【摘要】 本文简要地报导了47个高配箍率的方格网套箍混凝土的局部承压强度试验。试验的主要参数为:局压面积比、配箍率、套箍指标、试件的相对高度和局压方式。试验结果表明,套箍混凝土局压强度提高系数与局压面积比之间的关系服从众所周知的平方根规律;在高配箍率的情况下,现行规范公式的计算结果,有可能不安全。文中给出了适用于各种配箍牢的局部承压强度计算公式和修改补充现行规范条款的建议。更多还原

【Abstract】 Large concentrated forces acting over limited contact areas of concrete is frequently occured in civit engineering practice. Provision of lateral confinement steel can improve greatly the ultimate bearing strength of concrete. The current Chinese Code provisions for predicting bearing strength of such confined concrete are proposed and based on the experiment performed in the 50-60’s with lower volumetric ratio of confinement steel （less than 3%）. However, in recent years, there has been a rapid development of confined concrete with higher volumetric ratio of lateral reinforcement due to construction of high-rise building columns and increasing usage of large post-tensioned tendons （especially unbonded tendons）in some fields such as the nuclear reactor pressure vessels and containments etc. As loading cases are concerned （Fig.1）, the previous experimental investigations were concentrated on central loading only and the case of strip loading has not been studied.In the present paper, in order to suppement existing information on the subject, as well as to verify the Code provisions, an experimental investigation into the influences of lateral steel amounts and loading cases on the bearing strength of confined concrete has been conducted. Formulas for predicting the ultimate bearing strength of confined concrete with various volumetric ratio of confinement steel are suggested and recommendations for improving the Code provisions are also presented.The specimens were all 350 mm square prisms. The main experimental parameters studied were: the ratio of total area to bearing area, the volumetric ratio of reinforcement, the confinement index and the relative height of specimen. The details of reinforcement are given in Fig.2 and Table 1.Typical load-displacement curves of bearing plates are shown in Fig.3, from which it can be seen that the confined concrete blocks under concentrated loading behaved in a ductile manner at failure. The measured ultimate loads and first cracking loads are listed in Tables 2 and . The tests showed that the influence of relative height of specimen on the ultimate bearing strength is not pronounced. Provision of net reinforcement has some beneficial effect on the cracking resistance of specimens.The position of the first cracks is shown in Fig.4. The general patterns of cracks and modes of failure are shown in Fig.5. It can be seen clearly that in the case of strip loading, the pattern of cracks coincides more or less with the pressure spread lines.The ultimate bearing strength ratio is determined by equation（1）. The ultimate strength of confined concrete is computed by formulas（2） and（3）. It can be seen from Fig.6 that the measured ultimate bearing strength ratio for confined concrete in both central and strip loading cases is directly proportional to the square root of the bearing area ratio, A_cor/A_b, i. e. the increase in ultmate bearing strength with bearing area ratio for both plain and confined concretes obeys the well-known square-root law. Thus the ultimate bearing strength ratio for confined concrete with net reinforcement can be predicted by the formula （4）.For calculating the ultimate bearing strength of confined concrete with various amounts of transverse reinforcement, the formulas（5） and（3） or formulas（7） and（3） are proposed. The confining effect factor, α, computed by Eq. （7） with experimental results show satisfactory agreement, with that depicted by formula（3） （Fig.7）.For all 47 tests the average ratio of ultimate load observed and computed by Eqs.（7）and（3） for both central and strip loading cases are 1.08 with coefficient of deviation 0.106 and 0.061 respectively（see Tables 2 and 3）.The formula（7） is formally identical to that proposed in the earlier paper and adopted first in the previous Chinese design Code and subsequently in the current design Code, the factor of confining effect, α, being α constant of 1.5 and 2 respectively. It is to be noticed that the current Code formula taking α=2 may be unsafe in the case of higher volumetric steel ratio or higher confinement in更多还原