Disaster means occurrence of uncontrolled, painful and serious conditions. There are various natural disasters like:
Earthquakes
Volcanic eruptions
Cyclones
Fire
Landsliding
Tsunami (a long high sea wave
generated by an earthquake)
Flood.
Earthquakes, cyclone and fire needs special considerations in building design and construction since they are more frequent, widespread and more disastrous. In this chapter this aspect of building design and constructions are discussed.
EARTHQUAKES RESISTANT BUILDINGS
An earthquake is a sudden, rapid shaking of the earth surface caused by the breaking and shifting of rocks beneath. During earthquake, ground motion occurs in a random fashion in all directions radiating from a point within earth crust, called epicentre. It causes vibrations of structures and induce inertia forces on them. As a result structure may collapse resulting into loss of property and lives.Earthquakes do not kill people, vulnerable buildings do so. Hence there is need of designing earthquake resistant buildings, especially in the earthquake prone areas.
TYPES OF EARTHQUAKES
Depending upon the possible causes, the earthquakes may be classified as:
1. Natural earthquake
2. Earthquakes due to induced activities.
Natural Earthquakes
Natural earthquakes may be due to
(i) active faults (ii) movement of tectonic plates or (iii) due to volcanic eruptions.
In earth’s crust there are some faults which are not yet settled. The displacement of rocks along faults cause earthquake.Tectonic means large scale process affecting the structure of the earthcrust. This process causes gradual movement of material within the crust of earth. Sometimes it shakes the earth crust.Volcano is a mountain or hill having a crater through which lava, rock fragments, hot vapour and gas are or have been erupted from the earths crust. Occasionally the volcanoes become active and create earthquake near the mountain crater.
Earthquakes due to Induced Activities
These are caused by vibrations induced by atomic explosions and collapse of ground due to faulty mining.
TERMINOLOGY
1. Focus: The point on the fault
where slip starts is the focus. It is also known as hypocentre
2. Epicentre: The point
vertically above the focus on the surface of the earth is the epicentre.
3. Focal Depth: The depth of
focus from the epicentre is called the focal depth.
4. Epicentral Distance: Distance
from epicentre to any point of interest on the surface of earth is
IMPROVING STEPS FOR EARTHQUAKE RESISTANCE OF BUILDINGS
The earthquake resistance of small buildings may be increased by taking some precautions and measures in site selections, building planning and constructions as explained below:
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| Natural disaster mitigation |
1. Site
Selection: The
building constructions should be avoided on
(a) Near unstable
embankments
(b) On sloping ground with
columns of different heights
(c) Flood affected areas
(d) On subsoil with marked
discontinuity like rock in some portion and soil in some portion.
2. Building
Planning: Symmetric
plans are safer compared to unsymmetrical. Hence go for
3. Foundations: Width of
foundation should not be less than 750 mm for single storey building
4. Masonry: In case of stone masonry:
1. Place each stone flat on its broadest face.
2. Place length of stones into the thickness of wall to ensure interlocking inside and outside faces of the wall.
3. Fill the voids using small
chips of the stones with minimum possible mortar.
4. Break the stone to make it
angular so that it has no rounded face.
5. At every 600 to 750 mm distance
use through stones.
In case of brick
masonry:
1. Use properly burnt bricks only.
2. Place bricks with its groove
mark facing up to ensure better bond with next course.
In case of
concrete blocks:
1. Place rough faces towards top
and bottom to get good bond.
2. Blocks should be strong.
3. Brush the top and bottom faces
before laying.
In general walls of more than 450
mm should be avoided. Length of wall should be
5. Doors and
Window Openings:
1. Walls with too many doors and
windows close to each other collapse early.
2. Windows should be kept at same
level.
3. The total width of all openings
in wall should not exceed 1/3 rd
4. Doors should not be placed at
the end of the wall. They should be at least at 500 mm from
5. Clear width between two openings should not be
less than 600 mm.
6. Roof:
1. In sloping roofs with span greater than 6 m use trusses instead of rafters.
2. Building with 4 sided sloping
roof is stronger than the one with two sided sloping, since
7. Chejjas:
1. Restrict chejja or balcony
projections to 0.9 m. For larger projections use beams and
8. Parapet: Masonry parapet
wall can collapse easily. It is better to build parapet with bricks
9. Concrete and
Mortar: Use
river sand for making mortar and concrete. It should be sieved
10. Bands: The following
R.C. bands should be provided
(a) Plinth band
(b) Lintel band
(c) Roof band
(d) Gable band.
For making R.C. bands minimum
thickness is 75 mm and at least two bars of 8 mm diameters
11. Retrofitting: Retrofitting means preparing a structure in a scientific manner so that all elements of a building act as an integral unit.It is generally the most economical and fastest way to achieve safety of the building. The following are some of the methods in retrofitting:
1. Anchor roof truss to walls with
brackets.
2. Provide bracings at the level
of purlins and bottom chord members of trusses.
3. Strengthen gable wall by
inserting sloping belt on gable wall.
4. Strengthen corners with seismic
belts.
5. Anchor floor joists to walls
with brackets.
6. Improve storey connections by
providing vertical reinforcement.
7. Induce tensile strength against vertical bending of walls by providing vertical reinforcement at all inside and outside corners.
8. Encase wall openings with reinforcements.
CYCLONE RESISTANT BUILDINGS
A cyclone is a storm accompanied by high speed whistling and howling winds. It brings torrential rains.A cyclone storm develops over tropical ocean and blows at speed as high as 200–240 km/hour. It is usually accompanied by lightning, thunder and continuous downpour of rain. Cyclones extend from from 150 km to 1200 km in lateral directions with forced winds spiralling around a central low pressure area.The central region of light winds and low pressure, known as the ‘eye’ of cyclone has an average diameter of 20 to 30 km. This central eye is surrounded by a ring of very strong winds extending up to 40 to 50 km beyond centre. This region is called ‘wall cloud’. In this region strongest winds and torrential rains occur. Beyond this region winds spiralling extend outwards to large distances, which goes on reducing with the distance from the centre of the cyclone.
The following care should be taken in designing buildings in cyclone prone areas:
1. Foundations should be deeper
2. R.C.C. framed structures are
to be preferred over load bearing structures
3. Sloping roofs should be avoided.
4. Cantilever projections should
be avoided.
5. Roof and parapet wall should
be properly anchored to the columns and walls.
6. Height of the buildings should
be restricted.
7. Suitable wind load should be
considered in the building design.
8. Openings in the wall should be
less.
9. Structure should not rest on loose soil.
FIRE RESISTANT BUILDING
It is reported that in USA fire kills more people each year than all other natural disasters combined including floors, cyclones and earthquake. The fire load in a building should be kept to the minimum possible. The term fire load indicates the amount of heat liberated in kilo joules per square metre (kJ/m2) of floor area of any compartment by the combustion of the content of the building including its own combustible part. It is determined by multiplying the weights of all combustible materials by their
respective calorific values and dividing that with floor area.A building may be made more fire resistant by
1. Using suitable materials
2. Taking precautions in building
construction
3. By providing fire alarm systems and fire
extinguishers.
Using Suitable Materials
The fire resisting material is
having the following characters:
(a) It should not
disintegrate under the effect of heat
(b) It should not expand
under heat so as to introduce unnecessary stresses in the building
(c) The material should
not catch fire easily
(d) It should not lose its
strength when subjected to fire.
Fire resisting characters of some
of the commonly used building materials are given below:
Stone: It is a bad
conductor of heat. Sand stones with fire grains can resist fire moderately.
Brick: Bricks can
resist heat up to 1200°C. At the time of construction, if good quality mortar
is
Timber: Any structure made of timbers is rapidly destroyed in fire. Timber enhances the intensity of fire. Use of heavy sections of timber in buildings is not desirable. To make timber more fire resistant the surface of timber is coated with chemicals such as ammonium phosphate and sulphate, boric acid and borax. Sometimes fire resistant paint is applied to timber used in the building.
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