Seismic response of frp strengthened rc frame

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308 (Print),
INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND
ISSN 0976 – 6316(Online) Volume 3, Issue 2, July- December (2012), © IAEME
TECHNOLOGY (IJCIET)
ISSN 0976 – 6308 (Print)
ISSN 0976 – 6316(Online)
Volume 3, Issue 2, July- December (2012), pp. 305-321
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SEISMIC RESPONSE OF FRP STRENGTHENED RC FRAME
Shaikh Zahoor Khalid 1 S.B. Shinde2
1 2
P.G. Student Dept. of Civil Associate Professor Dept. of Civil
Engineering, J.N.E.C., Aurangabad Engineering, J.N.E.C., Aurangabad
(M.S.) India. (M.S.) India.
E-mail : szahoor555@gmail.com E-mail : sb_shinde@yahoo.co.in

ABSTRACT

The use of fiber-reinforced plastic (FRP) materials is becoming attractive solution to
retrofitting, strengthening and constructing column-like structural systems. The method is
considered superior to conventional concrete and steel jacketing methods in terms of
confinement strength; post-retrofit ductility, sectional area, weight, corrosion resistance;
application ease and overall project costs.
Axial strength and ductility increase of concrete columns is needed whenever repair and
strengthening are involved. Repair may be required when columns are damaged under
excessive external loads or due to erosion in exposed environments. Strengthening may be
required when there is a change of structural use or removal of some adjacent load bearing
structural members. Concrete jacketing, though has a lower cost, simply adds weight and
cross sectional area to the original structure and may be undesirable. On the contrary, FRP
composites, initially developed for aerospace and automobile applications, are found to be a
very promising material for civil engineering applications because of their high
strength/weight ratio, high corrosion resistance, ease of installation, and relatively low cost of
maintenance.
The aim of the present research was to recover the structural properties that the Frame had
before the seismic action by providing both column and beams cracks with FRP laminate and
to prove that FRP can be used for retrofitting for cracked sections. The driving principals in
the design and the outcomes of the study are presented in the paper. Comparisons between
original and repaired structures are discussed in terms of global and local performances. In
addition to the validation of the proposed technique, the results will represent a reference
database for the development of design criteria for the seismic repair of RC frames using
FRP.

Keywords: Fiber reinforced plastic (FRP), Axial Strength, Concrete Jacketing, Retrofitting,

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1. INTRODUCTION

Fiber-reinforced plastic (FRP) confined concrete under static axial, flexural and cyclic or
seismic lateral loads have been under investigation to develop retrofit technologies and new
construction methods. Over the past twenty years, a few models were developed for FRP
confined concrete. No one model can be applied directly for design with confidence.
Moreover, all models for the prediction of ultimate strength of the hybrid column are
developed based on the first confinement model that is developed by Richart et al. in 1927.
The only changes were the difference in constant coefficient and the power coefficient
corresponding to the confining pressure. They are all developed from regression analysis of
the researchers’ experimental data. A few stress-strain relationships were also developed so
as to model the load-deformation behavior of the confined concrete. These models mainly
relied on previously developed models for concrete or soil behaviors under triaxial stresses or
confinement stresses. Subsequent modifications were made to fit in the use of FRP
confinement.[1-7]
Finally, it should be pointed out that all these models are only valid, if accurate, for the use
of FRP encased concrete construction method, but not for retrofitting or strengthening of
concrete. The reason is that from the preceding experiments, FRP were used to wrap on
sound concrete instead of cracked concrete, which can have a very different behavior upon
initial loading. The FRP materials were also wrapped on initially unstressed concrete
specimens. When the specimens were prepared and loaded, both concrete and FRP were
stressed together from scratch. In reality, for the case of strengthening or retrofitting,
concrete columns are already under stress. The FRP is used to wrap around stressed columns
afterwards. The strength increase by FRP confinement may or may not be the as much as
predicted because the Poisson’s ratio of damaged concrete is much larger than that of
concrete stressed in the elastic region, hence activating the FRP confinement much earlier
than one might expect due to substantial radial dilation. Plastic zone of concrete is entered not
long after additional loading after wrap. Ultimate strain will definitely be different in this
case. [9-12]
Amirr M Malik, Hamid Saadatmanesh[8] presented analytical models to calculate the
stresses in the strengthened beams, and the shear force resisted by the composite plates before
cracking and after formation of flexural cracks. The anisotropic (orthotropic) behavior of the
composite plates or fabric has been considered in the analytical model. The companion paper
extends this discussion into post cracking behavior at the ultimate load, where the diagonal
shear cracks are formed. The method has been developed assuming perfect bond between
FRP and concrete (i.e. no slip), and using compatibility of the strains in the FRP and the
concrete beam.
Amer M. Ibrahim, Mohammed Sh. Mahmood[13] presented an analysis model for
reinforced concrete beams externally reinforced with fiber reinforced polymer (FRP)
laminates using finite elements method adopted by ANSYS. The finite element models are
developed using a smeared cracking approach for concrete and three dimensional layered
elements for the FRP composites. The results obtained from the ANSYS finite element
analysis are compared with the experimental data for six beams with different conditions
from researches (all beams are deficient shear reinforcement). The comparisons are made for
load-deflection curves at mid-span; and failure load. The results from finite element analysis
were calculated at the same location as the experimental test of the beams.
Baris Binici, Guney Ozcebe, Ramazan Ozcelik[14] Proposed that an urgent need to
retrofit deficient mid-rise reinforced concrete (RC) frame buildings in Turkey. For this
purpose, an efficient FRP retrofit scheme has been developed previously, in which hollow
clay brick infill walls can be utilized as lateral load resisting elements after retrofitting. The

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main premise of this practical retrofit scheme was to limit inter-storey deformations by FRP
strengthened infill walls that are integrated to the boundary frame members by means of FRP
anchors. Based on the analytical model that was previously verified extensively by
comparison with test results, a simplified model was proposed for use in displacement based
design of FRPs for deficient RC frame buildings.
Researchers have been going on in the last two decades in the United States, Canada,
Japan, Singapore, and some other countries. Significant contributions were mainly founded in
the past ten years in the US and Japan. Besides the above mentioned studies there have been
several studies on FRP, all these studies are based on testing columns under different load
conditions or testing beams under different load conditions. Experimental testing and
analytical models have been studied but differently for beams and columns and not as a
single unit i.e RC frame. Hence there is a gap in knowledge of the behaviour of FRP wrapped
RC frame when studied as a unit [15-16].

2. PRESENT STUDY

This paper presents findings of a programme where RC frame has been analysed using
finite element analysis in STAAD PRO software. Different number of models with different
thicknesses of FRP has been made and results have been compared for various location of
cracks.
2.1 Design Parameters Considered:-
Beam cross Section: 300mmx500mm
Column Cross Section: 300mmx500mm
Beam length: 5m
Column Length: 4m (Floor to Floor)
Diameter of Steel Bar: 20mm
Thickness of FRP Sheet: 1mm

2.2 Material Constants :-
a. Concrete: Density= 25kn/m3
Elasticity= 2.5x107kn/m2
Poisson’s Ratio=0.15
b. Steel: Density= 78.5kn/m3
Elasticity= 2.1x108 kn/m2
c. FRP: Density= 16kn/m3 (As per ACI 440.2R-02)
Elasticity= 3.6 x107kn/m2

2.3 Method of Analysis:-

Different frame models namely 2bay 3storey and 3 bay 5 storey are made using
STAAD.PRO.V8i software and are analysed for different crack locations and various
thicknesses of FRP. Line models of 2bay 3storey and 3bay 5storey can be seen in fig 1
and fig 2 respectively. For analysis purpose each frame model i.e. 3bay 5Storey and
2bay 3storey is distinguished into three groups namely Model I, Model II and
MODELIII depending upon the different crack patterns. Each group consists of
• Sound RC Frame

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• RC Frame Cracked During Earthquake Load
• RC Frame Retrofitted Using FRP Laminate

Fig. 1: 2 Bay 3 Storey line Model Fig. 2: 3 Bay 5 Storey line Model

2.4 3Bay 5Storey:-
In order to validate the numerical representation of the reinforced concrete beams and
columns strengthening with Fiber Reinforced Plastic (FRP), lateral loading is considered for
each storey for the analysis of Frame. Results and comparisons are based on this analysis.
For the present paper the analysis is done at different crack location and the thickness of FRP
used for wrapping the cracked section is 1mm. Three different frame models with different
crack location will be analysed using the proposed STAAD PRO finite element analysis.

Model I

A) Sound RC frame:
This is a solid RC frame which is subjected to design
lateral loads on each storey. The analysis of the behaviour of
this frame for the stresses developed and storey displacement
due to the applied lateral load is observed. Refer Fig. 3

Fig. 3 : Sound RC Frame
B) RC frame cracked due to earthquake load:
This frame consists of three different cracks developed due
to lateral load. The first crack at an integration point is shown
with a red circle outline, the second crack with a green outline
and the third crack with a pink outline.(refer fig4).The
analysis of the behaviour of this frame for the stresses
developed and storey displacement due to the applied lateral
load is observed.
This frame consists of three different cracks developed due to
lateral load. The first crack at an integration point is shown with a
red circle outline, the second crack with a green outline and the
third crack with a pink outline.(refer fig4).The analysis of the Fig. 4 : R.C. Frame Cracked
behaviour of this frame for the stresses developed and storey During Earthquake Load
displacement due to the applied lateral load is observed. (Pattern I)

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C) RC frame retrofitted by FRP laminate:
In this frame the cracked sections mentioned in the above
frame are wrapped using 1mm thick FRP laminate. The
cracked sections are wrapped as a single unit long the
periphery. Again the analysis of the behaviour of this frame
for the stresses developed and storey displacement due to the
applied lateral load is observed. Refer Fig. 5
Fig. 5: R.C. Frame Retrofitted
(Wrapped) by F.R.P. Laminate
(Pattern I)

The stress pattern and storey displacement obtained from the analysis of RC frame cracked
due to earthquake load and RC frame retrofitted by FRP laminate are compared with the
stress pattern and storey displacement obtained from the analysis of Sound RC frame.
As the frames of Model I are analysed and compared, similar analysis and comparison is
done for Model II and Model III frames for different crack locations. The Sound RC frame is
same as mentioned above in Model I for both Model II and Model III. RC frame cracked due
to earthquake load and RC frame retrofitted by FRP laminate for Model II is shown in fig 6a
and 6b respectively, similarly RC frame cracked due to earthquake load and RC frame
retrofitted by FRP laminate for Model III is shown in fig 7a and 7b respectively.

Fig. 6a : R.C. Frame Cracked During
Earthquake Load (Pattern II)
Fig. 6b: R.C. Frame Retrofitted (Wrapped) by F.R.P.
Laminate (Pattern II)

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Fig. 7a : R.C. Frame Cracked During Fig. 7b: R.C. Frame Retrofitted (Wrapped) by F.R.P.
Earthquake Load (Pattern III) Laminate (Pattern III)

3. RESULT AND COMPARISON

Results obtained from the different frame models for stresses and storey displacements are
compared. For different crack locations, the cracks developed are in beam as well as in
columns. So foe better comparison of the results the beam stresses and the column stresses
are compared individually
Model I
3.1 Beam Stress Pattern Comparison

Fig. 8a: Sound R.C. Frame Fig.8b: R.C. Frame Cracked During Earthquake Load
(Pattern I)

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Fig. 8c: R.C. Frame Retrofitted (Wrapped) by F.R.P. Laminate (Pattern I)

It can be seen from the beam stress pattern developed in the sound RC frame due to lateral
loads (fig. 8a), these stresses are disturbed when the cracks are developed due to earthquake
load (fig. 8b), and the variations in the stress pattern can be observed near the crack locations.
But when the same frame i.e. RC frame cracked due to effect of earthquake load is retrofitted
with FRP laminate of 1mm thickness, the original stress pattern and strength as that of the
sound RC frame is regained (fig. 8c).

3.2 Column Stress Pattern Comparison

Fig.9b: R.C. Frame Cracked During Earthquake Load
Fig. 9a: Sound R.C. Frame (Pattern I)

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Fig. 9c: R.C. Frame Retrofitted (Wrapped) by F.R.P. Laminate (Pattern I)

It can be seen from the column stress pattern developed in the sound RC frame due to
lateral loads (fig. 9a), these stresses are disturbed when the cracks are developed due to
earthquake load (fig. 9b), and the variations in the stress pattern can be observed near the
crack locations. But when the same frame i.e. RC frame cracked due to effect of earthquake
load is retrofitted with FRP laminate of 1mm thickness, the original stress pattern and
strength as that of the sound RC frame is regained (fig. 9c).

3.3 Storey Displacement
In the table given below the storey displacement in x-direction of Sound RC frame, RC
frame cracked due to earthquake load and RC frame retrofitted (wrapped) by FRP laminate
are compared.
z

Table I: Comparison for storey displacement in x-direction for Model I

Storey Displacement in x-direction (mm)
Storey RC frame cracked RC frame retrofitted
Sound RC frame
during earthquake load by FRP laminate
0 5.281 5.283 5.281
1 36.072 36.105 36.077
2 66.764 66.848 66.777
3 92.466 93.043 92.536
4 111.369 114.533 111.711
5 122.464 134.453 123.485

From the table above it can be observed that when the lateral load is acted upon the RC
frame cracked due to earthquake load, the storey displacement varies as compared to the
sound RC frame. The variations of the storey displacement can be seen on the storeys where
the cracks are developed in the beams or columns. In the above case the cracks are located on
the 4th and 5th storey, but when the crack frame is retrofitted with FRP laminate the original

312


storey displacement is observed as that of sound RC frame. Considering the storey
displacement the graph is plotted (fig. 10).

Fig10: Storey Displacement In X-Direction

Model II

(Pattern II)

313


Fig. 11c: R.C. Frame Retrofitted (Wrapped) by F.R.P. Laminate (Pattern II)


Fig. 12a: Sound R.C. Frame (Pattern II)

314


Fig. 12c: R.C. Frame Retrofitted (Wrapped) by F.R.P. Laminate (Pattern II)

are compared.

Table II: Comparison for storey displacement in x-direction for Model II

Sound RC frame
0 5.281 5.283 5.281
1 36.072 36.037 36.071
2 66.764 66.836 66.774
3 92.466 91.482 92.468
4 111.369 118.739 112.056
5 122.464 130.529 123.237

315



Model III


(Pattern III)

316


Fig. 13c: R.C. Frame Retrofitted (Wrapped) by F.R.P. Laminate (Pattern III)


(Pattern III)
Fig. 14a: Sound R.C. Frame

317


Fig. 14c: R.C. Frame Retrofitted (Wrapped) by F.R.P. Laminate (Pattern III)

are compared.

Table III: Comparison for storey displacement in x-direction for Model III

Sound RC frame
0 5.281 5.442 5.295
1 36.072 34.887 36.010
2 66.764 70.753 67.310
3 92.466 99.872 93.459
4 111.369 118.890 112.404
5 122.464 131.520 123.677

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As the stresses in beams, stresses in columns and storey displacement are obtained and
compared for Model I, similar analysis and comparison is done for Model II (fig. 11a, 11b,
11c and Fig. 12a, 12b,12c) and Model III(fig.13a, 13b, 13c and 14a, 14b, 14c ). In all the
models after retrofitting the RC frame cracked due to earthquake load, the stress pattern and
the strength obtained is in good agreement with that of the original sound RC frame.
Similarly, the storey displacement for Model II (table 2, fig. 14) and Model III (table 3, fig.
15) can be regained for the RC frame cracked due to earthquake load after retrofitting it with
FRP laminate.

4. DISCUSSION AND CONCLUSION

Different techniques have been developed in order to achieve local modification of
structural components. Reinforced concrete jacketing, steel profile jacketing and steel
encasement have been widely used in the past. All of them were characterised by
disadvantaged related to constructability (i.e. difficulty of ensuring perfect bond and
collaboration between new and old parts, loss of space, construction time and high impact on
building function) and durability issues. Innovative techniques based on FRP material appear
to be interesting alternatives to those solutions, along with high structural effectiveness. FRP
is light and easy to install, their application does not imply loss of space and in some cases it
can be performed without interrupting the use of structure.
This numerical solution adopted to evaluate the ultimate shear strength of RC frame
wrapped with FRP laminate is a simple, cheap and rapid way compared with experimental
full scale set test. The results obtained demonstrate that the use of FRP laminate is far more
effective and easy than reinforced concrete jacketing and steel profile jacketing in
strengthening RC frame. The present models can be used in additional studies to develop
design rules for strengthening RC frame using FRP laminates.
The main outcomes of the present study can be summarised as follows:

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• The results confirmed that FRP laminate allowed the RC frame retrofitted, to
withstand the lateral loads as the original RC frame.
• RC frame retrofitted with FRP laminates showed a large displacement capacity
without exhibiting any loss of strength and was able to provide energy dissipation
very similar to that of the original sound RC frame.
• The cyclic behaviour of RC frame retrofitted with FRP laminates was stable and no
significant effect of cumulative damage was observed on the strengthened elements
• FRP laminates are a better option for retrofitting because the can withstand the
seismic loads.

ACKNOWLEDGEMENT

The authors wish to thank the Management, Principal, Head of Civil Engineering
Department and staff of Jawaharlal Nehru engineering College, Aurangabad and Authorities
of Dr. Babasaheb Ambedkar Marathwada University for their support. The authors express
their deep and sincere thanks to Mr. Karim M. Pathan (Consulting Structural Engineer,
Aurangabad) for his tremendous support and valuable guidance from time to time.

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Seismic response of frp strengthened rc frame

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