Dolce et al. 2004
In most seismic regions, buildings have very low resistance capabilities. Many RC framed buildings in seismic areas were designed with no seismic criteria, considering only gravity loads. One approach to this problem is applying passive control mechanisms to existing structures. In this paper, a new technique to seismically upgrade a RC frame with SMA re-centering and dissipating braces is evaluated. Analyses of the experimental outcomes, including seismic measurements, are presented.
System Concept
The upgraded structural system consists of four special braces with SMA and NiTi properties increase energy dissipation of the braces. The SMA and NiTi wires are used in their austenite phase so as to exhibit superelastic characteristics and fatigue resistance fat large deformations. The SMA based devices transform any deformation of the brace through tensile deformation of the SMA wires. Wires are pre-stressed to give the brace a high initial stiffness. The tangent stiffness of the device depends on the stiffness of the SMA wire group when an external force becomes greater than the pre-stress force. This wire arrangement yields nonlinear behavior and a great re-centering capability. Moreover, austenitic wires can also dissipate energy, increasing the global damping.
Experimental Study, Results and Discussion
A 2-story, twelve bay existing building was used for the experimental testing. In order to avoid any interaction with the structural elements, all internal and external infill masonry panels were demolished. The twelve bays were divided into individual modules so that single bay structures could be tested for different retrofitting methods and configurations. The module, in the presented test in the paper, was strengthened with the SMA bracing system in its weak direction.
Pushover analyses were conducting using the DRAIN-3DX finite element program in order to evaluate the number of wires required in each brace. The final number of 1 mm diameter SMA in each device was found to be 64. Moreover, the length of the wires was also calculated from elongation of the braces and their deformations.
Cyclic dynamic and dynamic release tests were performed on the retrofit structure.
From tests results, there was some small energy dissipation in the structure, due to both the characteristics of the SMA devices and flexibility of the RC frame, despite high imposed displacement. The structure exhibited excellent re-centering capacity and no residual deformation when the load was removed.
Reference
Dolce, M., Cardone D., Marnetto R., Mucciarelli M., Nigro D., Ponzo F.C., and Santarsiero, G. (2004). “Experimental Static and Dynamic Response of A Real R/C Frame Upgraded With SMA Re-Centering and Dissipating Braces,” Proceedings of the 13th World Conference on Earthquake Engineering, Vancouver, B.C., August 1-6.