Effects And Calculation Mathod Of The Second And Third System On Fatigue Stress At The Deck-to-rib Joint Of Orthotropic Steel Decks

Orthotropic steel decks(OSDs)have many advantages,but the fatigue problem of them is very obvious.The cracks on the deck-to-rib joint not only account for a large proportion,but also cause serious damages.Nowadays,the studies on this type of crack mainly rely on experiments and finite element calculations.The understanding of the corresponding physical and mechanical background(such as the effects of the second and the third systems)is still insufficient.However,in fact,the third system effect is the main cause of the present fatigue problem.The theory on OSDs can hardly be applied to the fatigue problems due to a lack of the knowledge about the third system.This thesis analyzed the bending effect caused by the second and third systems on the deck-to-rib joint.It proposed an analytical solution of the bending stresses.Such a solution can help to understand the role of different systems in the overall effect on OSDs.The results can provide some reference for the design and parameter optimization of OSDs.The core work is as follows:(1)The thesis summarized the fatigue phenomenon of OSDs.The mechanical causes of deck-to-rib cracks were analyzed.It also introduced the methods of fatigue life evaluation.The research status about fatigue details,design parameters and third system stress were reviewed.(2)The finite element model of OSD segment was established.The thesis analyzed the influence of different parameters on the equivalent structural stress amplitude at deck-to-rib joints.The results showed that increasing the thickness of the deck could effectively reduce the stress amplitude.Reducing rib opening width also reduced the stress amplitude,while increasing the rib thickness had the opposite effect.According to the mechanical characteristics of deck-to rib joints,the validity of ignoring membrane stress was verified when using structural stress approach.(3)The thesis analyzed the role of the second and third system effects in the overall mechanical behaviors.The maximum compressive bending stress is related to both the second and third system,and the third system is the chief one.The influence range of the third system is 400 mm around the focused joint.The maximum tensile bending stress is only related to the second system.(4)The thesis proposed the calculation formulas of the bending stress caused by the second and third systems,respectively.The influence of different structural parameters on the stress caused by the two systems was studied.The results showed that the width of the rib opening and the thickness of the deck affected the overall stress by affecting the effect of the third system.The sensitivity of the deck thickness is greater than that of the rib opening width.The diaphragm spacing,the rib height and the rib thickness affect the overall stress by affecting the stress caused by the second system.The sensitivity of these four parameters decreases sequentially.(5)The thesis compared the equivalent structural stress calculated with formulas with the value obtained from the finite element analysis.It was found that the two results agreed with each other.Hence the formula can help to optimize the parameters of OSDs.The formulas were employed to calculate the fatigue stress at the deck-to-rib joint of the steel-UHPC orthotropic composite bridge deck with large longitudinal ribs.It was found that the present fatigue stress amplitude was much lower than that of the traditional OSDs.Therefore,the fatigue performance of the rib-to-deck joint does not control the design of the UHPC thickness and opening width of the large longitudinal ribs.