Mechanical Behavior Of Joints And Its Influence On Stability And Progressive Collapse Resistance Of Single Layer Latticed Shells

The single-layer latticed shell is widely used in large public facilities because of its beautiful appearance,flexible shape and good mechanical performance.The joints in single-layer latticed shell can be divided into welded joints and assembled joints according to different connecting mode.The traditional welded joints have the merits of high initial stiffness and bearing capacity,but suffer from the problems of long construction period,difficulties in assembling and residual welding stress.The emergence of assembled joint systems solved the above problems to a certain extent.However,the commonly used assembly joints are semi-rigid or hinged,and the stability bearing capacity of single layer latticed shell will be reduced.Moreover,since the redundancy of the single-layer latticed shell is relatively low,the progressive collapse of the single-layer latticed shell may occur under extreme weather or human factors.As the intersection of load-transferring path,the mechanical behavior of the joint also have great influence on the progressive collapse resistance of the single-layer latticed shell.Based on the above problems,a new type of Assembled Hub joint system for rectangular steel tube single-layer latticed shell was proposed in this paper.The mechanical behavior of newly developed Assembled Hub joints(AH joint)and the widely used Welded Hollow Spherical joint(WS joint)were studied and compared.The methods of theoretical analysis,numerical simulation and full-scale test were adopted and the stiffness and bearing capacities of the AH joint subject to axial load,bending moment,eccentric load and multi direction load were studied in detail.The nonlinear spring model of joints was established to simulate the mechanical behavior of joints.The stability and progressive collapse resistance of single-layer latticed shells were analysed.The main research contents and conclusions are as follow:(1)The initial axial stiffness,bending stiffness and torsional stiffness of the WS joint was analysed and theoretical formulas were derived.Numerical analysis is carried out on the initial stiffness and stiffness degeneration of the WS joint.Axial stiffness-deformation curve,rotational stiffness-rotation curve and torsional stiffness-rotation curve of the WS joint were obtained.The practical formulas of the initial stiffness of the WS joint were proposed,the method for calculating the critical elastic deformation(rotation)and critical elasto-plastic deformation(rotation)of the WS joint was proposed.(2)A new type of Assembly Hub joint(AH joint)was designed and studied.The stiffness and bearing capacities of the AH joint subjected to axial force and out-ofplate bending moment were studied according to component method.Through fullscale test and numerical simulation,the load transfer mechanism and failure mode of the joint under axial force,bending moment and eccentric load were studied,the accuracy of the theoretical analysis result was verified.The influence of stiffeners on the mechanical behaviors of joints was also investigated.The in-plane bending and torsional properties of assembled hollow hub joints were studied.The results show that the in-plane bending stiffness of the joints is 11% to 20% of the out-ofplate bending stiffness,the torsional stiffness of the joints is 17% to 23% of the outof-plate bending stiffness,and may not be considered in design.The bearing capacity of the hollow hub would be reduced by 50% at most under multi-direction loads,and the reduction factor can be taken into account.(3)The nonlinear spring model considering the mechanical behaviors of joints was established.The accuracy of the spring model was verified by comparing the joint stiffness and bearing capacity results of the test,the solid element model and the spring model.The stability of single-layer latticed shells with WS joints and AH joints were analysed,respectively.The influence of the mechanical behavior of the joint on the stability of single-layer latticed shell was evaluated,and the measures for improving the stability bearing capacity were put forward.The results shows that:(1)For the single-layer latticed shells with WS joints,the degradation of joint stiffness is the main reason leading to the reduction of the stability bearing capacity.When the WS joints meet the requirements of the design code,the stability bearing capacity of single-layer cylindrical latticed shells and single-layer spherical latticed shells decreases by less than 7%.(2)The in-plane bending stiffness and torsional stiffness of the AH joints are low,when used in quadrilateral mesh latticed shells,the stability bearing capacity will be greatly reduced.The stability bearing capacity of Lamella(orthonormal)cylindrical latticed shells with AH joints is 51% of that of latticed shells with rigid joints.By modifying the angle of diagonal,the stability bearing capacity can be increased by about 0.8 times.The stability bearing capacity of Meridian type spherical latticed shells with AH joints is 47% of that of latticed shells with rigid joints and can be increased by about 2.0 times by arranging cross cables.When using in shells with triangular grids or shells with quadrilateral grids and cross cables,the stability bearing capacity of latticed shells can reach over 74% of the shells with fully rigid joints.(4)The progressive collapse resistance of single-layer latticed shells with WS joints and AH joints were analyzed.Through sensitivity analysis,the locations of sensitive components and key components were studied.The progressive collapse resistance of latticed shells with various grid types was evaluated by the progressive collapse critical load.The results shows that:(1)The main progressive collapse failure modes of single-layer cylindrical latticed shells with longitudinal supports are overall lateral collapse and local propagation collapse.With the same steel consumption and joint type,the progressive collapse ultimate load of orthogonal type cylindrial shells with diagonals is higher than that of orthogonal type cylindrial latticed shells with cross cables.The progress collapse critical load of the Lamella cylindrial latticed shells and the Three-way cylindrial latticed shells are the lowest.When member failure occurs,the progressive collapse critical load of single-layer cylindrical latticed shells with AH joints is 3% to 29% lower than that of the shells with WS joints;when joint failure occurs,the progressive collapse critical load is 11% to 32% lower.(2)The main progressive collapse failure modes of single-layer spherical latticed shells are radiation propagation collapse and circumferential propagation collapse.With the same steel consumption and joint type,the progressive collapse critical load of Geodesic and K6 latticed shells are the highest.The progress collapse critical load of Ribbed type and Meridian type shell with diagonals are lower than Geodesic and K6 latticed shells.The progress collapse critical load of Ribbed type and Meridian type shells with cross cables are lower than that of Ribbed type and Meridian type shells with diagonals.The progress collapse critical load of Meridian type shell is the lowest.When member failure occurs,the progressive collapse critical load of single-layer cylindrical latticed shells with AH joints is 15% to 18% lower than that of the shells with WS joints;when joint failure occurs,the progressive collapse critical load is 7% to 25% lower.