ISBN 978-9937-0-9019-3

Pushover Analysis of Beam Retrofitted Multi

Storey RCC Building using CFRP

Namaraj Kafle

Department of Earthquake Engineering,

Thapathali campus

Kathmandu, Nepal Namaraj.kafle@gmail.com

Abstract— In this study seismic response of two storey

reinforced concrete building is analysed by pushover analysis. Building frame is structurally analysed by using software SAP 2000 (version 14). Nonlinear pushover analysis is perform to check the performance point. Effect of external wrapping of carbon fiber reinforcement polymer (CFRP) on fail beam investigate the improvement on

performance of beam. CFRP is used as retrofitting technique. It has been concluded that after the use of CFRP for retrofitting of beam, the structure were found to be within the targeted performance level.

Keywords— CFRP, Capacity Curve, Performance Point, Seismic retrofitting, Pushover analysis

I. INTRODUCTION

Natural disaster are originated from natural event,

causes loss of life and property. The most common

natural disasters are earthquake, flood, storm, tsunami

etc. Nepal is the one of most seismic regions of the

world. Seismic retrofitting is considered one of most effective technique for earthquake risk reduction [1].

Retrofit refers to strengthening of existing structure.

The retrofit process is general term that may consist

of variety of treatment, including preservation,

rehabilitation, restoration and reconstruction.

According to the United Nations, Nepal is the 11th-

most earthquake prone country [2]. Therefore,

earthquake vulnerability in Nepal is great concern.

Generally, used retrofitting technique for reinforced

concrete structure are base isolation, seismic damper,

reinforced concrete jacketing, steel caging, fiber reinforcement polymer. For masonry structure are

plaster stitching, cement grouting, shotcreting, splint

and bandage.

In the recent decades, the availability of fiber

reinforcement polymer (FRP) with its favorable

property such as ease of application, high stiffness,

strength, light in weight, advanced fatigue and

corrosion resistances, etc., providing significant

functional and economic benefits, ranging from

strength enhancement and weight reduction to

durability features [3]. However, the FRP

strengthening technique has a few drawbacks, which are mainly associated with the use of epoxy resins—

namely, high cost, poor performance in high

temperatures, inability to apply on wet surfaces, and

incompatibility with substrate materials (concrete or

masonry). In an attempt to alleviate the problems

arising from the use of epoxies, researchers have

suggested the replacement of organic (epoxy resins)

with inorganic (mortar) matrix [4].

The efficiency of FRP retrofitting in

strengthening/repairing of structural beam column joints has confirmed in many studies worldwide.

Researchers have also investigated the related

problems such as FRP-concrete interface interaction

and creep behavior in FRP strengthened structural

members [5]. However, very few studies have

scrutinized the overall behavior of FRP rehabilitated

RC structures. Seismic performance of a full-scale

RC structure repaired with carbon FRP (CFRP)

laminates and wraps. Their experimental results

proved the existence of a large displacement capacity

in the repaired structure without any reduction of strength after the application of FRP at the beam-

column joints and walls. In addition, the energy

dissipation remains almost identical to the original

structure. On the contrary, a reduction in the

deformability of shear walls observed during the

experiments due to the presence of CFRP laminates

over the entire height. In another experimental study,

Di Ludovico et al. [6] Investigated seismic retrofitting

of an under-designed, full-scale RC structure with

FRP wrapping. In their study, a bi-directional test

with peak ground acceleration (PGA) equal to 0.2g applied to the original structure prior to retrofitting

under which the structure found inadequate. The

structure was then retrofitted in order to withstand a

50% higher PGA of 0.3g. The successful outcome of

the tests proved the effectiveness of FRP in

improving the global performance of the structure in

terms of ductility and energy dissipating capacity.

Improving the seismic behavior of deficient RC

structures with FRP composites has also confirmed

by Garcia et al. [7] Who through experimental tests

and numerical modelling found that FRP retrofit

results in substantial improvement of seismic performance of damaged RC frames. Following the

main trend of the argument, the current study

conducted to investigate to the seismic behavior of

FRP retrofitted RC buildings. To pursue this

objective, FRP sheets applied at the beams and

columns regions that are prone to the development of

KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”

Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal

KEC Conference 2021

198

ISBN 978-9937-0-9019-3

plastic hinges in such a way that increases the

flexural strength. As a result, FRP sheets applied at

the top and bottom flanges of members with fibers

oriented parallel to the longitudinal steel

reinforcements. Of particular interest was to compare the effects of GFRP and CFRP application. As the

case study, a 2-storey moment resisting RC building

selected representing the buildings. The seismic

behavior of the structure evaluated using the

nonlinear pushover method. In addition, the concept

of lumped plasticity with flexural hinges at both ends

of beams and columns implemented in the

characterization of nonlinear properties of the

structural members. The analyses carried out in SAP

2000, a commonly used finite element program by

the structural engineering profession. SAP2000 can

perform static or dynamic, linear or nonlinear analysis of structural systems.

II. METHODOLOGY

Two storey residential building is located in

seismic zone v. 3-d model of two storey building

structure model as shown in fig 1. there are two

number of bay in x and y direction. 27 fit in x

direction and 24 fit in y direction in plan. The

analyses carried out in SAP 2000. To perform

pushover analyses in SAP2000, users can create and

apply hinge properties. In SAP2000, a frame element

is modelled as a line element having linearly elastic

properties and nonlinear force displacement

characteristics of individual frame elements are

modelled as hinges represented by a series of straight

line segments. There are three types of hinge

properties in SAP2000. They are default hinge

properties, user-defined hinge properties and generated hinge properties. Studies show that user

defined hinge model gives better results than default

hinge model [8]. Moment-curvature relationship is

used to model plastic hinge behaviour in non-linear

analysis. The seismic performance of a structure can

be evaluated in terms of pushover curve, plastic hinge

formation etc. The maximum base shear capacity of

structure can obtained from base shear versus roof

displacement curve.

Equivalent compressive strength of the section

confined by CFRP was calculated using the equation formulated by (Riad Benzaid, 2013).Non-linear static

pushover analysis was done by using the equivalent

compressive strength and the behavior of the

structure at maximum roof displacement of 300 mm

was studied. The design base shear of the building

was calculated using the IS 1893:2016 and is

compared with the performance base shear obtained

from analysis.

S.N. Materials Properties

Materials/section Grade/size Unit

1 Concrete grade M20

(Beam/column)

2 Concrete grade (slab)

M15

3 Steel grade Fe500

4 Modulus of elasticity E (concrete)

19364.92 for m15 22360.68 for m20

N/mm2 N/mm2

5 Modulus of elasticity E (steel)

200000 N/mm2

6 Column size 500*500 mm*mm

7 Beam size 230*300 mm*mm

8 Slab thickness 150 Mm

9 Floor height 1.5 for plinth level 3 for story height

M

10 Wall thickness 230 Mm

11 Density of concrete

25 KN/m3

12 Density of brick 20 KN/m3

13 Thickness of CFRP 0.167 Mm

14 Modulus of elasticity (E) of CFRP

230000 N/mm2

15 Tensile strengthof CFRP

3400 N/mm2

16 Strain (CFRP) 0.014

17 Number of layer of CFRP

1 Number

1. CALCULATION OF EQUIVALENT

COMPRESSIVE STRENGTH The maximum value of the confinement pressure that

the FRP can exert is attained when the

circumferential strain in the FRP reaches its ultimate

strain and the fibers rupture leading to brittle failure

of the cylinder. This confining pressure f1 is given by:

F1=

Where,

E = Modulus of elasticity of CFRP

Ɛ = Ultimate CFRP tensile strain

T= Thickness of CFRP n= number of wrap of CFRP

b=Dimension of section

The effective CFRP strain coefficient (𝞰=0.68)

represents the degree of participation of the CFRP

jacket, and the friction between concrete and CFRP

laminate. Type bond, geometry, CFRP jacket

thickness, and type of resin affect the effective CFRP

strain coefficient

F1=

KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”

Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal

=1.74 The Equivalent confined compressive strength is

given by

FCC = FUC + 3.3 f1

FUC =Unconfined Compressive Strength of Concrete

KEC Conference 2021

199

ISBN 978-9937-0-9019-3

FCC = 20 + 3.3*1.74

= 25.74 N/MM2

2. CALCULATION OF DESIGN BASE SHEAR

The design base shear of the building is calculated

from IS1893 (Part 1):2016 Vb=AH*W

Ah=

Where,

Vb=Design Base Shear

W=Seismic Weight of the Building=3852.31KN

Deadload and 1.5KN/m live load is used.

AH=Design horizontal Acceleration Coefficient

Z=Seismic Zone Factor=0.36

I=Importance Factor=1

R= Response Reduction Factor=5

Sa/g=Design Acceleration coefficient=2.5 for soft soil

site with time period 0.27 sec

Therefore equation (3) becomes Vb=346..71KN

Figure 1 plan and elevation of model

III. RESULT AND DISCUSSION

Pushover analysis is iterative analysis and design

process continues until the design satisfies a pre-

established performance criteria. The performance

criteria for pushover analysis is generally established

as the desired state of the building given a rooftop or

spectral displacement amplitude. Pushover analysis is carried out by vertical loading (gravity load) followed by a gradually increasing

displacement con-trolled lateral load in both +x and +y direction. The design base shear calculated as per IS

specifications is compared with the overall capacity of the structure obtained from the pushover curve.

Moment-curvature parameters are used as the input

for modeling the hinge properties and it can be

idealized as

shown in Fig. AB represents the linear elastic range

from unloaded state A to its effective yield B,

followed by an inelastic but linear response of

reduced (ductile) stiffness from B to C. CD shows a sudden reduction in load resistance, followed by a

reduced resistance from D to E, and finally a total

loss of resistance from E to F.

Figure 2 Idealized moment-curvature relationship

Table indicate the ordinary buildings are designed as

frequently in fully operational, occasionally life

operational, rarely in safe and very rarely in near

collapse zone. Essential buildings are designed as

occasionally fully operation, rarely life operational

KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”

Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal

KEC Conference 2021

200

ISBN 978-9937-0-9019-3

and very rarely safe. In the similar way hazardous

facilities are rarely fully operational and very rarely

life operational. For the ordinary building frequently

life operational, safe and near collapse is not possible

as shown in figure. Such portion are inacceptable performance.

At a roof displacement of 300mm, the hinge

formation at different part of the structure and beam

members are fails. These all beams, after retrofit with

one layer of CFRP and analysis carried out then all

member are pass.

On the above retrofitted building frame the non-linear

static pushover analysis was also performed to

investigate the performance point of the building

frame in terms of base shear and displacement. After

pushover analysis the demand curve and capacity curves are plotted to get the performance point of the

structure. The performance point is intersection of

capacity and demand curve, obtained as per ATC 40

capacity spectrum method. The base shear for PUSH

X load case is 1105.908 KN and for PUSH Y base

shear at performance point is at 1164.632 KN as

shown in Fig.3. & Fig.4.

The design base shear of the building frame is found

to be 346.71 KN as per calculation. After performing

the analysis the base shear at performance point is

found to be 1105.908 KN for X directional loading and 1164.632 KN for Y directional loading, which is

greater than design base shear. Since at the

performance point base shear is greater than the

design base shear the building frame is safe under the

earthquake loading.

Figure 3 capacity curve in x direction

Figure 4 capacity curve in Y direction

Design base shear distribution

Floor level

Load (Wi) KN

Height (hi) m

Wi*hi2

Qi=Vb* Storey shear force (KN)

2 1615.07 3 14535.62 69.34 69.34

1 1615.07 6 58142.48 277.37 346.71

total 72678.11

IV. CONCLUSION

Pushover analysis is an ideal method to explore the

non-linear behaviour of structure. Moment-curvature

relationship is an essential tool to define the user

defined plastic hinge properties of the sections. Load–moment interaction curve is required for defining

column and beam hinges. As a result of the work that

was completed it is concluded that the building frame

used for pushover analysis is seismically safe,

because of the performance point base shear is greater

than design base shear.

KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”

Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal

KEC Conference 2021

201

ISBN 978-9937-0-9019-3

REFERENCES

[1] N. Giordano, A. Norris, V. Manandhar, L. Shrestha, D. R.

Poudel, N. Quinn, E. Rees, N. Giordano "Life-Cycle analysis

of incremental seismic retrofitting of traditional," NRA article.

[2] R. B. Malla, k. kayastha, S. sharma, S. P. Ojha, Earthquake

preparedness and disaster relief in Nepal, American Society of

Nepalese Engineers, 2014.

[3] Y. Ou, D. Zhu, H. Zhang, L. Huang "Mechanical

characterization of the tensile properties of glass fiber and its

reinforced polymer (GFRP) Composite under Varying Strain

Rates and Temperatures," polimers, p. 8, 2016.

[4] L. N. Koutas, Z. Tetta, D. A. Bournas, T. C. Triantafillou

"Strengthening of concrete structures with textile reinforced

mortars," ASCE, 2020.

[5] V. Berardi, L. Feo, A. Giordano " An experimental study on

the longterm behavior of CFRP pultruded laminates suitable to

concrete structures," Engineering, p. 39, 2008.

[6] D. Ludovico, E. Cosenza " Seismic strengthening of an under-

designed RC structure with FRP," Earthquake Engineering &

Structural, p. 37, 2008.

[7] R. Garcia, K. Pilakoutas " Seismic behaviour of deficient RC

frames," Engineering structure, p. 32, 2010.

[8] D. M. Daniel, S. T. john "Pushover analysis of RC building,"

International journal of scientific and Engineering research,

vol. 7, no. 10, p. 88, 2016 october.

KEC Conference 2021, April 18, 2021“3rd International Conference On Engineering And Technology”

Kantipur Engineering College, Dhapakhel, Lalitpur, Nepal

KEC Conference 2021

202