Experimental and Theoretical Studies on Behavior of Unbonded PC Beams at the Ultimate State

本研究包括两个测试系列。A系列试验的目的是考察预应力钢用量、非预应力粘结钢用量、混凝土抗压强度和剪跨长度等基本参数对无粘结预应力钢梁和粘结梁基本抗弯性能的影响。在B系列试验中，以侧约束钢的比例和预应力钢的初始张拉应力为主要试验变量，研究了侧约束对无粘结和粘结梁的极限抗弯强度、预应力钢的最大可得应力和延性的影响。A系列试验结果表明，各试验变量对无粘结钢在最大抗弯强度前的受载应力增量有显著影响，实测应力增量与计算结果吻合较好，当应变协调系数为0.2 ~ 0.3时，随剪跨长度的变化而变化。根据ACI规范的建议，为无粘结梁提供最少量的非预应力钢，被证实对改善极限状态下的抗弯性能和裂缝分布非常有效。高强混凝土的使用可以很好地提高无粘结钢的应力增量和抗弯延性。B系列试验发现，与粘结梁相比，未粘结梁的侧向约束使其最大抗弯强度和最大可得应力显著增加，且随着钢材初始张拉应力的减小，这种趋势更为显著。在未粘结的梁中，施加1%体积的侧约束不能很好地增加预应力钢的应力增量和梁的极限状态强度，但可以显著提高梁的延性。利用上述应变协调系数可以很好地估计最大强度后无粘结梁的性能，包括落支区域。少

The present study consists of two test series. The purpose of series A tests is to examine the effect of basic parameters such as amount of prestressed steel, amount of nonprestressed bonded steel, compressive strength of concrete and length of shear span on the fundamental flexural properties of unbonded beams without bond of prestressing steel in comparison with those of bonded beams. In the series B tests, on the other hand, the ratio of lateral confinement steel and initial tensioning stress of prestressing steel were chosen as the main test variables and then the effect of lateral confinement on the ultimate flexural strength, maximum attainable stress in prestressing steel and ductility of unbonded and bonded beams were studied.The results of series A tests showed that the stress increment of unbonded steel due to applied load up to the maximum flexural strength is significantly affected by all the test variables and that the measured stress increment agreed well with the calcula … More ted one when the strain compatibility factor was assumed to be 0.2-0.3, depending upon the shear span length. The provision of the minimum amount of nonprestressed steel for the unbonded beams, as recommended by the ACI Code, was confirmed to be very effective for improving the flexural properties at the ultimate state as well as the distribution of cracks. Use of high- strength concrete could increase well both of stress increment of unbonded steel and flexural ductility. And then, it was found from series B tests that the maximum flexural strength as well as the maximum attainable steel stress was significantly increased due to lateral confinement in the unbonded beam compared to bonded beam and that such tendency became remarkable with decreasing initial tensioning stress of steel. In the unbonded beams, the provision of lateral confinement with 1% by vol. could not well increase the stress increment of prestressing steel and the beam strength at the ultimate state but significantly improve the beam ductility. The behavior of unbonded beam including the falling branch region after the maximum strength could be estimated well using the strain compatibility factor as mentioned above. Less