Wang, Nishiyama, Zhu, Tani, and Jiang 2021
Components of the self-centering (SC) steel coupling beams are experimentally tested individually in order to determine the behavior of the components in tension and compression, then this data was used to model a SC steel coupling beam in finite element analysis. Four different finite element systems with varying prestress of shape memory alloy bolts and steel angle sizes were modeled and tested cyclically to determine the effects of these changes on the behavior of the SC steel coupling beams.
System Concept
Self-centering (SC) steel coupling beams are used in structures with reinforced concrete (RC) walls in order to provide a self centering force to the system under seismic loads. The SC steel coupling beam consists of two elastic segments connected to either wall and joined in series with a rocking segment in the middle. The rocking segment remains stiff under lateral forces and is connected through bolted connections to allow for opening and closing gaps on either side of the rocking segment. The beams employ shape memory alloys (SMA) for use in the dog-bone shaped bolts connecting the elastic component of the self centering beam and the rocking component of the beam, which contract to an original position after loads are applied. The rocking segment is also connected to the elastic segment through steel angles, which deform under seismic loads and act to restrain movement of the rocking component of the SC steel coupling beam.
Lateral forces are resisted by the SC steel coupling beam through the use of the SMA bolts. The bolts will deform under lateral loads when a gap opens between the rocking and the elastic segments of the beam. As the gap opens, the SMA bolts restrict movement by contracting and closing the gap due to the shape memory components. Likewise, the steel angles act to maintain a static angle between the elastic and rocking component of the SC beam, which provide an additional energy dissipating component to the beam. Using both SMA bolts and steel angles, along with shear keys to prevent beam elongation under cyclic loading, provide a restoring force that acts as a self centering component of a structure under seismic loading and dissipate seismic energy.
Experimental Study, Results and Discussion
Individual testing was performed for each of the components of the SC steel coupling beam. First, the dog bone shaped SMA bolts were tested in a standard tensile test, in which they were able to exhibit stable hysteretic responses up to a 7.3% strain with no fracture. Afterwards, the steel angle was tested under tensile and compressive loads. The steel angles deformed at three plastic hinges, and withstood a maximum tensile load of 15.7 kN and a maximum compressive load of 11.2 kN.
After loading the individual components, a SC steel coupling beam was modeled using the information from the specimen tests. The design system utilized ABAQUS to model the individual components using the parameters from the component testing, and then test the entire system under cyclic loading up to a target rotation of 0.08 radians. The SMA bolts were loaded with a prestrain of 1%, or a load of 50 kN. The results indicate a flag shaped hysteretic curve, which shows stable responses of the system under cyclic loading without any significant permanent deformation. Any deformation or fracture in the steel angles can be easily replaced following loading by unbolting the damaged component and replacing it with a new one. The beam also demonstrated negligible beam elongation under the cyclic loading.
Four additional finite element models were formed after the initial test in order to determine the effects of the prestrain level of the SMA bolts and the size of the steel angles. All four specimens demonstrated flag shaped hysteretic responses. It was found that wider steel angles increased energy dissipation, but decreased self centering. It was also observed that the SMA bolts must be less than the starting strain of forward transformation in order to ensure a balance between lateral force, self centering, and energy dissipation.
Reference
Wang, B., Nishiyama, M., Zhu, S., Tani, M., and Jiang, H. (2021). “Development of novel self-centering steel coupling beams without beam elongation for earthquake resilience,” Engineering Structures, 232. pp. 1-13.