Local and lateral torsional buckling of 36 in. deep beams with IMF end-plate moment connections

Previous testing has shown that four-bolt extended unstiffened end-plate moment connections can be used with beams up to 24 in. deep and develop sufficiently ductile performance to satisfy seismic qualification. It is desirable to extend this depth limit to allow deeper beams for intermediate moment frames. Three moment connection specimens using built-up 36 in. deep beams with a four-bolt extended unstiffened end-plate moment connection, were tested to determine if they satisfy the intermediate moment frame qualification criteria given in AISC 341. The web of the specimens satisfied the moderately ductile section criteria for web slenderness, which is required for intermediate moment frames, but not highly ductile section criteria required for special moment frames. All three specimens passed qualification criteria for intermediate moment frames (IMF) as given in AISC 341-16 by retaining at least 80% of the nominal plastic moment strength through 2% story drift. The observed failure modes included lateral torsional buckling of the specimen, flange local buckling, fracture of the flanges, and failure of the flange-to-web fillet weld. The results of these tests support current moderately ductile limits associated with lateral bracing and cross-section slenderness given that the associated specimens reached IMF qualification criteria, but not special moment frame (SMF) criteria. It was concluded that four-bolt extended unstiffened end-plate moment connections satisfy intermediate moment frame requirements with a beam depth of 36 in. and that a modification is needed for beam web to flange welds in built-up sections near the moment connection.

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