| Modern high-rise buildings, especially super high-rise buildings are emerging toshape the skylines of our modern cities due to fast economic development. In thesehigh-rise buildings, however, axial forces in the lower columns, besides shear walls, areincreasing significantly and the plastic deformation capacity of such columns may bereduced remarkably as a result. In practice, limiting the axial force of the columns tosome extent, that is, the axial force ratio, is an efficient way to ensure these columns tohave enough plastic capacity during major earthquakes. An immediate result by doingso is the size of the section of the column is increasing, leading the shear span ratio ofthe column less than3, as the height of the column is usually unchanged. Therefore, theso called short columns are inevitably formed as a result. If they are inappropriatelydealt with in proportioning and detailing, the short columns may fail in the manner ofbrittle shear failure. Numerous in-situ earthquake reconnaissances have demonstratedthat such brittle shear failure of short columns is a vital factor to cause severe localdamage, even final collapse of a reinforced concrete building. Therefore, in practicaldesign, effective measures have to be taken to increase the plastic deformation capacityof such short columns alongside the necessary stable ultimate capacity, if the formationof such short columns is enevitale in the sense of geometry. Current studies havealready shown that one of the effective measures is to confine the central core concretein the column by using hoops, especially hoops made with high strength steel.At present, a large amount of experiments of effectiveness of high-strengthhoops/stirrups on seismic behavior of reinforced concrete columns, including shortcolumns, have been carried out home and abroad, a major of them are published inJapanese literature. On this basis, analytical models to predict ultimate shear capacityand design equations have been developed as a result. But up to now, a polular modeland corresponding design equations are still lacking. In this paper, an experimentaldatabase of RC columns was firstly built by collecting the testing data of columnsformally published. Then the most correlated factors influencing flexural and shearcapacities of the columns were figured out. On this basis, comparison and calibration ofthe available analytical models and design equations were undertaken and suggestionson future development were presented.Firstly, a total of865RC short columns, including short columns with high strength stirrups were collected and integrated into the testing database. Then, effect ofsuch parameters as high axial force ratio, yield strength of stirrups, stirrups ratio, onultimate shear capacity, was investigated based on the database. At the same time,correlation between these parameters was analyzed. On this basis, a series of typical testdata were chosen to compare and calibrate the available analytical models and designequations. In particular, the so called Minoru–Minami truss-arch model was undertakenand suggestions on future development were presented.The main contents of research in this paper are as follows.①A total of865RC short columns, with the shear-span ratio be less than3,including short columns with high strength hoops/stirrups were collected and integratedinto the testing database.②Effect of such parameters as high axial force ratio, yield strength of stirrups,stirrups ratio, on ultimate shear capacity, was investigated. Correlation between theseparameters was analyzed.③Effect of such parameters as high axial force ratio, yield strength ofhoops/stirrups, stirrups ratio, on ultimate shear capacity, was investigated. Correlationbetween these parameters was analyzed.④Calibration of different analytical methods of flexural ultimate capacity of RCcolumns, that is, the Japanese superposed strength method, Mander model consideringconfined concrete), was undertaken. Evaluation on these models in predicting theflexural ultimate capacity of columns was presented.⑤Comparison of different analytical models to predict ultimate shear capacity,including the A method and B method on the Japan Design Guidelines of RCstructures based on Ductility concept, Minoru–Minami truss-arch model, MASUOKiyoshi method, and the Nielsen Method was undertaken.⑥Comparison of shear equations of RC columns in different design codes, suchas China design code of reinforced concrete structures (GB50010-2010),TechnicalSpecifications of high strength stirrups in RC structures(Draft), Design code ofConcrete structures of New Zealand, NZS3101－06, ACI318-08, EC8, and JapanDesign code of concrete structures(Arakawa Equation）was presented.⑦Comparison of adaptation of design equations of ultimate flexural capacity indifferent design codes to high strength concrete and high axial compression ratio wasundertaken, the testing data was used for calibration.The main conclusions are as follows: ①The shear capacity increases with the increase of the effective stirrup stress,p w wy.This increasing rate is becoming flat whenp w wyis3.5~4MPa, while the shearspan ratio is1.5. Similar tendency occurs whenp w wyis5.0MPa while the shear spanratio is2.0.②The failure mode is in close correlation with the effective stirrup stress,p w wy.For RC short columns with the shear span ration is1.5, shear failure occurs when theaxial compression ratio is greater than0.5, the effective stress in longitudinalreinforcementpt yis greater than4~5, and the effective stirrup stress,p w wyis lessthan3.5MPa; while flexural failure happens when the axial compression ratio is lessthan0.5, the effective stress in longitudinal reinforcementpt yis less than4, and theeffective stirrup stress,p w wyis greater than3.5MPa.③The superposed strength method of Japan predicts lower ultimate flexuralstrength when high strength concrete is used, while the Mander method obtains muchbetter results for such high strength concrete when the axial compression ratio is in therange of0.2~0.6due to significant effect of confinement.④When high strength stirrups are used, both the A method and the MasuoKiyoshi method predict the shear strength conservatively due to no considerations aretaken to the axial compression ratio and the longitudinal reinforcement ratio. The Bmethod overestimates the shear strength due to no consideration is taken to reduction ofconcrete strength. Both the Nielsen method and the Minoru-Minami truss-arch methodoverestimate the shear strength, while the prediction of the Nielsen method is muchgreater than that of tested results. In general, the Masuo Kiyoshi method and the Bmethod present better predictions.⑤The shear strength predicted by equation (4.5) in the China design code islower due to strict limit on the value of yield strength of stirrups, while that of theArakawa equation of Japan is much better. The technical specifications of high strengthstirrups in RC structures predicts less shear strength.⑥When the axial compression ratio is low, predictions by different design codesare almost the same. The China design code (GB10) and ACI318present lowerpredictiions in the context of high strength concrete and high axial compression ratio. |
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