STMF 1.1 - Cyclic Loading Test of Special Truss Moment Frame with Double-Channel Chord Members

The chord members of STMF 1.1 were double channels (2C8 x 18.75). 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) and the clear spacing of stitch was 20" which was greater than AISC 341-10 requirements of 13.9 in. 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 proposed new configuration.
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. STMF 1.1 exhibited stable and ductile behavior up to the 1st cycle of the 3% story drift. Moderate yielding at each end of the chord members in the special segment was observed at the end of the 1% story drift. Strength of STMF 1.1 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 and propagated into the chord members. By the 2nd cycle of 3% story drift, fractures at plastic hinges were so severe that some members were completely torn at the plastic hinge. As a result, strength of STMF started to drop and chord members in the special segment started to twist. At 4% story drift, the majority of the chord members in special segment were torn and the capacity of STMF drastically dropped to approximately 20% of the peak strength. The specimen first yielded at about 0.5% story drift, which indicates that its overall stiffness was not compromised by increasing the aspect ratio. Preliminary results showed that the obtained Vne was close to the one predicated by AISC 341-10.