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EARTHQUAKE-RESISTANT MASONRY BUILDINGS
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Foreword
The 1990 World Conference of Education for All set the challenge of progress
towards basic education for all, but man-made and natural disasters have been a
barrier to the achievement of this goal. Natural disasters such as floods, hurricanes
and earthquakes destroy hundreds of school buildings and other educational facilities
every year, causing death and injury to children and disruption to the ongoing process
of education.
We cannot prevent natural disasters from striking, but we can prevent or limit their
impact by making buildings strong enough to resist their destructive forces. In the
case of earthquakes, it is possible, as this study demonstrates, to neutralize their harm
by applying basic engineering and planning principles that are inexpensive and not
beyond the skills of most building industries.
Prologue
Masonry is the most important construction material in Iran. It has been used for
public and residential buildings in the past several thousand years. A great number of
well-preserved old masonry structures still exist proving that this form of construction
can successfully resist loads and environmental impact.
Although some specific features have been invented during the course of time to
improve the seismic behaviour of masonry buildings, such as connecting stones and
tying of the walls, masonry construction remains, even today, the most venerable part
of existing building stock. This is not only the case in developing countries, but it is
also the case in the most developed regions of Europe and the USA.
How Buildings Respond to Earthquakes
An earthquake is the vibration of the earth’s surface that follows a sudden release of
energy in the crust..
During an earthquake, the ground surface moves in all directions. The most damaging
effects on buildings are caused by lateral movements which disturb the stability of the
structure, causing it to topple or to collapse sideways. Since buildings are normally
constructed to resist gravity, many traditional systems of construction are not
inherently resistant to horizontal forces. Thus design for earthquakes consists largely
of solving the problem of bracing a building against sideways movement. The actions
illustrated in Figue 1 demonstrate combinations of the vertical gravity effects with the
lateral effects of earthquakes.
Lateral Force-Resisting Systems
Most of our techniques of building construction focus on direct resistance of gravity
forces. This is a natural result of our own experience with gravity acting on our bodies
and of our handling the elements with which we build. We thus tend to produce
constructions that are vulnerable to the effects of horizontal forces. Figure 7-a
illustrates such a structure, a fence post, designed to resist gravity loads but is
vulnerable to the horizontal load conditions shown in Figure 7-b.
Stiffness of Non-structural Elements
12
Stiffness of Non-structural Elements
It is recommended to reinforce non-structural partition walls with 4-6 mm diameter
bars placed in the bed joints with a vertical spacing of not more than 600 mm.
Where weather conditions necessitate the use of reinforced concrete pitched roofs the
masonry gable end walls should be anchored to the uppermost tie beams. If the height
of the gable wall exceeds 4 m, intermediate tie beams should be added at intervals not
exceeding 2 m, as shown in Figure 9.
Designing Masonry Buildings for Earthquakes
Masonry has been popular through the ages for its fire resistance, its thermal capacity
and its durability. However the combination of weight, stiffness and weakness against
tensile forces makes traditional masonry buildings highly vulnerable to earthquakes.
This is not only the case in developing countries but it is also the case in the most
developed regions of the world.
Vibrations caused by earthquakes generate additional loading. Shear stresses develop
which cause damage to structural elements. Since masonry, which can be stressed
relatively high in compression, is weak in resisting bending and shear, collapse is
often the result. Consequently, masonry has, for a long time, been considered
unsuitable in earthquake resisting constructions.
Floors and Roofs
Floors and Roofs
During earthquakes, floors and roofs should act as rigid horizontal diaphragms, which
distribute the seismic forces among structural walls in proportion to their stiffness.
One of the main reasons for the poor behaviour of existing masonry buildings is a
lack of proper horizontal diaphragm action of floor and roof structures and or lack of
proper connections between them and the structural walls which carry them.
Use of timber floors and roofs in high-risk seismic zones is only recommended where
the requisite carpentry skills exist and if specially designed details to ensure the
integrity of these elements and their anchorage to the supporting walls.
Annex
The earthquake destroyed twenty-eight schools completely and damaged 137 others.
A total of 16,526 pupils were affected.
The effect was felt most in the in rural areas where schools were built of unreinforced
masonry. Structural failure resulted mainly from wall separation due to lack of lateral
resistance and use of non-ductile materials and from dislocation of lightly attached
heavy roofs. These forms of failure would have been avoided had the buildings been
adequately conceived and constructed against earthquakes.
The photographs illustrate some of the positive and negative aspects of school
building practice observed during the survey. They were taken during four days of
fieldwork in the provinces of Qasvin and Hamadan in the first week of December
2002, five months after the earthquake.
Further Reading
Most of the ideas presented in this booklet are based on laboratory tests and site
observations illustrated in the following references.
Design for Earthquakes,
Authors: Ambrose and Vergun,
Publisher: John Wiley & Sons, 1999.
Protection of Educational Buildings Against Earthquakesi
Author: Prof. A.S. Arya
Publishers: UNESCO Regional Office for Asia and the Pacific, Bangkok
Eurocode 8: Design Provisions for Earthquake Resistance of Structures
British Standards Institute, 1996
Structural Principles,
Author: I. Engel
Publisher: Prentice Hall, Inc., 1984