Analysis of Reinforced Concrete (Rc) Frames Under Lateral Loads Using Steel Bracings

Abstract

The preliminary prerequisite of a structure is to satisfy serviceability conditions, stiffness and safety. The shape and proportions of the structure effects the distribution of forces. Due to irregularities in the structure it will get deficient stiffness at continuous load path and tension capacity will be re-entrant at corners. Steel bracings are retrofitted to increase the strength of RC frame structure and are good rehabilitation technique. Bracing provide high stiffness due to horizontal shear and is primarily absorbed and resist the lateral forces by developing the internal axial action and relatively small flexural actions. Steel bracings are economical, less self weight, good appearance, less spaced occupied, compared with other retrofitting techniques. Steel bracings are known for its efficient structural system for buildings under high lateral loads. The applications of steel bracings are faster to execute. Seismic study is carried out by considering G+15 unsymmetrical RC structures in zone III with bare frame and compared with X-braced frame, single diagonal braced frame, V-braced frame, K-braced frame bracings and static analysis is carried out using ETABS software. These bracing are used at specific locations and results are obtained for Base Shear, Story Displacement, Story Drift, and Time Periods are compared with bare frame models in seismic zone III. Keywords:-Bare Frame (BF), X-Braced Frame (XBF), Single diagonal Braced Frame (SDBF), V-Braced Frame (VBF), K-Braced Frame (KBF).

Full text

Volume 3, Issue 11, November – 2018 International Journal of Innovative Science and Research Technology
ISSN No:-2456-2165
IJISRT18NV328 www.ijisrt.com 364
Analysis of Reinforced Concrete (Rc) Frames Under
Lateral Loads Using Steel Bracings
Puneeth K, Rudresh A N
Asst.Professor, Department of Civil Engineering
ATMECE, Mysore, India
Abstract:- The preliminary prerequisite of a structure is to
satisfy serviceability conditions, stiffness and safety. The
shape and proportions of the structure effects the
distribution of forces. Due to irregularities in the structure
it will get deficient stiffness at continuous load path and
tension capacity will be re-entrant at corners. Steel
bracings are retrofitted to increase the strength of RC
frame structure and are good rehabilitation technique.
Bracing provide high stiffness due to horizontal shear and
is primarily absorbed and resist the lateral forces by
developing the internal axial action and relatively small
flexural actions. Steel bracings are economical, less self
weight, good appearance, less spaced occupied, compared
with other retrofitting techniques. Steel bracings are
known for its efficient structural system for buildings
under high lateral loads. The applications of steel bracings
are faster to execute. Seismic study is carried out by
considering G+15 unsymmetrical RC structures in zone III
with bare frame and compared with X-braced frame,
single diagonal braced frame, V-braced frame, K-braced
frame bracings and static analysis is carried out using
ETABS software. These bracing are used at specific
locations and results are obtained for Base Shear, Story
Displacement, Story Drift, and Time Periods are compared
with bare frame models in seismic zone III.
Keywords:- Bare Frame (BF), X – Braced Frame (XBF),
Single diagonal Braced Frame (SDBF), V- Braced Frame
(VBF), K – Braced Frame (KBF).
I. INTRODUCTION
Earthquake is a natural disaster which occurs with
various magnitude and intensities causing damage to
engineering properties and infrastructures which leads to
economical loss & loss of life. It is mainly based on focal
depth, location, magnitude, origin, and epicentral. Earthquake
may be caused by tectonic plate movements. Human activities
like coal mining, oil drilling, constructing enormously heavy
building, and large amount of water stored in a dam leads to
earthquakes. Global warming is also reason for increase in the
frequency and intensity of the earthquake. Earthquake damage
the buildings fully or partially.
Rehabilitation for damaged buildings has two technique
levels: Structural level & member level rehabilitation. In
structural level, modification can be done with respect to
whole structure, steel bracings is the best example. Seismic
performances of Beams, columns, walls are improved in
member level. Different rehabilitation techniques to improve
the seismic performance of the existing non-ductile RC
structure: adding new structural member such as steel bracing
or new structural walls, FRP wrapping and steel jackets, use of
base isolation or dampers. There are two generally used
procedure specifying seismic design of lateral forces (a)
Equivalent static analysis, (b) Dynamic analysis, Dynamic
analysis has two types (1) Response spectrum, (2) Time
history analysis. In Static analysis Base shear is calculated
approximately by assuming that structure is rigid, acceleration
is same at every point on the structure, also structure and
foundation as ideal fixity. It has only one mode of vibration on
the structure. An irregular structure leads to erroneous results.
Time history method based on suitable ground motion.
Response spectrum analysis based on design response
spectrum specified in Indian code or on site specific design.
Steel bracings or shear walls are commonly used to
increase the seismic performance. Steel bracing system is most
commonly used instead of shear elements because it’s
economical and easiness of construction. Steel bracings are
usually used in steel structure. From last two decades the use
of steel bracings in RC frames came into practice. Bracings
reduced the earthquake induced torsion in the structure. It acts
has a truss that provides lateral stiffness to buildings. Steel
bracing system contributes less mass, when compare to other
retrofitting techniques. Seismic forces are proportional to the
weight of the building. Structures designed with bracings can
improve the energy absorption and resists the forces generated
by ground waves. It provides good ductile property with
respect to earthquake loads. This system reduces lateral
displacement considerably and also decreases the lateral drift,
they decreases shear forces and bending moments in columns.
Nitin Bhojkar and Mahesh Bagade [2015] Use of steel
bracings in high raised building to resist against lateral loads.
G+9 stories R C building situated in zone III. Main parameters
taken into consideration are base shear axial force, story drift,
lateral displacement. From the verdict bracing system is the
better retrofitting technique. No major change in the total
weight of the structure after the addition of bracings. About
65% of lateral displacement is minimized. Increase in stiffness
of the structure.
Vishwanath K G, Prakash K B, and Anath Desai [2010]
Analytical research work on use of concentric steel bracing in
existing R C building to resist the seismic loads. The R C

Volume 3, Issue 11, November – 2018 International Journal of Innovative Science and Research Technology
ISSN No:-2456-2165
IJISRT18NV328 www.ijisrt.com 365
building of four stories is conceded for analysis. The
peripheral columns are offered with steel bracings. The
building is located in zone IV and analysis is carried out in
STAAD.PRO software. The number of stories is increased to
8, 12, and 16. After the evaluation of results it is found that X-
type bracing s are minimize the lateral displacement, story
drift and bending moments.
The objective of the present work is to findout the
performance of G+15 storied building under lateral loads and
RC buildings with steel bracing such as X, V, diagonal, K in
static condition for seismic zone III and soil type 2 using
ETABS software. The parameters such as storey drift,
displacement, base shear, and time period are compared with
the different bracing systems with above parameters and
selecting the best among the different steel braces.
II. METHODOLOGY
T-shaped R C building of G+15 stories is considered for
modeling with bare frame and different types of bracings (X,
V, single diagonal, K). Designed as per Indian standard codes,
and linear static analysis is carried out for all models. The
performance of RC structure with bracings and without
bracings for storey displacements, drifts, base shear & modal
time period are compared with the performance of bare frame.
III. MODELLING
Various parameters such as load intensities, material
properties, dimension of the structural member and the seismic
data considered in the modelling of different types of
buildings considered for analysis are mentioned below.
Storey height (including base)
3.5 m
Number of story’s
G+15
Type of structure
RCC
Width of bay in X-direction
5m
Width of bay in Y-direction
5m
Bare frame Support condition
Fixed
Braced frame support condition
Pinned
Thickness of slab
175 mm
Wall thickness
230 mm
Cross section property of steel bracing
ISMB 200
Table 1:- Geometrical details of T-shaped building
Numbers of storie
s
Beam size in m
m
Column size in m
m
Ground to 5
300×450
600X600
6 to 10
300X450
450X450
11 to 16
300X450
375X375
Table 2:- Beam and Column cross section details
Grade of concrete
M35
Grade of steel
Fe500
Density of concrete
25 KN/m³.
Poisons ratio of concrete
0.2
Table 3:- Material properties of T-shaped building
Dead load
Self weight multiplier
1
Wall load
14.03 KN/m²
Parapet wall load
4.6 KN/m²
Floor finish
1.5 KN/m²
Live load
Floor load
2.5 KN/m²
Table 4:- Load cases
Earthquake code
IS 1893:2002
Seismic zones
III
Importance factor
1
Response reduction
5
Soil type
Medium
Live load reduction
25%
Table 5:- Earthquake parameters
Fig 1:- Plan of framed building
Fig 2:- 3-D view of structure

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ISSN No:-2456-2165
IJISRT18NV328 www.ijisrt.com 366
Fig 3:- Top view of braced models (darken part shows
location bracings are placed)
Fig 4:- Elevation view of X-Bracing
Fig 5:- Elevation view of V-Bracing
IV. RESULTS AND DISCUSSIONS
Comparison of various parameters such as time period,
displacement, storey shear, storey drift ratio of different types
of reinforced concrete (RC) buildings with respect to existence
and non existence of bracings has been characterized in the
below tables.
Models
Base Shear in
X-direction
KN
Base Shear in
Y-direction
KN
Bare Frame
1718.09
1718.09
X- Braced Frame
1726.10
1726.10
Single Diagonal
Braced Frame
1722.10
1722.10
V- Braced Frame
1723.74
1723.74
K- Braced Frame
1724.80
1724.80
Table 6:- Maximum Base Shear in X and Y direction for
Linear static Analysis (ZONE III)

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ISSN No:-2456-2165
IJISRT18NV328 www.ijisrt.com 367
Fig 6:- Graphical representation of Maximum Base Shear in X and Y direction for Linear static Analysis (ZONE III)
Story displacement in mm
Story Level
BF
XBF
SDBF
VBF
KBF
X
direcn
Y
direcn
X
direcn
Y
direcn
X
direcn
Y
direcn
X
direcn
Y
direcn
X
direcn
Y
direcn
16
93.7
93.6
46
41.6
53.3
49.7
50.8
46.5
56.6
53.1
15
91.8
91.8
43.6
39.6
51.1
47.8
48.4
44.5
54.5
51.3
14
88.5
88.4
40.8
37.3
48.3
45.4
45.6
42.1
51.7
48.9
13
83.8
83.7
37.8
34.7
45.1
42.6
42.4
39.3
48.4
46
12
77.8
77.8
34.5
31.9
41.5
39.4
38.9
36.3
44.7
42.6
11
70.9
70.8
31
28.8
37.7
35.9
35.1
32.9
40.6
38.9
10
63
63
27.4
25.6
33.5
32
31.2
29.4
36.2
34.8
9
56.3
56.3
24
22.5
29.6
28.4
27.5
26
32.1
31
8
49.2
49.2
20.5
19.4
25.6
24.7
23.7
22.5
27.9
27
7
41.8
41.8
17.1
16.2
21.6
20.9
19.9
19
23.6
22.9
6
34.2
34.2
13.8
13.2
17.7
17.2
16.2
15.5
19.3
18.8
5
26.4
26.4
10.7
10.3
13.8
13.5
12.6
12.2
15.1
14.8
4
20.1
20
8
7.7
10.5
10.3
9.6
9.2
11.5
11.3
3
13.8
13.8
5.5
5.3
7.3
7.1
6.6
6.4
8
7.8
2
7.8
7.8
3.2
3.1
4.2
4.2
3.9
3.8
4.6
4.6
1
2.6
2.6
1.2
1.1
1.5
1.5
1.4
1.4
1.7
1.6
Table 4.2: Storey displacement in X and Y direction for Linear static Analysis (ZONE III)
Story Drift Ratio
Story
BFZ3
XBF
SDBF
VBF
KBF
X
direcn
Y
direcn
X
direcn
Y
direcn
X
direcn
Y
direcn
X
direcn
Y
direcn
X
direcn
Y
direcn
16
0.000548
0.000526
0.000756
0.000559
0.000695
0.000529
0.000755
0.000565
0.000665
0.000512
15
0.000974
0.000945
0.000857
0.000657
0.000851
0.00068
0.000881
0.000686
0.000844
0.000685
14
0.001385
0.001348
0.00094
0.000741
0.000981
0.000809
0.000984
0.000789
0.000996
0.000835
13
0.00174
0.001697
0.00101
0.000814
0.001092
0.000922
0.001073
0.000879
0.001124
0.000964
12
0.002042
0.001993
0.001061
0.000872
0.00118
0.001014
0.001141
0.000951
0.001228
0.001071
11
0.002284
0.002232
0.0011
0.000921
0.001255
0.001098
0.001198
0.001016
0.001318
0.001166
10
0.001972
0.001915
0.001049
0.000887
0.001175
0.001038
0.001131
0.00097
0.001226
0.001095
9
0.002085
0.002026
0.001055
0.000904
0.001208
0.00108
0.001151
0.001
0.001268
0.001144
8
0.002178
0.002118
0.001033
0.000895
0.001208
0.001091
0.001141
0.001001
0.001276
0.001162
7
0.002244
0.002183
0.000989
0.000868
0.001187
0.001084
0.00111
0.000985
0.001263
0.001161
6
0.002272
0.002213
0.000936
0.000833
0.00116
0.001073
0.001071
0.000963
0.001244
0.001157
5
0.001875
0.001817
0.000804
0.000724
0.00099
0.000924
0.000913
0.000831
0.001062
0.000997
4
0.001849
0.001795
0.00076
0.000692
0.000963
0.000907
0.000878
0.000807
0.00104
0.000984
3
0.001764
0.001716
0.000688
0.000634
0.000906
0.000863
0.000817
0.000759
0.000984
0.000939
2
0.001507
0.001473
0.000593
0.000555
0.000801
0.000773
0.00072
0.000678
0.000866
0.000835
1
0.000757
0.000744
0.000344
0.000328
0.000453
0.000443
0.000417
0.000398
0.000481
0.000468
Table 4.3: Storey drift in X and Y direction for Linear static Analysis (ZONE III)

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Time Period in Seconds
Modes
BF
XBF
SDBF
VBF
KBF
1
3.583
2.39
2.628
2.541
2.725
2
3.579
2.294
2.555
2.45
2.659
3
3.167
1.668
1.962
1.847
2.082
4
1.265
0.763
0.874
0.822
0.916
5
1.264
0.742
0.856
0.802
0.899
6
1.128
0.545
0.662
0.609
0.707
7
0.734
0.401
0.478
0.44
0.507
8
0.733
0.397
0.474
0.436
0.503
9
0.659
0.293
0.37
0.333
0.4
10
0.515
0.269
0.328
0.298
0.349
11
0.515
0.267
0.326
0.296
0.347
12
0.463
0.22
0.257
0.229
0.277
Table 4.4: Time Period in Seconds for Linear static Analysis (ZONE III)
V. CONCLUSION
In the current study T-shaped G+15 stories building is
considered and made an effort to evaluate the seismic
performance of bare frame and different types of braces are
used under zone III by considering medium soil condition.
Linear static analysis (LSA) is carried out, and compared the
results. Major conclusions are as follows:
 Base shear
 Base shear of the structure directly depends on the self-
weight of the building.
 Braced frame structure has baser shear value compared to
bare frames.
 Base shear of braced frames increase because of bracings
are added extra self-weight to the bare frame structure.
 Over all X-braced frame has more base shear value
compared to bare frame and other types of bracings.
 K-braced frame is an alternative for X-braced frame with
slightly less base shear value.
 Story Displacement
 The top story of the T-shaped building has maximum
displacement for all types of structures.
 X-braced frame performed better in reducing story
displacement compared to all other models.
 K-braced frame has almost similar displacement values
compared to X-braced frames, seems to be a better
alternative.
 Story Drift
 The story drift ratio is maximum at middle level of the
story and decrease towards the roof level.
 Story drift is more in bare frame due to be short of stiffness
in the frame compared to brace frame.
 Time Period
 The braced frame structure reduces time period and
increases stiffness of the braced frame compared to bare
frame.
 A braced frame model has less time period values
compared to bare frames.
 X-braced frame has less time period compared to other
models.
 On the basis of time period building is flexible (T > 1 sec).
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