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Experimental study on seismic behavior of new beam - column joints of high - rise steel frame

In the multi-high-rise steel frame, the typical rigid-coupled beam-column type is the beam flange and the column butt welding, and the beam web and the column are connected with high strength bolts. In the 1994 Northridge earthquake and the 1995 Kobe earthquake, steel structures with such nodes did not collide, but there were many serious cases at the node site, although the nodes had good seismic performance in the past. Brittle damage. The study of the Northridge earthquake in the United States has pointed out that the three main factors influencing the bearing capacity and ductility of the joints are: fracture toughness of welded metal, shape and size of welding hole, deformation control of joint plate [7]. Further research has proposed a number of measures to improve the ductility of the nodes [2]. The main methods include increasing the bearing capacity of the joints by adding axils and capping plates, weakening the beam section and moving the plastic hinges from the node area to the beam. Node area welding hole structure type, ease the local stress concentration. The results show that the improved joint structure of the joint area can make the plastic hinge move out, which can effectively enhance the ductility of the node, but also has little effect on the bearing capacity of the node area. The production process is basically the same as the traditional node, Ideal node structure [4]. Therefore, this paper focuses on the welding hole expansion node were studied. According to the different shape and size of the welding hole proposed three kinds of node structure, and with the traditional node structure of the comparative experimental study.


2. Test overview


2. 1 node specimen design

In this paper, four types of nodes (Figure 1) were studied, that is, standard nodes and three different types of welding holes to expand the type of node. Each node two test pieces, completed a total of eight test pieces. The material used for the test piece was @ 235 steel. The material yielded the material with a yield strength of 260 MPa and an elastic modulus of 2.0 x 10 5 Mpa. Each type of node two specimens, the difference is that a test piece on the side of the connection plate and the beam between the web attached to a fillet weld, the other is not. The dimensions of the beam are the same. Shaped beam section height 400mm, web thickness 8mm; flange width 150mm, thickness 12mm. The beam web consists of four 10. 9 M20 high strength bolts connected to the column flange through the shear plate.

2. 2 test device

The test device and the measuring point are arranged as shown in Fig. The test piece is flat and the beam is perpendicular to the ground. The beam ends are horizontally placed by servo-operated two-way jacks that impose a horizontal pull and press force. In the test, the cyclic reciprocating load is applied to the beam end until the member is completely destroyed. Limited to loading conditions without axial load applied to the beam. All specimens have the same beam-column section size, loading position and constraints. The measurement items include the applied load, the horizontal displacement of the loading point, the relative rotation of the beam and column, the deformation of the nodal domain, the rotation and translation of the shear plate relative to the web, the stress distribution along the midline of the beam and the stress distribution on the web. The load and displacement data are obtained by the sensor, and the strain is measured by the strain gauge. All data were obtained by electrical measurement.

2. 3 loading system

According to the finite element analysis, the elastic limit load of the specimen is about 120kN. The load is divided into two stages, the maximum load of the elastic stage is 120kN, and the load increment control is adopted. The plastic yield is controlled by displacement increment. In order to study the effect of loading order on the bearing properties of the specimen, four different loading sequences were used in the experiment (Table 1). Where loading sequence I is directly loaded at 42 mm displacement amplitude to specimen failure. TS3 and TS4a are loaded with sequence I, TS1 is loaded with sequence II, and TS2a uses loading sequence III, TS1a, TS2, TS3a and TS4 using loading sequence IV. In fact, in the test, when the beam end of the horizontal load to 120kN has been observed when the local patent leather wrinkle, that has occurred local material yield.

3 test phenomenon and destruction of the form

3. 1 test phenomenon

In the test, the horizontal load is first applied along the direction in which the upper flange of the beam is pulled and then loaded repeatedly. The TS1 of the specimen has a shortening due to the lack of external constraints, and the torsion occurs during the elastic loading stage. Plastic stage, welding hole end beam web and flange contact at the upper edge of the web compression cracking. The shear plate appears to slip, and the flanges are almost completely cracked when the damage occurs, and the displacement of the loading point is about 5 cm. In the plastic stage, the local buckling was observed on the compression side of the web, and the out-of-plane displacement of the beam-loading point was up to 7 cm, and the shear plane of the shear plate was significantly cracked. And finally by the side of the wing edge of the weld suddenly destroyed, the rapid decline in load, the beam under the flange part of the pull off, while the upper flange was crushed.

TS2 shear plate slips appear earlier, the lower flange welds crack and continue to expand, shear plate slipping increases. The crack suddenly extends to the axis of the beam, and the out-of-plane displacement increases abruptly, and the residual deformation and out-of-plane displacement of the TS2a elastic phase are small. Plastic phase, the outermost bolts first slip, and soon the bolt slip increases, the component suddenly destroyed, the beam flange welds almost to the full flange width, shear plate additional weld cracking almost to full length, and shear Slip slides. The lower end of the upper flange is crushed. TS3 is loaded with displacement control, the first cycle load point displacement of 44mm (about l / 50 beam length), the load is l40kN yield, shear plate and bolt slip, beam flange, beam web, node area partial yield. After the welding hole at the end of the beam to the upper edge of the flange resistance to cracking, load down, while the beam under the flange was crushed, butt weld cracks. TS3a elastic stage of the compression flange to reverse the more serious, the web in the vicinity of the shear plate at the corner of the skin. Followed by the flange, web, shear plate, the node plate area local yield, shear plate additional weld cracking, the lower flange weld cracks suddenly extended to the full flange wide, component damage. TS4 plastic phase appears obvious node stiffness decreased. The first cycle of the shear plate at both ends of the relative rotation. The lower flange opening section appears distorted outside the plane, then the upper flange first buckling, there is a half-wave, out-of-plane displacement and shear plate slippage increases. After the two flanges repeatedly buckled and straightened, and finally the lower flange weld crack initiation test. TS4a is also loaded with displacement control. Shear plate and beam up and down the flange, beam web, node domain has skin. Since then the out-of-plane displacement is increasing, and the two flanges are alternately pressed and straightened. The webs are deformed planeally at the corners of the shear plate, and the shear plate slips. And finally pull the flange full section of the flange, the pressure flange local buckling, and the web junction cracking, the upper and lower flange welds are fatigue damage.

Figure 3 for the node damage to the photo, you can see the beam edge of the local buckling phenomenon.

3.2 Failure model and influencing factors The test results show that the factors that affect the failure mode include welding defects, stress distribution and stress concentration. Significant welding missing