Download Effect of Diaphragm Discontinuity on Seismic Response

International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 12, December 2015)
Effect of Diaphragm Discontinuity on Seismic Response of
Multistoreyed Building.
Osama Maniar1, Roshni J John2
1
PG Student, Dept of Civil Engg, 2Head of Civil Engineering Department, Saraswati College of Engineering, Kharghar, India
In structural engineering, a diaphragm is a structural
system used to transfer lateral loads to shear
walls or frames primarily through in-plane shear stress.
These lateral loads are usually wind and earthquake loads,
but other lateral loads such as lateral earth pressure or
hydrostatic pressure can also be resisted by diaphragm
action.
Two primary types of diaphragm are rigid and flexible.
Flexible diaphragms resist lateral forces depending on the
area, irrespective of the flexibility of the members that they
are transferring force to. Rigid diaphragms transfer load to
frames or shear walls depending on their flexibility and
their location in the structure. Flexibility of a diaphragm
affects the distribution of lateral forces to the vertical
components of the lateral force resisting elements in a
structure
Abstract— In multi-storeyed framed building, damages
from earthquake generally initiates at locations of structural
weaknesses present in the lateral load resisting frames
Diaphragms with abrupt discontinuities or variations in
stiffness, which includes those having cut-out or open areas
greater than 50 percent of the gross enclosed diaphragm area,
or changes in effective diaphragm stiffness of more than 50
percent from one storey to the next. In structural engineering,
a diaphragm is a structural system used to transfer lateral
loads to shear walls or frames primarily through in-plane
shear stress. Lateral loads are usually wind and earthquake
loads. This paper focuses the general effects of diaphragm
discontinuity on seismic response of multi-storeyed building
on various structural parameters.
Keywords-- Diaphragm discontinuity, Liner static
I. INTRODUCTION
In multi-storeyed framed building, damages from
earthquake generally initiates at locations of structural
weaknesses present in the lateral load resisting frames. This
behaviour of multi-storey framed buildings during strong
earthquake motions depends on the distribution of mass,
stiffness, strength in both the horizontal and vertical planes
of buildings. In few cases, these weaknesses may be
created by discontinuities in stiffness, strength or mass
along the diaphragm. Such discontinuities between
diaphragms are often associated with sudden variations in
the frame geometry along the length of the building.
Structural engineers have developed confidence in the
design of buildings in which the distributions of mass,
stiffness and strength are more or less uniform. There is
less confidence about the design of structures having
irregular
geometrical
configurations
(diaphragm
discontinuities).
In the present project, the effect of diaphragm
discontinuity on the seismic response of a selected multi
storey building is studied.
According to IS 1893-2002 part 1, diaphragm is a
horizontal, or nearly horizontal system, which transmits
lateral forces to the vertical resisting elements, for example,
reinforced concrete floors and horizontal bracing systems.
Fig 1 Types of Diaphragm
II. EARLIER RESEARCH
According to IS-1893:2002: Diaphragms with abrupt
discontinuities or variations in stiffness, which includes
those having cut-out or open areas greater than 50 percent
of the gross enclosed diaphragm area, or changes in
effective diaphragm stiffness of more than 50 percent from
one storey to the next.
R.G. Herrera and C.G. Soberon (2008) showed an
analytical description of the damages caused by different
plan irregularities, during seismic events of different
magnitudes. In all the studied systems, effects of different
irregularities are analyzed based on the variation of
displacements, with respect to regular systems.
130
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 12, December 2015)
They concluded that A summary of important seismic
events from 1980 to 2008, where it is observed building
damaged due to different irregularities causes. They
conclude that constructions are more vulnerable when they
are irregular are. The linear analyses provide important
information for torsion behaviour of weak structures like
the studied. Despite we understand that elastic analysis
underestimates the inter-story drifts when the
superstructure enters in nonlinear performance, and the
behaviour is adopted torsion mode
Turgut Ozturk (2011) analysed earthquake codes of
several other countries along with Turkish Earthquake
Code (TEC) and the conditions that it brings for structural
irregularities and slab discontinuities were mentioned. In
the building models formed for different positions and
ratios of the gaps, the results of the analysis made by
keeping in mind the effects of the number of storey, beam
continuity, earthquake zone, soil type and rigid diaphragm
work on the structural system have been given as graphs.
They concluded that the maximum torsion values occur for
the buildings in which the slab openings are not
symmetrical and the continuity of the beams is not enabled;
lateral displacements also do increase in such buildings.
The increase in the number of storeys, the largeness of the
earthquake zone, and the poor nature of the soil do increase
the negative effects of the slab openings on the structural
system behaviour.
Morteza Moeini and Behzad Rafezy (2011) reviewed
about the provisions of some modern seismic codes for the
analytical modeling of the floor diaphragm action is made
and a methodology using finite elements models, taking
into consideration the in-plane flexibility, for monolithic
floor is suggested. Using this method with comparative
response-spectrum dynamic analyses, some reinforced
concrete structures with different plan shapes like Tshape, L-shape, U-shape and rectangular according to 2800
(Iranian seismic code) are analyzed. Then, the efficiency of
codes provisions is investigated
Floor to Floor height: 3 m.
Slab is modeled using rigid diaphragm.
Wind load is considered as per IS: 875. (Part III)
Earth quake load is considered as per IS: 1893-2002.
(Moment resisting frame with response reduction factor of
4, zone III & 5% damping is provided.)
The building is analyzed for static load using The load
combinations are considered as per IS: 875 (part 5) for DL,
LL, WL & EQ loads. Twenty five percent of imposed load
has been accounted along with dead load for seismic
weight calculation of building as per IS: 1893(2002).
Fig. 2 Plans of G+ 7 storey building.
III. MODELING
The structure is analyzed in ETABS software and
following design parameters are to be consider i.e.
Dead load: 1.5KN/m2
Live load: 2.0KN/m2 (live load of 3kN/m2 is provided for
passage and stair case slab.)
Bricks of density 20KN/m3 are used for walls.
Number of stories: 7
Fig 3-3D view (Type 1)
131
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Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 12, December 2015)
Fig 4-3D view (Type 2) .
Fig 7-3D view (Type 5)
Fig 5-3D view (Type 3)
Fig 8-3D view (Type 6)
IV. RESULTS AND DISCUSSION
Time Period
Table 1
Shows Time Period comparison of three modes for the six considered
G+7 storey models
Fig 6-3D view (Type 4)
Mode
Type 1
Type 2
Type 3
Type 4
Type 5
Type 6
1
1.704
1.701
1.674
1.692
1.783
1.723
2
0.944
0.934
0.912
0.927
0.896
0.916
3
0.863
0.835
0.899
0.846
0.892
0.860
132
International Journal of Emerging Technology and Advanced Engineering
Time period in seconds
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 12, December 2015)
Frequency is the reciprocal of time period. The
frequency for various modes for the six considered models
are given in table above. Frequency is directly proportional
to stiffness of the building. More the frequency stiff is the
building.
2
type
1
type
2
type
3
type
4
1.5
1
0.5
0
0
1
2
3
Base Shear
Table 3
Base Shear comparisons for the six considered G+7 storey models
4
Type
EQX
EQY
WIND X
WIND Y
1
-2843.78
-1444.88
-1375.93
-1031.95
2
-3037.39
-1492.44
-1379.67
-1034.76
3
-2826.78
-1509.37
-1375.93
-1034.76
4
-3085.56
-1539.44
-1379.67
-1034.76
5
-2901.93
-1459.2
-1379.67
-1034.76
6
-2952.05
-1473.88
-1375.93
-1029.15
Mode Shape
Fig 9 Time Period comparison for the six considered G+7 storey
models
It is observed that the time period of Type 5 is more as
compared to other models in 1st mode , in 2nd mode time
period of Type 1 is more and time period of Type 3 is more
in 3rd mode as compared to other models. The behaviour of
building is better when diaphragm discontinuity is closer to
the Centre of the building.
Frequency
Table 2
Shows Frequency comparison of three modes for the six considered
G+7 storey models
Mode
Type 1
Type 2
Type 3
Type 4
Type 5
Type 6
1
0.586
0.587
0.597
0.590
0.560
0.580
2
1.058
1.070
1.096
1.077
1.114
1.090
3
1.152
1.196
1.112
1.181
1.21
1.162
Frequency
1.5
Fig 11 Base Shear comparison for the six considered G+7 storey
models when considered earthquake
Type 1
Type 2
1
Type 3
Type 4
0.5
Type 5
Type 6
0
1
2
Mode
3
Fig 10 Frequency comparison for the six considered G+7 storey
models
Fig 12 Base Shear comparison for the six considered G+7 storey
models when considered wind
133
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 12, December 2015)
It is observed that base shear is more in Type 4 for
earthquake in X direction and Y direction, on the other
hand it is less in Type 3 and Type 1 for earthquake in X
and Y direction respectively. When considering wind in X
and Y direction base shear is almost same for all Types.
More the base shear stiff the building since member will
attract more forces.
Displacement
Table 4
Displacement comparisons for the six considered G+7 storey models
Type
X direction
Y direction
1
0.0217
0.0406
2
0.0208
0.0407
3
0.0201
0.0398
4
0.0211
0.0402
5
6
0.0221
0.0218
Fig 14 Displacement comparison for the six considered G+7 storey
models in Y direction
It is observed that when considering earthquake and
wind in X and Y direction, displacement of Type 5 is more
as compared to other types and the displacement is least in
Type 3. More the displacement flexible is the structure.
Table 5
Shows Modal Mass Participation comparison for the six considered
G+7 storey models
Mode
Type1
Type2
Type3
Type4
Type5
Type6
1
84.48
84.26
84.59
84.51
84.52
84.51
2
0
0
0
0
0
0
3
80.61
80.91
73.42
80.85
80.95
79.89
0.0423
0.0415
Fig 13 Displacement comparison for the six considered G+7 storey
models in X direction
Fig 15 Modal Mass Participation comparison for the six considered
G+7 storey models in X direction
134
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 5, Issue 12, December 2015)
4. More the displacement flexible is the structure and
vice versa. Therefore model type 3 is more stiff as
compared to other models.
5. For G+7 building with diaphragm discontinuity modal
mass participation is almost same for all models.
Therefore diaphragm discontinuity does not have
much effect on modal mass participation
REFERENCES AND STANDARDS
[1]
[2]
Fig 16 Modal Mass Participation comparison for the six considered
G+7 storey models in Y direction
[3]
It is observed that when considering modal mass
participation in X direction, the participation is zero for all
types in 1st and 2nd mode and in 3rd mode it is less in Type 3
when compared with other Types. On the other hand the
modal mass participation is zero for all types in 2 nd and 3rd
mode and it is almost same for all types in 1 st mode.
Diaphragm discontinuity does not have much effect on
modal mass participation.
[4]
[5]
[6]
[7]
V. CONCLUSION
From the linear static analysis of G+7 storey building
with plan irregularity following conclusions can be made
1. The behaviour of building is better when diaphragm
discontinuity is closer to the centre of the building.
2. Frequency is directly proportional to the stiffness of
building. Therefore model type 3 is more stiff than
other model considered.
3. More the base shear stiff the building since member
will attract more forces. Model type 3 is more stiff
than other models considered
[8]
[9]
135
Indian Standard code IS 18932-2002
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