Maurya and Eatherton 2015
The authors examined the performance of a self-centering beam (SCB) designed to retain the advantages of a typical self-centering system while improving upon some of the limitations. Large scale experiments were conducted on the SCB in order to determine the effectiveness of the post-tensioning (PT) strands as a self-centering mechanism and the replaceable elements meant to dissipate seismic energy.
System Concept
The SCB is designed to minimize, and if possible eliminate, residual drift and concentrate structural damage in replaceable elements during large earthquakes, while being shop fabricated to allow for conventional field construction. To accomplish this, an outer tube of a HSS section is welded to the bottom flange of a W-section beam, so that it moves laterally, in unison with the beam. The inner tube of the HSS section is connected to columns so that it telescopes as the columns rotate due to story drift. PT strands encourage the alignment of these two tubes. Free floating anchorage plates at the ends of each tube, held in place by the PT force, slide over the end connections. Under lateral loading, a gap opens between the tubes and plates on either end. The energy dissipating fuse is connected to the inner tube on one side and the outer tube on the other. Telescopic movement of the tubes cause the bars to axially deform resulting in the dissipation of energy.
Experimental Study, Results and Discussion
Three 2/3 scale SCBs were subjected to a lateral load corresponding to a 9% story drift while a gravity load simulator applied a constant gravity load to the system at 1/3 points. One of the SCBs (SCB-2) displayed negligible residual drifts at zero force even after loss in self-centering capacity. The other SCBs (SCB-1 and SCB-3) exhibited small drifts at zero force and did not have full self-centering. Each SCB was subjected to a 6% story drift without causing any noticeable damage to the system, columns or connections. They all displayed adequate initial stiffness and high deformation capacity.
Fuses in the SCBs displayed stable and full hysteretic behavior. SCB-1 and -2 are smaller beams and had smaller deformations in their fuses compared to SCB-3. Only during the 8th cycle of 6% drift (after which the tests were stopped) did SCB-2 experience a decrease in strength associated with local buckling. However, the fuse in SCB-3 saw a 25% decline in strength at the start of the 2nd cycle of 6% drift and the fuse in SCB-1 experienced a decrease in strength during the 4th cycle. Overall, the fuses used in the SCBs displayed 20-35% higher peak strength in compression compared to tension cycles.
SCB-1 was post-tensioned to 41% of the ultimate tensile strength of the PT strands and began to yield during the 6% drift cycles. SCB-2 was post-tensioned to a higher level of initial PT stress and therefore yielded earlier than SCB-1. This resulted in a greater loss in the PT stress which contributed to the decrease in the self-centering capacity of the SCB.
Reference
Maurya, A., and Eatherton, M. R. (2015). “Large-Scale Experimental Results for Self-Centering Beams with Resilient Seismic Performance.” Structures Congress 2015, 1326–1337.