Design of FRP Systems for Strengthening Concrete Girders in Shear (2011) / Chapter Skim
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2.2 Field Applications Although there are several field projects related to FRP strengthening systems, detailed information on these projects is not available and most of these projects were strengthened for flexural rehabilitation. The following six projects were identified as directly related to FRP shear strengthening of concrete bridge girders: • A single span, reinforced concrete T-beam bridge in New York State was strengthened in flexure and shear with externally bonded FRP laminates in November 1999 (Hag-Elsafi et al., 2001b)
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2.4.1 Studies on the Behavior of Concrete Girders Strengthened in Shear with Externally Bonded FRP The review included 49 experimental studies, encompassing more than 500 test specimens. The review provides information on the objectives, the methodology, the experimental program, the test method, the FRP used and its orientation, as well as the strengthening scheme used (configuration)
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6.134 0.580ln f ft E feL e − = and 6 1 110 10C constant strain rate of mm− −= × cot se se s v dV E A s θ ε= where , sinse f ave f fs syε ε α γ ε= ≤ Table 2.2. Models based on an effective FRP strain.
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10 Khalifa and Nanni (2000)
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The previously developed analytical models were based on the studies listed in Table 2.5, the majority of which considered only small-scale testing. Therefore, this research aimed at expanding the experimental database with results from tests on full-scale T-beams, which are more representative 11 Pellegrino and Modena (2002)
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12 Reference Author (Year) Equations Chen and Teng (2003a and 2003b)
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13 Reference Author (Year) Equations Malek and Saadatmanesh (1998)
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h,d.min zff { } , min 0.9 , sin / f rid eq w e fdd f s z d h l f E β= − − U-wrap or complete wrapping: ( ) , 1 0.9 2 cot cot fRd f fed f Rd f w V d f t p θ β γ = ⋅ ⋅ ⋅ ⋅ + ⋅ design effective stress for U-wrapping: { }−⋅= w e fddfed h,d.min sinlff 903 11 β design effective stress for complete wrapping: { } { }−⋅−+−⋅= w e fddfdR w e fddfed h,d.min sinl)
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15 Properties and Parameters Concrete Type of Geometry Type of Beam and Steel FRP Strengthening Scheme Author Year N um be r o f T es ts R ec ta ng ul ar S ec tio n TSe ct io n B ea m S pa nn i n g L< 7 ft B ea m S pa nn in g 7 ft |
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debonding, the bond stress-slip relationship, the effective bond length, and the bond strength is required for the development of improved shear design equations. The most important role of the interface bond between the FRP sheets and concrete is to transfer shear stresses from existing concrete structures to externally bonded FRP sheets for both shear and flexural strengthening.
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These methods include single-lap-type, double-lap-type, bending-type, and insertedtype tests, as shown in Figure 2.1. Among the interface parameters evaluated are average shear bond strength, effective bond length, maximum shear bond stress, interfacial fracture energy, and the local bond stress-slip relationship.
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Figure 2.4 shows the effective bond lengths calculated by analytical models and equations stipulated in many current code and design guidelines versus the rigidity of FRP reinforcement (Ef ρf)
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Lo ca l b on d st re ss es Free 99.99 % of Max Load 80 % of Max Load 50 % of Max Load 20 % of Max Load Le Loaded end 99.9 % of Max Load 96.0 % of Max Load Figure 2.3. Concept of effective bond length based on stress distribution (Ueda and Dai, 2005)
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conducted a project to strengthen four continuous reinforced concrete beams in shear and flexure, by supporting them with masonry columns. Bond/anchorage tests indicated a 44% increase in anchorage capacity with the use of steel bolted connections.
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Anchoring methods of CFRP sheets. nally bonded FRP based on failure modes.
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The Canadian Design and Construction of Building Composites with Fiber Reinforced Polymers (CAN/CSA S806, 2002) is a design code that addresses externally bonded FRP reinforcement for concrete.
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2.6.1.2 Effect of Internal Transverse Steel Reinforcement Recent studies have shown that the contribution of externally bonded FRP to shear resistance is less for beams containing internal transverse steel than for beams without such reinforcement (Li et al., 2002; Pellegrino and Modena, 2002; Chaallal et al., 2002; Bousselham and Chaallal, 2004; and Czaderski, 2002)
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Thus, the shear contribution of externally bonded FRP is less for deep beams than for slender beams. 2.6.1.5 Influence of FRP Configuration and Anchorage The frequency of each mode of failure occurrence for different FRP configurations (side bonding, U-wrap, or complete wrap)
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tested six specimens under fatigue loading that varied between 35% and 65% of the respective static capacity of the specimen. Three of the beams had no internal shear reinforcement, and the other three had internal transverse steel reinforcement.
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2.6.2.1 RC T-Beams The experimental program was conducted to investigate the shear performance of full-scale RC T-beams strengthened with externally bonded FRP sheets. Tests were performed on eight full-scale RC beams, seven of which were designed to provide two distinct test regions and one beam was designated for fatigue testing.
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Test results showed that beams with slight corrosion damage can be effectively repaired in shear by externally bonded FRP sheets since cracks due to corrosion do not influence the effectiveness of FRP shear strengthening. The stirrups in the beams with pre-existing cracks yielded at a lower shear force than those in the beams without pre-existing cracks.
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2.6.2.2 PC Girders Tests were conducted on full-scale AASHTO type PC girders to investigate the effects of FRP shear strengthening. Table 2.8 lists the test parameters for the PC girders.
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. A shear component analysis showed that externally bonded FRP provides a significant contribution to the total shear resistance of a PC girder.
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0.6" dia. tendons prestressed to 40% of ultimate #3 confinement bar 9-7/8" 5-3/16" Horizontal Shear Studs #5 bar spaced @ 12" o.c.
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However, accurate prediction of cracking and ultimate loads, similar crack patterns, consistent ductility, and similar strain/stress variations in each component are indications of the developed FE model's efficiency. In terms of failure strength, the average ratio of experimental shear strength to analytically evaluated shear strength of the PC girders (Vexp/VFE)
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The FE analysis allowed investigation of local behaviors that could not be examined through experiments such as the interface behavior between concrete and FRP sheets. The FE analysis also provides the stress and strain variations for concrete, steel, FRP, and interface regions that were used to investigate each component contribution to the shear transfer mechanism.
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Vf,test is the experimentally measured strength of a test beam with FRP reinforcement minus the experimentally measured strength of the corresponding (control) beam without FRP reinforcement, and Vf is the strength calculated from each model.
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34 M o de l Af fili at io n Al Su la im a n i e t al .
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The parameters proposed in a Vf relationship for use in the LRFD specifications should recognize that FRP shear reinforcement is most likely to be used for strengthening of an existing old structure for which full design details may not be available.
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The figure shows that the reliability index is nearly the same for all girder spacings and is about 3.50 for shorter span lengths but decreases for longer span lengths. 36 Random Variable Bias COV Material and Fabrication Tolerances, αMF Varies (see Appendix)
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