An earthquake is the sudden shaking of the earth which releases the energy in the earth’s lithosphere causing the creation of seismic waves. It may be due to volcanic eruptions, moving of tectonic plates, the formation of the cave in small area or explosions. It is also called quake( or tremor or temblor). It is measured with a Richter Magnitude scale.
Earthquake proof buildings are those buildings that have been specially designed to withstand the earthquake forces imposed to some extent. Despite the continuous efforts of constructing earthquake-proof structures, it is practically not possible to provide complete immunity to the buildings from earthquake due to its unpredictable intensity of occurrence. It may be said that the motive of constructing earthquake-proof buildings is to erect buildings that have better resistance to seismic forces during seismic activity.
With the growing cities, the risk of collapsing of buildings during the earthquake has increased and due to this, constructing earthquake-resistant buildings have become a prime area of concern for engineers throughout the world leading to the development of a new field of engineering i.e. Earthquake Engineering.
Why do buildings fall during the earthquake?
The main causes behind the falling of buildings during an earthquake can be summarized as follows:
1. Failure of Soil:
Earthquake causes shaking of the ground simultaneously particularly due to the passage of the seismic wave. The lateral forces imposed are so strong that it can easily turn the soft soil into the loose mass of sand-like particles abandoning its ability to bear weight.
Such sand-like mass can transform sloppy sides into mudslides posing the risk of landslide. Thus, buildings that have been constructed in such soft soil or sloppy areas have a greater threat of collapse during the earthquake.
During the motion of ground in the event of an earthquake, the building also moves in the back and forth direction. After the shaking stops, the buildings may sometime slump into the ground.
The buildings that are well-built and intact may remain erect but will topple in case of unstable soil on account of the failure of the soil. According to the reports, during the Mexico earthquake,1985; about four billion $ damage was caused due to the collapse of buildings owing to the failure of soil.
2. Failure of Foundation:
One of the main cause behind the collapse of buildings during an earthquake is the failure of the foundation. When the foundation is not able to withstand the seismic stresses imposed, it fails thereby causing falling of building.
The mass of a building can resist normal lateral forces like force exerted by the wind. However, in most of the cases, the buildings are not designed to resist the intensive multi-directional lateral forces. The foundations of the buildings, in particular, may shake from its original position and may not be able to hold the structure above.
3. Failure of Soft Floors:
Soft floors are those floors that consist of large open spaces, minimal shear walls on the interior side and additional floor-to-floor height.
In many cases, it has been observed that often the upper floors remain intact but the lower floors are either crushed or crumbled. This is because the seismic forces are maximum at the ground floor where most of the soft floors are located. The soft floors are also less intact than the building structure as a whole and thus are more prone to failure.
One such example of the collapse of building due to failure of soft soil was the Leaning Tower collapse during the Taiwan earthquake.
4. Failure of Building Itself:
The strength of the building depends upon the materials that have been used during its construction. In general practice, buildings made up of wooden materials are less susceptible to collapse than the concrete buildings. This is because concrete buildings lose rigidity.
Features of Earthquake Proof Buildings
Some of the features of earthquake proof buildings are:
1. Strengthened Diaphragm:
The diaphragm is one of the important horizontal components of the building including floors. Earthquake-proof buildings have their diaphragms placed on their deck and strengthened horizontally to share the forces with vertical components.
2. Cross-Braced System:
Earthquake-proof buildings are designed with a set of properly braced columns, braces and beams to reverse the seismic forces back to the ground. Cross brace incorporates mainly two diagonal sections in an X like a shape.
3. Stronger Shear Walls:
To resist the sway during an earthquake, vertical walls known as shear walls are erected in earthquake-proof buildings. It helps to enhance the stiffness of the structural frame of the building. It is used in addition to the brace system.
4. Moment-Resisting Frames:
Earthquake-proof buildings may be designed with moment-resisting frames as an alternative to shear walls as shear walls somewhat limit the flexibility of the buildings.
Moment-resisting frames function same like the shear walls. In addition to this, moment-resisting frames allow more flexibility to the designers for constructing exterior walls, ceilings and also for the arrangement of various building components.
5. Lighter Roofs:
One of the prominent features of earthquake-proof buildings is that they have lighter roofs. Most of the designers use profiled steel cladding on light-gauge steel purlins or double skin with insulators and purlins.
This feature of earthquake-proof buildings is concerned with the movement of buildings in a lateral direction. It must be ensured that during the earthquake the building moves equally in both directions and dissipate equal forces on both the sides avoiding excessive force on a single side.
Earthquake proof buildings must have adequate vertical as well as lateral stiffness.
Redundancy is perhaps the most important feature of earthquake-proof buildings on account of safety. Redundancy ensures that during an earthquake even if one method of prevention fails, other alternative methods or strategies of safety come into play. Due to this reason, earthquake engineers focus on equally distributing the masses and strength throughout the building.
9. Stronger Foundation:
A strong and stable foundation is an important characteristic of earthquake-proof buildings. A strong foundation is vital for resisting the large earthquake forces as well as for the long life of buildings. In most of the earthquake-proof structures, foundations are well driven deeper into the ground i.e. deep pile foundations are used.
10. Continuous Load Path:
While designing the earthquake-resistant buildings, the designers must ensure that a continuous load path is maintained. The structural and non-structural components of the buildings must be tied together so that the inertial forces dissipate. If the structure is not properly tied, the components will move independently making the structure prone to collapse. Ensuring continuous load path is a must for dissipating large seismic forces.
Making Buildings Earthquake Proof
The buildings that can resist earthquake and its effects is simply called earthquake-resistant building. Making building earthquake-proof results decrease in loss of lives, properties, etc.
The process of increasing the earthquake resistivity of buildings can be divided into 2 parts:
1. Increasing Earthquake resistivity of small buildings( Earthquake resistant buildings – small buildings)
2. Increasing Earthquake resistivity of big buildings( Earthquake resistant buildings – big buildings)
1. Increasing Earthquake Resistivity of Small Buildings
By taking some precautions and measures in site selections, building planning and constructions small buildings can be made earthquake resistance. Some of the precautions and measures in site selection, building plannings and constructions are explained below:-
The building constructions should be avoided on:
a)Near unstable embankments
b)On the sloping ground with columns of different heights
2) Building Planning
Symmetric plans are safe compared to unsymmetrical plan. Hence we should go for square or rectangular plans rather than L, E, H, T shaped. Rectangular plans should not have length more than twice of width.
Width of the foundation must not be less than 750 mm for single-storey buildings and not less than 900 mm for storeyed buildings. Depth of foundation should not be less than 1.0 m for soft and 0.45 m for rocky ground. Before laying of the foundation, remove all loose materials, including water from the trench and compact the bottom. After the laying of the foundation, back-filling of the foundation properly and then compacting of foundation should be done.
In case of stone masonry
1. Place each stone flat on its broadest face.
2. Place the length of stone into the thickness of the wall to ensure interlocking inside and outside faces of the wall.
3. Voids should be filled with the small chips of the stones with minimum possible mortar.
4. The stone should be a break to make it angular so that it has no rounded face.
5. At every (600 -700) mm distance use through stones.
|Read: Stone Masonry|
In case of brick masonry
1. Use properly burnt bricks only.
2. Bricks should be placed with its groove mark facing up to ensure better bond with the next course.
In case of concrete masonry
1. Place rough face towards top and bottom to get a good bond.
2. Blocks should be strong.
3. Brush the top and bottom faces before laying.
Length of the wall must be restricted to 6 m. Cross walls make the masonry stronger. It is better to build partition walls along the main walls interlinking the two.
5)Doors and windows openings
1. Walls with too many doors and windows near to each other may collapse early. Windows should be kept at the same level.
2. The total width of all openings in the wall should not exceed one -third the length of the wall.
3. Doors should not be placed at the end of the wall. They should be at least 500 mm from the cross wall.
4. Clear width between two openings should not be less than 600 mm.
1.In slopy roofs with a span greater than 6 m use trusses instead of rafters.
2. Building with 4 sided sloping roof is stronger than that with two-sided sloping, since gable walls collapse early.
|Read: Gable Roof|
Restrict chejjas or balcony projections to 0.9 m. For larger projections use beams and columns.
Masonry parapet wall can collapse easily so it is better to build parapet with bricks up to 300 mm followed by iron railings.
|Read: Parapet Wall|
9)Concrete and mortar
Use river sand for making mortar and concrete. It should be sieved to remove pebbles. Silt must be removed by holding it against the wind. Coarse aggregate of size more than 30 mm should not be used. Aggregates should be well-graded and angular. Before adding water cement and aggregate should be dry mixed thoroughly.
The following R.C. bands should be provided:-
For making R.C. bands, the minimum thickness is 75 mm and at least two bars of 8 mm diameters are required. They must be tied with steel limbs of 6 mm diameter at 150 mm spacing.
If wall size is large, vertical and diagonal bands also may be provided.
|Read: Lintel Band|
|Read: Plinth Band|
Retrofitting simply means, scientifically preparing a structure or building so that all elements of a building act as an integral unit. It is generally the fastest and economical way to achieve the safety of the building. The following are some of the methods in retrofitting:-
1. Anchor roof truss to walls with brackets.
2. Provide bracing at the level of purlins and bottom chord members of trusses.
3. Gable wall is strengthened by inserting sloping belt on the gable wall.
4. Strengthen corners with seismic belts.
5. Anchor floor joints to walls with brackets.
6. Improve storey connections by providing vertical reinforcement.
7. Introduce tensile strength against vertical bending of walls by providing vertical reinforcement at all inside and outside corners.
8. Encase wall openings with reinforcement.
2. Increasing Earthquake Resistivity of Big Buildings
Tall buildings are subjected to heavy horizontal forces due to inertia during the time of the earthquake. Hence they need shear walls. Shear wall is usually built around the lift room.
Shear walls should be provided evenly throughout the buildings in both directions as well as from bottom to top. Apart from providing shear walls, the given following techniques are also used for making tall buildings earthquake-resistant:
This is an idea behind isolation to detach (isolate) the building from the ground in such a way that earthquake motions are not transmitted up through the building, or at least greatly reduced.
The concept of base isolation is explained through an example of building resting on the roller. When the ground is shaking, the roller freely rolls but the building above does not move. If the gap between the building and the vertical wall of the foundation pit is small, the vertical wall of the pit may hit the wall. Hence 100% frictionless rollers are not provided in practice. The building is rested on flexible pads which offer resistance. This helps in reducing some effects of ground shaking to the building. The flexible pads are called base-isolator, whereas the structures projected utilizing these devices are called base-isolated buildings.
2)Using seismic dampers
Another method for controlling seismic damage in buildings is by installing seismic dampers in place of structural elements, such as diagonal braces. When the seismic energy is transmitted through them, dampers absorb part of it and thus damp the motion of the building. There are 3 types of seismic isolation bearings:-
a)High-density rubber bearing
b)Laminated rubber bearings
c)Friction pendulum bearings
|Read: Seismic Dampers|
New Techniques: Earthquake Proof Buildings
Following techniques are some techniques for making earthquake-resistant buildings are discussed below:-
As we know joints are most vulnerable during the earthquake and most of the structures fail due to failure of joints. Thus by increasing the strength of joints, some resistance can be achieved. Strength of joints can be gained or achieved by simply using high strength or fibre reinforced concrete, or just by the increasing section near joints or provide haunches. This might be work as a knot as in bamboo. And thus provide stiffness to the joints.
As we all know secondary and love types of waves are most destructible among other earthquake waves. And the secondary waves can not pass through water media. Thus by the provision of hollow type raft foundation filled with water can be used for reducing some destructible effects of the earthquake. It may be filled with some viscous fluid, worked as a damper to reduce earthquake effects.
Two belts are to be provided within a bituminous layer in between. In experimental setups, it was found that the damage to the building is decreased very much. The upper belt is moved concerning the lower belt by a few centimetres.
(Note: During an earthquake, do not hide under the staircase because it is the weakest part of the buildings and structures.)
|Read Also: Lintel Level|
|Read Also: Parapet Wall|