Seismic Evaluation of Framed Structures Reinforced by FRP
Corresponding author: Sara Mogahed1, Suzan Mustafa 2 and Hilal Hassan 3, Department of Structural Engineering, ZagZiag University, Egypt; E-mail: firstname.lastname@example.org; email@example.com ; Habdelkader@zu.ed.eg
Nonlinear static pushover analysis has become a popular analysis tool for seismic performance of reinforced concrete structures. Standards guidelines such as ATC 40 , FEMA 273 followed by FEMA 356 have given detailed guidelines to perform the nonlinear static pushover analysis and to use it to obtain the performance of the structures under a given lateral loads. To determine the performance of the structure against a given earthquake, are different for different method, FEMA 356 describing displacement coefficient method and FEMA 440 describing improvements to both capacity spectrum and displacement coefficient methods. In the last years , extensive research studied the behavior of FRP as strengthening material or as reinforcing for beams (exposed to flexural and shear loads), also a limited research work has been examined the axial behavior of RC columns with FRP, Afifi.M, et al. carried an investigation on the circular column under monotonic axial load until failure, the specimens were reinforced with GFRP and steel bars and stirrups , The model takes into account the effect of many parameter such as; type of reinforcement, longitudinal reinforcement ratio; transverse reinforcement configuration; and the volumetric ratio. The results showed the compression failure of longitudinal FRP bars was by buckling or crushing, The amount and distribution of longitudinal FRP reinforcement significantly affected column ductility, with a slight strength gain. on the analytical side , S. Cimilli Erkmen and Saatcioglu  studied the behavior of a 5-storiey – 3-bay plane frame under seismic load and reinforced by FRP. Column and beams were confined with FRP Grid to ensure inelastic deformability, From analytical investigation CFRP reinforced concrete buildings can be used for seismically active regions.
In Egyptian code ECP208 and ACI-440-1R-06, it isn’t recommend to use FRP in compression members or in seismic zones . FRP sections are designed according to ACI-440, ISIS.No3and CAN/CSA-S806-12. The objective of this study is to investigate the effect of FRP on the seismic behavior of frame structures. Three RC frames with different heights representing low-, medium-, and high-rise structures, respectively, were designed and investigated frame. The pushover analysis was conducted based on SeismoStruct software to evaluate the seismic performance of the reinforced frames.
The 3D RC structures are assumed to represent high-rise, medium- rise and low-rise RC structures. The structures have three bays along X-direction and three bays along Y-direction moment-resisting frame of reinforced concrete. The concrete floors are modeled as rigid Fig.1. The details of RC –sections in Fig.2, Fig.3 and Fig.4.The details of study case are given as:Number of stories were taken to be 6 ,9 and 12 .Number of bays along X-direction were taken to be 3.Number of bays along Y-direction were taken to be 3 Story height was taken to be 4000 mm.Bay width along X-direction was taken to be 8000 mm .Bay width along Y-direction was taken to be 8000 mm.
Figure1: 3D models of symmetric-plan 6-story structure
Figure2: Typical details For Steel-RC sections
Figure 3: Typical details For CFRP-RC sections
Figure 4: Typical details For GFRP-RC sections
The letters S,C and G identify model as being reinforced with steel ,CFRP or GFRP bars respectively .The letters N ,F refer to number of story and Compressive strength respectively. The FEM considered a range of concrete compressive strength from 30 to 60 MPa with 10 MPa increments for each run.
For materials definition, a mean concrete compressive strength of 30 MPa , a steel yield strength and FRP tensile strength in Table.1, Table.2 respectively were considered, The concrete was modelled using the proposal of Mander et al. For Reinforcement steel was modelled with the Menegotto & Pinto, and FRP was modelled and assumed by SeismoStruct Fig.5 .
a) Concrete b) Steel c) FRP
Figure 5: Stress-Strain model for the structural materials adopted in SeismoStruct
Table 1: Steel Reinforcement Properties
Table 2: FRP Reinforcement Properties
Incremental static loads were applied according to ECP 203 at different floor levels of frames in a predefined parabolic pattern according to equation (1). Also, a permanent load was added (live load, floor cover).
Pf = (X/H) 0.5 
Where Pf: is the factor multiplying to lateral force
Beam-column elements is then obtained through the integration of the nonlinear uniaxial stress-strain response of the individual fibres (typically 100-150) in which the section has been subdivided Fig.6. FB is used for simulating connection between beams and columns , this Element infrmFB (the inelastic force-based element type) is the most accurate among the four inelastic frame element types of SeismoStruct, since it is capable of capturing the inelastic behaviour along the entire length of a structural member. To predict the P-delta effects on the structural seismic analysis, geometric nonlinear was modeled .
- a) Fiber element model b) Integration sections
Figure 6: Beam-column elements
- Results and Discussions
4.1 General behavior and modes of failure
It was observed that failure mechanism steel reinforcement control frame, failed in flexure by yielding of the steel bars. Concrete crushing was the most common failure mode, occurring in the models of over-reinforced section for carbon and glass fiber reinforced models, and then following by rupture FRP in T-beam section Fig.7.
Figure 7: Location of plastic hinges/ damage elements for 6-story and 12-story
With increasing the height of frame, the performance level changes its stage. For Steel-RC frame, based on target displacement, the worst plastic hinge is located in immediate occupancy level for 6-story and life safety level for 9-story and 12-story. Because of no yielding for FRP bars, the target displacement is taken at intersection with capacity curve, where the area above and below the curve is equal Fig .8, the worst virtual plastic hinge is located near collapse prevention level. Thus, frames under parabolic lateral load are not safe for re-occupancy and could not be technically practical to repair.
Figure 8: Capacity Curve and its Idealized force-displacement curve.
4.2 Inter-story drift
The maximum inter-story drift ratio of Steel-RC frames have reached to 2% and the limit state criteria at this level are life safety according to FEMA 356.the maximum inter story drift ratios for CFRP and GFRP reinforced frames were 1% almost and the limit state criteria at this level are life safety according to FEMA 356 ,Fig.9.
Figure 9: Inter-story drift at failure point
4.3 Parametric study
Effect of reinforcement type: FRP-RC frames started to failure early than Steel-Rc frame . Thus, in this time, Steel –RC frame still in operational level ,FRP-RC frames started inelastic range Fig.10
Figure 10: Capacity curves for Reinforcement type.
Effect of number of story: In general it was observed that the ductility ratio and over strength decreased with increasing the number of stories .the over strength and ductility ratio as shown in Fig.11.
Figure 11: Over strength-ductility relationship.
Effect of concrete strength: It was observed from Fig.12.that increasing the concrete compressive strength enhanced the lateral load capacity and the corresponding drift level. By dictating the target displacement (performance point), it clearly show, that behavior of frames enhanced.
Figure 12: capacity curves for various compressive concrete.
- Pushover analysis is a good tools to get information about drifts which are indicators of damage ,but in high- rise structures becomes little useful, which before formation plastic hinges on upper stories the plastic hinges in lower stories turn to life safety and even to collapse prevention.
- Because of the brittle characteristics of FRP reinforcement, the sections are designed to be over –reinforced with failures initiating through the crushing of concrete. T-beam sections were designed to be under-reinforced because of its large concrete compression zones, which required very large areas of FRP reinforcement.
- Target displacement is calculated according to displacement coefficient method. The frames analyzed to the target displacement limit have shown no failure.
- By increasing number of stories, the damaged elements is increased, thus the elements change performance levels to higher stage.
- Increasing of compressive strength of concrete has significant effect on the behavior of frames reinforced by FRP, unlike Steel-RC frames was found to be a little benefits to structural response of frames.
- SeismoStruct assumed that FRP has no resistance in compression, thus, the seismic behavior of FRP –RC frames started to failure early as not usual. Also frames influenced with acceptance limit for performance level different than Steel –RC frames.
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