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Eccentrically braced frames (EBF) by covering the advantages of moment-resisting frames (MRF) and concentrically braced frames (CBF) have been used as seismic load resisting systems in buildings for more than two decades. In eccentrically braced frames (EBFs), the link beams transmit bracing forces through themselves into the columns and other bracings and, in the end, create dominant forces in the bracings. Link beams, similar to ductile fuses, in addition to avoiding bracing buckling, attract earthquake energies. In eccentrically braced system, failure and yielding should happen in the link beams, and other members of the structure must remain in elastic behaviour. On the other hand, link beams prevent transmitting of more forces to the other members by yielding, therefore, link beams are so important. Typically, the link beams, which are relied upon for energy dissipation through inelastic deformation, have had a wide-flange or I-shaped cross-section that requires lateral bracing to prevent lateral torsional buckling. This has limited the use of wide-flanges or I-shaped cross-sections in bridge piers and towers, as lateral bracing is difficult to provide in those situations. Tubular cross-sections of link beams have substantial torsional stability, making them less susceptible to lateral torsional buckling than I-shaped cross-sections in eccentrically braced frames, and may thus not require lateral bracing. Long link beams due to providing proper conditions for the openings performances have architectural advantages. Nevertheless, the behaviour of long link beams within sever seismic loads is not comparable to short link beams in stiffness, strength, rotation capacity and energy dissipation capacity, i.e. it is at lower level. Therefore, using long link beams is not recommended in buildings and particularly besides the columns. In this study, a method is presented for arraying the stiffeners of long tubular link beams to improve the behavior of long tubular link beams in eccentrically braced frames. Long link beams at the distance of 1.5b from both ends of the link beams make flange buckling. Now if this distance strength by any way, the flange buckling delay and the rotation capacity of link beams are increased. For this purpose in this investigation, the stiffeners have been used in the middle of the flange vertically in this diatance and the link beam web has been considered as a stiffener that its distance from the middle stiffeners of link beam flange is 0.5b. In long tubular link beams when the middle stiffeners of the link beam flange do not present, Tension-Field will not create and link beam flange will buckle because of moment. When the middle stiffeners of the link beam flange present at its both ends, then Tension-Field will be created. In this investigation, the formulas presented for determining the stiffener sizing of long tubular link beam flange. In this investigation, non-linear dynamic behavior for 6-stotories eccentrically braced frames with different two length of link beams (shear-flexural and flexural), tubular cross sections with two arraying of stiffeners under the influence of three records of far-fault and three records of near-fault are studied. The result of investigations indicates that flange stiffeners of long tubular link beams have important influence in decrease of displacement demand of eccentrically braced frames that approximately 19% for far fault earthquake records and 32% for near fault earthquake records.

Keywords: Eccentrically braced frame, Tubular link beam, Far and near records of fault, Stiffener

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