Nowadays there is much talk about the earthquake-resistant design of buildings and we should know the fundamental of the subject. For the disaster-resistant design of buildings (against earthquakes or cyclones), we divide the structures into three categories as follows:
1.
Fully resistant structures which will behave elastically under
the worst loads, for example, hospitals, electrical stations, etc. which should
survive the event.
2.
Partially resistant structures which may undergo damages
that can be subsequently repaired, for example, other important buildings.
3.
Survival structures may get very much damaged during the
disaster but there should be features which will exclude any loss of life, for
example, temporary structures and low-cost buildings.
Again,
depending on the design and construction of buildings in India, we divide buildings
into another three categories as follows:
1.
Engineered buildings: These buildings are fully designed by
engineers and architects like tall multi-storey buildings which have to be designed
by engineers.
2.
non-engineered buildings: These buildings are fully conceived
and constructed by artisans such as carpenters and masons. (A great part of
residential houses built in India are built on the basis of empirical
knowledge.)
3.
Semi-engineered buildings: These buildings are not designed
fully, but unlike the non-engineered buildings, the codal provisions of IS 4326
(COP of Earthquake-Resistant Design of Buildings) and IS 13828 (Improving
Earthquake-Resistant Design of Low Strength Masonry Buildings) are incorporated
in their construction. (The necessary features of these buildings are fully
explained in a pamphlet "Guidelines for Improving Earthquake Resistance of
Housing" published by Building Materials and Technology Promotion Council,
New Delhi.) IS
13828, 1993, "Indian Standards for Improving Earthquake Resistance of
Low Strength Masonry Buildings", also explains the features to be
incorporated in such buildings.
DESIGN AND CONSTRUCTION OF EARTHQUAKE-RESISTANT
BUILDINGS
Multi-storey buildings have
to be built as engineered buildings (framed structures). We should strictly
adhere to codes in their planning (provision of shear walls and other features
with frame construction), in their analysis and design (by providing ductile detailing)
as well as in their construction. However, most of the one- or three-storey
buildings can be built as semi-engineered buildings to provide seismic
resistance.
As unreinforced or
unconfined masonry will crack under large earthquake forces, earthquake
resistance can be provided by confining masonry with RC bands at the plinth,
lintel and roof level according to the codes already given in the article Design
of Brick Masonry Walls. The provision of lintel bands for the various
zones is given in Table 8.1 in Chapter 8. The amount of reinforcement will
depend on the probable intensity of the earthquake or the zone in which the
building is built.
NEED FOR DUCTILE DETAILING
We must have a clear idea
about the need of ductile detailing of structures for earthquake resistance. We
must remember that it is extremely difficult to predict the magnitude of the
earthquake forces. In the case of cyclonic wind forces, we can estimate the full
force of the wind and we assume these full forces as static forces for
analysis. However, in the estimation of the seismic forces, what the codes give is
an estimate of the probable forces, and the actual earthquake forces are
dynamic and can be much larger. The
objectives of ductile detailing are as follows:
(a) The structure should resist the moderate intensity of
earthquakes without any damage.
(b) The structure should resist the exceptionally large
intensities of earthquakes which can occur without collapse.
We have to incorporate the ductile behaviour of the structure so that it can take a large amount of
overloading beyond the design load and does not collapse, thus ensuring no loss
of lives during a large earthquake. This is carried by the following rules laid
down for ductile detailing by the code IS
13920.
RECOMMENDED METHODS OF DESIGN
From the above discussion,
we can arrive at the following conclusions:
1. In regions of low earthquake intensity zone 2, normal
design according to the general codes IS
456-2000 for RCC and IS 1905 for masonry will give enough built-in safety.
2. If we have to get an absolutely safe building in other
zones, we should adopt a framed fully engineered building according to the
codes IS
1893 (1984) and IS
13920 (1993).
3. Otherwise we can have semi-engineered masonry
buildings having not more than 4 storeys in zones 3 and 4 and not more than 3
storeys in zone 5. In such buildings, we must use the prescribed guidelines
published in the references given in above which are given in below
BRIEF REVIEW OF RECOMMENDATIONS FOR SEMI-ENGINEERED
BUILDINGS
The
main recommendations for keeping a building intact are as follows:
1. Provision of good foundations
2. Provision of plinth beams to tie together the
foundation
3. Provision of lintel band
4. Provision of roof/floor band
5. Vertical bar at corners
6. Reinforcing around door and window openings
The details of the above recommendations can be obtained
from the IS codes and references given in above
Key Components: Confined Masonry Building
Conclusion
Tall and other buildings
which should be fully resistant to earthquake forces should be designed and
built as fully engineered buildings. However, the ordinary masonry low-rise
buildings, which are the majority of houses built in India, can be built as
semi-engineered buildings to be detailed according to the recommendations of IS
codes and "Guidelines for Improving Earthquake Resistance of
Buildings" published by the Building Materials and Technology Promotion
Council, Government of India, referred in Section above
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