预应力纤维带混锚加固窄梁界面应力分析

Interfacial stresses in narrow beams hybrid strengthened with prestressed FRP straps

预应力纤维带混锚加固结构方法已被成功应用于工程实践,其纤维带端部存在由端锚系统提供的轴向拉力,可有效防止普通粘贴加固梁端部界面应力集中引起的剥离问题,大幅提升材料利用率。如何评价其应力缓解程度是预测抗剥离能力的关键问题。参考纯粘加固梁界面应力分析方法,考虑端锚系统抗剪刚度和预应力作用,推导预应力纤维带混锚加固梁的界面应力计算公式,通过预应力纤维带混锚加固木梁四点弯曲试验及有限元分析验证理论公式的准确性,并在此基础上进行关键参数分析,明确不同参数对加固梁端部界面应力的影响。参数分析结果表明:通过适当增加胶层厚度,减小胶层弹性模量、纤维带厚度和弹性模量及原梁高宽比,可以不同程度地降低端部界面应力,其中胶层厚度影响最大,当胶层厚度由0.2 mm增加到6 mm时,预拱混锚梁端部界面剪应力和正应力分别降低83%和92%,且其降低幅度大于无端锚工况。随原梁高宽比增大,加固梁端部界面应力增大,当原梁高宽比等于3时,附加端锚的预拱混锚梁与预拱粘贴梁相比,端部界面应力降低28%,因此纤维带端部附加端锚系统可有效降低界面剥离风险。预应力纤维带混锚加固窄梁基于三阶段的界面应力分析方法对其剥离防治设计具有参考价值。

The hybrid strengthening method with prestressed fiber reinforced polymer(FRP)has been successfully applied in engineering.Owing to the axial tension provided by the end anchorage system at the FRP end,the interfacial debonding failure at FRP strap end usually occurred in purely bonded beams due to the stress concentration can be effectively prevented,and the material utilization ratio can be largely improved.However,how to evaluate the reduction of interfacial stress due to the anchorage system remains unclear,which is a key problem to the prediction of interfacial debonding.Based on a three-stage analysis of the interfacial stresses in an externally bonded beam,considering the shear stiffness of the end anchorage system and the effect of prestressing,the expressions for interfacial shear and normal stresses in a beam hybrid strengthened with prestressed FRP strap were determined.Meanwhile,four-point bending tests and finite element analysis for strengthened timber beams were carried out to verify the theoretical results.Employing the theoretical model,the effect of certain parameters on the maximum interfacial stresses was examined.The parametric analysis shows that the maximum interfacial stresses can be reduced in different degrees by properly increasing the thickness of the adhesive layer,and decreasing the elastic modulus of the adhesive layer,the thickness and the elastic modulus of the FRP strap and the height-width ratio of the beam.The thickness of the adhesive layer is the most significant to the maximum interfacial stresses.When it increases from 0.2 mm to 6 mm,the maximum interfacial shear and normal stresses decrease by 83%and 92%,respectively.Moreover,the reduction of interfacial stress in FRP end of the hybrid strengthened beam is greater than that of the purely bonded beam.The maximum interfacial stress of the strengthened beam increases with the height-width ratio.When the height-width ratio is 3,the maximum interfacial stress of the hybrid strengthened beam is 28%lower than that of the purely bonded beam as the contribution from the end anchorage system.Therefore,the risk of interfacial debonding can be effectively reduced.The proposed method can provide the reference to the design of anti-debonding measures for the narrow beams hybrid strengthened with prestressed FRP strap.