Ultimate Load-carrying Capacity Study Of Concrete-filled Steel Tubular Arch Bridge

Concrete-filled steel tubular (CFST) arch bridge are widely used in the construction ofcity and highway arch bridge in the world, because of their advantages such as long-span,high load-carrying ability, low engineering cost, little maintenance cost, convenienceconstruction, the lightness and beautiful shape. Taking account of the traffic flow, sitelandform and investment scale, we often face the design of long-span narrow bridge, lessthan a width-to-span ratio of 1/20, during the highway construction. The plane truss typesection rib, which made of two concrete-filled steel tubular (CFST) chords and webmember system, as a new solution was carried out for the long-span narrow arch bridge.And it has obvious rationality and superiority. However the lateral stability is the key issueto this type bridge because the lateral stiffness of the ribs themselves was rather low.Combined with the engineering pratice of 238m span Tongwamen Bridge and 202mMoonIsland Bridge, yielding width-to-span ratio of 1/23.8 and 1/22.44 respectively, thispaper gives the study and discussion about analytic method of lateral stability, stabilityreliability and ultimate load-carrying capacity for CFST arch bridge. The main researchwork covers the following aspects:1. With the energy principle, utilized a symmetric half-wave and two antisymmetrichalf-waves as displacement functions for the first time, taken the ribs center distance ofdeck as the width of bridge, considered the deck position of the half-through arch bridge, auniform lateral stability calculation formula including deck, half-through and throughX-type double parabola ribs is established, subjected to the orientedly conservativeloadings and non-orientedly conservative loadings. Meanwhile the structure parametersinfluencing on lateral stability, such as the torsional stiffness and vertical flexural rigidityof arch rib, flexural rigidity and numbers of crossbeams, span-depth ratio, width-to-spanratio, incline inward degree of ribs, deck position and lateral flexural rigidity of the deck,are discussed. And the corresponding numerical charts are given. The methods to improvestability of X-type double ribs arch are presented.2. Based on the CFST main theory, the performance function of local stability failuremode is established, considering material performance, structure geometrical parameterand indefinite calculation model. Five performance functions of typical arch rib forTongwamen Bridge and MoonIsland Bridge are put forward for a maximum load case.According to the analysis result, the reliability meets the necessity of object reliabilityindex to Gradeâ… , Ductile Fracture, Bridge Structure of Highway Enginering. As for theproblem of limit state function that can not be expressed explicitly to the complex bridgestructures, a new strategy for positioning sampling points of the response surface method (RSM), based on the composite method combining RSM with geometric method forstructural reliability analysis, is adopted to obtain the reliability index of whole lateralstability. And in addition the correlated parameters were discussed in detail to find themajor factors of lateral stability.3. Based on the theory of non-linear finite element analysis method and adoptedstress-strain relationship of core concrete for eccentric compression, the software toanalyse CFST arch bridge utilmate load-carrying capacity is developed, which can performwith material and geometrical nonlinearity (in U.L. formulation). Then, through analysisof utilmate load-carrying capacity for Tongwamen Bridge, it is found that for the loadingcase of the whole span and half span, the live load coefficient of ultimate load-carryingcapacity are 10.73 and 5.6 respectively. Meanwhile the factors influencing on utilmateload-carrying capacity, such as load mode, steel ratio of arch rib chords, core concretegrade, plasticity of web member are discussed in detail. The further study shows that uponcollapse of the bridge, the result based on elastoplasticity and elasticity, no matter fordeformation or internal force, are greatly different, showing that the structure comes to theelastoplasticity stage with a significant redistribution of internal force due to the rigiditychange of cross section. The elastic ultimate load-carrying capacity of the structure is muchgreater than that of the elastoplastic method. Based on the practice of engineeringappliation, the ultimate load-carrying capacity is not increased dramaticly with the rising ofsteel ratio. As to the selection of external diameter to thick ratio, the code and the actualstress state should be considered at the same time for the designers. Therefore theappropriate steel ratio utilized for arch ribs, it is not only ensuring structure ultimateload-carrying capacity, but also obtaining considerable economy on the premise of systemstability and safe.