Abstract for: Punching Shear Upgrade of Reinforced Concrete Flat Plates- CMS proposal 0301395
PI: Oguzhan Bayrak, UT-Austin.
Existing reinforced concrete slab-column connections may require strengthening to prevent catastrophic punching shear failure and/or to improve the ductility of connections. Change of the intended use of the structure and load increases due to changing function, modification of the structural system due to openings cut through slabs, partial loss of the concrete contribution to shear strength due to cracking, spalling, and delamination of the cover concrete due to corrosion of reinforcing bars and/or concrete deterioration due to alkali silica reaction and delayed ettringite formation, compliance with the current building code, and compensation for design errors or construction deficiencies are some of the reasons that might necessitate strengthening of slab-column connections to prevent punching shear failures. In addition to the aforementioned reasons, the need to upgrade such slabs in earthquake-prone regions for strength and ductility is also clear. Examples of punching shear failures have been observed following the 1964 Alaska earthquake, the 1985 Mexico City earthquake, the 1989 Loma Prieta earthquake, and the 1994 Northridge earthquake. Finally, in the wake of recent terrorist activities, the need to strengthen various reinforced concrete buildings is also obvious.
The objective of the proposed research is to experimentally and analytically investigate the behavior of reinforced concrete flat plates strengthened with fiber reinforced polymers to enhance punching shear strength. The efficiency of various repair techniques in which fiber reinforced polymers are used in a new and innovative manner will be investigated. Design guidelines and analytical procedures will be developed based on the experimental findings of the research program. It is well established that the use of stud rail systems in new construction helps increase the punching shear strength and the response to dynamic loads. However, no established scheme for increasing punching shear strength has been reported in the literature. Flexibility of fiber reinforced polymers during the application process makes them very suitable for the purpose of establishing .external stirrups through the holes drilled in reinforced concrete flat plates.
In order to investigate the feasibility of the proposed technique, pilot tests were conducted by one of the PIs. Reinforced concrete slabs tested during this study were scaled laboratory models of typical two-way flat-plate construction. During the pilot tests it was observed that the failure surface shifted away from the loading plate in all specimens strengthened by the proposed technique, indicating the effectiveness of the method. Examination of the load-deformation responses of the specimens indicated that punching shear strength of a strengthened specimen increased as much as 50 %, relative to the control specimen, using one of the proposed repair schemes. For the specimens tested, the punching shear strength could have been increased further if more transverse reinforcement had been used in an efficient configuration. Preliminary analyses results suggest that size of the strengthened zone and exact location of vertical FRP strands influence strength, ductility and stress flow pattern in strengthened reinforced concrete flat plates. In light of the results from the pilot test series and preliminary analyses, the PIs propose to conduct an extensive experimental and analytical program to investigate the efficiency as well as the limitations of the proposed upgrade technique. Monotonic and reversed cyclic tests will be conducted on twenty-four full-scale test specimens to investigate punching shear behavior of strengthened flat plates. Behavior of the test specimens will be investigated analytically to identify the most appropriate methods of analysis. Nonlinear analysis techniques will be developed to capture the behavior more accurately, should the existing analysis techniques and/or tools be deemed inadequate. Analytical models will likely evolve over the course of the study as additional test data become available. As models become more refined, they will be more useful for aiding in the design of future strengthened specimens.