Workshop Report: Development of FRP Retrofit Guidelines for Deficient Reinforced Concrete Horizontal Lateral Force Resisting Systems

This report presents a summary of the “Development of FRP Retrofit Guidelines for Deficient Reinforced Concrete Horizontal Lateral Force Resisting Systems” workshop held virtually on 17 August 2021. The meeting was a part of the research initiative led by Virginia Polytechnic Institute and State University involving the use of externally bonded fiber-reinforced polymer (FRP) in strengthening deficient horizontal lateral force resisting systems (hLFRS) in older reinforced concrete buildings. The meeting was attended by the research team from the university and an advisory panel comprising engineers, practitioners, and producers. The meeting included a presentation from the research team and brainstorming sessions in which the advisory panel provided their insights into the issues and gaps put forward. The discussions were centered around the three primary research tasks – development of preliminary design approach, experimental evaluation of hLFRS diaphragm shear strengthening, and preparation of design recommendations. The majority of discussions were directly related to the first task. The discussion sessions addressed gaps in areas such as current design procedure, inadequate diaphragm shear capacity, axial tension strengthening, axial compression strengthening, joint strengthening, anchorages, and detailing issues based on the experience of using FRP for diaphragm strengthening. The panel shared consensus that there is a lack of adequate guidelines for the design and detailing of FRP strengthening of deficient hLFRS. Despite the lack of provisions, there exist the widespread application of the FRP for strengthening. In the present scenario, several designs and analysis approaches have been followed by engineers for the retrofit design, force-controlled and displacement-controlled, beam analogy, strut and tie methods, finite element analysis. Further, in the design process, the FRP fibers are treated as reinforcing steel and are mainly unidirectional except at locations of high shear transfer; Carbon FRP fibers are preferred over other types. The limit of 0.0015 on the FRP design strain would be reasonable. Owing to the lack of design expressions for flexural or shear strengthening concrete diaphragms developed for beams, slabs, and walls are adopted even though they do not accurately represent the behavior of diaphragms. The FRP can be laid orthogonally or diagonally depending on the contribution of existing steel. FRP anchors and embedded steel anchors bolted to steel plates are used for anchorage purposes which should be provided at areas of high stress concentration and termination points. For anchorage design, it was recommended to compare anchors between different manufacturers however, as a result of lack of test data at a scale that remotely simulates the size and magnitude of forces being developed, there still exist inconsistencies in the detailing of anchors. The meeting successfully provided recommendations and expert opinions on the diaphragm strengthening using FRP. The issues discussed and the expertise provided by the panel shall be incorporated into the research process.