Elastic Stiffness and Yield Strength of High Strength Steel Square Tube Column To Beam Flange Connections With Exterior Diaphragm

Abstract eng:
In recent years, high strength steels, whose tensile strength is more than 780 MPa, have been developed in Japan, and are used mainly for columns of high rise building structures. However, it is difficult to secure not only quality but also workability of welding at beam-to-column connections with high strength steel. In Japan, most well used shape of columns is a square hollow section but this type of closed cross section is necessary to stiffen the section by a diaphragm at the location of the joint with beam flanges to restrain local deformation. In this study, we focus on exterior diaphragm type connection. Because a diaphragm plate is attached only from outside of a column and columns need not to be cut, this type of diaphragm has advantage to overcome the problem about workability of welding of high strength steels. Furthermore, the exterior diaphragm is suitable for applying concrete filled steel tubes with high strength steels. For improving convenience of construction and transportation efficiency, this study regards a square diaphragm of thick steel plate, whose depth of protrusion is smaller than a conventional exterior diaphragm. In case of the exterior diaphragm, out-of-plane deformation of steel tube wall occurs. Accordingly, the methods to evaluate elastic stiffness and yield strength of beam-to-column connection are necessary to design the connection. In this study, we focus on tension sides of moment connection where tensile force is transmitted from a beam flange. To evaluate elastic stiffness and yield strength theoretically, only the beam flange connection is picked up and a mechanical model consisted of two parts, i.e. "exterior diaphragm model" and "column model", is used. The exterior diaphragm model is made of elasto-plastic wire elements having the same mechanical properties as diaphragms, and the column model is made by rigid-body spring model of the column. Then, we consider that elastic stiffness of the beam flange connection occurs under same deformations of two parts, and presume that elastic stiffness of the connection is equal to the sum of stiffness of two parts. We consider yield strength of the beam flange connection as the sum of resistant forces of two parts at the instant when the diaphragm model yields under bending moment and shear force, or axial force, while the column model is still under elastic range. FEM analysis and loading tests are conducted on beam flange connections, and calculated values of elastic stiffness and yield strength were compared with results of analysis and tests. Investigated parameters are the thickness and the width of columns, width of beam flanges, and the thickness and depth of the projection of diaphragms. The beam flange is subjected to force in one direction, and elastic stiffness and yield strength of beam flange connections were obtained. As a result, it is confirmed that calculated values correspond with results of many analysis and tests well and the validity of the proposed method is confirmed.

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