Experimental Study On Fatigue Properties Of Orthotropic Steel Bridge Deck Under Combined Bending And Torsion Loading

Orthotropic steel bridge deck is widely used in long-span cable-stayed bridges andsuspension bridges because of its advantages such as lightweight,high strength,good applicability and fast erection speed.In actual operation,due to its construction characteristics,orthotropic steel bridge deck is often subjected to combined bending-shear-torsion force under the action of vehicle loads,resulting in fatigue cracking at the welded joints and reducing the bearing capacity and service life of bridge deck,which is an common fault for this kind of bridge deck.However,the existing researches are mainly focused on the bridge decks with symmetric loading.The corresponding results cannot fully reflect the stress caused by torsion at the weld region,and the effect of bending and torsion on the fatigue properties of bridge deck welds.Therefore,it is necessary to study the fatigue properties of orthotropic steel deck under combined bending and torsion loading.Based on the support of National Basic Project(973 project)“The Fundamental Research of Extra Large-span Bridge Safety Design and Evaluation” topic 3 “Extra Large-span Bridge properties Evolution Characteristics under the Action of Various Factors”(2015 CB057703),this study conducted an experimental study on the fatigue properties of orthotropic steel deck under combined bending and torsion loading.The main contents and conclusions are as follows:(1)18 full-sized specimens with decks,U-ribs and diaphragms were designed and manufactured according to the data of an actual bridge.The finite element analysis of the specimens under combined bending and torsion loading was performed using ANSYS.Based on the stress distribution of specimens predicted with ANSYS,the measuring points were determined.After the loading device,test procedure and measurement content were determined,the high-cycle fatigue loading test was performed on three groups of specimens.(2)The distribution of fatigue stress,final failure pattern,crack propagation and stiffness change of the three groups of specimens were systematically analyzed.The results show that with increasing of the specimen length,the stress of measuring point at the joint of deck-diaphragm decreases,but the stress of the measuring point at the joint of the deck-U ribs increases gradually;crack forms will change following a trend of arc line to arc line plus straight line to straight line with increasing of the specimen length;the fatigue crack propagation process can be roughly divided into four stages: crack initiation stage,steady crack propagation stage,crack penetrating deck thickness stage and fatigue fracture stage;thefatigue cracks of deck bottom,deck thickness and deck top present similar propagation trend:expanded slowly at the early stage,then faster at later stage and fastest at last stage;in the whole loading process,the stiffness change is similar to the crack propagation trend,which degrades not obviously at the early stage,then degrades steadily and faster and faster at last stage;for the specimen with long length,the stiffness degradation accelerates with the main crack propagating,but decelerates in the later stage.(3)After analyzing the fatigue life of specimens,two criterias based on the strain change of measure point and the visible crack were established,and the fatigue properties of different connection details was evaluated.The results show that under the same fatigue load spectrum,the fatigue life of specimens generally presnts a decrease trend with increasing of the size;the length of specimen has little effect on the fatigue life of each stage of crack propagation;the fatigue strengths of the joints of both deck-diaphragm and deck-U rib are higher than those of corresponding fatigue details recommended by International Institute of welding IIW,European Code Eurocode 3,American Code AASHTO and British Code BS 7608 respectively under two criterias;the S-N curve fitting is performed on the test data under two different failure criterias for the details of deck-diaphragm and deck-U rib connection.The slope for the S-N curve of fatigue detail is close to the slope value of the S-N curve recommended by the specifications.