Reliability-based Investigation On Design Of Flexural Concrete Beams Reinforced With GFRP Rods

As one kind of new composite materials, fiber reinforced polymer (FRP) rods have the characteristics of light weight, high tensile strength and excellent corrosion resistance. FRP rods can be a good alternative to the solution of problems, e.g. corrosion, durability and etc. in conventional steel reinforced concrete structures. But the relative low modulus of elasticity of FRP rods could inevitably result in considerable cracking and deflection of FRP-reinforced concrete structures even in serviceability limit states (called SLS thereafter). So, it is necessary to carry out some research not only on load-carrying ability in ultimate limit state (called ULS thereafter), but also on SLS. Some work has been done in this field by foreign research institutions, and some relevant design guidelines have been already released or updated. Unfortunately, the relevant work done by domestic institutions is obviously lagged. In China, the first design guideline titled as "Technical specification for strengthening concrete structures with carbon fiber reinforced polymers laminate" (CECS 146:2003) was released in 2003. As an updated and extended version of CECS 146, the "Chinese technical Code for the application of fiber reinforced polymers in civil engineering" (GB×××××,2006) is under development. The design methods in those guidelines are mostly based on the revision of those in current codes for steel-reinforced concrete structures. Although adequate safety margin is taken into account in the guideline, there is lack of strict theoretical investigation and sufficient test data as well as economic considerations. So, design recommendations based on systematic reliability assessments would be very helpful for updating current design guidelines.Reliability assessments are carried out using the Rackwitz-Fiessler method on the flexural design provisions in the latest guideline ACI 440.1R-06 titled as "Guide for the design and construction of concrete reinforced with FRP bars" both on ULS and SLS for GFRP-reinforced concrete beams. The global average reliability indexes for tension-controlled failure mode and compression-controlled failure mode are found to be 5.6 and 4.5, respectively. In SLS, for short-term loading, the global average reliability indexes for deflection-controlled and crack-controlled cases are 0.75 and 0.57, respectively. Together with the investigations on the effects of design variables associated with resistance, load effect ratio, computational uncertainty factor, the coefficient of variation of GFRP rod and limits on average reliability level in ULS and SLS. The changing of sectional unload stiffness is studied by the reversibility factor is defined in the paper in which nonlinear sectional analysis is adopted. The study has revealed that the reversibility factor is close to 1.0, indicating that the mechanical behavior of GFRP-reinforced concrete beams under SLS is almost completely reversible. As load effect ratio increases, the reversibility factor also increases.On the basis of the relationship between resistance reduction factor and global average reliability index, the value of 0.8 is suggested for the factor under the determination of target reliability indexes of 4.0 for tension-controlled failure mode and 3.5 for compression-controlled failure mode, respectively. According to the specification on ISO2394:1998 that the target reliability index for SLS ranges from 0 to 1.5, depending on the reversibility of the state under consideration, the deflection limit is suggested as l/120 (l is the span of beam) by the reliability assessment for GFRP-reinforced flexural concrete beams. By the consideration of the characteristics of GFRP-reinforced concrete flexural beams, and the consideration of visible tolerance, the maximum crack width of 0.7mm is accepted by this paper also through the assessment.