Legeron, Desjardins, and Ahmed, 2014
A concentrically braced frame consisting of diagonal cross bracing is equipped with steel fuses at different connections between the braced members. In this study, eight different cross sections were studied under tensile forces to determine if the steel fuses functioned to absorb tensile stress, while limiting the reduction in compressional strength of the system.
System Concept
The system described in this paper involves a concentrically braced frame in which the system deforms according to the tensile and compressional forces applied to the system. In order to achieve this effect, the resistance of the bracing must be greater than the strength of the members, such that the brace deforms prior to the deformation of the steel members. In order to achieve this effect, a fuse is connected to different bracing connections in order to deform and alleviate the plastic deformation of the connection itself.
The fuses in this study are connected in different locations to the legs of the frame. The fuse system is tested by loading eight different concentrically braced frames in order to determine which orientation will absorb the most seismic loading and prevent inelastic system deformation, while limiting reduction in compressional strength. The fuse is connected to a section of the angled members and functions as the strengthening component of the system. The goal of the fuses is to absorb the seismic energy in order to prevent deformation of the bracing of the system in order to concentrate deformation of the system into a ductile, replaceable component.
Experimental Study, Results, and Discussion
Eight different full-scale test specimens were created with two different angles for the cross bracing on the steel frame. The specimens were loaded by a 500 kN actuator on the top right brace of the system. Strain gauges were also connected to the test specimens in order to determine the axial and out of plane strain on the system, and therefore calculate the force distributions on the frame. The loading protocol chosen for the system was decided upon in order to result in high levels of inelastic deformation during a severe earthquake. First, the systems were loaded with 150 kN of shear force in order to account for the effects of bolt displacement prior to testing. Then, two full cycles were applied to each bracing at each level of displacement at 2, 3,4, and 5 times the yield displacement until the failure of the specimen.
In all experimental specimens, failure occurred at the connections between the cross bracings and the frame, but the failure was often brittle due to the high ultimate strength compared to the yield strength of the system. If there were two fuses placed on both ends of the brace, the tension criteria were met, and the fuse was effective in absorbing seismic energy and deforming in order to increase the tensile strength of the frame. However, these systems also displayed a reduction in compressive strength when the fuse offered an increase in tensile strength at the connections. Those equipped with four fuses did not lose as much compressional strength, but had a lower ultimate tensile strength, but still met all requirements for tension and compression design.
The inclusion of fuses also reduced the tensile properties of the cross bracing of the system, but the fuse system did allow for the desizing of the cross bracing without a reduction in the energy dissipation capacity of the system. Therefore, the fuses allow for the concentrically braced frame system to comply with design requirements with less material than the traditional concentrically braced frame configuration. Likewise, the strength of the bracing components is shown to have little effect on the strength of the overall system, since much of the seismic energy is dissipated in the connections, which is facilitated by the inclusion of the fuses at the connections of the system.
Reference
Legeron, F., Desjardins, E., and Ahmed, E. (2014). “Fuse performance on bracing of concentrically steel braced frames under cyclic loading,” Journal of Constructional Steel Research, 95, pp 242-255.