Research On The Failure Mechanism And Ultimate Bearing Capacity For The New Wind Turbine Tower Joint

With the increase of the single unit installed capacity of wind turbine,the supporting frames are required for greater height and higher strength and stiffness.In order to gain better working performance,more convenience in dismounting and transportation,and more saving in material consumption,a new type of concrete-filled steel tubular lattice wind turbine tower structure has been invented.As the key parts of new type of wind turbine tower,the joints have very complex stress state,and once they failed,the transmission path of the inner forces for the tower structure will be changed or broken,which will cause the collapse of whole wind turbine.At present,the available research results for the concrete-filled steel tubular joints are mainly for trusses,bridges,transmission towers,lattice columns,ocean platform and other structures,which cannot be directly used to guide the design and calculation of joints in wind turbine tower.Therefore,it is of great theoretical significance and practical value to study the failure mechanism,ultimate bearing capacity of the wind turbine tower joints and to determine the control index and key parameters.In this paper,the failure mechanism,bearing capacity and design method for the concrete-filled steel tubular joints and joints with gusset plate connections are studied systematically,and the main contents and results are as follows:This thesis took the tower structure of a 2MW horizontal-axis variable-pitch wind turbine in Bayan Obo in Inner Mongolia as the research object.Based on the calculation results of the loads obtained from the?Design guide and interpretation for the wind power equipment tower structure?in Japan Society of Civil Engineering and the software of GH bladed,the differences between the forces of the concrete-filled steel tubular lattice wind turbine tower(which is a typical?Tall chicken leg dumbbell?type high-rise structure)and those of the other kinds of towers were analyzed.The results have shown that for the concrete-filled steel tubular lattice wind turbine tower the axial forces and stiffness of the column limbs are much larger than those of the web members,the axial forces of the horizontal web members is very small and hence are only the"redundancy bars"after the failure of the compressed oblique webs.The internal force ratios of joints in the tower system are quite different.Experimental study and finite element analysis were carried out for the concrete-filled circular steel tubular K-joints subjected to axial loads to investigate the influence of the parameters,such as the column limb diameter-thickness ratio?,the diameter ratio?,the thickness ratio?and the angle?between diagonal web members and column limb,on the mechanical properties.The results were compared with those of the circular hollow section joints.No obvious stress concentration was observed at the intersection part of the concrete-filled circular steel tubular K-joints in all the experiments,and the failures mainly occurred on web members.The finite element analys is results indicated that the main failure modes for the concrete-filled circular steel tubular K-joints included the buckling and yielding failure for the web members,and the punching shear failure for the column limb,while the control failure mode mainly depended on the thickness ratio?and the angle?between diagonal web members and column limb.The criterion for the ultimate bearing capacity of the concrete-filled circular steel tubular K-joints were completely different from that of the circular hollow section joints.From the safe and economical point of view,it is suggested to take the value of??1 and??45°to avoid the occurrence of punching shear failure.The static responses of concrete-filled circular steel tubular K-joints with gusset plates to axial loads were experimentally and numerically investigated.Considered parameters included the column limb diameter-thickness ratio?,the diameter ratio?,the thickness ratio?between diagonal web members and column limb,the thickness ratio t_g/t_i between the gusset plate and web member.The test results have shown that the joints failed due to overall buckling or local buckling of the compression webs or the local buckling of the gusset plate,which is completely different from that of the circular hollow section joint with gusset plate.The stress concentration was of great less pronounced in the intersection area.The finite element analyses have shown that the web failure and the gusset plate failure were the control failure modes for the concrete-filled circular steel tubular K-joints with gusset plates.The thickness ratio t_g/t_i between the gusset plate(which transfers the forces from the connecting secondary members to the main structure steel tube,i.e.the"transition member")and the web directly determined the failure pattern of joint.t_g/t_i>2 is suggested to avoid the early failure of gusset plate.The analyses on bearing efficiency for the above two kinds of joints have shown that the load-bearing efficiencies of the joints were higher when the web failed while were very low if the punching shear failure of the column limb or the buckling of the gusset plate occurred.It was indicated that in order to ensure the normal operation of the wind turbine,it is necessary to change the geometrical parameters and the structural parameters at joint area to ensure that the web failure occurred firstly,and to guarantee that the wind turbine tower system still works normally after the redundancies are reduced.Finally,the designing method for the prediction of the ultimate bearing capacity of the concrete-filled steel tubular K-joints and that of the K-joints with gusset plates were established accounting for the typical failure modes and was benchmarked against finite element results.It has been shown that the proposed designing equations can accurately predict the ultimate bearing capacity for the two types of joints,and can provide reference for engineering design.