Design criteria of FRP Reinforced Concrete Flexural Members

某些FRP增强筋的抗压强度和刚度在反复拉伸荷载作用下降低。疲劳寿命受环境温度和重复载荷时间间隔的影响。在设计时应考虑到这些特点。经计算，抗弯承载力的阻力系数约为0.8。然而，仍然存在对失败的安全边际的讨论。本研究未考虑FRP筋抗压强度小于抗拉强度、反复拉伸载荷对抗压强度的降低等问题。特别是在地震荷载作用下，考虑FRP筋的抗压强度，应计算其抗力系数。对网格型FRP加固受弯构件进行了短期加载试验和长期加载试验，得到了使用极限的设计准则。通过对实测挠度和裂缝宽度的评估，可以推导出能较好地再现试验结果的预测公式。当开裂后刚度退化较大时，工作应力将被限制在开裂弯矩水平下。在这种情况下，长期挠度的增加与FRP筋的应力水平成正比。然而，总挠度与跨长相比并不大。裂缝宽度的增加也与FRP筋的应力水平成正比。然而，开裂后立即产生的裂缝宽度相当大。因此，在工作应力水平下，裂缝宽度不能再增加，这是设计准则。在这种情况下，我们必须知道，当裂缝宽度达到使用极限时，FRP加固的应力水平将被限制在相当低的应力水平。

The compressive strength and the stiffness of some FRP reinforcements are reduced by the repeated tensile load. The fatigue life is affected by the environmental temperature and the time interval of repeated load. These characteristics should be taken into consideration of design criteris.The resistance factor for flexural capacity was calculated to be 0.8 approximately. However, there still remains the discussion of safety margin against the failure. In this study, not considered are the smaller compressive strength of FRP reinforcement than the tensile strength, the reduction of compressive strength by the repeated tensile load, etc. Specially the resistance factor should be calculated, considering the compressive strength of FRP reinforcement, when the structure would be subjected to earthquake load.The short term loading test and the long term loading test of flexural members with grid type FRP reinforcement were conducted to obtain the design criteria for serviceability limit. As a result of evaluation of measured deflection and crack width, the prediction formulae, that could well reproduce the experimental results, could be postulated.When the stiffness degrades much after cracking, the working stress would be limited under the cracking moment level. In this case, the increase of long term deflection is proportional to the stress level of FRP reinforcement. However, the total deflection is not large comparing the span length. The increase of crack width is also proportional to the stress level of FRP reinforcement. However, the crack width immediately after cracking is considerably large. So, the design criteria should be needed such as no more increase of crack width could be permitted under the working stress level. In this case, We have to know that the stress level of FRP reinforcement would be limited to a considerably low stress level when the crack width would be a serviceability limit.