On the importance of 3D analysis for zipper frames structures

The theoretical failure mechanism in a suspended K braced frame (also named "zipper frame") is the successive buckling and yielding of braces of higher stories after the buckling and yielding of braces at first story level until reaching an upper story purposely designed to remain elastic. The success of such mechanism depends on the capability of the "zipper column" to transmit to the upper floors the unbalanced vertical force generated in a buckled story. A shaking table test of a 3 stories suspended zipper frame model was performed at the University at Buffalo NEES facility. Analytical predictions from 2D models showed the development of the described theoretical failure mechanism. The test however, showed that the braces buckled out of plane and induced torsional moment and rotations in the beam above the braces, which was not restraint to prevent torsion. More importantly, measurements in the zipper column did not record any significant stresses. The preliminary conclusion from the initial test is that if the braces buckle out of plane and the beam twists because it is not braced properly, the theoretical failure mechanism of the zipper frame may not develop. At the moment of preparing this manuscript, one more test was performed (with lateral restraint) and a second is planned without torsional beam restraint, to verify and confirm the preliminary conclusion. Results of those tests are presented which are helpful identify the importance of considering out-of-plane behavior in the design of planar frames with braces and to provide recommendations for the restraint design of the beam.