SEISMIC street level. In this project a

SEISMIC
BEHAVIOUR OF BUILDING WITH AND WITHOUT SET BACKS

 Dr.G V V Satyanarayana 1    A Maheswar Reddy2

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1 Professor, Dept. of Civil Engineering
,Gokaraju Rangaraju Institute of Engineering and
Technology, Hyderabad, Telangana, India

2 Post Graduate Student, Dept. of Civil
Engineering,  Gokaraju Rangaraju Institute of Engineering and
Technology, Hyderabad, Telangana, India  

ABSTRACT

Generally
irregularities in buildings are plan and elevation irregularities. Geometrical
irregularity in building structures by the reduction of the lateral size of the
building at certain levels of the elevation is known as setback building. A setback, sometimes called step-back, is a step-like recession in building walls.

Setbacks were used by ancient builders to
increase the height of masonry structures
by distributing gravity loads produced by the building material such as clay,
stone, or brick. This was achieved by regularly reducing the footprint of each
level located successively farther from the ground. Setbacks also allowed the
natural erosion to occur without compromising the structural integrity of the
building. In heavily developed places, setbacks also help get more daylight and
fresh air to the street level. In this project a multi
storied building with 15 to 25 floors are taken and seismic analysis is carried
to that building with different set back ratios according to the dimensions of
the building. Results like nodal displacements, member deflections, drift
ratios and story shear are compared among models of different setback ratios
and the model with more resistant to seismic activity is figured out.

Keywords: Staad
Pro V8i, Set Backs Building Regulations, RCC Designs.

1.
INTRODUCTION

Earthquakes
are the most unpredictable and devastating of all natural disasters, which are
very difficult to save over engineering properties and life, against it.

1.2
SEISMIC MAPS

Seismicity
trend of India classifies it as a highly varied country in terms of seismic
activities. Fig 1.1 shows the seismicity map of India, it can be seen that the
Deccan plateau region being prone to less seismicity.

The
latest version in code IS 1893:2002 uses CIS-64 scale for its ground shaking
Intensity (I) considerations. Both MMI and CIS-64 scales are considered for donation
due to the absence of instrumentally recorded data.

Figure
1.2 Seismic Zoning maps of India from (1962 – 2002)

1.3
GLOBAL DEFICIENCIES

Global deficiencies can broadly be
classified as plan irregularities and vertical irregularities, as per the Code.
Some of the observed irregularities are as follows:

Plan
Irregularities:

·        
Torsional irregularity
due to plan symmetry and eccentric mass from water tank.

·        
Frequent re-entrant
corners.

·        
Diaphragm discontinuity
due to large openings or staggered floors, along with the absence of collector
elements.

·        
Out-of-plane offset for
columns along perimeter.

·        
Nonparallel lateral load
resisting systems (not observed in the building studied).

Vertical
Irregularities:

·        
Stiffness irregularity,
soft storey due to open ground storey.

·        
Mass irregularity (not
observed in the building studied).

·        
Vertical geometric
irregularity from set-back towers.

·        
In-plane discontinuity
for columns along the perimeter of the building.

·        
Weak storey due to open
ground storey.

In
architectural design it is usual to choose irregularly shaped plans as an
answer to the various factors, such as functional, spatial, environmental,
conceptual, formal, etc., which are part of the conception of a building.
However, in engineering the use of such plans is criticized due to their
inappropriate seismic behavior.

1.4
OBJECTIVES OF THE STUDY

·        
Creation
of 3D building model for both linear and non-linear
dynamic method of analyses.

·        
Understanding the seismic
behaviour of Setback buildings.

·        
Co-relating the seismic
behaviour of the Setback building with that of a building without Setback.

·        
Comparing the seismic
behaviour of building with setback at every two levels to that of building with
setback at each floor level.

·        
Study the influence of
vertical irregularity in the building when subjected to earthquakes.

1.5. SCOPE OF THE PRESENT WORK

·        
The design is limited to
reinforced concrete framed structure to designed for seismic loads (DL, LL
& EL).

·        
The seismic behaviour of
three 8-Storied buildings with and without setbacks was studied.

·        
The buildings were
analyzed using Response Spectrum Method and Time History Analysis.

·        
The effect of Setback is
studied considering the parameters such as Time Period, storey drifts,
Displacements, Storey Shears, Bending Moments and Shear Forces and correlated
with the building without setback.

2. LITERATURE REVIEW

The seismic response of vertically
irregular buildings, which has been the subject of numerous research papers,
started getting attention in the late 1970s. A large number of papers have
focused on plan irregularity resulting in torsion in structural systems.
Vertical irregularities are characterized by vertical discontinuities in the
distribution of mass, stiffness and strength. Very few research studies have been
carried out to evaluate the effects of discontinuities in each one of these
quantities independently, and majority of the studies have focused on the
elastic response.

OBJECTIVE
OF STUDY

·        
Creation
of 3D building model for both linear and non-linear
dynamic method of analyses.

·        
Understanding the seismic
behaviour of Setback buildings.

·        
Co-relating the seismic
behaviour of the Setback building with that of a building without Setback.

·        
Comparing the seismic
behaviour of building with setback at every two levels to that of building with
setback at each floor level.

·        
Study the influence of
vertical irregularity in the building when subjected to earthquakes.

3. IRREGULARITIES
IN STRUCTURES

3.1 GENERAL

The
building configuration has been described as regular or irregular in terms of
size and shape of the building, arrangement of structural elements and mass.
Regular building configurations are almost symmetrical (in plan and elevation)
about the axis and have uniform distribution of the lateral force-resisting
structure such that, it provides a continuous load path for both gravity and
lateral loads. A building that lacks symmetry and has discontinuity in
geometry, mass, or load resisting elements is called “irregular”. These
irregularities may cause interruption of force flow and stress concentrations.
Asymmetrical arrangements of mass and stiffness of elements may cause a large
torsional force (where the centre of mass does not coincide with the centre of
rigidity).

3.2
Architect, constructor and initial building design

The
initial building is usually proposed by an architect who should harmonize the
needs of investor with his own ideas and concepts, as well as with static and other
technologies requirements. It is also useful to adapt the working of the
building, to define the major dimensions of the structure and to propose the
arrangement of the rooms in the way that 
correspond best to the given location, as well as to the needs of the
investor and user.

3.2.1
Reasons for irregularities in buildings

·        
Construction in Hilly
areas

·        
Modern/new trends in
commercial complexes

·        
Thickly populated areas

CLASSIFICATION
OF IRREGULARITIES

The
structural irregularities are categorized in three types as:

a)       Plan
Irregularities

b)       Vertical
Irregularities

c)       Other
Irregularities

Plan
Irregularities

Plan
Irregularities refers to asymmetrical plan shapes (e.g. L, T, U, F, +) or
discontinuities in the horizontal resisting elements (diaphragms) such as cut-outs,
large openings, re-entrant corners and other abrupt changes resulting in
torsion, diaphragm deformations and stress concentration.

Fig
4.2 Plan Irregularity

If,
structural walls are placed on one side of a building while the other side has
open frames, the eccentricity between the centers of the mass and resistance
causes torsional vibration during an earthquake. Larger damage develops in
members away from the centre of resistance. The structural wall is reducing
lateral deformation and resisting large horizontal forces, especially when they
are distributed in plan.

Fig
4.3 Code Plan Irregularity

4.3.2
Vertical Irregularities

Vertical
Irregularities refer to sudden change of strength, stiffness, geometry and mass
results in irregular distribution of forces and/or deformation over the height
of building.

When
the stiffness is reduced in a storey along the height, earthquake-induced
deformations tend to concentrate at the flexible and weak storey. The
concentration of damage in a storey leads to large deformations and excessive
deformation in vertical members often leads to failure of the storey.

4. NUMERICAL STUDIES

Most building codes prescribe the method of analysis based on
whether the building is regular or irregular. 
Almost all the codes suggest the use of static analysis for symmetric
and selected class of regular buildings. 
For buildings with irregular configurations, the codes suggest the use
of dynamic analysis procedures such as response spectrum method or time history
analysis.

In the present work, the test structure is an
eight storey re-entrant corner building with setback provided at every two
levels. The building is compared with the normal building without any setbacks
and building with setback at each level. Experimental observations of both buildings
are supplied with series of analytical methods, response spectrum method
(linear dynamic) and time history analysis (non-linear dynamic).  STAAD PRO has been used to perform the above
mentioned analysis. The effect of setbacks is studied considering the
parameters such as Time period, Storey Drifts, Storey Shears, Displacements,
Bending Moments and Shear Forces for identical columns of both the buildings
for comparison.

5.2
DESCRIPTION OF SAMPLE BUILDING

The plan layouts
for the building modals with and without Setbacks are shown in figures.

Symmetric
Building Models

Model
1: Building
modeled with Re-entrant corner and without Setback.

Model
2: Building
modeled with Re-entrant corner and Setback at every two levels.

Model
3: Building
modeled with Re-entrant corner at Setback at each level.

Plan
Layout

Model
1:

Fig 5.1 Plan Layout

Fig
5.2 Elevation of Building without Setback (Model 1)

 

Fig
5.33D View of Building without Setback (Model 1)

 

 

 

 

 

 

 

 

Model
2:

 

Fig 5.4 Plan Layout for 1st and 2nd Floors

 

 

Fig 5.5 Plan Layout for 3rd and 4th Floors

Fig 5.6 Plan Layout for                                                  

Fig
5.7 Plan Layout for

5th
and 6th Floors, 7th and 8th Floors

 

Fig 5.8 Elevation of Building with Setback (Model 2)

Fig 5.9 3D View of Building with Setback (Model 2)

Model
3:

 

Fig 5.10Plan Layout for 1st Floor

RESULTS AND DISCUSSIONS

6.1
INTRODUCTION

All
the selected building models with different setback irregularities are analyzed
for linear dynamic behavior using commercial software ETABS analysis results
and relevant discussions.

6.2.1.
FUNDAMENTAL TIME PERIOD FOR SETBACK BUILDINGS

1.
The fundamental time periods of all the 90 selected setback buildings were
calculated using different methods available in literature including code based
empirical formulas.

2.Fundamental
period of these buildings were also calculated using modal analysis. Modal
analysis procedure is explained.

3.The
fundamental periods presented here are computed as per different code empirical
equations such as IS 1893:2002,UBC 95, ASCE 7 as well as Rayleigh Method, and
period obtained from modal analysis.

Conclusions

Buildings with the setback
irregularity have not received much attention in previous researches, and in
the formulation of seismic design methodologies. In the present study, a
detailed analytical study has been carried out to overcome these shortcomings.
The main conclusions were as follows:

1. To quantify the setback
irregularity a parameter called ‘irregularity index’, is proposed. The proposed
irregularity index accounts for mass and stiffness changes due to the presence
of setbacks along the building height.

2.The proposed parameter is
based on dynamic response of the building, and is found to be quite simple.

3.
This study could not conclude on the appropriate parameter defining the
irregularity in three-dimensional multi-storied setback buildings.

4.
There is a scope to investigate different parameters either geometrical or
structural or combination of both to define the setback irregularity.

5.
The present study is limited to reinforced concrete (RC) multi-storied building
frames with setbacks only in one direction.

6.
There is a future scope of study on three dimensional building models having
setbacks in both of the horizontal orthogonal directions

REFERENCES

1.      
ABAQUS user’s manual, Version 6.11.
(2011). Dassault Systemes Simulia Corp., Providence, RI, USA.

2.      
Tim Stratford, John Cadei (2006) “Elastic
analysis of adhesion stresses for the design of a strengthening plate bonded to
a beam”, Construction and Building Materials 20 (2006) 34–45.

3.      
Narayanamurthy V, J.F.Chen, J.Cairns,
A.Ramaswamy (2011) “Effect of shear deformation on
interfacial stresses in plated beams subjected to arbitrary loading”,
International Journal of Adhesion & Adhesives 31 (2011) 862–874.

 

 

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