Special Truss Moment Frame Component Test 9 - 2C12x20.7

Special truss moment frame component test 9, made of double channels 2-C12x20.7, was tested under the same cyclic history loading as experiment 4. The test setup was still a representative of half of the chord in the special segment of the prototype special truss moment frame. In experiment 4, web bulging was heavily concentrated between the first two pairs of the web stiffeners which was the location of the plastic hinge. In this experiment, the third pair stiffeners were removed and the second pair stiffeners were placed in the middle between the end of the horizontal weld and the first pair of the stitches. This resulted in spacing of 4.25 in. between the first and the second pair of the stiffeners. This larger spacing between the stiffeners aimed to relax the unbraced length of the flanges between the first two pairs of stiffeners that caused premature failure in experiment 5. One noted change in this specimen was the presence of the vertical members welded to the gusset plate and the bottom flanges of the specimen which represent a welded connection between the vertical members near special segment and the chord members of the prototype STMF. The number of horizontal stitches was one pair placed at 1 in. away from the edge of the gusset plate. The other stitches were conventional. Weld crack started early in this experiment at 1% story drift where the vertical member and the specimen were welded. Crack propagation progressed slower than in experiment 5 but the bottom flange and web of the east channel fractured at 1.5% story drift which was equivalent to 4.19% member drift, the same as that of experiment 5. The hysteretic behavior was also identical to that of experiment 5. Premature fracture was believed to be a result of the restraint on the channel flanges between the stiffeners. Even though the unbraced length of the flanges was relaxed but the vertical members welded to the bottom flanges of the specimen along with the first pair of the stiffeners may have rigorously hindered the plastic flow.