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EARTHQUAKE, ITS EFFECTS, DESIGN & CONSTRUCTION OF BUILDINGS
Prof. Y. P.
Gupta,
Chairman,
ICI UP
Allahabad
Center &
Materials Consultant, BCEOM-LASA JV
Allahabad
By
Pass Project
INTRODUCTION
Earthquake is a natural phenomenon, which can occur any
time anywhere. Therefore, the Buildings must be built in such a way that they are
safe during such occurrence. IS: 1893, Code Of Practice For “CRITERIA FOR EARTHQUAKE
RESISTANT DESIGN OF STRUCTURES” give the guidelines for the design of structures.
This has been revised (2002) taking care of the experiences of several Earthquakes,
which occurred after 1984 when the code was last revised. During various Earthquakes,
large number of buildings failed and many lessons are learnt especially after Gujrat
Earthquake (2001). This paper describes in brief the phenomenon of Earthquakes,
various provisions that can be made for calculating the Design forces. It also brings
out many provisions, which can be implemented in design and/or construction of buildings
to make them earthquake resistant.
HOW EARTHQUAKES OCCUR?
Earth is a Spherical body made up of mainly four layers consisting of Inner &
Outer Core, Mantle and Crust as shown in Figure 1. The Core of Earth is very hot.
There is enormous temperature and
pressure gradient between Core (innermost layer)
and Crust (outermost / top layer), which generates convection currents in the Mantle.
These currents cause the Crust and a portion of the Mantle to slide over the outer
Core. It also gives rise
to the development of strain energy, which causes crust
to slide over mantle. Earth’s crust consists of 7 major tectonic plates (that contain
continents and ocean basins), which keep moving in different directions. As a result
of such movements, strains are building up at plate interfaces. In course of time, when this cumulative strain energy becomes excessive for the rock plates to sustain,
it gets released in the form
of a slip occurring between adjoining plates. The interface
of plates is called fault or fault zones. Some of these faults are active, whereas
others are not. When the amount of energy stored inside Earth’s body increases and
exceeds the breaking strength of the Earth’s crust in a active fault zone, then
it suddenly breaks and releases the energy stored inside the Earth causing vibrations
or Earthquake. There are many other explanations about the occurrence of earthquakes.
Tsunami:
When the epicenter of earthquake is on Sea floor, then vibrations are
also caused on the body of seawater. Therefore, earthquakes on sea floor are the
origin of tsunami waves. After an earthquake has occurred beneath the sea floor
at shallow depth, it takes some time (say few minutes to few hours) for dynamic
waves of large heights to be formed. The shock embedded wave travels at a speed
of about 1000 km per hour or so and reaches Coastline.
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Fig. 1 – Earth’s Core & Crust Showing 4 Layers of Earth’s Body
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MAGNITUDE AND SCALE OF AN EARTHQUAKE
Rupture of earth’s crust generally commences at a deep point, somewhere
inside the Earth’s body on the fault plane, which is called Focus or Hypocenter
of the quake. At this point, the first rupture of the fault surface takes place.
The point right above the focus is called the Epicenter as shown in Figure -2. The
magnitude of an earthquake is the measure of the energy released from the Focus.
It is commonly measured in terms of Richter’s scale.
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EPICENTER FOCUS Fig. 2 - Typical Position of Epicenter & Focus (Hypocenter)
inside earth's body
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The relation between the energy released and magnitude is:
Where,
M - Magnitude of earthquake
E - Amount of energy released during an Earthquake.
By definition, Magnitude is the logarithm to the base 10 of
the maximum trace amplitude, measured in microns (10-6 m), which the Standard Short
Period Torsion (Wood – Anderson) Seismometer would register due to the earthquake
at an epicentral distance of 100 km.
It is seen from above relation that energy released by an earthquake
increases by a factor of 32 for each unit increase in magnitude. That is, each increase
in magnitude number is a 10 - fold bigger wave and has 32 - fold increase in energy
released by that wave. It is said that Bhuj Earthquake (2001) at magnitude 7.7 released
so much energy, which was about 100 times the energy given by Atom Bomb dropped
on Hiroshima during World War II.
The magnitude of earthquake by itself is not
sufficient to indicate
whether Structural damage at any place can be expected. This is only a measure of
the size of earthquake and energy released at its source. The effect of an earthquake
in an area depends on the distance of the structure from the focus and nature of
the Earth’s crust at the location of the Structure. Thus, how an individual, positioned
on the earth’s surface in different parts of the World, feels the severity of an
Earthquake is called Earthquake Intensity. This is a measure of damage occurred
to a structure and is defined by Modified Mercalli Scale. The intensities are classified
on 12 level scales in IS: 1893-2002 ranging from bare minimum to total destruction
of structure.
SEISMIC ZONING MAP OF INDIA
As per IS: 1893 – 2002 Pt I code of practice for “Criteria for
Earthquake Resistant Design of Structures”, India is now divided into four earthquake
zones as shown in Figure – 3. In this revision, Zone I has been eliminated. Some
areas of Zone II like Killari area in Maharastra etc have been upgraded and shifted
to Zone III. Approximately 12 percent of India (Bhuj, Jammu and Kashmir, Uttaranchal,
the North-East, north Bihar and Andaman and Nicobar Islands, etc) falls under the
seismic zone V – the highest risk category.
EFFECT OF EARTHQUAKE ON BUILDINGS
Earthquakes directly effects ground shaking and in turn can generate Landslides, Tsunamis, Liquefaction of ground and may also cause damage
to Structure by way of shaking etc. Structural response affecting the buildings
is described here.
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Fig. 3 - Seismic Zoning Map of India – The four Earthquake Zones (IS: 1893-2002) |
Why does a Building Collapse?
The response of a Structure to an Earthquake is a function of
the nature of the foundation soil; material, form, size & mode of construction;
and the duration and characteristics of ground motion. A typical response of any
building during an Earthquake is shown in Figure – 4(a). This response depends on
the natural period of vibration & absorbing Character (damping) of the structure.
During an earthquake, amplitude of vibration generally buildup in a few cycles.
A typical diagram showing the amplitude build up of any object during few cycles
of earthquake excitations is given in Figure -4(b).
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Fig. 4(a) – Typical Response of a Structure to Earthquake Vibrations (with varying
damping ζ)
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Fig. 4(b) – Typical Amplitude Build up during continued Vibrations |
Effect of earthquake on some of the buildings during Latur
(1993) and Gujrat (2001) earthquakes is shown in figures-5 & 6, as few typical
examples of structural behavior. It is to be noted that RC framed buildings during
Latur Earthquake suffered less damage as compared to the Gujrat Earthquake. This
may be because of poor quality of design / construction and absence of proper beam
column connections in the Gujrat area.
Earthquakes cause motion to the ground in random direction.
The horizontal vibration is predominant & more damaging. The amplitude of motion
of any structure normally builds up over a period of time in a few cycles i.e. duration
of Earthquake as shown in Figure -4(b). Thus, if the Earthquake lasts longer, the
amplitude of vibration is more, i.e. the structure will deflect more and get damaged.
The violent ground motion pushes the building rapidly from
one direction to another making it difficult for the super-structure to constantly
balance its load due to inertial effects. Result: while columns can bend, the swaying
motion, when intensified, snaps the building like matchsticks and collapses.
A superstructure can be damaged, not only on account
of the shaking which results from quakes but also due to chain effects like fire,
land slide etc caused by earthquake.
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Fig. 5(a) - An undamaged Reinforced Concrete building after the Latur Earthquake
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Fig. 5(b) - A RC Hospital Building suffered only diagonal cracking in the walls during the Latur Earthquake (1993).
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Fig. 5(c) –A beam-column junction in a multi – storied building failed during the
Latur Earthquake (1993) – Bad placement of reinforcement at the junction. |
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Fig. 6(a) – A RC apartment building in Ahemdabad failed during the Gujrat Earthquake
(2001) – Bad design
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HOW TO MAKE BUILDING QUAKE RESISTANT ?
There are two essential features to make a building earthquake resistant
i.e. safe design and quality construction. To achieve this, the desirable factors
required in design of any structure for better Earthquake resistance are:
• Stiffness / Ductility and
• Damping.
The stiffness is an ultimate effect of structural design &
material characteristics while ductility and damping comes directly from material
used for construction. Thus, it is desirable that the material used for construction
is ductile, especially at locations where damage is expected like at Beam-Column
junction. Normally Reinforced Concrete is a good ductile material.
CHOICE OF CONSTRUCTION MATERIALS
A) Reinforced Concrete:
Construction material is crucial for the earthquake resistance and
durability of structure. The safest building will be the one made of all steel (though
very heavy – attracting more earthquake force), as it is an extremely strong material.
Reinforced Concrete is the next most suitable material for earthquake resistant
construction of buildings. It is a good, durable and economic material of construction,
but the condition is that the quality of construction should be good. It was seen
during the Latur earthquake, that most buildings made with concrete, remained standing
without suffering much damage. But during the Gujrat earthquake many buildings made
in RCC also got damaged or collapsed because of poor quality of construction.
B) Other Materials:
A brick, stone or mud house cracks even with moderate tremors. However,
these materials can be effective when strengthened with RCC elements at critical
points. Masonry buildings become brittle when large deflections take place, so RCC
bands can strengthen them at regular intervals. A wooden frame building is good
as it absorbs shock evenly and vibrates along the quake and unlikely to collapse.
The danger with wooden frame structures is that it is highly inflammable and has
limited use i.e. only up to one or two stories.
DESIGN OF BUILDINGS FOR EARTHQUAKE EFFECT
The behavior of a building during ground vibration is a function of
the nature of foundation soil and natural period of vibration of the structure,
which depends on the material used in construction, its form, size, and mode of
construction etc. The structure also gets affected with the duration & intensity
of the earthquake. IS: 1893 -2002 specifies seismic coefficients for calculating
the design forces for simple structures standing on soil which will not settle or
slide much, due to loss of strength (like Liquefaction effect). In the design of
buildings, horizontal force due to earthquake is considered simultaneously along
with the vertical forces.
Normally, the natural period of vibration of any structure should
not coincide with the predominant period of earthquake excitations, otherwise resonance
may occur and even the strongest structure may collapse. Thus, while designing the
building, following aspects should be looked into:
a) Magnitude & Type of Earthquake Excitations.
b) Natural Period of Vibration of Structure along with its material &
mode of construction.
Response of Structure to earthquake Design forces, to which
the building elements will be subjected, can be calculated by any one of the following
methods.
1. Seismic Coefficient Method
2. Response Spectrum Method (Modal Analysis)
3. Time History Analysis.
Depending upon the complexity and importance of Structure, any one of the above
three methods can be adopted. Here only seismic coefficient method is described,
as this is the most common method. Earthquake forces can be calculated in any direction
of Structure, but the most damaging direction is horizontal (Least lateral direction).
The horizontal earthquake force can be calculated as:
VB (or F) = Ah W …..(1)
Where,
VB (or F) = Total Design force generated due to earthquake or Design
Seismic Base Shear
W = Seismic weight of the Building i.e. Sum of the Seismic weight
of all floors (DL + appropriate amount of live load as per IS: 1893)
Ah = Design horizontal seismic coefficient.
Vertical acceleration coefficient, Av can be taken as 2/3 Ah
I. SEISMIC COEFFICIENT METHOD (AS PER IS: 1893 - 2002)
The Design Horizontal Seismic Coefficient ‘Ah’ for a structure is
determined by the following expression as per IS: 1893 - 2002:
Ah = (Z/2) x (Sa/g) / (R/I) …(2)
Subject to the condition that for any Structure having T ≤ 0.1 sec,
Ah will not be less than Z/2 for any value of I/R. Here description of various parameters
is given below. a) Seismic Zone Factor, Z: The Values of Seismic Zone Factor ‘Z’
reflect more realistic values of effective peak ground acceleration under maximum
considered earthquake (MCE) in each Seismic Zone. These values are given in table
1 as per revised 1893 code. The factor ‘2’ in the denominator of Z is used so as
to reduce the maximum considered earthquake (MCE) zone factor to the factor for
design basic earthquake factor (DBE).
Table: 1 Seismic
Zone Factor, “Z”
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Seismic Zone
II
III
IV
V
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Seismic Intensity
Low
Moderate
Severe
Very severe
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Zone Factor,
Z 0.1
0.16
0.24
0.36
Zone factors for some important cities have also been modified. These are given
in Annexure E of the code. For example for
Lucknow
, Kanpur etc, it is 0.16.
b)
Importance Factor, I:
The
design of a building should be carried out, based on its
functional use before and after an earthquake. For example
important services and community structures like hospitals,
schools, important bridges, power houses, monumental
structures, telephone exchange, fire stations, assembly halls,
sub-ways etc. are given an Importance factor of 1.5 as per
IS-Code and they should be designed accordingly. For houses
and general buildings its value can be taken as 1.0.
c)
Concept of Response Reduction Factor, R:
Code
adopts the procedure of first calculating the actual force
that may be experienced by the structure during the probable
maximum earthquake, if it were to remain elastic. Then the
concept of response reduction due to ductile deformation or
frictional energy dissipation in the cracks is brought into
action in the code explicitly by introducing the response
reduction factor ‘R’ in place of the earlier performance
factor. Some typical values of the response reduction factors
are given in Table 2.
Response
reduction factor, R, depending on the perceived seismic damage
performance of the structure is characterized by ductile or
brittle deformations, with the condition that the ratio (I/R)
shall not be greater than 1.0 d) Average Acceleration Response
Coefficient, Sa/g: The acceleration response of a structure to
ground vibrations is a function of the nature of foundation
soil, material, size and mode of construction of structure and
characteristics of ground motion. The Response Spectra is now
specified for three types of foundation strata viz. one for
Rock or hard soil, second for Medium Soil and third for soft
Soil, as given in three different curves of Figure -7. Fill
type of soil is not considered suitable for construction
activity in earthquake zones.
d)
Average Acceleration Response Coefficient, Sa/g:
The
acceleration response of a structure to ground vibrations is a
function of the nature of foundation soil, material, size and
mode of construction of structure and characteristics of
ground motion. The Response Spectra is now specified for three
types of foundation strata viz. one for Rock or hard soil,
second for Medium Soil and third for soft Soil, as given in
three different curves of Figure -7. Fill type of soil is not
considered suitable for construction activity in earthquake
zones.
Table: 2 – Some Values of Response Reduction Factor ‘R’ for Building Systems
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S. No.
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Lateral Load Resisting System
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R
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A.
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Building Frame System Alone
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1.
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Ordinary RC Moment-Resisting Frame
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3
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2.
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Special RC Moment-Resisting Frame
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5
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3.
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Steel Frame with Concentric Braces
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4
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4.
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Steel Frame with Eccentric Braces
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5
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5.
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Steel Moment Resisting Frame
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5
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B.
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Load Bearing Masonry Wall Buildings
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1.
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Un-reinforced
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1.5
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2.
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Reinforced with horizontal RC Bands
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2.5
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3.
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Reinforced with horizontal RC Bands
and vertical bars at Corners & Jambs
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3
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C.
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Ordinary Reinforced Concrete Shear Walls
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3
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D.
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Ductile Shear Walls
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4
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The average acceleration response coefficient Sa/g, for 3
types of soil sites as given in Figure- 7, is based on the
appropriate natural period and 5% damping value of the
structure. Natural period of vibration can be calculated by
usual methods or as given below for multi-story building. A
normal structure in concrete will have a damping value of
about 5% for which the curves are given. For other damping
values, a multiplying factor is given in IS: 1893, and
reproduced here in Table – 3. Some empirical relations can
also give values of Sa/g .
![]() | | Fig. 7 - Shape
of Response Spectra curves at 5% Damping
Level |
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Table 3- Multiplying Factor for Other than 5% Damping level
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% Damping
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0
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2
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5
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7
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10
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15
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20
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25
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30
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Factor
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3.20
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1.40
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1.00
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0.90
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0.80
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0.70
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0.60
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0.55
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0.50
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E) Building Category:
After finding the values of all parameters given in equation
2, the value of Ah can be found. Then
depending upon the value of seismic Coefficient,
Ah, the category of Building can be defined as given in Table 4.
Table 4 – Classification of Building Categories
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Range Of Ah
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Building Category |
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Less than 0.05 |
A
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0.05 to 0.06 |
B |
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0.06 to 0.08 |
C |
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0.08 to 0.12 |
D |
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> 0.12 |
E
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APPROXIMATE RELATIONS FOR FUNDAMENTAL PERIOD OF VIBRATION
The empirical expression for estimating the fundamental natural period
‘Ta’ of multistory buildings having regular moment resisting frame can be found by following relations (as given in IS: 1893):
a) The approximate fundamental natural period of vibration ‘Ta’ of moment resisting
frame buildings without brick infill panels is: Ta = 0.075 h 0.75 - for RCC frame
Building …3(a) = 0.085 h 0.75 - for Steel frame Building …3(b)
b) The approximate fundamental natural period of vibration ‘Ta’ of all other buildings
including moment resisting frame buildings with brick infill panels may be estimated
by: Ta = 0.09 / d …4 Where, Ta = Fundamental period of vibration in seconds h =
Height of Building in meters. D = Base dimension of building at plinth level in
‘meters’, along the considered direction of the lateral force.
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