Earthquake Resistant Design of Structures: Version 2 CE IIT, Kharagpur
Earthquake Resistant Design of Structures: Version 2 CE IIT, Kharagpur
Earthquake Resistant Design of Structures: Version 2 CE IIT, Kharagpur
16
Earthquake Resistant
Design of Structures
Version 2 CE IIT, Kharagpur
Lesson
39
Seismic Effects,
Material Behaviour and
General Principles of
Earthquake Resistant
Design of Structures
Version 2 CE IIT, Kharagpur
Instructional Objectives:
• explain the behaviour of plain concrete, steel and reinforced concrete with
high intensity repeated axial cyclic loads,
• determine the horizontal seismic coefficient, design seismic base shear and
distribution of design force,
• explain the need for ductility and ductile detailing of reinforcement in the
earthquake resistant design of structures.
16.39.1 Introduction
Earthquakes, Tsunamis, Seiches, Landslides, Floods and Fires are
natural calamities causing severe damage and sufferings to persons by
collapsing the structures, cutting off transport systems, killing or trapping
persons, animals etc. Such natural disasters are challenges to the progress of
development. However, civil engineers as designers have a major role to play in
minimising the damages by proper designing the structures or taking other useful
decisions. Because of the vastness of the topic, “Disaster management and
mitigation”, this module includes understanding the earthquakes, behaviour of
the materials of construction and structures and the extent to which structural
engineers make use of the knowledge in taking proper decisions in designing the
structures made of reinforced concrete.
As the state of the art of this subject is still developing, integrated field
inspection of structures damaged by earthquakes and their analyses are
useful in further adding/improving the expert knowledge in the seismic
It is essential that the whole structure and the foundation should work as a
unit especially for the seismic resistant design and construction of
structures. For this the superstructures should be anchored properly to the
foundation.
B. Indirect effects
Tsunamis, seiches, landslides, floods and fires are the indirect effects of
earthquakes. These may occur either alone or in combinations to add to
the damages during an earthquake.
Plain concrete has tensile strength less than twenty per cent of its
compressive strength. The tensile strength of plain concrete is normally not
obtained from the direct tension test because of difficulties in holding the
specimens and the uncertainties of developing secondary stresses due to
holding devices. Hence, this is measured indirectly either by split cylinder test or
by bending test conducted on plain concrete prisms. Prism tests give the tensile
strength of concrete in flexure, known as the modulus of rupture. Plain concrete
specimens are not tested under repeated axial cyclic tensile loads.
(b) Epicentre
(c) Focus
Critical damping is the damping beyond which the free vibration motion
will not be oscillatory (cl. 3.3 of IS 1893 (Part 1): 2002).
(h) Liquefaction
2. IS 1893 has other four parts: (a) Part 2 for liquid retaining tanks–
elevated and ground supported, (b) Part 3 for bridges and retaining
walls, (c) Part 4 for industrial structures including stack like structures
and (d) Part 5 for dams and embankments. However, they are yet to
be finalised. Hence, provisions of Part 1 will be read along with
relevant clauses of IS 1893: 1984 for structures other than buildings.
a) Ground motion
(b) Assumptions
The design horizontal seismic coefficient Ah (cl. 6.4.2 of IS 1893 (Part 1): 2002)
for structure is determined from
Ah = (Z I Sa / 2 R g) (16.1)
where Z = the zone factor as given in Table 2 of IS 1893 (Part 1): 2002, based on
classifying the country in four seismic zones,
It is further stipulated in cl.6.4.2 of IS 1893 (Part 1): 2002, that for any
structure with undamped natural period of vibration of the structure (in seconds)
T ≤ 0.1 second, the value of Ah will not be taken less than Z/2 whatever be the
value of I/R.
The total design lateral force or design seismic base shear VB along any
principal direction (cl.7.5.3 of IS 1893 (Part 1): 2002) shall be determined from
the following equation:
VB = Ah W (16.2)
where Ah is as given in Eq. 16.1, and W is the seismic weight of the building
as given in cl. 7.4.2 of IS 1893 (Part 1): 2002.
The design base shear VB of Eq. 16.2 shall be distributed along the height of
B
the building as per the following equation (cl.7.7 of IS 1893 (Part 1): 2002):
n
Qi = VB (Wi hi2 ) / ∑ Wi h 2j (16.3)
j =1
where
1. Dead load and imposed loads are considered as per cl.6.3 of IS 1893
(Part 1): 2002.
3. Bending moments and shear forces are determined on columns and floor
beams in a manner similar to that of frames subjected to wind loads.
4. All parts must be efficiently bonded together so that the structure acts as a
unit.
Structures designed on this simple elastic principle may even survive when
subjected to severe earthquake due to the following:
(b) Irregular buildings – All framed buildings higher than 12 m in Zones IV and
V and those higher than 40 m in Zones II and III.
Though not mandatory, the code also recommends dynamic analysis for
buildings lesser than 40 m for irregular building in Zones II and III (see Note
below cl.7.8.1 of IS 1893 (Part 1): 2002).
The dynamic analysis may be carried out either by Time History Method or by
Response Spectrum Method. More about the dynamic analysis is beyond the
scope of this module.
Accordingly, the design approach adopted in IS 1893 (Part 1): 2002 is stated
in cl 6.1.3 of the standard which is as follows:
Q.2: Explain the behaviour of plain concrete with high intensify repeated axial
cyclic loads.
Q.3: Explain the behaviour of steel with high intensity repeated axial cyclic loads.
Q.4: Explain the behaviour of reinforced concrete with high intensity repeated
axial cyclic loads.
Q.5: Define the following terminologies as per IS 1893 (Part 1): 2002: (a) Design
Basis Earthquake (DBE), (b) Epicentre, (c) Focus, (d) Intensify of
earthquake, (e) Magnitude of earthquake, (f) critical damping, (g) Maximum
Considered Earthquake and (h) Liquefaction.
Q.7: Write the expression for determining (a) Horizontal seismic coefficient, (b)
Design seismic base shear and (c) Distribution of design force.
Q.8: Write the steps of performing static elastic design of earthquake resistant
structures.
Q.11: Explain the need of ductility and ductile detailing of reinforced concrete
structures subjected to seismic forces.
16.39.13 References
TQ.2: Explain the behaviour of reinforced concrete with high intensity repeated
axial cyclic loads.
(10 Marks)
A.TQ.2: Part (c) of sec. 16.39.3
TQ.4: Write the steps of performing static elastic design of earthquake resistant
structures.
(10 Marks)
A.TQ.4: Section 16.39.8 is the complete answer.
This lesson explains the direct and indirect effects of earthquakes. Behaviour
of plain concrete, steel bars and reinforced concrete are discussed when
subjected to high intensity repeated axial loads. Some common terminologies are
defined as given in IS 1893 (Part 1): 2002. Different BIS standards pertaining to
earthquake design are given. General principles and assumptions are given in
this lesson. Determinations of horizontal seismic coefficient, design seismic base
shear and distribution of design force are explained. Steps of the static elastic
design are given mentioning the particular situations when dynamics analysis
should be done. The objectives of earthquake resistant design of structures are
explained. The need for ductility and ductile detailing of reinforcement is
explained in this lesson.