Reliability Analysis And Safety Factor Assessment For Stability Of CFST Arch Ribs

The concrete-filled steel tubular (CFST) has been widely used in arch bridges in recent years due to the advantages of light weight, high ultimate compressive strength, convenience of construction and aesthetic appearance. According to incomplete statistics, more than400CFST arch bridges have been constructed worldwide. CFST arch bridges have been constructed in China since the1990s and developed fast in the past two decades. Consequently, investigations on theory and experiment of CFST arch bridges have attracted some attentions from researchers recently and some remarkble conclusions have been obtained. However, the present investigations of CFST arch bridges are concentrated on the applications of new bridge types and new techniques. The gap between the rapid development of practical engineering and the slowly development of theoretical research becomes more and more serious. Up to now, more than300CFST arch bridges have been constructed in China, accounting for3/4in the world. Moreover, CFST arch bridges have become a member of large span bridges in China. For the properties of high strength and large span, the arch ribs are commonly quite slender, and the problem of stability becomes more significant. Although considerable contributions on time-independent stability of CFST arches have been devoted over the past decades, studies of time-dependent stability with and without considering uncertainties due to the concrete creep have been limited. Moreover, the stability design method of CFST arch ribs, which directly employs the items in the design code of reinforced concrete (RC) arch bridges, is not reasonable. Meanwhile, the stability safety factors of RC arch bridges, based on engineering experiences and expert advices, still require further verifications. In view of the above questions, the contents and conclusions of this paper are as follows:(1) Investigations on time-independent stability of CFST arch ribs. The elastic stability and nonlinear stability of CFST arch ribs are investigated by the finite element method (FEM) and verified by analytical formulas or experiments. Parametric studies are conducted using the FEM, and influences of slenderness ratios, rise-to-span ratios and steel ratios on stability are discussed. Comparison studies are conducted to distinguish the differences between elastic stability and nonlinear stability. The results show that the FEM models are of good accuracy and efficiency in this study.(2) Investigations on time-dependent stability of CFST arch ribs. The creep-induced resistance deterioration for stability of arch structures is discussed. For the in-plane stability of circle arches, a creep integral operator is introduced to derive the formula for the creep stability, and the influences of steel ratios and concrete loading ages are discussed. For the out-plane stability, the time-dependent initial geometric imperfection is introduced to express the creep deformation of CFST arch ribs at any time. Then the out-plane creep stability could be obtained via the nonlinear finite element analysis by introducing the above time-dependent imperfection. A fixed parobolic arch is taken as an example to discuss the influences of loading levels, slenderness ratios, rise-to-span ratios, lateral loads, steel ratios and concrete loading ages. The results show that the influences of concrete creep on the in-plane and out-plane stability of CFST arch ribs are significant.(3) Investigations on the time-independent reliability for the stability of CFST arch ribs. The time-independent limit state function for the stability of CFST arch ribs is established. The general FE software and the reliability toolbox are combined via the application programming interface (API) to realize the finite element reliability method (FERM). The influences of various random variables on the reliability index and sensitivity index are obtained. The Monte Carlo Simulation (MCS) and Latin Hypercube Sampling (LHS) methods are utilized to obtain the statistical information of the stability. The results show that the influences of resistance and load uncertainties on reliability for stability of CFST arch ribs are significant.(4) Investigations on the time-dependent reliability for the creep stability of CFST arch ribs. The time-dependent limit state function for the creep stability of CFST arch ribs is established. The stastical information of creep coefficient of concrete is obtained based on the MCS method. Based on the models of in-plane and out-plane creep stability, the time-dependent reliability method and the FERM are combined to investigate the reliability deteriotion for the stability of CFST arch ribs. Influences of various random variables on reliability index and sensitivity index are discussed. The MCS and LHS methods are utilized to obtain the statistical information of in-plane creep stability and out-plane creep deformation. The results show that the influences of concrete creep on the in-plane and out-plane reliability index of CFST arch ribs are significant.(5) Investigations on the time-independent and time-dependent stability safety factor assessment of CFST arch ribs. For the time-independent stability, the in-plane and the out-plane limit state design equations are established. Given the target reliability indices, the in-plane and the out-plane safety factors for the time-independent stability are calculated based on the inverse FERM. For the time-dependent stability, a safety factor for creep stability is introduced into the time-independent limit state design equation. Given the target reliability indices, the in-plane and the out-plane safety factors for creep stability are calculated. The influences of various parameters on the time-independent and time-dependent safety factors are discussed. The results show that the influences of uncertainties on stability safety factors of CFST arch ribs are significant, and it will be not safe without considering uncertainties.