Evaluation Of The Performance Of Buckling Restrained Brace Encased In Only Steel

The restraining member frequently proposed in previous studies was a mortar-filled steel section, which made an extremely rigid member. In such types of BRBs, the brace member and the buckling restrained mechanism were integrated and overall buckling did not occur. However, in all-steel BRBs, which are considered to be a new generation of BRBs, the brace system is completely made of steel, and the Buckling Restrained Mechanism (BRM) system is lighter in comparison with conventional BRBs, which leads to a high potential for brace overall buckling caused by the low rigidity and stiffness of the BRM. In common all-steel BRB, the steel inner core is sandwiched between buckling restraining mechanisms made entirely of steel components, thus avoiding the cost of mortar needed in conventional BRBs. This eliminates the fabrication steps associated with pouring and curing the mortar or concrete, significantly reducing manufacturing time and costs. In addition, such a BRB can easily be disassembled for inspection after an earthquake.

Furthermore, Usami et al. (2008) claimed that when a mortar filled BRB is applied to a bridge structure, it inevitably becomes large-scale compared with the frame structure of buildings, accordingly the influence of the deflection by the dead weight of that BRB grows and the construction becomes difficult. They believed there was need for the development of light-weight BRBs. In that case, however, the examination of overall buckling becomes indispensable.

BRBs composed of only steel members without filling mortar was proposed, and its performance experiment conducted. The examination of prevention of buckling and the strength of the restraining member was the main study criteria for Usami et al. (2008). To obtain the hysteretic characteristic on the compression side similar to the tension side, a condition of not causing overall buckling (flexural buckling) on the restraining member together with the brace member is necessary. The target maximum deformation of the brace member (ratio of axial displacement and yield displacement, δ/δy) was set to be 20.0 (almost 3% of strain in the case of a mild steel SS400), and the condition of not causing overall buckling before the target maximum deformation was examined. In the study, cyclic loading tests were carried out on four test specimens for the verification. Of which, three specimens used flat bar type (hw = 0) restraining member and one used a T-type restraining member. The test specimen was connected by bolts with an angle of 45° between the perpendicular pillar of the pin support (height =1.9m from the pin) and the plinth without causing the eccentric axial force as much as possible.

In this experiment, the actuator was controlled by the axial displacement δ of the test specimen, and the gradually increasing cyclic loading of tension and compression alternating was performed. The loading pattern began from 0.5 δy, and increased evenly by displacement increment; for each amplitude one time of cyclic loading on both sides was performed.

After the experiment was carried out, the results were as follows; among the four test specimens, two of the flat bar restraining member (plate thickness = 10.2, 12.3mm) with low strength and rigidity experienced overall buckling failure under compression. The other test specimens did not experience overall buckling until reaching the target ductility (δ/δy= 20.0). The strength and energy absorption ability decreased rapidly by overall buckling, and the deformation performance under tensile load was also influenced.

Experimental Results

One of the required performances of the buckling restrained brace (BRB) is the performance of not causing overall buckling until the brace member reaches enough plastic deformation. This required performance becomes important as BRB is lightened, and the strength and rigidity of the restraining member becomes lower. Furthermore, Hoveidae et al. (2012) carried out more research on the all-steel type of Buckling Restrained Brace with the proposed BRBs having identical core sections but different buckling restraining mechanisms (BRMs). The objective was to conduct a parametric study of BRBs with different amounts of gap (between the core and the BRM) and initial imperfections to investigate the global buckling behaviour of the brace. The finite element analysis method has been used to predict the buckling response of the core plates in BRB members with tubes filled with mortar. Subsequent studies have been conducted by Tremblay et al. to investigate the core buckling behaviour in all-steel BRBs. These studies also provided a description of the complex interaction that develops between the brace core and BRM.

Hoveidae et al. investigated the finite element analysis studies of overall buckling behaviour of all-steel BRBs regarding the effect of the gap amplitude between the core and BRM and the initial imperfection of an entire BRB member. Numerical studies were conducted on 13 proposed all-steel BRBs. All models consisted of a constant 100×10 mm2 core plate with various cross sections for BRM members. The yield strength of the core was kept constant when the stiffness and strength of the BRMs were altered. The effect of the generation of a gap between the core and the BRM was considered in the analysis. The total length of the BRBs, L, was assumed to be 2000 mm. The core plate's yield load, Py, was calculated by multiplying the yield stress by the cross-sectional area, and the buckling load of the BRM, Pe, was calculated from the Euler buckling load formula.

An analysis of the elastic buckling of a composite brace composed of a steel core encased by a restrainer showed that the critical load ofthe entire brace member under compression could be found by solvingan equilibrium equation as follows: EBIB* d2v dx2 þ v þ v0 ð ÞNmax ¼ 0 ð2Þin which EBIB is the flexural stiffness of the BRM, Ny represents the brace yielding load, and v and ν0 denote the transverse and the initial.

Connections

The purpose of buckling-restrained braces is to dissipate lateral forces from columns and beams. Therefore, the connection of the braces to beams and columns can greatly affect the performance of the brace in the case of a seismic event. Typically, the brace is attached to a gusset plate, which in turn is welded to the beam and/or column that the brace will be attached to. Usually three types of connections are used for BRBs:

• welded connection – the brace is fully welded to the gusset plate in the field. Although this option requires additional man-hours on-site, it can increase the performance of the brace itself by improving the force transfer mechanism, and potentially lead to smaller braces.
• bolted connection – the brace is bolted to the gusset plate in the field.
• pinned connection – the brace and gusset plate are both designed to accept a pin, which connects them to each other and allows for free rotation. This can be beneficial to the design engineer if he or she needs to specify a pinned-type connection.

In addition to the connection type, the details of the connection can also affect the transfer of forces into the brace, and thus its ultimate performance. Typically, the brace design firm will specify the proper connection details along with the brace dimensions.