A Model for Increasing the Distances and Decreasing the Dimensions of Stiffeners for Tubular Link Beams in Eccentrically Braced Frames

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 EBF 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، these 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 eccentrically braced frames 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. Web of I-shaped cross-sections because of having clamped boundary conditions in its four sides than web of tubular cross-sections that have simply supported boundary conditions in its four sides، having advantage. Web buckling of link beams in eccentrically braced frames cause rapid strength and stiffness degradation، and this significantly impedes the energy dissipation capabilities of the system. Web stiffeners can be used to delay web buckling beyond a certain rotation level. Stiffener spacing of link beam is of important causes for the delay of web buckling. Stiffener spacing is based on the boundary condition of web that is between the flanges and the stiffeners. In this study، link beams with different tubular and I-shaped cross-sections in non-elastic limits are investigated and has been tried by changing link beams stiffeners array cause increasing the distances and decreasing the dimensions of them. For this purpose nonlinear finite element software and AISC-2005، loading protocol are used. The numerical result ofdifferent link beams models indicate that tubular and I-shaped cross-sections if having same geometric profile، I-shaped cross-sections have higher rotation capacity because ofbetter performance of their web، this increasing is approximatelly twice. So study of results indicate that new arraying of stiffeners of tubular link beam inceases the distances of them approximatelly 30% and decreasing the dimensions of them approximatelly 15% anddoesn’t have any bad effect on rotation angle capacity of them. An investigation of the effect of stiffener dimensions indicated that stiffeners dimensions if having minimum area and moment of inertia requirements don’t effect on rotation angle capacity.