4.6 effect of transverse reinforcing on circular columns confined with frp

7/30/2019 4.6 Effect of Transverse Reinforcing on Circular Columns Confined With FRP

1/6

Challenges, Opportunities and Solutions in Structural Engineeringand Construction Ghafoori (ed.)

2010 Taylor & Francis Group, London, ISBN 978-0-415-56809-8

Effect of transverse reinforcing on circular columns confined with FRP

G. Ghodrati AmiriCenter of Excellence for Fundamental Studies in Structural Engineering, School of Civil Engineering, IranUniversity of Science & Technology, Tehran, Iran

A. Jaberi Jahromi & B. MohebiSchool of Civil Engineering, Iran University of Science & Technology, Tehran, Iran

ABSTRACT: In recent years retrofitting of bridges against earthquake is interested and many works have beendone for strengthening every part of bridges. One of very important parts of a bridge is piers. Piers in manybridges are main earthquake resisting system. Therefore they play an important rule during earthquakes and

failure of them can cause collapse of entire deck and other parts of bridge. Strengthening of bridges piers can bedone by several methods. One of the most common methods is retrofitting RC columns with FRP jacketing. Anypiers of a bridge can be improved against lack of strength or ductility. Usually RC columns in bridges are circleshaped. In this study it has been tried to investigate effect of amount of transverse reinforcing before retrofittingcolumns on nonlinear behavior of confined columns with FRP. For performing this research, 4 Finite elementmodels with different amount of transverse reinforcing has been created and results have been presented in thispaper.

1 GENERAL INSTRUCTIONS

When reinforced concrete columns are subjected toseismic loading, the large lateral cyclic earthquakeforce will degrade the concrete and the reinforcingbar very quickly, and the columns will fail prema-turely. Investigations of bridge failures during recentearthquakes, such as the 1987 Whittier, 1989 LomaPrieta, 1994 Northridge, and 1995 Kobe show thatinadequate lateral reinforcement and insufficient laplength of the starter bars are among the major catas-trophic causes of failure (Priestley & Whitter 1988;Buckle 1994; Priestley & Seible 1996; Parvin & Wang2002). The main resisting system against earthquakesin many bridges is piers. In this kind of bridges alllateral force induced to bridge will be transferred to

piers. Therefore any failure in this member resultsthe overall collapse in bridge. In recent years, the useof externally bonded fiber-reinforced polymers (FRP)has been become increasingly popular for civil infras-tructure applications, including wrapping of concretecolumns. Significant research has been devoted tocircular columns retrofitted with FRP and numerousmodels were proposed (Khaloo et al. 1991; Fardis &Khalili 1981; Miyaushi et al. 1997; Samman et al.1998; Spoelstra et al. 1999). FRP wrapping of exist-ing circular columns has proven to be an effectiveretrofitting technique (Seible et al. 1997; Chaallal

et al. 2003). Retrofitting bridge columns is donebecause several reasons. One of these reasons is thelack of transverse reinforcing. In this case the shear

capacity of section is notsufficient forresisting againstearthquake forces, transferred to thecolumn from deckand other masses of bridge. On the other hand lackingof transverse reinforcing can cause decrease in defor-mation capacity in plastic hinges of bridges and it willlead to collapse of columns in plastic hinge regions.As mentioned before, for overcoming this deficiency,many engineers use FRP jacketing. FRP jacketing canincrease strength of column and deformation capacityof it simultaneously. It has been shown that confine-ment with fiber reinforced polymer (FRP) improvesthe behavior of columns submitted to seismic loading(Priestley et al. 1992; Katsumata et al. 1987). FRPfabric wraps consisting of carbon, aramid, or glassfibers bonded by an epoxy resin have been success-fully applied for seismic rehabilitation of bridge piers

in the U.S. and Japan (Mufti et al. 1992; Kasei 1993).The growing use of FRP composites as confinementelements is attributed to the important mechanical andchemical properties of these materials. Some of theseadvantages are light weight, high-tensile strength andmodulus, corrosion resistance, and durability. Theseadvantages make FRP composites suitable for use incoastal and marine structures like river bridge piers.In additions, their low density is important becauseit adds less weight to the existing structures, andbecause the use of heavy equipment for repair withFRP composites is not necessary during rehabilita-

tions. It has been shown that wrapping FRP fabricsaround the perimeter of both circular and rectangu-lar concrete columns to create a confinement effect

229


2/6

improves ductility and strength (Katsumata & Kimura1990; Picher et al. 1996). Other FRP confinementtechniques have been shown to improve the behav-ior of normal and high-strength concrete (Harmonet al. 1995). Retrofitting of concrete columns by lat-eral confinement with FRP wires has also resulted

in an increase in strength and ductility under uni-axial compression (Nanni et al. 1994; Houssam &Toutanji 1999). Amount of lacking of transverse rein-forcing is different in different case studies. In thisresearch 4 samples with different amount of transversereinforcing have been retrofitted by FRP jacketingmethod. The kind and thickness of FRP in all spec-imens are similar and the only difference betweenthem is the amount of transverse reinforcing. In nextpart of this paper the properties of FRP is presented.After that specification of samples andanalyzemethodhave been explained. Finally the comparison between

results has been done.

2 DESCRIPTION OF MECHANICALPROPERTIES OF MATERIALS

2.1 Mechanical properties of concrete

Essentially concrete is an inelastic material thusbehaves elastic in manyrespects. Compressionstrengthis only one specification of concrete materials. Forexplanation of concrete behavior, the strain-stresscurve could be useful. This curve explains that in

low stresses, concrete behaves elastic. This curveis inclined horizontally in higher stresses and strainreaches to 0.002 in ultimate stress. After ultimatestress, the curve generally is descending to rupturepoint. As much as concrete strength would be low, theductility property is higher.

The concrete used in the present study for all thespecimens is 25 MPa and the module of elasticity isthe same with value of 210 MPa. Thickness of con-crete cover is 2.5 cm in models. Stress strain curvethat have been used for confined concrete in finiteelements model is mander curve which is shown in

Figure 1 (Mander et al. 1988).

Mander

0

50

100

150

200

250

0 0.0080.002 0.004 0.006

Figure 1. Stress-strain curve for confined concrete.

2.2 Mechanical properties of steel bars

It is unavoidable to use of steel bars because the con-crete is weak in tension. Strength and deformationproperties of steel bars usually obtain from strain-stresscurves. Hot rolled steel bars often have a specific

yield point and at the time of rupture show significantstrain; forthis reason sometimescalled mild steels. Forsteel modeling, usually apply three plasticity models(Ansys Standard Users Manual):

2.2.1 Steel with isotropic hardeningThis model is very simple and often used when loadsare monotonic. In the mentioned model, Bauschingereffect is neglected and yield area is changed isotropi-cally.

2.2.2 Steel with kinematic hardeningThis state is a little more complicated than the iso-tropic model considers the Bauschinger effect and con-sisting of bilinear hardening model. In this case, onlythe center of yield area is transformed in space and notchanged isotropically.

2.2.3 Steel with combined hardeningThis model is the most exact of the steel models and iscombination of two previous models. This means thatyield area is changed isotropically and transformedin space. It results to improve the loadings cycles inaddition to consideration of Bauschinger effect. Fordetermination of this behavior is needed much param-

eters and their calibrations are difficult. Because steelis the secondary material (Concrete is the primarymaterial), then kinematic hardening model is used toconsideration of cyclic loads and it is not complicatedsuch as compound model.

2.3 Mechanical properties of FRP

Usually a composite material is defined as a phys-ical combination of two or some different materi-als in macroscopic scale. These materials keep theirmechanical and chemical properties and form a spe-

cific boundary with each other. These materials havebetter specification than their components.

Fiber Reinforced Plastic (FRP) products were firstused to reinforce concrete structure in the mid 1950s.Thoday, these FRP products take the form of bars,cables, 2-D and 3-D grids, sheet materials, plates,etc. FRP products may achieve the same or betterreinforcement objective of commonly used metallicproducts such as steel reinforcing bars, prestress-ing tendons, and bonded plates. Application andproduct development efforts in FRP composites arewidespread to address the many opportunities for

reinforcing concrete members (ACI 1996). Somemechanical properties of FRP is shown in Table 1.All the details about FRP material that used in the

present modeling have been shown in Table 2.

230


3/6

Table 1. Specifications of carbon, glass and aramid fibers.

Modulus of Ultimateelasticity Strength tensile

Material (GPa) (MPa) strain %

CarbonHigh strength 215235 35004800 1.42Ultra high Strength 215235 35006000 1.52.3High modulus 115130 35004000 2.53.5Ultra high modulus 500700 21002400 0.20.4

GlassE 70 19003000 34.5S 8590 35004100 4.55.5

AramidLow modulus 7080 35004100 4.35High modulus 115130 35004000 2.53.5

Table 2. FRP material details.

Modulus of elasticity in fibers direction 120 GPaTensile strength in f iber direction 1.5 GPaModulus of elasticity in vertical fibers direction 3 GPaTensile strength in vertical fiber direction 40 GPaThickness 0.5 mmUltimate strain 0.012

3 DESCRIPTION OF SPECIMENS

All the models have similar geometrics properties.All models were circular with diameter of 70 cm andheight of 300 cm. These properties have been shown inTable 3. Thicknesses of CFRP in all models are similar(0.5 mm).

4 FINITE ELEMENT MODELING

Concrete has been modeled by an 8-noded SOLID65element, which consist of a single solid material andup to three smeared reinforcing materials in three

different orientations. The solid material, i.e., plainconcrete, is treated as an initially isotropic homoge-neous material with different tensile and compressivestrengths. It is also capable of cracking in tensionand crushing in compression. Cracking can occurin any of the three orthogonal directions. The ele-ment can also accommodate plastic deformations andcreep. The jacket is modeled by 4-noded linear elas-tic membrane SHELL41 element, which is a threedimensional shell element with membrane stiffnessand three translational degrees of freedom per node.However, it does not have any bending stiffness, nor

any rotational degree of freedom. The element canaccommodate variable thickness, orthotropic behav-ior, stress stiffening, large deflection, and cloth option,the last of which is a nonlinear feature that constitutes

Table 3. Geometric details of models.

Height of columns 300 cmDiameter 70 cmConcrete cover thickness 205 cm

Figure 2. Three-dimensional modeling with its mesh.

wrinkling of the element in compression in one orboth orthogonal directions (Mirmiran et al. 2000).ANSYS (Ansys Structural Nonlinearities Manual),used in this study, has a parametric design languagethat is useful for parametric input and automatic mesh

generation. The following steps were taken in themodeling:

1. Geometric input consists of core diameter, speci-men height, and number of elements.

2. Concrete, FRP, and reinforcement bars propertiesinclude compressive strength, tensile strength, yieldstress, Poissons ratio and modules of elasticity.

5 LOADING AND ANALYZE METHOD

After making four 3-D finite element models of RCcolumns, confined with FRP, by ANSYS, axial loadstatically submitted to the top of the columns, thenlateral load as cyclic load submitted to the model withdisplacement control. As mentioned before all thecharacteristics of four models are the same, excepttransverse reinforcement ratio, as it has shown inTable 4.

6 VALIDATION BY EXPERIMENTAL MODEL

The validityof the proposedanalytical model is checkedthrough extensive comparisons between analyticaland experimental results of RC columns confinedwith FRP under compression load. The experimental

231


4/6

Figure 3. Cyclic load.

Table 4. Transverse reinforcing of models.

Model name Transverse reinforcement ratio

Model I 0Model II 0.004Model III 0.006Model IV 0.008

Table 5. Maximum lateral displacement in models.

Models Maximum lateral displacement

I 30 mmII 47 mmIII 48 mmIV 68 mm

Figure 4. Comparison between finite element model andSamaans model.

data used herein were derived from Samaan et al. test(1998). For this aim, Samaans model, exactly mod-eled in finite element software (ANSYS).comparison

between analytical and experimental model have beenshown in Figure 4. As it is shown, there is a goodrelation between them.

7 RESULT AND COMPARISONS

A comparison between four finite element modelshas been done. Force-Lateral Displacement curveshave been shown in Figure 5. Also hysteresis curvesderived from finite element nonlinear analysis have

been shown in Figures 69. As it shown in Figure 5

Figure 5. Comparison between results.

Figure 6. Hysteresis curve for model I.

Figure 7. Hysteresis curve for model II.

232


5/6

Figure 8. Hysteresis curve for model III.

Figure 9. Hysteresis curve for model IV.

in range 0.004 and 0.006 in transverse reinforcement,there is no difference. After the ratio of 0.006 thereis a significant increase in the strength and maximumdisplacement of specimen. Hysteresis loops of mod-els II and III have the same shape, but in model IV,there exist a good hysteresis model with more energydissipation.

8 CONCLUSIONS

As it shown in figures, the amount of transverse rein-forcing of FRP retrofitted column has no significanteffect in columns behavior, if amount of transversereinforcing be in a regular range (e.g. 0.0040.006).If amount of transverse reinforcing be more than usual,it has a good effect on columns behavior. In fact,

if transverse reinforcing of a column be sufficient,FRP retrofitting has no significant effect on columnsbehavior.

REFERENCES

ACI Committee 440R. 1996. State of the Art Report onFiber Reinforcement for Concrete Structures. AmericanConcrete Institute, Detroit.

ANSYS Standard Users Manual Help,Ver.11 and ANSYS

Structural Nonlinearities Manual.Ahmad, S.H., Khaloo, A.R. and Irshaid, A. Behavior of con-

crete spirally confined by Fiberglass Filaments. Magazineof Concrete Research, V. 43, No.156, 1991, pp. 143148.

Buckle IG (Ed). The Northridge, California earthquake ofJanuary 17, 1994: performance of highway bridge. Tech.Rep. NCEER-94-0008, Nat. Ctr. for earthquake Engrg.Res., state University of New York at Buffalo, NY, 1994.

Chaallal, O., Shahawy, M. and Hassan, M. ConfinementModel for Axially Loaded Short Rectangular ColumnsStrengthened With Fiber-Reinforced Polymer Wrapping,ACI Structural Journal, MarchApril 2003.

Fardis, M.N. and Khalili, H.H. Concrete Encased in Fiber-glass-Reinforced Plastic, ACI Journal, Proceedings V. 78,

No. 6, Nov/Dec. 1981, pp. 440446.Harmon, T., Slattery, K. and Ramakrishnan, S. The Effect

of Confinement for Concrete Structures, Proceedingsof the second International RILEM symposium, 1995,pp. 584592.

Houssam A, Toutanji: Stress-Strain Characteristics ofConcrete Columns Externally Confined with AdvancedFiber Composite Sheets.ACI Material Journal, 1999, pp.397404.

Kasei, M. Carbon Fiber Reinforced Earthquake-ResistantRetrofitting, Mitsubishi Kasei Crop., Japan 1993.

Katsumata, H. and Kimura, K. Applications of RetrofitMethod with Carbon Fiber for Existing Reinforced Con-crete Structures, 22nd joint UJNR Panel Meeting, U.S.-Japan Workshop, Gaithersburg, MD, 1990, pp. 128.

Katsumata, H., Kobatale, Y. and Takeda, T. A Study onthe Strengthening with Carbon Fiber for Earthquake-Resistance Capacity of Existing Reinforced ConcreteColumns, Proceedings of the seminar on Repair andRetrofit of Structures, U.S.-Japan Panel on wired andSeismic Effects, UJNR 1987, pp. 18161823.

Mander, J.B., Priestley, M.J.N. and Park, R. 1988: Theoreti-cal Stress-Strain model for confined concrete, Journal ofstructural engineering, V. 114, No. 8, pp. 18041826.

Mirmiran A., Zagers K. and Yuan, W. Nonlinear finite ele-ment modeling of concrete confined by fiber composites.Elsevier 2000, pp. 7996

Mufti, A.A., Eriki, M.A. and Jaeger, L.C. Advanced Com-posite Materials in Bridge and Structures in Japan, Cana-dian Society of Civil Engineering, Montreal, Canada,1992.

Nanni, A. Norris, M.S. and Bradford, N.M. Lateral Con-finement of Concrete Using FRP Reinforcement, FiberReinforced Plastic Reinforcement for Concrete Struc-tures, SP-138, American Concrete Institute, FarmingtonHills, Mich., 1994, pp. 193209.

Parvin azadeh, Wang Wei. Concrete columns confined byfiber composite wraps under combined axial and cycliclateral loads, 2002.

Picher, F. Rochette, P. and Labossiere, P. Confinement ofConcrete Cylinders with CFRP, Fiber Composites in

Infrastructure, Proceedings, First International Confer-ence on Composites in Infrastructure, 1996,pp. 829841.

233


6/6

Priestly, M.J.N., Seible, F. and Fyfe. E. Column SeismicRetrofit Using Fiber Glass/Epoxy Jackets, Proceedingsof the first International Conference on Advanced Com-posite Material in Bridge and Structures, Sherbrooke,Canada, 1992, pp. 287298

Priestley MJN., Seible F., Calvi GM. Seismic design and

retrofit of bridges. New York: Wiley; 1996.Priestley MJN. Whittier narrows, California earthquake ofOctober 1, 1987-damage to the I-5/I-605 separator. Earth-quake Spectra J 1988; 4(2): 389405.

Samaan, M., Mirmiran, A. and Shahawy, M. 1998: Model ofconcrete confined by fiber composites., J. of Stru. Eng.,ASCE, V. 124, No. 9, pp. 10251031, September 1998.

Seible, F., Priestly, N., Hegemier, G.A. and Innamorato, D.:Seismic Retrofit of RC Columns with Continuous Car-bon Fiber Jackets, Journal of Composites for Constructon,

V. 1, No. 2, 1997, pp. 5262.Spoelstra, M.R., and Monti, G. FRP-Confined ConcreteModel, Journal of composite for construction, V. 3, No. 3,August 1999, pp. 884888.

234

4.6 effect of transverse reinforcing on circular columns confined with frp

Documents