Seismic Behavior Of Steel Frames With Novel Types Of Unbonded Steel Brace Encased In Panel

Unbonded steel plate brace encased in panel (panel BRB) is a typical type ofbuckling-restrained brace (BRB) in which the panel is used to provide restrainingaction to the encased brace. Unbonded materials or gaps are generally employedbetween outer panel and inner steel brace. The inner steel brace resists all of thestory shear force applied to a panel BRB, and the outer panel is only used to preventthe inner brace from large amplitude overall or local buckling. Therefore, the innersteel brace in a panel BRB can yield in axial tension and compression, and thecompressive strength and energy dissipation capacity of steel brace can be usedefficiently. The panel BRBs can work well in anti-seismic structures and the panelscan also work as partition walls. Thus, the prospect for applications of panel BRBsis good.Nowadays, the studies on the seismic performance of the panel BRBs are lesspopular than those of the bar shaped BRBs. According to disadvantageous aspects ofthe steel frames braced by the panel BRBs (panel BRBFs), quasi-static tests forthree panel BRBFs with one span and two stories were conducted. Furthermore, theseismic performance of ten-story and thirty-story panel BRBF models was studiedand therefore seismic design methods and suggestions were proposed.For the preparations of the tests, two type panels with good performance wereadopted to avoid bending cracks or punching shear failure occurred in reinforcedconcrete panels. One type is the profiled steel sheet-concrete composite panel, inwhich the profiled steel sheets with stiffening ribs replace steel bars used inreinforced concrete panels, act as components to resist tension forces and enhanceflexural capaciy and ductility of panel. Moreover, the punching shear capacity of thecomposite panel can be improved by perforated channels. The other type is thelight-weight steel panel assembled by high-strength bolts, self-drilling screws, etc.The lateral restraining actions of panel were provided by the steel tube welded withthe steel plate with bolt holes. This assembled steel panel can reduce the weight ofpanels and facilitate the fabrication and installation of panel BRBs. Also, it isbeneficial to control the initial crookedness of brace, as well as the gaps betweenpanel and brace. Thus, the assembled panel has good performance.Quasi-static tests for a rigid frame braced by diagonal panel BRBs, a hingedframe braced by chevron panel BRBs and a rigid frame braced by chevron panelBRBs were conducted. The effects of the construction details, including theconnections between braces and frames, the connections between beams andcolumns, the shape of braces and the type of panels, on the hysteretic behavior of the panel BRBFs were examined. The tests reveal that, compared with the diagonalpanel BRBs, the chevron panel BRBs have higher lateral stiffness when the heightand span of the panel BRBFs are the same. Compared with the profiled steelsheet-concrete composite panels, the light-weight assembled steel panels exhibitedbetter performance. Especially, the assembled panels were reused for the tests.When connecting a brace to a column, the force transfer route is direct and thedesign of a connection is simplified because inner forces in the beam are reduced.The hinged frames braced by chevron panel BRBs have smaller inner forcescompared with the rigid frames braced by chevron panel BRBs when the lateraldisplacement is the same. During the tests, beams and columns in the hinged framedo not have noticeable failure phenomena.The tested panel BRBFs have been simulated by the ANSYS program in whichthe material and geometrical nonlinearity are modeled in order to examine thehysteric behavior of the panel BRBFs and the interaction between the braces and theframes. According to the analysis results, the simplified simulation method for thepanel BRBs has been investigated. The hysteretic curves acquired from the analysis,which are composed of horizontal load and displacement at the top floor, have goodagreement with those from the tests, indicating that the simplified simulationmethod is acceptable and it can provide a reference for the elasto-plastic seismicanalysis of the steel frames braced by the panel BRBs.The seismic performance of five ten-story chevron panel BRBFs and threethirty-story chevron panel BRBFs has been investigated by the ANSYS program inwhich the material and geometrical nonlinearity are taken into account, and in eachstructure hinged connections are used for two ends of the beams braced by chevronBRBs. The analysis results indicate that all ten-story chevron panel BRBFs can meetthe limit of inter-story drift under frequence excitation of the earthquakes. Undersevere excitation of the earthquakes, all ten-story chevron panel BRBFs can meetthe limit of inter-story drift. Except for the structure in which hinged connectionsare used for all beams, the other ten-story chevron panel BRBFs can also meet thelimit of inter-story residual drift. All thirty-story chevron panel BRBFs can meet thelimit of inter-story drift under frequence excitation of the earthquakes. Under severeexcitation of the earthquakes, all thirty-story chevron panel BRBFs can meet thelimit of inter-story drift. In general, the thirty-story chevron panel BRBFs designedas dual systems can also meet the limit of inter-story residual drift.The seismic analysis for chevron panel BRBFs indicates that the inter-storydrift, as well as the inter-story residual drift, decreases but the amount of steel usedincreases with section areas of the frame members increasing. Considering thefabrication cost of connections and the amount of steel used, a structure composedof hinged frames braced by the panel BRBs and rigid frames can be used in medium and low-rise structures, in which the requirements of dual systems are not necessary.As to high-rise buildings or buildings with strict requirements for the inter-storydrift, it is suggested that rigid frames braced by the panel BRBs be designed as dualsystems. In other words, rigid frames in a structure designed as a dual system shouldindependently resist25%of the total base shear force of the structure, which can beapplied to the rigid frames as lateral distributed forces or a lateral concentrated forceat top of the frames.