Disaster means occurrence of uncontrolled, painful and serious conditions. There are various natural disasters like:
Tsunami (a long high sea wave generated by an earthquake)
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:
- Natural earthquake
- Earthquakes due to induced activities.
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 earth crust. 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 earth’s 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.
- Focus: The point on the fault where slip starts is the focus. It is also known as hypocentre
- Epicentre: The point vertically above the focus on the surface of the earth is the epicentre.
- Focal Depth: The depth of focus from the epicentre is called the focal depth.
- Epicentral Distance: Distance from epicentre to any point of interest on the surface of earth is called epicentral distance.
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:
- 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.
- Building Planning: Symmetric plans are safer compared to unsymmetrical. Hence go for square or rectangular plans rather than L, E, H, T shaped. Rectangular plans should not have length more than twice the width.
- Foundations: Width of foundation should not be less than 750 mm for single storey building and not less than 900 mm for storeyed buildings. Depth of foundation should not be less than 1.0 m for soft soil and 0.45 m for rocky ground. Before foundation is laid remove all loose materials including water from the trench and compact the bottom. After foundation is laid back-fill the foundation properly and compact.
- Masonry: In case of stone masonry:
- Place each stone flat on its broadest face.
- Place length of stones into the thickness of wall to ensure interlocking inside and outside faces of the wall.
- Fill the voids using small chips of the stones with minimum possible mortar.
- Break the stone to make it angular so that it has no rounded face.
- At every 600 to 750 mm distance use through stones.
In case of brick masonry:
- Use properly burnt bricks only.
- Place bricks with its groove mark facing up to ensure better bond with next course.
In case of concrete blocks:
- Place rough faces towards top and bottom to get good bond.
- Blocks should be strong.
- Brush the top and bottom faces before laying.
In general walls of more than 450 mm should be avoided. Length of wall should be restricted to 6 m. Cross walls make the masonry stronger. It is better to build partition walls along main walls interlinking the two.
- Doors and Window Openings:
- Walls with too many doors and windows close to each other collapse early.
- Windows should be kept at same level.
- The total width of all openings in wall should not exceed 1/3 rd the length of wall.
- Doors should not be placed at the end of the wall. They should be at least at 500 mm from the cross wall.
- Clear width between two openings should not be less than 600 mm.
- In sloping roofs with span greater than 6 m use trusses instead of rafters.
- Building with 4-sided sloping roof is stronger than the one with two-sided sloping, since gable walls collapse early.
- Restrict chejja or balcony projections to 0.9 m. For larger projections use beams and columns.
- Parapet: Masonry parapet wall can collapse easily. It is better to build parapet with bricks up to 300 mm followed by iron railings.
- Concrete and Mortar: Use River sand for making mortar and concrete. It should be sieved to remove pebbles. Silt should be removed by holding it against wind. Coarse aggregates of size more than 30 mm should not be used. Aggregates should be well graded and angular. Before adding water cement and aggregates should be dry mixed thoroughly.
- 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 are required. They should be tied with steel limbs of 6 mm diameter at 150 mm spacing. If wall size is large, diagonal and vertical bands also may be provided.
- 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:
- Anchor roof truss to walls with brackets.
- Provide bracings at the level of purlins and bottom chord members of trusses.
- Strengthen gable wall by inserting sloping belt on gable wall.
- Strengthen corners with seismic belts.
- Anchor floor joists to walls with brackets.
- Improve storey connections by providing vertical reinforcement.
- Induce tensile strength against vertical bending of walls by providing vertical reinforcement at all inside and outside corners.
- 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:
- Foundations should be deeper
- R.C.C. framed structures are to be preferred over load bearing structures
- Sloping roofs should be avoided.
- Cantilever projections should be avoided.
- Roof and parapet wall should be properly anchored to the columns and walls.
- Height of the buildings should be restricted.
- Suitable wind load should be considered in the building design.
- Openings in the wall should be less.
- 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
- Using suitable materials
- Taking precautions in building construction
- 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. Granite disintegrate under fire. Lime stone crumbles easily. Most of the stones disintegrate during cooling period after heated by fire.
Brick: Bricks can resist heat up to 1200°C. At the time of construction, if good quality mortar is used, fire resistance is extremely good.
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.