STMF 3 - Cyclic Loading Test of Special Truss Moment Frame with Double-HSS Chord Members

The chord members of STMF 3 were double-HSS channels (2HSS8x4x1/2) while the web members outside special segment were 2HSS5x5x9/16. The overall length (center-to-center of the columns) was 31'-9". The special segment length was 9'-11" long and the depth of the truss was 4'-0". The aspect ratio was approximately 2.5 (greater than the upper limit in the AISC 341-10). The connection detailing of the chord members in the plastic hinge regions, which were located at the ends of the chord members in special segment, was done according to the component test specimen. Unlike STMF with double-channel chord members, it did not require extended weld-free gusset plate or horizontal stitches because there was no lateral torsional buckling.

The STMF specimen was cyclically loaded according to a loading protocol similar to the AISC 341-10 loading sequence for beam-to-column moment connection. Hysteretic behavior of STMF 3 exhibited stable and ductile behavior up to the 1st cycle of the 3% story drift. Strength of STMF 3 increased but stiffness decreased from 1.5% to 3% story drift due to the increasing intensity of yielding as well as fractures initiated at the end of the welds. By the 2nd cycle of 3% story drift, fractures at plastic hinges were so severe that strength of STMF 3 dropped noticeably. At 4% story drift, the chord members in special segment were torn across the flanges and most part of the webs. Consequently, the capacity of STMF 3 drastically dropped to approximately 20% of the peak strength.