Mechanical Behaviors And Reliability Analyses Of Fastener Steel Tube Full-hall Formwork Support Systems

With increasing investment of civl infrastructure in China,collapse accidents during construction periods often appear.As important load transfer systems for construction,fastener steel tube full-hall formwork support systems are usually used and their safety is of great concern.In recent years,research on such systems has been carried out by domestic and foreign scholars.And some design specifications were also developed based on the research fruits,which plays an important role in improving system safety.However,there are still some problems to be solved.For example,the nonlinear characters of rotational fasteners,as well as the effects due to top structures are seldom considered when analyzing the bearing capacity of an entire system.Meanwhile,the stability problem is generally simplified by calculating the stability of a single tube.Moreover,methods for reliability evaluations are not mentioned in current specifications.Due to thess drawbacks,this thesis first studies the mechanical behaviors of fastener steel tube full-hall formwork support systems.Then the reliability evaluation methods are also proposed based on the previous study.By this means,the safety of such systems has been investigated in detail with the research fruits listed as follows:Firstly,a semi-rigid connection assumption was employed for the numerical and experimental analyses of rotational performance of fasteners.The M-? curve was determined through the experimental data.Then the experimental curve was simplified into a tri-linear model for the purpose of practical applications.Meanwhile,a fixed value for the rotational stiffness of the semi-rigid connection was taken for subsequent theorecial analysis.The fixed value was determined through curve-fitting on the tri-linear model.Secondly,the overall stability(buckling capacity)of fastener steel tube full-hall formwork support systems was studied.Two different models with or without cross bracings(S1 and S2)were established and the linear buckling analysis were first performed.And the fundamental buckling modes were used as the geometric imperfection of the support systems.Then combined with the material nonlinearity,nonlinear buckling analyses were performed to obtain the load-displacement curve,based on which the ultimate bearing capacity was determined.Meanwhile,the semi-rigid steel frame stability theory was utilized to calculate the bearing capacities of models S1 and S2.And the calculation results were verified against a single-span model.Thirdly,according to abundant on-site survey data,the probability distributions and statistical properties of some uncertain factors such as materials,geometries,loads and other quantities were determined.The final failure mode of a system was defined by the overall instability,by which means the reliability of the system could be investigated.The stability formula from the existing Chinese specification was adopted as the functional equation for reliability analysis.And the corresponding reliability index was calculated by using the Monte Carlo sampling method.For practical usage,a simplified formula of reliability index was obtained by fitting all the samples.The simplified formula may be regarded as a supplement to the current specification.Subsequently,parametric analyses were implemented and response surface method was also used to calculate the reliability index for comparison.Lastly,in order to validate the reliability of the proposed formula,Monte Carlo based stochastic finite element analysis provided by ANSYS/PDS was used for further study.The reliability and sensitivity of systems were systematically investigated.Besides these aspects,the effects of load variations,different initial defects and human errors on system reliability were also discussed.And the curve embodying the variations of the reliability index under different factors was obtained.