The use of FRP stirrups has been hindered by their limited availability. Also, bending FRP bars to make stirrups has to be performed in production plants with special care and equipment. Fibre-reinforced polymers were also used in slabs in the form of composite grids for instance by Banthia et al.
The general conclusion was that FRP grids are a suitable material for reinforcing concrete slabs and that the punching shear strength is lower than that of similar steel-reinforced slabs [14].
Also, studies recommended that flexural design of FRP-reinforced slabs and beams should aim at an over-reinforced section in order to achieve a compression failure [15]. This recommendation was also adopted by design codes such as ACI Grira and Saatcioglu [16] investigated the use of both steel and CFRP grids as stirrups for confinement of columns with longitudinal steel reinforcement.
Several configurations of transverse reinforcement were tested under cyclic loading. They concluded that the performance of columns reinforced with CFRP stirrups was comparable to that of columns reinforced with steel stirrups. They also argued that the use of grids in general, beside ease of construction, provided a near-uniform distribution of the confinement pressure along the column, without congesting the reinforcement cage. Fukuyama et al.
RA11S aramid-bars were used for the main columns reinforcement, RA7S bars were used as flexural reinforcement for beams and slabs, while RA5 bars were used as shear reinforcement. It was argued that frame deformations would govern the design. It was also noted that the rehabilitation of such a frame was easier than that of conventional RC frames since residual deformations were smaller. However, the frame was not tested up to failure and its behaviour under excessive deformations was not reported.
Thus, testing a steel-free FRP-reinforced standard frame up to failure should provide valuable information. These include concrete bridge decks, barrier walls, water tanks, slabs-on-grade, curtain walls, underground tunnel linings, rock storage cavities, etc. The advantages of FRP grids include suppression of delamination problems, equal longitudinal and transverse reinforcement depth, built-in redundancy [19], and high durability and fatigue resistance [12].
In general, the grid structure being lightweight and manufactured into either curved or flat plates, drastically reduces assembly work at construction sites. Offsite construction also allows for better quality control and assurance. Because of its corrosion resistance, FRP grids constitute a promising technology for reinforcing concrete structures in offshore and coastal regions and in environments where corrosive de-icing salts are used.
Furthermore, the magnetic neutrality characteristic of FRP makes it an ideal reinforcing material for instance in structures housing hospitals where magnetic resonance imaging MRI equipment is in common use. The fibres are impregnated with a resin polyester, vinylester or epoxy. Using a layering process, individual FRP laminates are formed into rigid 2D rectangular grid shapes. The longitudinal and transverse bars are orthogonal and continuous at the intersection points so that there is a 2D symmetry of mechanical and geometric properties.
Bars have a rectangular cross-section, with smooth top and bottom surfaces and rough fibrous sides. The beam of the current test unit is taken to the mid-span of the bay, while the column is taken from the mid-height of one storey to the mid-height of the next storey. It has sufficient shear reinforcement in the joint area, in the column hinging area, and in the beam hinging area.
Dimensions and reinforcement details for the standard specimen J1 are shown Fig. The rebars used in the column had 5 nodes per meter. All GFRP longitudinal rebars had nodes 50 mm wide from end to end. The longitudinal reinforcement configuration aimed at providing a similar bending moment capacity to that of the standard steel-reinforced specimen, thus inducing a comparable level of joint shear input.
The transverse reinforcement in specimen J4 consisted of 3-branched vertically and horizontally G10 77 mm2 of cross-sectional area stirrups. This provides built in redundancy since the failure of a branch is not complete until both of its two vertical portions fail. Column stirrups detail Dimensions in mm. Table 1. The stirrups, being taken from a manufactured grid, were dimensionally identical. Accordingly, the longitudinal reinforcement needed very little rearrangement.
The much lighter weight of the GFRP rebars allowed easier manipulation of the reinforcement cage. For the steel-reinforced specimen J1 , extra work was required to fit steel rebars in place, especially in the joint area. Test Setup and Procedure The beam-column joint specimens were tested under a constant axial load of kN applied on the column and reversed cyclic load quasi-static applied at the beam tip.
The selected loading pattern applied at the beam tip was intended to cause forces that induce high levels of deformations usually experienced by structural frames during severe earthquakes. For the steel-reinforced specimen, the selected load history consisted of two phases. This was followed by two load cycles reaching the concrete flexural cracking load in the beam at the column face.
These in turn were followed by two cycles at the load causing initial yield in the beam. For each load increment, two consecutive cycles were applied at the same loading level to verify the stability of the specimen. A different loading routine was selected for the GFRP-reinforced specimen J4 since unlike conventional steel-reinforced sections, those reinforced with GFRP do not undergo yielding.
A displacement-controlled load history similar to the one used by Fukuyama et al. The very first drift was applied in one cycle, while all other subsequent drifts were applied in two cycles. Further details about test setup are available elsewhere [23]. First flexural cracking of the beam section subjected to maximum bending moment appeared at a beam tip load of 15 kN corresponding to a drift of 0.
The onset of diagonal cracks in the joint area took place at a beam tip load of 50 kN corresponding to a drift of 0. Beam tip load-storey drift relationship for the standard steel-reinforced specimen J1. Additional cracks in the joint area appeared thereafter as loading progressed, but remained within a very fine width throughout the test.
The test was stopped as the beam capacity dropped but the axial load in the column was maintained and the joint areas remained still intact, except the presence of fine cracks. The final crack pattern of the standard specimen J1 is shown in Fig. After the test termination, two longitudinal rebars one top and one bottom were detected failing in tension. A permanent beam-tip deformation of 1.
As the test progressed, several distinct cracks extended through the depth of the beam section at specific locations corresponding to grid nodes in the longitudinal reinforcement, while several smaller cracks formed along the beam. This took place since bars, which are originally cut from grids, are not deformed and the bond with concrete is predominantly supplied by the nodes. Load — Storey Drift Angle Envelope Relationship For the tested beam-column joint specimens, the envelope of the beam tip load-storey drift angle relationships are shown in Fig.
Comparing the two envelopes shows a lower load capacity and stiffness for the GFRP-reinforced specimen, which is due to the lower stiffness of GFRP compared to that of steel.
The envelopes started at comparable stiffness, but as soon as cracking took place a distinct difference between the two appeared and was significant for the remainder of the tests. The GFRP-reinforced specimen J4 had an essentially elastic envelope, whereas the steel-reinforced specimen J1 had a typical elastic-plastic envelope. Beam tip load-storey drift envelopes for the tested specimens. Cumulative Dissipated Energy The capability of a structure to survive an earthquake depends on its ability to dissipate the energy input from ground motion.
Although it is difficult to estimate such an energy input during a ground movement event, a satisfactory design should ensure a larger energy dissipation capability of the structure than the demand.
The cumulative energy dissipated by the beam-column joint specimens during the reversed cyclic load tests was calculated by summing up the energy dissipated in consecutive load-displacement loops throughout the test. The energy dissipated in a cycle is calculated as the area that the hysteretic loop encloses in the corresponding beam tip load-displacement plot.
Results displayed in Fig. This is clear from the shape of individual hysteretic loops of the tested specimens Figs. The ductility of steel reinforcement allowed higher plastic deformations to occur in the beam, thus increasing the area of each individual loop.
The damage levels that the specimens sustained at failure, shown in Fig. Yielding of steel is a major mechanism for RC structures to dissipate energy, whereas plastic deformations and friction along cracks in concrete usually have lower contribution to the total energy dissipated. Cumulative energy dissipated for the tested specimens. The storey shear-joint deformation plots for specimens J1 and J4 are traced in Fig. Comparing the behaviour of both joint panels, it is clear that the steel-reinforced panel of specimen J1 had higher stiffness and smaller joint deformation compared to that of the GFRP-reinforced panel.
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