Strengthening of Multibay Reinforced Concrete Flat Slabs to Mitigate Progressive Collapse Kai Qian, Ph.D., M.ASCE 1 ; and Bing Li, Ph.D., M.ASCE 2 Abstract: Unable to generate sufficient ductility and continuity, RC flat slab structures are vulnerable to progressive collapse, in which the relatively brittle failure mechanism attributable to punching shear failure may lead to catastrophic consequences. Thus, it is necessary to evaluate the effectiveness of approaches for improving the ability of flat slabs to mitigate progressive collapse. Previous studies have indicated that integrity reinforcement may enhance the behavior, particularly postpunching behavior, of newly designed flat slab structures. However, limited tests have been conducted to determine reliable approaches for strengthening existing flat slab structures to resist progressive collapse. In this study, a series of seven multibay flat slab substructures were cast and tested to assess the effectiveness of proposed glass fiber- reinforced polymer (GFRP) strengthening schemes for improving the progressive collapse behavior of existing flat slab structures, owing to its low density, high strength, rigidity, and excellent resistance to corrosion. Three specimens without strengthening were used as control specimens and the remaining four specimens were strengthened by GFRP strips. Test results indicated that proposed strengthening schemes effectively improved the initial stiffness and flexural resistance of flat slab structures. However, they did not sufficiently enhance the postfailure resistance and deformation capacity: there was significant debonding of the GFRP strips from the concrete interface in the large displacement stage, even when specially designed fiber anchors were employed. DOI: 10.1061/(ASCE)ST.1943-541X.0001125. © 2014 American Society of Civil Engineers. Introduction When an initial local failure causes the loss of gravity load capacity of building structures, it may lead to the eventual collapse of the entire building, or a large part of it. This type of collapse is defined as progressive collapse or disproportionate collapse. Given the catastrophic consequences of progressive collapse, there is signifi- cant interest in developing methods to avoid such instances. Recently, extensive research has examined the behavior of struc- tures following the loss of a column. Most previous tests regarding progressive collapse have focused on beam-slab structures with or without slabs (Sasani et al. 2007; Sasani and Kropelnicki 2008; Su et al. 2009; Yap and Li 2011; Qian and Li 2012, 2013; Qian et al. 2014). Although these tests provided valuable insights into the behavior of beam-slab structures to mitigate progressive collapse, many researchers such as Kunz et al. (2008), Knoll and Vogel (2009), Mirzaei (2010), and Habibi et al. (2012) have noted that flat slab structures are highly vulnerable to progressive collapse be- cause a punching shear failure mechanism may dominate in the failure mode. However, these conclusions were drawn in the con- text of studies focusing on initial punching shear failure caused by insufficient maintenance, unexpected overload, and earthquake sce- narios; minor studies have examined the propagation of punching shear failure caused by other extreme events such as the loss of a column. As shown in Fig. 1, the column failure due to unexpected extreme loading may significantly increase the moment and shear force at adjacent slab-column connections, which can trigger punching shear failure at these connections resulting in the dispro- portionate or entire collapse of the flat slab structures. Thus, several studies had been carried out to evaluate the possibility of develop- ing reliable resisting mechanisms which could be utilized for mit- igating disproportionate collapse of flat slabs. Mitchell and Cook (1984) discussed the possibility of developing compressive mem- brane action (CMA) and tensile membrane action (TMA) for resisting the progressive collapse of flat slabs based on modeling and analysis. It was indicated that flat slabs with rebar of sufficient integrity passing through the column cages developed considerable TMA in the large displacement stage to mitigate progressive col- lapse. Keyvani et al. (2014) also numerically studied the increase in punching shear capacities of CMA developed in multipanel flat slabs. It was found that CMA significantly enhanced the punching shear capacity of isolated slab-column connections with lateral con- straints. For multipanel flat slab structures, CMA may prevent the propagation of punching shear failure and avoid progressive collapse. To investigate the ability of flat slab structures to resist progressive collapse, a series of single story multipanel flat slab substructures were tested by Qian and Li (unpublished data, 2014). Flat slabs, especially flat plates without drop panels, were highly vulnerable to progressive collapse although considerable CMA and TMA were observed. Strengthening schemes to improve the resis- tance capacity of such structures are highly necessary. In recent years, strengthening or rehabilitation of existing struc- tures has been an important challenge in civil engineering. Steel plates have been utilized in flexural strengthening of concrete beams for several years (MacDonald and Calder 1982; Zhang et al. 2001). The primary disadvantage of using steel plates is their pro- pensity for corrosion, which adversely affects the effectiveness of bonds at steel and concrete interfaces. Fiber-reinforced polymers (FRPs) have recently been utilized because these materials are 1 Associate Professor, College of Civil Engineering, Hunan Univ., Changsha, Hunan 410082, China; formerly, Research Fellow, School of Civil and Environmental Engineering, Nanyang Technological Univ., 50 Nanyang Ave., Singapore 639798 (corresponding author). E-mail: qiankai@ntu.edu.sg; qian0024@e.ntu.edu.sg 2 Associate Professor, School of Civil and Environmental Engineering, Nanyang Technological Univ., 50 Nanyang Ave., Singapore 639798. E-mail: cbli@ntu.edu.sg Note. This manuscript was submitted on November 6, 2013; approved on May 30, 2014; published online on July 28, 2014. Discussion period open until December 28, 2014; separate discussions must be submitted for individual papers. 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