Reliability Design And Study On Prestressed Partial Single-Layer Cylinder Shell

Compared to traditional double-layer cylinder shells, prestressed partial single-layer cylinder shell is a new type of spatial structure, which is based on the combination of equal strength shell theory and prestress technology. This type of structure has a good technical-economic index and a bright prospect, because it can not only bring us a graceful indoor visual , take a short duration to construct , but also enhance the stiffness and ultimate bearing capacity of the lattice, decrease the whole steel consumption.Through the check of reliability index of one practical engineering, the shortcome of current code is pointed out and reliability-based stability design method for prestressed partial single-layer cylinder shells was proposed, which would make the structure stability design more safety.In view of the heavy calculation costs in PDS module of current FEM program, a more efficient reliability analysis method based on iterative response surface approach is proposed, which is suitable for reliability calculation for large-scale structures. Under current structure design codes, multi-factor combinatorial design theory is adopted.The complex relationship between the range of single-layer lattice, the gauge of double-layer lattice, the value of prestress and the ultimate bearing capacity, the total amount of steel used was obtained. The regression analisys results showed that there existed a high nonlinear mapping among the design parameters, and an obvious interaction between the gauge of double-layer lattice and the value of prestress which indicated that single factor design method may lead to unreliable design results. The best design was derived from the optimized solution to the regression equation, which satisfied the elastic stability requirement in technical specification for latticed shells. The results showed that the design scheme was quite similar to the engineering practice.Random defect mode method and consistent defect mode method was compared.In the solution process of reliability index, four load random variables and eight resistance random variables are adopted. Four kinds of failure modes are selected, including relaxation of steel cables, strength failure of materials, excessive displacement of nodes, buckling failure of whole structure. Analytical results showed that among various failure modes, the most unfavorable load case is comprised of half-span load, and large deformation collapse buckling is the main failure mode and stability reliability index in individual circumstances was rather low. The above conclusions are still valid when the value of prestress and live load-dead load ratio have changed, and stability becomes a dominating factor during the design. Subsequent sensitivity analysis shows that among all of the random variables, the half-span snow load, the thickness of pipe and material strength of steel are the most sensitivity factors, while cable area, material strength of cable are the least sensitivity factors.Based on structure reliability theory, the stability design method for this kind of prestressed latticed shells is studied to avoid the ambiguous stability reliability indices. According to target reliability index, liveload-deadload ratio, distribution pattern and type of variable load, the practical design expressions and envelope curves of overall stability coefficient are obtained, which possess definite reliability meanings and second-order inelastic analysis. Finally, the optimization models based on stability reliability are established for structures of various span and rise-span ratios, and the reliability index of optimum designs all meet the requirement.