Research On Reliability, Sensitivity And Failure Process Of Large-span Space Frame Structures

Reliability and sensitivity analysis of engineering structures are paid much attention nowadays. A great number of large-span space structures have been erected in succession, especially with the host of the 2008 Beijing Olympics Games. However, engineering accidents of space steel structures occurred simultaneously. The research on reliability and sensitivity of space structures just started at home and abroad, and even no systematical investigations on them, hence only few of literatures can be referred.Humankind always suffers from natural disasters, e.g., earthquake. Especially, earthquakes occurred time and again all over the world in recent years, which caused plenty of engineering destruction and loss of life and property. As the symbols of city (e.g., Beijing National Stadium, known as the Bird's Nest, Beijing National Aquatics Center, known as the Water Cube) and emergency shelters (e.g., Niigata Gymnasium in 2011 catastrophic Ms 9.0 earthquake, Mianyang Jiuzhou Stadium in 2008 Wenchuan Ms 8.0 earthquake and New Orleans's Louisiana Superdome in Hurricane Katrina) after disaster's attack, the safety and seismic performance of large-span space structures got much attention in scholar and engineering fields at all times. This kind of structures, accommodating lots of people and important facilities, once destroyed, catastrophic accident will be occurred. Therefore, it is meaningful to research on reliability, dynamic failure and destruction mechanism of space structures.Based on above issues, reliability and sensitivity analysis of large-span space structures have been systematically carried out. Failure process and failure mechanism of double-layer reticular dome under static critical loads and three-dimensional earthquake motions have been intensively studied. The main contents in this thesis are concluded as follows:(1) Two issues are sometimes come forth when carry out nonlinear FE reliability analysis of large-span space frame structures, one is that certain material models may cause gradient discontinuity, which will lead to failure of the search algorithm to converge; the other is that the search algorithmes may generate trial points too far in the failure domain, which causes difficulties to obtain reasonable results. To settle the above two issues, the former issue is addressed by introducing smoothed bi-liner material model and Bouc-Wen material model, while the latter one is settled by introducing advanced searching algorithms, i.e. improved HL-RF Algorithm, the Gradient Projection Method (GPM), Sequential Quadratic Programming (SQP) and Polak-He algorithm. Furthermore, efficiency, accuracy and robustness are of the above four searching algorithms are elaborately illustrated and extensively compared, and gives some advices on how to choose the value of parameters that influence the computation efficiency and robustness of these algorithms, which can offer some valuable guidance to reliability evaluation in space structures.(2) Effect of initial imperfection on reliability of space grid structures is extensively investigated. Seven trusses with different thicknesses and six different magnitudes of initial imperfection are taken into considerations, respectively. Layout of initial imperfection is chosen from the lowest order buckling mode of truss structures. Subsequently, initial imperfection's influence on reliability of grid structures is systematically researched; to comprehensively investigate reliability of truss under different thresholds, parametric reliability evaluations are carried out for these trusses; in order to research the effect of correlation coefficients of random variables on reliability of truss structures, six correlation coefficients are selected.(3) Sensitivity analysis is an important aspect in reliability evaluation of engineering structures because its results can rank the random variables based on their relative importance. Different Difference Method (DDM) is adopted in the sensitivity analysis. Three numerical examples (e.g., a simple truss, a double-layer cylindrical reticular shell and a plane truss structure) are demonstrated, Four parameters (α,γ,δandη) are taken as importance measures to identify the sensitivities of random variables, their means and standard deviations. Cross-sectional area, Young's modulus, yielding strength, strain hardening rate and loads etc. are taken as random variables in space truss, and reliability and sensitivity are evaluated by FORM. Nodal coordinates are usually treated as invariants in reliability analysis, which will cause significant error in some cases. Thus, the variations of nodal coordinates are considered into reliability and sensitivity analysis of space grid truss structures. For large and complicated structures, much time and effort have to be paid to carry out reliability and sensitivity analysis. Hence, random variable reduction technique is introduced to reduce computation effort, and it facilitates the reliability and sensitivity analysis in space structures.(4) First-order Reliability Method (FORM) is widely applied for its better efficiency and accuracy, however, it will oscillates around the design point and fails to converge in iteration process when the curvature is too large; large reliability index will impedes to converge. To settle the above issues, Performance Measure Approach (PMA) is firstly introduced into nonlinear FE reliability evaluation in space structures. Compared with Reliability Index Approach (RIA), constraint function can be treated as the problem that searching prescribed minimum performance target point in PMA. To verify the efficiency and robustness of PMA, reliability and sensitivity of four cylindrical reticulated shells with different rise-to-span ratios and four spherical reticulated shells with different rise-to-span ratios are evaluated by RIA and PMA, respectively. Numerical results indicate that PMA has better efficiency and robustness in reliability and sensitivity analysis of space structures. (5) To identify the influences of different types and parts of random variables on reliability and sensitivity of space structures, four cylindrical reticulated shells with different rise-to-span ratios and four spherical reticulated shells with different rise-to-span ratios are selected. Based on the type and location of random variables, vertical loads, cross-sectional area of members, Young's modulus and yielding strength are treated as one or more random variables, respectively, to identify the sensitivity of these variables; several performance functions are selected to identify the effect of random variables on them and research their correlations; variation rules of sensitivity of all random variables are investigated with the change of rise-to-span ratio; the effect of different probabilistic distribution types of random variables on reliability, sensitivity and correlation is also extensively researched.(6) Failure process and destruction mechanism of double-layer spherical reticulated shell under statically critical loads and three-dimensional earthquake motion are systematically investigated. Four double-layer spherical latticed domes with different rise-to-span ratios are selected, and deformation capability, number of yielding members, total strain energy, total elastic strain energy, total plastic strain energy, total elastic strain and total plastic strain of reticular domes are elaborately investigated. Failure processes of these domes under statically critical loads and dynamic excitations are captured, furthermore, failure sequence of members is illustrated in detail. Finally, similarities and differences of failure mechanism of four reticulated shells are developed. Numerical results can give some reasonable suggestions and advices for the design of double-layer spherical reticular shell.