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Original Paper

International Journal of Informative & Futuristic Research ISSN (Online): 2347-1697

Volume 2 Issue 7 March 2015

Abstract

The present study centers on the use of retrofit technique which involve X steel bracing with centrally located ductile shear panel named as braced ductile shear panel(BDSP) system for RC building. The purpose of the work is to study the suitability of BDSP system used in reinforced concrete building during earthquake

ground motion. A G+4 storey and G+10 storey building is analyzed for seismic

zone IV as per IS 1893-2002 by response spectrum method using ETABS. The

effectiveness of BDSP bracing system in rehabilitation of G+ 4 storeys and G+10 storey building is examined. The performance of the building is evaluated in terms of storey drifts, fundamental time period and stresses developed in bracing system members. It is found that both BDSP system and pure X bracing system reduces the maximum inter storey drift significantly. Pure X bracing system

reduces drifts 3%-5% more than BDSP system. But the stresses developed in X bracing members are beyond the permissible limit and in BDSP system bracing member stresses are under permissible limit. In BDSP system only shear stresses in shear panel exceeds permissible limit. Therefore it is concluded that by using

BDSP system with the compramisation of 3%-5% drifts we can achieve significant improvement in safety of the structure.

Seismic Evaluation Of RC Building

By Using Steel Bracing With And

Without Shear Panel Paper ID IJIFR/ V2/ E7/ 069 Page No. 2106-2115 Subject Area

Structural

Engineering

Key Words BDSP Bracing System, X Bracing System, Seismic Protection, R/C Frames,

Response Spectrum Analysis

Aparna S. Patil 1

M.E. (Structure) Student, Department of Civil Engineering Rajarshi Shahu College of Engineering , Tathawade, Pune, Maharashtra-India

G R. Patil 2

Assistant Professor Department of Civil Engineering Rajarshi Shahu College of Engineering , Tathawade, Pune, Maharashtra-India

2107

ISSN (Online): 2347-1697 International Journal of Informative & Futuristic Research (IJIFR)

Volume - 2, Issue - 7, March 2015 19th Edition, Page No: 2106-2115

Aparna S. Patil, G R. Patil :: Seismic Evaluation Of RC Building By Using Steel Bracing With And Without Shear Panel

1. Introduction

Structures in high seismic risk areas may be susceptible to severe damage in a major earthquake.

For the variety of structures and possible deficiencies that arise, several retrofitting techniques can

be considered. Adding steel bracing is one of the retrofitting techniques and in past few years this

concept has been extended to strengthen reinforced concrete frames. Use of steel braces have

potential advantage over other schemes such that the bracing system adds much less weight to the

existing structures, opening for natural light can be made easily and disturbance to occupants may

be minimized during retrofitting work. Many researchers have studied on different bracing systems

such that external bracing system [10], internal bracing system [7].[8],[9], concentric bracing

system[1],[5],[8],[12],[13] and eccentric bracing system[3].

Vishwanath K. G., Prakash K. B., Anant Desai [13] have studied the seismic performance of

reinforced concrete building rehabilitated using concentric steel bracing. They carried out analysis

of 4, 8, 12, 16 storey building for seismic zone IV as per 1893-2002 with different types of steel

bracing system using STAAD Pro. They found that X type of steel bracing significantly contributes

to the structural stiffness and reduce maximum interstorey drift of the frame. Marco Valente [11]

proposed a new alternative dissipative bracing system named as braced ductile shear panel (BDSP)

bracing system for improving seismic performance of R/C frames carried out numerical

investigation. He performed nonlinear dynamic analysis on a 4 storey R/C 2D frame designed only

for gravity load through the computer code SIMQKE. He found that BDSP bracing system can

protect the primary structural elements of the frame preventing them from the damage under severe

seismic action.

The purpose of this paper is to study numerically the seismic performance of reinforced concrete

building constructed as ordinary moment resisting frame(OMRF) by using BDSP bracing system in

comparison with pure concentric X bracing system as a retrofitting technique..

2. Description Of Bracing System

In pure X bracing system two I shaped braces are provided. One brace act as a compression member

and other acts as a tension member when lateral load is applied. BDSP bracing system proposed by

Marco Valente consists of 4 short I shaped braces with centrally located shear panel. The ductile

shear panel consists of non-slender in plane plate elements stiffened around the perimeter by

boundary flanges and capable of achieving high levels of ductility when strained in elastically in a

shearing mode. Braces transfers the lateral displacements arising from the lateral load on the frame

to the shear panel. Braces and panel are connected using bolted connection.

Figure 2.1 : Schematic overview of the dissipative bracing system: (1) ductile shear panel,

(2) braces, (3) bolted connection [11]

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3. Model data of building and selection of bracing members

The buildings adopted consist of reinforced concrete and brick masonry elements. The frames are

assumed to be firmly fixed at the bottom and the soil–structure interaction is neglected.

Table 3.1: Model data of the buildings

Structure OMRF

No. of stories G + 4, G+10

Storey height 3.00

Type of building use residential

Foundation type Isolated footing

Seismic zone IV

Material properties

Grade of concrete M-20/M30

Grade of steel Fe 415

Density of reinforced concrete 25KN/

Modulus of elasticity of brick masonry

Density of brick masonry 19.2 KN/

Member Properties

Thickness of slab 0.125

Beam size 0.23m x 0.3m

Column size 0.23m x 0.6m

Thickness of wall 0.23 m

Dead Load Intensities

Floor finishes 1.00 KN/

Live Load Intensities

Roof and Floor 3.00 KN/

Earthquake LL on slab as per Cl. 7.3.1 and 7.3.2 of IS 1893(part 1)2002

Roof 0 KN/

Floor 0.25 x 3.0 = 0.75kN/

Seismic Zone IV

Zone factor, Z 0.24

Importance factor, I 1.00

Response reduction factor, R 3.00

The load cases considered in the seismic analysis are as per IS 1893 – 2002.

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Building to be considered for analysis has symmetrical plan. Plan dimension of building to be

modelled is 15m x 9m. It consists of five bays of 3m width at longer dimension side and three bays

of 3m width at shorter dimension side. Bracing member has been selected from pure brace system

such that the slenderness ratio should not exceed 180. ISMB 225 is selected for the bracing

members for both bracing system. Bracing configuration is considered in alternate bays at periphery

of the building .Shear panel in BDSP system is in pure shear. Therefore aspect ratio of shear panel

should be match with aspect ratio of structural frame. Shear panel of plate size 500mm x 500mm

with thickness 7.5mm is selected. Stiffeners having depth 100mm and thickness 7.5mm are selected

to provide at perimeter and middle of shear panel.

4. Model Description

For the analysis total six models in ETAB are prepared. Three for G+4 storey building and three for

G+10 storey building including bar frame model considering infill or wall load, model retrofitted

using concentric X bracing and model retrofitted using BDSP bracing system. Plan dimension,

column and beam size, bracing members are kept similar to both G+4 and G+10 Storey building

model. Models for G+4 storey building are shown below.

Figure 4.1(a): Plan of building

Figure 4.1(b) Elevation of the building

Figure 4.1(c): Building with X bracing Figure 4.1(d): Building with BDSP bracing

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5. Method of analysis

G+4 storey and G+10 storey building models with bare frame including wall load using X bracing

system and using BDSP bracing system are analyzed in ETABS software by response spectrum

analysis.10 number of modes are considered for the analysis so that above 90 % of total mass is

participated . Response spectrum functions are given as per 1893-2002 Clause no. 6.4.5 (Fig.2) for

type 1 soil that is hard soil and 5% damping. Load combinations are defined as per IS456-2000

Clause no. 36.4.1(Table 18). All possible load combinations for limit state of collapse and limit

state of serviceability are considered for analysis. Fundamental time period for both G+4 and G+10

storey buildings are defined as per IS 1893-2002 Clause no. 7.6.2.

6. Result Analysis

6.1: Modal Period Comparison

Figure 6.1 (a): Modal period comparison G+4

Figure 6.1 (b): Modal period comparison G+10

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

0 1 2 3 4 5 6 7 8 9 10

BARE FRAME X BRACING

BDSP

Mode Number

00.20.40.60.8

11.21.41.61.8

22.22.42.62.8

3

0 1 2 3 4 5 6 7 8 9 10

BDSP

X BRACING

BARE

Tim

e p

erio

d

Mode number

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It is observed from fig.6 and fig.7 fundamental time period of X braced model and BDSP model is

reduced upto 50%to 55% in G+4 and 40% to 45% in G+10 than that of bare frame model.BDSP

model has 2% to 2.5% more values of time period than X braced model.

6.2: Storey Drift Comparison

Figure 6.2(a): Storey drifts comparison G+4

Figure 6.2(b): Storey drifts comparison G+10

It is observed from figure 8 and figure 9 that storey drifts of bare frame model is reduced by 85% to

95% for G+4 and 75% to 80% in G+10 storey building in both X bracing and BDSP bracing model.

BDSP bracing model has 3% to 5% more drift values than X bracing model.

0

1

2

3

4

5

6

0 0.002 0.004 0.006 0.008

bare

X bracing

BDSP

Inter-storey drift

Sto

rey

Nu

mb

er

0

1

2

3

4

5

6

7

8

9

10

11

0 0.002 0.004 0.006 0.008 0.01

bare frame

X bracing

BDSP bracing

Sto

rey

nu

mb

er

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6.3 : Roof Point Displacement Comparison

Figure 6.3: Roof point displacement comparison

By using X bracing and BDSP bracing system the displacements of bare frame model are

significantly reduced. It is observed from figure 10 that the value of roof point displacement of bare

frame model are reduced by 80% to 85% in G+4 storey building and 65% to 70% in G+10 story

building. BDSP bracing model has 3% to5% more displacement value than X bracing model.

6.4: Comparison between stresses in bracing members of X bracing model and BDSP bracing

model

Permissible tensile and compressive stresses in bracing members are calculated as per IS 800-2007

Clause no.11.2 and 11.3. For reference bracing members in single bay shown in figure are taken

into consideration. Bracing members that exceeds permissible limit are highlighted in table 6.4(a)

Table 6.4: Stresses in bracing members of X Bracing model ( G+4 )

Storey Brace Load P

Actual

stress

Permissible

stress

ROOF. D7 ENV COMB -13.33 -3.356 -30.1

STORY4. D7 ENV COMB -98.64 -24.834 -30.1

STORY3. D7 ENV COMB -156.33 -39.358 -30.1

STORY2. D7 ENV COMB -195.5 -49.22 -30.1

STORY1. D7 ENV COMB -222.35 -55.979 -30.1

ROOF. D8 ENV COMB -42.43 -10.682 -30.1

STORY4. D8 ENV COMB -133.73 -33.668 -30.1

STORY3. D8 ENV COMB -190.55 -47.973 -30.1

STORY2. D8 ENV COMB -225.2 -56.697 -30.1

STORY1. D8 ENV COMB -238.95 -60.159 -30.1

0

20

40

60

80

100

120

140

160

180

200

G+4 G+10

BARE

BDSP

X BRACING

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From table 6.4(a), it is observed that in G+4 storey building many of bracing members in pure

X braced model have greater stress values than the permissible stresses in compression. Almost

all bracing members in storey 1, 2, 3 and 4 are overstressed. Also from table6.4(b), it is

observed that in G+4 storey building , all bracing members in BDSP model have stress values

lesser than permissible limit. Similar results are observed in G+10 storey building.

Table 6.4 (b): Stresses In Bracing Members Of BDSP Model (G+4 )

Story Brace Load

Actual

Stresses

Permissible

Stress

ROOF D31 ENV COMBO -3.6883 104.5

STORY4 D31 ENV COMBO -24.882 104.5



















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From Table 6.4(c) it is observed that for G+4 storey building, shear panels from storey 1 to

storey 2 exceeds permissible shear stress value. That means in BDSP bracing system bracing

members are under permissible limit and shear panels are exceeds permissible shear stress

probably fail in shear during earthquake motion. Similar results are observed in G+10 storey

building.

7. Conclusion and Future scope

Dynamic analysis of bare OMRF frame including infill effect for G+ 4 stories and G+10 story

building shows that the drift values exceeded allowable drift. Building frame using pure X

bracing system and BDSP bracing in alternate bays at perifery of the building significantly

reduced drift as well as time period .Drifts of BDSP system model are 3% -5% more than that

of X braced model. But the drifts are very less than allowable drift. In pure X braced model

most of bracing members are overstressed in compression i.e. the members fail in compression

and the stresses may be transferred in structural elements of frame during earthquake motion. In

BDSP bracing model introduction of panel centrally in X bracing reduces the length of bracing

member. The bracing member in BDSP remains in elastic range as not exceeds permissible

stress limit. Stresses in shear panel exceed the permissible limit i.e. ductile shear panel probably

fails in shear during earthquake motion means energy dissipation occurs in shear panel and

bracing members remains safe in BDSP bracing system. As thickness of shear panel reduced

shear stress values in shear panel increases. In BDSP system only shear panel is damaged so

after earthquake damage we have to replace only shear panel rather than whole bracing member

in X bracing. Thus retrofitting of damaged building is easy and economical.

Experimental tests are required to assess the potential of BDSP to validate the computational

simulation results and to address the design detailing requirements which play important role in

system performance. Future development for the numerical model can be including the addition

of damage failure of steel panel. Experimental investigation for the design of connections

between braced member and RC frame as well as connection between braced member and

ductile shear panel is required. Design of economical sections for bracing members and ductile

shear panel need to be studied.

Table 6.4(c): Shear stresses in braced ductile shear panel for G+4 storey

Story

Shear

Panel

Load

Case

S11

Top

S22

Top

Permissible

Shear Stress

ROOF W61 ENV COMBO -4.62 -16.18

100

STORY4 W61 ENV COMBO -66.91 -66.76

100


100


100


100

2115




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Research article ISSN 0976 – 4399

Notations:

ENV = Enveloping load combinations

S11= In plane shear stress values in local 11 axis

S22= In plane shear stress values in local 22 axis.

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