Research On Formation Mechanism And Performance Evaluation Of Dominant Fatigue Failure Modes Of Orthotropic Steel Deck

Orthotropic steel decks(OSDs)have become the primary bridge deck structure of steel bridges because of their outstanding advantages,such as light weight and high strength.However,due to the service environment,load conditions,structural system,structural detail design,manufacturing defects and other factors,the fatigue crack of OSDs is prominent,showing the characteristics of “universality,premature,concealment and reproducibility”,which affects the service quality of steel bridges.At the same time,it is difficult to reinforce the OSDs with fatigue crack and they are easy to crack again after strengthening.Therefore,it is a basic research topic to accurately evaluate the fatigue performance of OSDs during service life and the reinforced performance after degradation.Taking the dominant cracking mode as the mainline through the OSDs in service or the loading process of test model(without crack)and the reinforcing system(after strengthening),the relevant researches on the dominant cracking mode and fatigue performance evaluation of OSDs at different stages were carried out.The following works were carried out and completed:(1)Multiple cracking mode evaluation method of OSDs based on equivalent structural stressIn view of the fatigue characteristics of OSDs with multiple cracking modes,an evaluation framework based on the dominant cracking mode was established.Based on the basic theory of equivalent structural stress and combined with the stress characteristics and cracking modes of key structural details of OSDs,an analytical model of cracking mode Crack-Ⅲ was established and the thicknesses of crack surfaces of cracking mode Crack-Ⅵand Crack-Ⅶ were determined.The relationship between the stress and crack depth at key measuring points of rib-to-deck welded joints was established,and the fatigue failure criterion of the structure details was discussed.According to the above research,a unified evaluation method for multiple cracking modes of OSDs based on equivalent structural stress was developed.(2)Collecting and analyzing the fatigue test of OSDs and revealing the law of dominant cracking mode of various test modelsA total of 880 fatigue tests on key structural details of OSDs were collected,classified,and analyzed.The influence mechanism of the plate thickness,weld leg size,penetration rate,assembly error,and initial micro-cracks on the dominant cracking mode of rib-to-deck welded joints was clarified.The law of dominant cracking mode of various test models was revealed.On this basis,the fatigue tests were re-analyzed and the fatigue performance was evaluated based on the unified evaluation method of multiple cracking modes for OSDs.(3)Study on the critical penetration rate of single-side and double-side rib-to-deck welded jointsBased on the above research,the critical penetration rates of single-side and double-side rib-to-deck welded joints were investigated under the combination of common deck thickness and weld leg size.The effect of the deck thickness and weld leg size on the equivalent structure stress of each cracking mode was revealed.On that basis,the critical penetration rate of single-side and double-side rib-to-deck welded joints were quantized,which was verified by the relevant experimental models.(4)Study on fatigue reliability evaluation considering internal factors,such as initial micro-crack,and external factors,such as traffic loadThe probability and statistical characteristics of internal factors such as weld leg size,penetration rate and initial micro-crack of key structural details of OSDs were determined by process test.Meanwhile,the probability and statistical characteristics of external factors such as vehicle proportion and axle weight were determined based on the measured traffic information of a bridge.On this basis,a fatigue reliability evaluation model considering the random distribution characteristics of internal and external factors was established to evaluate the OSD’s fatigue reliability of a Yangtze River bridge,and the effect of key factors on the fatigue life was quantified.(5)Fatigue performance evaluation based on dominant cracking mode of reinforcing systemThe study on fatigue reliability of OSDs shows that it has a high risk of fatigue crack during its service life.According to the understanding about the reinforcing system of OSDs’ fatigue crack,an evaluation framework for the fatigue life of reinforcing system based on dominant cracking mode was established.Combined with the linear elastic fracture mechanics and equivalent structural stress method,the fatigue cumulative damage assessment method of cracking modes of key structural details before strengthening was established.Furthermore,four fatigue tests were carried out to study the internal weld reinforcement effect of Crack-Ⅱ at the rib-to-deck welded joints,and the fatigue performance of this reinforcing system was evaluated.