| The development of highway steel bridges in our country is very late, and the fatigue study is comparative lag, at the same time, the design criterion is relatively old. Foreign criterions and the criterion of railway steel bridges in our country were taken as reference in the building of few highway steel bridges in early time, and the states of these steel bridges are almost bad. Highway steel bridges in our country have developed rapidly after reform, but these bridges sometimes destroy accidently by fatigue because lack of sience guidance of criterion, the deficiency in particularity of highway steel bridges fatigue, and the improper management in details. Fatigue design and fatigue life evaluation of highway steel bridges have become very necessary, imminent and meaningful. The issues in highway steel bridge fatigue of our country are discussed in this dissertation.In fatigue design of steel bridges, a fatigue design method that can be the reference for highway steel bridges in our country is proposed, according to the defects of current code. Feasibility and applicability of the method are proved by a case study. In steel bridges, secondary internal forces and distortion induced stresses have become the dominant destroying factors because of bad structure and minus analysis of specific conditions. Aiming at this problem, stress characteristics of complex fatigue details have been analyzed. Out-plane distorsion at webgap which is the first cause of welded steel bridges'destroying is studied as an example. The sourse causing this kind of destroying is analyzed and different improving measures are proposed. The comparison and analysis of different improving measures are carried on. The analyzing method used can also be the reference when studying other complex details.In fatigue life evaluation of steel bridges, the methods applied to highway steel bridges of two applicable deterministic fatigue life evaluation methods, nominal stress method and fracture mechanics method, are analyzed. Stress concentration factor which is the key factor in fatigue life evaluation using nominal stress method is studied. Changes of stress concentration factors at wed toes of two common structures in highway steel bridges with different structure parameters are analyzed, and the regressive formulas are achieved for the convenience of enginnering use. Study on the influence of structure parameters to stress concentration can be the theory basis of improving design and processing technology, decreasing stress concentration, and at the same time, the precision of fatigue life evaluation by nominal stress method can be increased. Stress intensity factor which is the key factor of steel bridges'fatigue life evaluation by fracture mechanics method is studied. The stress intensity factors of two common structures containing cracks in highway steel bridges are calculated. The regressive formulas about crack length and structure width of geometry functions are achieved, and can be the theory basis of engineering application.There are no fatigue load models in current code for highway steel bridges in our country. The establishing method of fatigue load model is studied, according to the needs of fatigue design and fatigue life evaluation of highway steel bridges in our country. The aspects of vehicle load survey, model truck establishment and random truck load simulation by Monte-Carlo method are analyzed and reviewed. The fatigue damages in different bridge type and span caused by foreign fatigue trucks and strength design truck models in other bridge codes in our country are analyzed and compared. It is helpful to the truck selection in fatigue design and evaluation, and is meaningful to fatigue truck establishment of highway steel bridges in our country. In addition, the features and distribution forms of highway steel bridges in measured stress spectrums are studied.At the aspect of fatigue reliability, fatigue models based on traditional fatigue theory and fracture mechanics applicable to highway steel bridges are established. The random parameters in the models are studied. The target reliability index of highway steel bridges is studied. The models are applied in actual steel bridges to estimate the fatigue lives of these bridges, the feasibility and practicability are also validateed.
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