1. GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGN
Chapter 3
GENERAL CONCEPTS OF EARTHQUAKE
RESISTANT DESIGN
3.1 INTRODUCTION reinforcement as required.
Experience in past earthquakes has dem-
Chapter 2 has provided a good overview
onstrated that many common buildings
of structural action, mechanism of damage
and typical methods of construction lack
and modes of failure of buildings. From
basic resistance to earthquake forces. In
these studies, certain general principles
most cases this resistance can be achieved
have emerged:
by following simple, inexpensive princi-
ples of good building construction prac-
(i) Structures should not be brittle or
tice. Adherence to these simple rules will
collapse suddenly. Rather, they
not prevent all damage in moderate or large
should be tough, able to deflect or
earthquakes, but life threatening collapses
deform a considerable amount.
should be prevented, and damage limited
to repairable proportions. These principles (ii) Resisting elements, such as bracing
fall into several broad categories: or shear walls, must be provided
evenly throughout the building, in
(i) Planning and layout of the building both directions side-to-side, as well
involving consideration of the loca- as top to bottom.
tion of rooms and walls, openings
(iii) All elements, such as walls and the
such as doors and windows, the
roof, should be tied together so as to
number of storeys, etc. At this stage,
act as an integrated unit during
site and foundation aspects should
earthquake shaking, transferring
also be considered.
forces across connections and pre-
(ii) Lay out and general design of the venting separation.
structural framing system with spe-
(iv) The building must be well connected
cial attention to furnishing lateral
to a good foundation and the earth.
resistance, and
Wet, soft soils should be avoided, and
(iii) Consideration of highly loaded and the foundation must be well tied to-
critical sections with provision of gether, as well as tied to the wall.
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2. IAEE MANUAL
Where soft soils cannot be avoided,
special strengthening must be pro-
vided.
(v) Care must be taken that all materials
used are of good quality, and are pro-
tected from rain, sun, insects and
other weakening actions, so that their
strength lasts.
(vi) Unreinforced earth and masonry
have no reliable strength in tension,
and are brittle in compression. Gen-
erally, they must be suitably rein-
forced by steel or wood.
These principles will be discussed and
illustrated in this Chapter.
3.2 CATEGORIES OF
BUILDINGS
For categorising the buildings with the
purpose of achieving seismic resistance at
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3. GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGN
Soft: Those soils, which have allowable many projections Fig 3.2 (b). Tor-
bearing capacity less than or equal sional effects of ground motion are
to 10 t/m2. pronounced in long narrow rectan-
gular blocks. Therefore, it is desirable
Weak: Those soils, which are liable to large
to restrict the length of a block to
differential settlement, or liquefac-
three times its width. If longer
tion during an earthquake.
lengths are required two separate
Buildings can be constructed on firm blocks with sufficient separation in
and soft soils but it will be dangerous to between should be provided,
build them on weak soils. Hence appropri- Fig 3.2 (c).
ate soil investigations should be carried out
(iii) Separation of Blocks: Separation of a
to establish the allowable bearing capacity
large building into several blocks
and nature of soil. Weak soils must be
may be required so as to obtain sym-
avoided or compacted to improve them so
metry and regularity of each block.
as to qualify as firm or soft.
3.2.4 Combination of
parameters
For defining the categories of buildings for
seismic strengthening purposes, four cat-
egories I to IV are defined in Table 3.1. in
which category I will require maximum
strengthening and category IV the least in-
puts. The general planning and designing
principles are, however, equally applica-
ble to them.
3.3. GENERAL PLANNING AND
DESIGN ASPECTS
3.3.1. Plan of building
(i) Symmetry: The building as a whole
or its various blocks should be kept
symmetrical about both the axes.
Asymmetry leads to torsion during
earthquakes and is dangerous,
Fig 3.1. Symmetry is also desirable
in the placing and sizing of door and
window openings, as far as possi-
ble.
(ii) Regularity: Simple rectangular
shapes, Fig 3.2 (a) behave better in Fig 3.1 Torsion of unsymmetrical plans
an earthquake than shapes with
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4. IAEE MANUAL
For preventing hammering or in larger buildings since it may not
pounding damage between blocks a be convenient in small buildings.
physical separation of 3 to 4 cm
(iv) Simplicity: Ornamentation
throughout the height above the
invo1ving large cornices, vertical or
plinth level will be adequate as well
horizontal cantilever projections, fa-
as practical for upto 3 storeyed
cia stones and the like are danger-
buildings, Fig 3.2 (c).
ous and undesirable from a seismic
The separation section can be treated viewpoint. Simplicity is the best ap-
just like expansion joint or it may be proach.
filled or covered with a weak mate-
Where ornamentation is insisted
rial which would easily crush and
upon, it must be reinforced with
crumble during earthquake shaking.
steel, which should be properly em-
Such separation may be considered
Fig 3.2 Plan of building blocks.
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5. GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGN
bedded or tied into the main struc- (i) Stability of Slope: Hillside slopes li-
ture of the building. able to slide during an earthquake
should be avoided and only stable
Note: If designed, a seismic coeffi-
slopes should be chosen to locate the
cient about 5 times the coefficient
building. Also it will be preferable
used for designing the main struc-
ture should be used for cantilever
ornamentation.
(v) Enclosed Area: A small building en-
closure with properly intercon-
nected walls acts like a rigid box
since the earthquake strength which
long walls derive from transverse
walls increases as their length de-
creases.
Therefore structurally it will be ad-
visable to have separately enclosed
rooms rather than one long room,
Fig 3.3. For unframed walls of thick-
ness t and wall spacing of a, a ratio
of a/t = 40 should be the upper limit
between the cross walls for mortars
of cement sand 1:6 or richer, and less
for poor mortars. For larger panels
or thinner walls, framing elements
should be introduced as shown at
Fig 3.3(c).
(vi) Separate Buildings for Different
Functions: In view of the difference
in importance of hospitals, schools,
assembly halls, residences, commu-
nication and security buildings, etc.,
it may be economical to plan sepa-
rate blocks for different functions so
as to affect economy in strengthen-
ing costs.
3.3.2 Choice of site
The choice of site for a building from the
seismic point of view is mainly concerned
with the stability of the ground. The fol-
lowing are important: Fig 3.3 Enclosed area forming box units
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6. IAEE MANUAL
to have several blocks on terraces 3.3.4 Fire resistance
than have one large block with It is not unusual during earthquakes that
footings at very different elevations. due to snapping of electrical fittings short
A site subject to the danger of rock circuiting takes place, or gas pipes may
falls has to be avoided. develop leaks and catch fire. Fire could also
be started due to kerosene lamps and
(ii) Very Loose Sands or Sensitive Clays:
kitchen fires. The fire hazard sometimes
These two types of soils are liable to
could even be more serious than the earth-
be destroyed by the earthquake so
quake damage. The buildings should there-
much as to lose their original struc-
fore preferably be constructed of fire resist-
ture and thereby undergo
ant materials.
compaction. This would result in
large unequal settlements and dam-
3.4 STRUCTURAL FRAMING
age the building. If the loose
There are basically two types structural
cohesionless soils are saturated with
framing possible to withstand gravity and
water they are apt to lose their shear
seismic load, viz. bearing wall construction
resistance altogether during shaking
and framed construction. The framed con-
and become liquefied.
struction may again consist of:
Although such soils can be compacted,
for small buildings the operation may be (i) Light framing members which must
too costly and these soils are better avoided. have diagonal bracing such as wood
For large building complexes, such as hous- frames (see Chapter 6) or infill walls
ing developments, new towns, etc., this fac- for lateral load resistance, Fig 3.3 (c),
tor should be thoroughly investigated and or
appropriate action taken.
(ii) Substantial rigid jointed beams and
columns capable of resisting the lat-
Therefore a site with sufficient bearing
eral loads by themselves.
capacity and free from the above defects
should be chosen and its drainage condi- The latter will be required for large col-
tion improved so that no water accumu- umn free spaces such as assembly halls.
lates and saturates the ground close to the
footing level. The framed constructions can be used
for a greater number of storeys compared to
3.3.3. Structural design bearing wall construction. The strength and
Ductility (defined in Section 3.6) is the most ductility can be better controlled in framed
desirable quality for good earthquake per- construction through design. The strength
formance and can be incorporated to some of the framed construction is not affected
extent in otherwise brittle masonry con- by the size and number of openings. Such
structions by introduction of steel reinforc- frames fall in the category of engineered
ing bars at critical sections as indicated construction, hence outside the scope of the
later in Chapters 4 and 5. present book.
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7. GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGN
3.5 REQUIREMENTS OF The strengthening measures necessary
STRUCTURAL SAFETY to meet these safety requirements are pre-
As a result of the discussion of structural sented in the following Chapters for vari-
action and mechanism of failure of Chap- ous building types. In view of the low
ter 2, the following main requirements of seismicity of Zone D, no strengthening
structural safety of buildings can be arrived measures from seismic consideration are
at. considered necessary except an emphasis
on good quality of construction. The fol-
(i) A free standing wall must be de- lowing recommendations are therefore in-
signed to be safe as a vertical canti- tended for Zones A, B and C. For this pur-
lever. pose certain categories of construction in a
number of situations were defined in
This requirement will be difficult to
Table 3.1.
achieve in un-reinforced masonry in
Zone A. Therefore all partitions in- 3.6 CONCEPTS OF DUCTILITY,
side the buildings must be held on DEFORMABILITY AND
the sides as well as top. Parapets of DAMAGEABILITY
category I and II buildings must be
Desirable properties of earthquake-resist-
reinforced and held to the main
ant design include ductility, deformability
structural slabs or frames.
and damageability. Ductility and
(ii) Horizontal reinforcement in walls is deformability are interrelated concepts sig-
required for transferring their own nifying the ability of a structure to sustain
out-of-plane inertia load horizon- large deformations without collapse.
tally to the shear walls. Damageability refers to the ability of a struc-
(iii) The walls must be effectively tied
together to avoid separation at verti- Table 3.1 Categories of buildings for strengthening purposes
cal joints due to ground shaking. Category Combination of conditions for the Category
I Important building on soft soil in zone A
(iv) Shear walls must be present along
both axes of the building. II Important building on firm soil in zone A
Important building on soft soil in zone B
Ordinary building on soft soil in zone A
(v) A shear wall must be capable of re-
sisting all horizontal forces due to III Important building on firm soil in zone B
Important building on soft soil in zone C
its own mass and those transmitted Ordinary building on firm soil in zone A
to it. Ordinary building on soft soil in zone B
IV Important building on firm soil in zone C
(vi) Roof or floor elements must be tied Ordinary building on firm soil in zone B
together and be capable of exhibit- Ordinary building on firm soil in zone C
ing diaphragm action. Notes: (i) Seismic zones A, B and C and important buildings are defined
in Section 3.2.
(vii) Trusses must be anchored to the sup- (ii) Firm soil refers to those having safe bearing value more than
porting walls and have an arrange- 10 t/m2 and soft those less than 10 t/m2.
ment for transferring their inertia (iii) Weak soils liable to compaction and liquefaction under earth-
quake condition are not covered here.
force to the end walls.
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8. IAEE MANUAL
ture to undergo substantial damage, with- together so that excessive stress concentra-
out partial or total collapse. This is desir- tions are avoided and forces are capable of
able because it means that structures can being transmitted from one component to
absorb more damage, and because it per- another even through large deformations.
mits the deformations to be observed and
repairs or evacuation to proceed, prior to Ductility is a term applied to material
collapse. In this sense, a warning is received and structures, while deformability is ap-
and lives are saved. plicable only to structures.
3.6.1 Ductility Even when ductile materials are present
Formally, ductility refers to the ratio of the in sufficient amounts in structural compo-
displacement just prior to ultimate dis- nents such as beams and walls, overall
placement or collapse to the displacement structural deformability requires that geo-
at first damage or yield. Some materials are metrical and material instability be
inherently ductile, such as steel, wrought avoided. That is, components must have
iron and wood. Other materials are not proper aspect ratios (that is not be too high),
ductile (this is termed brittle), such as cast must be adequately connected to resisting
iron, plain masonry, adobe or concrete, that elements (for example sufficient wall ties
is, they break suddenly, without warning. for a masonry wall, tying it to floors, roof
Brittle materials can be made ductile, usu- and shear walls), and must be well tied to-
ally by the addition of modest amounts of gether (for example positive connection at
ductile materials, Such as wood elements beam seats, so that deformations do not
in adobe construction, or steel reinforcing permit a beam to simply fall off a post) so
in masonry and concrete constructions. as to permit large deformations and dy-
namic motions to occur without sudden
For these ductile materials to achieve a collapse.
ductile effect in the overall behaviour of the
component, they must be proportioned and 3.6.3 Damageability
placed so that they come in tension and are Damageability is also a desirable quality
subjected to yielding. Thus, a necessary re- for construction, and refers to the ability of
quirement for good earthquake-resistant a structure to undergo substantial damages,
design is to have sufficient ductile materi- without partial or total collapse
als at points of tensile stresses.
A key to good damageability is redun-
3.6.2 Deformability dancy, or provision of several supports for
Deformability is a less formal term refer- key structural members, such as ridge
ring to the ability of a structure to displace beams, and avoidance of central columns
or deform substantial amounts without or walls supporting excessively large por-
collapsing. Besides inherently relying on tions of a building. A key to achieving good
ductility of materials and components, damageability is to always ask the ques-
deformability requires that structures be tion, “if this beam or column, wall connec-
well-proportioned, regular and well tied tion, foundation, etc. fails, what is the con-
sequence?”. If the consequence is total col-
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9. GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGN
lapse of the structure, additional supports high frequency motions. Unfortunately, tra-
or alternative structural layouts should be ditional applications of this technique usu-
examined, or an additional factor of safety ally do not account for occasional large
be furnished for such critical members or displacements of this pin-connected
connections. mechanism, due to rare very large earth-
quakes or unusually large low-frequency
3.7 CONCEPT OF ISOLATION content in the ground motion, so that when
The foregoing discussion of earthquake- lateral displacements reach a certain point,
resistant design has emphasized the tradi- collapse results. A solution to this problem
tional approach of resisting the forces an would be provision of a plinth slightly be-
earthquake imposes on a structure. An al- low the level of the top of the posts, so that
ternative approach which is presently when the posts rock too far, the structure is
emerging is to avoid these forces, by isola- only dropped a centimeter or so.
tion of the structure from the ground mo-
tions which actually impose the forces on 3.8 FOUNDATIONS
the structure. For the purpose of making a building truly
earthquake resistant, it will be necessary to
This is termed base-isolation. For sim- choose an appropriate foundation type for
ple buildings, base- friction isolation may it. Since loads from typical low height
be achieved by reducing the coefficient of buildings will be light, providing the re-
friction between the structure and its foun- quired bearing area will not usually be a
dation, or by placing a flexible connection problem. The depth of footing in the soil
between the structure and its foundation. should go below the zone of deep freezing
in cold countries and below the level of
For reduction of the coefficient of fric- shrinkage cracks in clayey soils. For choos-
tion between the structure and its founda- ing the type of footing from the earthquake
tion, one suggested technique is to place angle, the soils may be grouped as Firm and
two layers of good quality plastic between Soft (see Section 3.2.3) avoiding the weak
the structure and its foundation, so that the soil unless compacted and brought to Soft
plastic layers may slide over each other. or Firm condition.
Flexible connections between the struc- 3.8.1 Firm soil
ture and its foundation are also difficult to In firm soil conditions, any type of footing
achieve on a permanent basis. One tech- (individual or strip type) can be used. It
nique that has been used for generations should of course have a firm base of lime or
has been to build a house on short posts cement concrete with requisite width over
resting on large stones, so that under earth- which the construction of the footing may
quake motions, the posts are effectively pin- start. It will be desirable to connect the in-
connected at the top and bottom and the dividual reinforced concrete column
structure can rock to and fro somewhat. footings in Zone A by means of RC beams
This has the advantage of substantially re- just below plinth level intersecting at right
ducing the lateral forces, effectively isolat- angles.
ing the structure from the high amplitude
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10. IAEE MANUAL
3.8.2 Soft soil footings are presented in Chapters 4 and 9
In soft soil, it will be desirable to use a plinth respectively.
band in all walls and where necessary to
connect the individual column footings by These should ordinarily be provided
means of plinth beams as suggested above. continuously under all the walls. Continu-
It may be mentioned that continuous rein- ous footing should be reinforced both in
forced concrete footings are considered to the top and bottom faces, width of the foot-
be most effective from earthquake consid- ing should be wide enough to make the
erations as well as to avoid differential set- contact pressures uniform, and the depth
tlements under normal vertical loads. De- of footing should be below the lowest level
tails of plinth band and continuous RC of weathering.
•••
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