Seismic Evaluation of Framed Structures Reinforced by FRP

Original Article

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: sara.megahed@zu.edu.eg; suzanaa1@hotmail.com ; Habdelkader@zu.ed.eg

Abstract

The primary using FRP as replacement material instead of steel is curing corrosion in concrete structures, which caused damage in concrete. The use of FRP as concrete reinforcement is relatively new, despite of common using it as rehabilitation and retrofitting purpose. There are lacks of researches in, whether, experimentally or analytically data of performance and design of new FRP reinforced concrete structures, especially for seismically region. The main objectives of this research were evaluation the seismic performances of frames with FRP bars. Three models are considered to represent low-rise, medium-rise and high-rise frames. Also, this research is needed to gain a better understanding of the behavior of FRP materials and their interaction with traditional materials. A three parametric studied were investigated type of reinforcement, concrete strength and number of stories .In this study, analyzes of RC frames has been performed using SeismoStruct software. This is base shear – drift curve and the cracking behavior of performance criteria where obtained. By comparing the obtained results, it can be declared that FRP-RC frames are started to failure early than Steel-RC frames, thus the limit of inter-story drift at collapse prevention is different for FRP than Steel. Increasing number of stories affected on changing the stage of performance level of frames, however, the FRP-RC frame not much affect with this parameter. Increasing the concrete compressive strength enhanced the lateral load capacity for FRP-RC frames and with a little benefit for steel-RC frames, although top drifts at failures increased in case of Steel-RC frames, unlike FRP-RC frames. In general, the extent of damage experienced by the frames at the target displacement was found to be between immediate occupancy and life safety when subjected to design lateral load.

Keywords: FRP; Reinforced concrete Frames; seismic evaluation; pushover analysis Inter-story drift; Performance level; SeismoStruct.

Introduction

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[2]. 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 [10] 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.

2.Modelling

 2.1 Geometry

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.

2.2 Material

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 [10].

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

2.3 Loading

Incremental static loads were applied according to ECP 203[6] 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  [9]

Where Pf: is the factor multiplying to lateral force

  1. SeismoStruct

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 [11].

 

 

  1. a) Fiber element model                                                                          b) Integration sections

         Figure 6: Beam-column elements[11]

  1. 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.

  • Conclusion

 

  • 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.

References

  1. 1R-06,2006,”Guide for the Design and Construction of Structural Concrete Reinforced with FRP Bars”,American Concrete Institute. 
  2. Afifi, M. M. Z. M.,[2013],”Behavior Of Circular Concrete Columns Reinforced With FRP Bars And Stirrups”,Department of civil,Sherbrooke,December. 
  3. ATC-40,1996,”Seismic Evaluation And Retrofit Of Concrete Buildings.

 

  1. CAN/CSA-S806-12,(2002 and 2012),”Design and Construction of Building Structures with Fiber-Reinforced Polymers”.

 

  1. ECP208,2005,”The Use Of Fiber Reinforced Polymer ( FRP) In The Construction Fields”,Egyptian Code Of Practice.

 

  1. ECP203,2001,”Egyption Code For Design and Constructions Concrete Structures “,Egyptian Code Of Practice.

 

  1. FEMA-273,1997,”NEHRP Guidelines For The Seismic Rehabilitation Of Buildings”,AGENCY, F.E.M.,Washington, D.C.

 

  1. FEMA-356,2000,”Prestandard And Commentary For The Seismic Rehabilitation Of Buildings”,Federal Emergency Management Agency,Washington, D.C.

 

  1. Khoshnoudian, F., Mestri, S. and Abedinik, F.,[2011],”Proposal Of Lateral Load Pattern For Pushover Analysis Of RC Buildings”,Computational Methods in Civil Engineering,Vol. 2,2,169-183.

 

  1. Cimilli Erkmen and Saatcioglu, M.,[2008],”Seismic Bahviour Of FRP Reinforced Concrete Frame Buildings”,The 14thWorld Conference on Earthquake Engineering,Beijing, China ,October 12-17.

 

  1. Seismostruct,v. 7.0.4,”SeismoStruct User Manual For version 6″,seismosoft, seismosoft.com

 

  1. V-ROD,Class Fiber Reinforced Polymer (CFRP) Rebar – Product Data Sheet”,vrod.ca.

 

  1. V-ROD,Glass Fiber Reinforced Polymer (GFRP) Rebar – Product Data Sheet”,vrod.ca.

Be the first to comment on "Seismic Evaluation of Framed Structures Reinforced by FRP"

Leave a comment

Your email address will not be published.


*