steel high-rise structures may experience large plastic strain and eventually lead to low-cycle fatigue damage.5.Study on fatigue crack growth life assessment of wind-induced fatigue of beam-to-column connections in steel high-rise structures.The innovation of solution method to obtain stress intensity factor is made and a method which uses strain energy density factors to calculate stress intensity factors in two-dimensional crack tips is proposed.The method is applied to the crack tips of mode I cracks,mode II cracks and mode I+II combined cracks and,by the comparison of the results and the theoretical solutions,it proves that the method can achieve good precision.The two-dimensional through-crack growth of welded components is discussed and the current method to use strain energy density to calculate the fatigue crack growth is improved and a method which is able to calculate strain energy density factors directly by finite element analysis and to predict fatigue crack growth life is proposed.The fatigue crack growth life assessment model of three-dimensional surface cracks on welded components is established and the propagation law of the surface side cracks and central cracks is studied.The model is improved to suit civil engineering structures and stochastic wind loading so that the fatigue crack growth life assessment model of three-dimensional surface cracks on beam-to-column connections in steel high-rise structures is established.The advantages and disadvantages of each fatigue life assessment method are comprehensively summarized and the suggestions on practical selection are put forward.The results show that the proposed method to calculate stress intensity factors based on strain energy density factors uses energy as the index and can be well applied to the solution of stress intensity factors of two-dimensional cracks.The proposed improved strain energy density factor method for calculating fatigue crack growth life can more effectively predict the life of two-dimensional cracks.The three-dimensional surface crack growth life prediction model is more consistent with the fatigue crack growth of real engineering cases so that there is a promising prospect of application in the fatigue crack growth life prediction of welded components and welded connections in steel high-rise structures;The steel high-rise structure with initial cracks in welded connections has the possibility of fatigue failure during its service period due to fatigue crack growth under the action of wind. |
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