Pinching-Free Connector in a Self-Centering Braced Frame

Three desirable features of a seismic-resisting system's hysteresis are ductile behavior, consistent dissipation during loading cycles, and negligible residual displacements after earthquakes. A self-centering, pinching-free connector (PFC) was developed to attempt to meet these demands. The tension-only PFC uses a linear ratchet made from unified/metric screw threads to absorb the slack generated when a dissipator deforms/elongates monotonically. Nondestructive ratcheting tests were conducted to support the equations derived to predict the ratcheting behavior. Although the PFC can recenter unimpeded during tension (half) cycles, the ratcheting persisted on the compressive stroke during full tension-compression cycles and subsequently hindered its ability to recenter. Therefore, additional hook and ratchet stoppers were incorporated into the PFC to control the ratcheting and allow recentering from any position. This was verified in a preliminary test of a diagonally braced frame that demonstrated self-centering when unloaded. An incremental numerical procedure was able to predict the cyclic response of the frame satisfactorily from just the backbone curve of the PFC, which is mostly a reflection of the dissipator's backbone. This paper highlights an alternate means to self-centering dissipative systems through a linear ratchet that precludes the need for buckling restraints or substantial prestressing/poststressing. One implication is that very slender braces could be used in moderate-to-high seismicity regions because the pinching is no longer an issue and the dissipative potential is restored for monotonic dissipators.