Experimental and analytical investigation of tubular links for eccentrically braced frames

Eccentrically braced frames have been used as seismic load resisting systems in buildings for more than two decades. Typically, the links, 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. This paper describes first an experimental, and then an analytical investigation into the use of members with hollow rectangular (i.e., tubular) cross-sections as eccentrically braced frame links that do not require lateral bracing. Using cross-sectional plastic analysis, the plastic shear and moment strength for a general tubular section with different web and flange yield strengths and thicknesses are derived. Equations are derived for maximum flange compactness ratio and minimum web stiffener spacing to prevent flange and web buckling. A proof-of-concept experiment involving a large scale eccentrically braced frame with a tubular link is then described. The link has a hybrid tubular cross-section composed of webs and flanges of different thicknesses, with full-penetration groove welds. Experimental results indicate that the link reached a rotation of 0.15 rad, almost twice the current 0.08 rad limit for wide-flange links, prior to suffering flange fracture. An investigation of the fracture surface indicated that flange fracture did not initiate in the full-penetration weld used to assemble the shape, but rather in the heat-affected-zone of the flange adjacent to a fillet weld used to connect a stiffener to the flange. Finally, a finite element model of the link is developed using shell elements, and reasonable agreement with the experimental results is observed.