Seismic Test Research And Analysis On Abnormal Knee Joints In Frame Structures

With the needs of architectural features,the situation of the ratio of height of beam to column being different of noamal joints widely appeared in actual projects. In the abnormal knee joints, negative bars could be cut off in the joint, its force characteristics is obviously different from normal joints, and there isn't treatment of such joint given by our country's current concrete code. So working on this kind of abnormal joints has theoretical values and practical useful values.Two specimens of abnormal knee joints in reinforced concrete frames under cyclic reversed loading designed basing on"Code for Design of Concrete Structures"(GB50010-2002) are conducted, discussion is made upon the problems of designing abnaomal knee joints with the existing standard,and appropriate solutions are proposed. The main points and conclusions are summarized as followed:â‘ Abnormal joints in this article are"big column and small beam".The ratio of the height of beam to column is 4:7. Double layers bars on the top of beam is used to achieve the design of eaqul moment in the joint. The results of test showed that when double layers bars is used on the top of the beam, the problem of anchor environmental weakening appears. It makes shear failure of the joint appear in low ductility.â‘¡FEM analysis is done by ABAQUS, and the simulation results are similar to the results of the test. One conclusion comes by ansysing the force of the stirrups in the joint. It's that the folding beam model is not applicable pecfectly in the abnormal knee joints, and the force region is near the end of the beam within the joint.â‘¢According the results of tests and FEM analysis, the design suggestion is given: U-bars are used in the range being next beam. In the second specimen U-bars is added in the joint,solving the problem. The eigenvalues of U-bars is 0.03, the spacing is not less than 200mm, and the range is calculated by the height of the column and the beam.â‘£The anchoring problem of small spcing rebars in the concrete specimens is not considered adequatly. The results of simulations and tests prove that the anchoring problem is serious when there is not enough stirups. The point is very important in all concrete speciments,so more tests and ansysis are needed.