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OCR for page 40
40 The effectiveness of FRP for shear strengthening is signifi- An interaction exists between the internal transverse steel cantly affected by the cross-sectional shape of the girder. reinforcement and externally bonded FRP shear reinforce- Debonding of FRP accompanied by peeling of the concrete ment, but there are insufficient data to quantify this inter- cover can result in a web crushing failure. action. Further investigations are needed to better quantify If the stresses in the stirrups are less than the yield strength, the mechanisms involved in this interaction and incorpo- the FRP strengthening can help delay yielding and prevent rate it into an enhanced model for the shear resistance of fatigue failure of the girder in shear. However, if the stirrups RC beams strengthened with externally-bonded FRP. have already yielded, the FRP strengthening system would The use of mechanical anchorage involving discontinuous not reduce the stresses but could help restrain steel stress CFRP plates attached with steel concrete wedge anchors or increases and prevent catastrophic failure of the girder. bolts through the web was found to delay or, in some cases, In the absence of a proper anchorage system, the externally prevent debonding of FRP. However, because these anchors bonded FRP sheets for shear strengthening could become and bolts are susceptible to corrosion, research is needed to debonded and result in no shear strengthening. explore alternative mechanical anchorage techniques that Limiting the stress in the stirrups to the yield strength will are not susceptible to such corrosion. assure that shear fatigue failure of the girder will not occur. The cross-sectional geometry of PC girders influences the The effective FRP strain used in evaluating the FRP shear effectiveness of externally bonded FRP. Also thin web and contribution can be expressed by two separate design expres- stiff flange geometry reduce the effectiveness of the FRP sions to consider the two predominant failure modes (i.e., shear strengthening. However, limited results are available debonding and FRP rupture). One expression is for mem- to fully understand the mechanisms involved in such behav- bers in which sufficient anchorage is provided (FRP rupture ior. Further research is needed to examine the effect of cross- failure mode), and the other is for members in which insuf- sectional geometry of PC girders. ficient anchorage is provided (FRP debonding failure mode). The effective strain concept was adopted for design guide- lines and codes to provide a simple and practical method for estimating the shear contribution of FRP. Research is 4.2 Suggestions for Future Research needed to investigate the non-uniform FRP distribution Based on the work performed in this research, the follow- and develop more reliable design equations. ing research efforts are needed to enhance understanding of Research is needed to investigate the long-term fatigue per- the mechanisms associated with the use of externally bonded formance of FRP systems for shear strengthening, particu- FRP for shear. larly the effects of cracks on bond characteristics.