Theoretical And Experimental Studies On The Slip Mechanism Between Main Cable And Saddle In Suspension Bridge

Main cable is an important load-bearing part of suspension bridge,the relative location between main cable and saddle is therefore critical to ensure the safety of whole bridge structure.Multi-pylon suspension bridge,developed from the traditional double-pylon suspension bridge,is regarded as an ideal option for crossing broad waters(e.g.,bays,straits)due to its strong spanning capacity as well as good economic benefit.For this type of bridges,however,the anti-slip problem between the main cable and the middle saddle is a core factor that restricts the selection of mid-pylon and even the establishment of the structural system.For this reason,it is necessary to carry out studies around that anti-slip problem.Based on both theoretical and experimental approaches,a systematic study,including the calculation of anti-slip friction,the mechanism of slip behavior,the design of anti-slip scheme and the reasonable framework for evaluating anti-slip performance,is carried out in the present work,which has comprehensively solved the issue of slip phenomena.Result of this study could offer a basis for the revision of relevant theories as well as the innovations in practical engineering.The main work of this study consists of the following parts:First,to solve the problem that the lateral frictional resistance between main cable and saddle cannot be calculated precisely and cannot be used sufficiently,theoretical and experimental studies on the lateral force of main cable were carried out.The theoretical basis and application limitations of the existing formulas were revealed.Based on the contact relationships between discrete bodies,a calculation model for interaction forces in the saddle groove was proposed.Subsequently,the theoretical calculation method for lateral force was established.By applying the least square's method to partitioned multivariate nonlinear regression analysis,the practical calculation formula was obtained.The calculation results obtained from the existing formulas,theoretical method and practical formula,respectively,were compared,and the effects of parameters were discussed as well.As an example,the frictional resistance of the main cable of an actual bridge was calculated using the practical formula.On the other hand,based on the exploration and practice in model design,loading and testing approaches,feasible experimental schemes for lateral force were proposed.By analyzing the test results,the distribution pattern and change rule of lateral force were further studied,and the theoretical research results were validated.To clarify the slip mechanism between the main cable and saddle and to provide essential foundations for relevant theoretical studies,the experimental study was carried out by taking each strand as a basic study object.The loading schemes were established since the equilibrium state of the main cable was simulated by stretching the strands at both ends,and the unbalanced loading states were produced by pushing the saddle at one side.The corresponding test models and the measurement methods were designed.Totally 15 test cases were carried out with consideration of the effects of lateral friction,the number of strands,and the profile of saddle.The mechanism problems,such as the development of strand forces,slip behavior,time-varying effects,and the nominal friction coefficient,were systematically investigated using measured data.Finally,the necessity and feasibility of reasonable of a reasonably determined friction coefficient between main cable and saddle were discussed.Furthermore,for the problems of insufficient anti-slip capacity of the mid-saddle in the multi-pylon suspension bridge,the general designs of the anti-slip scheme involving vertical friction plate(VFP)and horizontal friction plate(HFP)were investigated.A single strand was taken as a basic unit,yielding in the conception of strand element(SE).Subsequently,the physical model and normalized mechanical model of SE were proposed,and an analysis module of SE was then developed in order to facilitate the embedding and calling of the slip model of main cable.On this basis,combined with the structural characteristics of saddle and the profile of strand distribution when equipped with VFP or HFP,considering the layeredslipping behavior and the lateral friction effects,the layered slip analytical models of main cable incorporating with VFP or HFP were built by theoretical derivation.These analytical models were next validated by means of experimental verifications.Finally,regarding the insufficient suitability of the existing anti-slip mode,three indexes including the anti-slip safety coefficient,the unevenness of strand tensions and the rate of slipped strands were proposed,and the criterion for evaluating the anti-slip performance of main cable was then established taking into account the whole slip process.The Wenzhou Oujiang River North Estuary bridge was taken as a typical practical case.Based on the proposed layered slip analytical model and the evaluation criterion of anti-slip performance,detailed parametric studies were performed around the design of anti-slip scheme involving either VFP or HFP.The regularity characteristics of the strand slippages,the evaluation indexes,the forces at different directions,the combination mechanism of frictional resistance and the anti-slip effects were clarified.Two relative optimal schemes were found: setting full VFPs among columns and setting two HFPs on about 1/4 and 1/2 of the height,respectively.Subsequently,the construction feasibility of these two schemes was clarified.From the above studies,a methodology framework for the anti-slip performance of main cable was developed,providing references for the conception,design and implementation of the anti-slip scheme for the main cables in real cases.