Deformation Mechanism And Energy Dissipation Of Origami-basedbuckling-controlled Concentric Braces

A member with a relatively large slender ratio tends to globally buckle under axial compressive load.Commonly,a member with a biaxially symmetric cross-section buckles in a globally flexural pattern.Concentric steel braces usually suffer from stability caused by globally flexural buckling during earthquake actions,which seriously deteriorates its seismic performance.Considering the issue,a particular fold scheme is proposed in this dissertation based on curved Miura origami pattern.Local buckling is allowed to preclude the global buckling under axial compression by applying the scheme on concentric braces.The innovative brace is named as Origami-Based Buckling-Controlled Concentric Brace?OBBCCB?herein and hereafter.Numerical and experimental studies are conducted on the behavior of OBBCCBs under axial compression and reversed cyclic load.The experimental study includes the tests on a basic cell of the brace and a reduced-scale physical model of the brace.First of all,geometric relationship of a curved Miura origami-based tubular cell is derived by applying spatial trigonometry.Calculation equations of vertex coordinates,general closure condition and side lengths and proof of angular relationships are provided.In this way,a process forms via four independent geometric parameters of the cell?mid-height when fully deployed H,side length l,the number of sides n,fold angle?and inclined angle of folds?₁?or?₂??to determine the geometry of a curved Miura origami-based long tube for total length in demand L.Based on the theoretical derivations and the process for geometry determination,the folds are classified to Type?folds and Type?folds by mechanical bending resistance.Type?folds are hinge-like folds bearing no bending,whereas Type?folds have the same bending resistance as the panel.Four fold schemes are developed by assigning the two types of folds to different folds.Non-linear finite element simulations are conducted on the behavior of 3-level curved Miura origami-based tubes adopting the schemes under axial compression.The scheme having the horizontal folds and the inclined mountain folds as Type?folds and the inclined valley folds as Type?folds is distinguished with its greatest axial deformation capacity.The study results show that the scheme yields the greatest deformation compatibility of the panels among the schemes,which leads to 30%-40%greater axial deformation capacity than those of the others.The distinguished fold scheme is applied to OBBCCBs.Considering initial geometrical imperfections,non-linear numerical study is conducted on the behavior of those OBBCCBs using ANSYS.The study investigates the effects of the independent geometric parameters on the behavior of the braces under axial compression.Theoretically derived inertial moment of the brace is adjusted according to the simulation results and the calculation equation of the slenderness ratio is obtained.Also,a calculation equation of the stability coefficient under axial compression is proposed,yielding a stability calculation equation of the brace under axial compression.The study results show that three OBBCCBs?C-ORI-BC1,C-ORI-BC2 and C-ORI-BC3?postpones the ultimate strains to2.38%².74%by effectively controlling the deformation modes through prescribed folds.The braces do not experience instability within limited global buckling,exhibiting superior axial deformation capacity.Considering initial geometrical imperfections,non-linear numerical study is conducted on the behavior of the three OBBCCBs using ANSYS.The study investigates the behavior of the tubes under reversed cyclic load and compares it with that of the conventional square-section braces with the identical length,panel thickness and Euler load.Also,a bilinear simplified skeleton curve with hardening behavior is proposed by utilizing the stability coefficient.The study results indicate that the three OBBCCBs effectively control the deformation modes through prescribed folds.Generally,they achieve stable,symmetric,repeatable load-force hysteresis curves with positive incremental stiffness,accompanied by limited global flexural buckling within the average axial strain?_brw=2.0%.In comparison to the straight square-section counterparts,they show much greater deformation capacity,compression-tension balance and stable energy dissipation.The sensitivity to global flexural buckling is also improved,allowing the OBBCCBs to stay stable with limited global buckling.Three levels of load-carrying capacity are obtained using the same geometric parameters as the three OBBCCBs and setting the panel stiffness hierarchically to 6 mm,8mm and 12 mm.Those OBBCCBs with different load-carrying capacity effectively control the deformation modes through prescribed folds.They deform primarily in centric axis,accompanied by limited global flexural buckling within the average axial strain?_brw=2.0%,exhibiting superior axial deformation capacity.Generally,they achieve stable,symmetric,repeatable load-force hysteresis curves with positive incremental stiffness.Exploration for a feasible construction method was made with an attempt to construct a basic cell of Brace C-ORI-BC2 in an assembly way.Construction methods of Type?folds and Type?folds and the method of the other parts were explored and discussed and the cell was made tentatively.Then,with the code recommended loading protocol for BRB?Buckling Restrained Braces?,experimental study on the cell was conducted to investigate the behavior under reversed cyclic load and to check the feasibility and rationality of the tentative construction method.The study results show that although Type?folds could be constructed by specific grooving on a single face of cold-rolled steel plates and Type?folds by tappingly screwed connecting steel plates,a vertex where three Type?folds and one Type?fold meet was prone to fracture,which seriously deteriorated the energy dissipation of the specimen.Last,by improving the construction of the cell specimen,an updated method was used to make a 1/4-scale OBBCCB by assembling 8 panel groups.Experimental study was conducted on the specimen under reversed cyclic load.The test results show that the specimen could achieve full,stable,repeatable load-displacement hysteresis curves with positive incremental stiffness when the axial strain was no larger than 2.2%,indicating superior energy dissipation capacity.The introduction of a curved Miura origami pattern and the fold schemes decreased the sensitivity to global flexural buckling,allowing the OBBCCBs to stay stable with limited global buckling.