Reaearch On Ultimate Load Of Single-layer Reticulated Shell Considering Initial Imperfection Subjiected To Strong Earthquake

The single-layer reticulated shell structure has three-dimensional force in the space,has the advantages of large span,light weight,beautiful appearance,etc.,and is widely used in crowded large-scale public buildings.When this type of structure encounters earthquakes and collapses,it will cause major casualties.When studying the dynamic stability of a single-layer reticulated shell,the influence of initial imperfection cannot be ignored.In this paper,the single-layer reticulated shell is taken as the research object,based on finite element software OpenSEES,the influence of the initial imperfection on the ultimate load of the single-layer reticulated shell subjected to strong earthquake is studied with method of increment dynamic analysis(IDA),and the collapsing test of the scale model of the single-layer reticulated shell is conducted.Moreover,the finite element analysis of the test was conducted.The main research contents and results are as follows:(1)Finite element model of single-layer cylindrical reticulated shell with a span of 15 m and different length-span ratio was established.The influence of initial imperfection on its ultimate load and failure modes under strong earthquakes was studied.The results show that,compared with the ideal cylindrical shell,the ultimate load of cylindrical shells with a length-span ratio of 1.0,1.4 and 2.0 decreased by 23.1%,29.4% and 36.8% respectively after considering the geometric imperfection.On this basis,the structural ultimate load reduction within 10% after considering the member imperfection.The initial imperfection reduce the degree of plasticity of the structure,and the structure tends to lose stability after considering the initial imperfection.For single-layer cylindrical reticulated shell with a small span,the failure type of the structure after considering the initial defect does not change,and is still dynamic instability.(2)Finite element model of single-layer spherical reticulated shell with a span of 80 m and different rise-span ratio was established.The influence of initial imperfection on its ultimate load and failure modes under strong earthquakes was studied.The results show that,compared with the ideal spherical shell,the ultimate load of spherical shells with a rise-span ratio of 1/3,1/5 and 1/7 decreased by 25.0%,30%,and 46.4% respectively after considering the geometric imperfection.On this basis,the structural ultimate load reduction within 20% after considering the member imperfection.For spherical shells with small rise-span ratio,the failure type of the structure can change from strength failure to dynamic instability after considering the initial imperfection.(3)The shaking table test was performed on the scale model of the single-layer cylindrical lattice shell with a similar relationship of 1:6 and the collapse process of the structure under strong earthquakes was studied.The results showed that,the most unfavorable position of the model is located at the mid-span of the lateral edge.As the ground motion increases,the plastic zone of the structure continues to develop.When the peak acceleration reaches 1.4g,the two lateral edges collapsed.When the peak acceleration is 1.8g,the whole model collapsed and the cross-section of collapsed structure was “M” shaped.The plasticity of the structure before the structural damage was fully developed before collapse and the failure mechanisms of the shell is strength failure.(4)The shaking table test of the cylindrical reticulated shell was simulated with OpenSEES program.The results show that the simulation results of structural natural frequency and elastic displacement agree well with the measured values.The simulated structural ultimate bearing capacity and development process of plastic zone are consistent with the experimental results,verifying the accuracy of the analytical method and finite element model.