Static Behavior Research On Key Joints Of 750kV Concrete Filled Steel Tube Substation Truss Structures

Concrete filled steel tube(CFST)structure is widely used in the engineering construction for its good mechanical performance and low cost.With the “pumping jack-up method”,about 50% the weight of the single piece lifting and 30% engineering cost can be reduced compared with the 750 kV substation with pure steel pipe structure under same conditions.To widely promote the construction of the CFST structure in the 750 kV substation,and provide the theoretical basis for construction of the Qiaowan 750 kV substation in Gansu,China,the experimental,finite element(FE)analysis and theoretical study on satic behavior of two kinds key connection joints,including flange-plate connection and T-tubular joints,were carried out in depth.As the background of the Qiaowan 750 kV substation in Gansu,China,the static performance of two kinds of key joints for 750 kV CFST substation truss structures(flange and tubular joint)static performance were tested.Firstly,the test results of four CFST flange joint specimens indicate that,the failure pattern of connection designed not consider the filled concrete is accord with the design concept of “strong-joint and weak-member”,and the sufficient safety reserves shows that it can be used in Qiaowan substation.The axial tension,flange plate thickness and stiffener height influence the initial stiffness of the connection significantly,but little influence on the bearing capacity.The rotation axis of the bolt group would move to the compressive side gradually with the increase of eccentricity,but it was still closed to the wall of the steel pipe at the pressure side.Secondly,the static test results of four CFST tublar T-joint specimens with different reinforced components(a doubler-plate,a sleeved and a haunched reinforced joints and a no-reinforced joint specimen for comparison)were carried out.Test results indicate that,the failure patterns of the all joints are the branch pipe failure,which is accord with the design concept of “strong-column and weak-beam”,and the mechanical performance of the doubler-plate and sleeved reinforced joints are better than the other two.It is shown that the three joints specimens with different forms of reinforced components can significantly improve the bending stiffness and bending resistance of this joint.The in-plane bending stiffness of the three reinforced joints meet the requirements of the rigid joints of the Eurcode3,and their in-plane bending resistance can be achieved even more than the cross-section plastic resistance of branch pipe.In addition,the hot spot stress distribution along the intersection between chord and brace was tested during this experiment.Test results indicate that,the three specimens with different forms of reinforced components effectively reduces the peak stress concentration factors compared with the no-reinforced joint,and the SCFs of the doubler-plate and sleeved reinforced joints are the lowest.After the experimental verification of finite element modeling method,30 flange finite element(FE)specimens and 96 doubler-plate reinforced specimens were calculated and analyzed.Firstly,the main contents of FE analysis for the flange FE specimens include the bearing capacity and rotation axis of the bolt group.Finite element results showed that,the bearing capacity of CFST flange joints do not improve significantly,but its initial stiffness increase by about 50%,and the stress of bolts reduce significantly,compared with the steel pipe flange joints.The flange plate thickness,stiffener plate height and bolt grade have little help to improve the bearing capacity when these indexes meeting requirements.The bolt group rotation axis position basically in the compression side wall position under different eccentricity.Secondly,the main contents of FE analysis for the T-joint FE specimens include the bearing capacity,and the influence of parameters ?,?,?d and ?d for the failure mode and the ultimate bearing capacity were discussed in detail.Finite element results showed that,the detailing requirements of the doubler plate length should be 2 times the brace pipe diameter,and the thickness should be 2 times of the chord wall thickness.With the experimental and FE results,current design code and traditional T-stub analogy were introduced to obtain the flange bending capacity determined by the flange plate and bolts.Furthermore,the design model herein can provide useful reference for practical design of other kinds of bolted flange-plate connections.In addition,for the CFST T-joint with doubler-plate,its neutral axis was determined at the branch center,and the position of compression zone force action point was determined at the in compression side crown point.After summarizing all kinds of joints failure modes,the in plane flexural bearing capacity formula was derived using mechanical derivation.