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In this article, we will discuss Earthquake Proof Buildings. An earthquake is the sudden shaking of the earth that releases the energy in the earth’s lithosphere, causing the creation of seismic waves.
✔ It may be due to volcanic eruption, tectonic plates moving, cave formation in a small area, or explosions.
✔ It is also called a 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 earthquake forces.
✔ Despite the continuous efforts to construct earthquake-proof structures, it is practically impossible to provide complete immunity to the buildings from earthquakes due to their unpredictable intensity of occurrence.
✔ It may be said that constructing earthquake-proof buildings aims to erect buildings that have better resistance to seismic forces during seismic activity.
✔ With the growing cities, the risk of collapsing buildings during the earthquake has increased. Due to this, constructing earthquake-resistant buildings has become a prime area of concern for engineers worldwide, leading to the development of a new field of engineering, i.e. Earthquake Engineering.
1. Why do buildings fall during the Earthquake?
The leading causes behind the falling of buildings during an earthquake can be summarized as follows:
a. Failure of Soil:
Earthquake causes shaking of the ground simultaneously, mainly due to the passage of the seismic wave.
The lateral forces imposed are so strong that they can quickly turn the soft soil into a 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 landslides.
Thus, buildings constructed in such soft soil or sloppy areas have a greater threat of collapse during the earthquake.
During the motion of the ground in the event of an earthquake, the building also moves back and forth.
After the shaking stops, the buildings may sometimes slump to the ground.
The well-built and intact buildings 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, in 1985, about 4 billion $ damage was caused due to the collapse of buildings due to soil failure.
b. Failure of Foundation:
One of the leading causes behind the collapse of buildings during an earthquake is the failure of the foundation.
When the foundation cannot withstand the seismic stresses imposed, it fails, thereby causing the falling of the building.
The mass of a building can resist regular lateral forces like force exerted by the wind. However, in most cases, the buildings are not designed to withstand the intensive multi-directional lateral forces.
The foundations of the buildings, in particular, may shake from their original position and not be able to hold the structure above.
c. 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 on 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 a building due to the failure of soft soil was the Leaning Tower collapse during the Taiwan earthquake.
d. Failure of Building Itself:
The strength of the building depends upon the materials that have been used during its construction.
Generally, buildings made up of wooden materials are less susceptible to collapse than concrete buildings. This is because concrete buildings are highly rigid.
|Read Also: 30 Facts about Earthquake|
2. Features of Earthquake Proof Buildings
Some of the features of earthquake-proof buildings are:
a. Strengthened Diaphragm:
The diaphragm is one of the essential horizontal components of the building, including the floors. Earthquake-proof buildings have diaphragms on their deck and are strengthened horizontally to share the forces with vertical components.
b. Cross-Braced System:
Earthquake-proof buildings are designed with 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 shape.
c. 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.
d. 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 the same as the shear walls.
In addition, moment-resisting frames allow more flexibility to the designers for constructing exterior walls, ceilings, and the arrangement of various building components.
e. Lighter Roofs:
One of the prominent features of earthquake-proof buildings is that they have lighter roofs.
Most designers use profiled steel cladding on light-gauge steel purlins or double skin with insulators and purlins.
This feature of earthquake-proof buildings concerns 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 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 crucial feature of earthquake-proof buildings on account of safety.
Redundancy ensures that even if one method of prevention fails during an earthquake, other alternative methods or safety strategies come into play.
Due to this reason, earthquake engineers focus on equally distributing the masses and strength throughout the building.
i. Stronger Foundation:
A strong and stable foundation is an essential 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 earthquake-proof structures, foundations are well driven deeper into the ground, i.e. deep pile foundations are used.
j. Continuous Load Path:
While designing earthquake-resistant buildings, the designers must maintain a continuous load path.
The structural and non-structural components of the buildings must be tied together so that the inertial forces dissipate.
If the structure is not tied correctly, the components will move independently, making the structure prone to collapse.
Ensuring a continuous load path is a must for dissipating large seismic forces.
3. Making Buildings Earthquake Proof
A building with capabilities to resist the effects of earthquakes is called an earthquake-proof building.
Making building earthquake-proof results decrease in loss of lives, properties, etc.
A. Increasing Earthquake Resistivity of Small Buildings
Small buildings can be made earthquake-resistant by taking some precautions and measures in site selections, building planning, and construction.
1. Site selection
The building constructions should be avoided on:
a) Near unstable embankments
b) On the sloping ground with columns of different heights
c) Flood-affected areas
2. Building Planning
Symmetric plans are safe compared to unsymmetrical plans.
Hence we should go for square or rectangular plans rather than L, E, H, and T shapes.
Rectangular plans should not have a length more than twice of width.
The width of the foundation must not be less than 750 mm for single-story buildings and not less than 900 mm for multi-story buildings.
( Note: Storey in British English and story in American English)
The depth of the foundation should not be less than 1.0 m for soft and 0.45 m for rocky ground.
Before laying the foundation, remove all loose materials, including water, from the trench and compact the bottom. After laying the foundation, back-filling and compacting of the foundation should be done.
In the 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 broken 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 the case of brick masonry:
1. Use properly burnt bricks only.
2. Bricks should be placed with their groove mark facing up to ensure a better bond with the next course.
In the 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.
The length of the wall must be restricted to 6 m. Cross walls make the masonry stronger. Building partition walls along the main walls and interlinking the two is better.
5. Doors and windows openings
1. Walls with too many doors and windows near 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 of 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.
Fig: Hip Roof
1. Slopy roofs with a span greater than 6 m use trusses instead of rafters.
2. Building with a 4-sided sloping roof is stronger than that with two-sided sloping since gable walls collapse early.
|Read: Gable Roof|
Restrict chhajjas or balcony projections to 0.9 m. For larger projections, use beams and columns.
Masonry parapet walls can collapse quickly, so it is better to build parapets 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 and mixed thoroughly.
10. Bands ( Levels )
The following R.C. bands should be provided:-
Fig: Different Bands
a) Plinth band
b) Lintel band
c) Roof band
d) Gable band
For making R.C. bands, the minimum thickness is 75 mm, and at least two bars of 8 mm diameters are required.
If the wall size is large, vertical and, diagonal bands also may be provided.
|Read: Lintel Band|
|Read: Plinth Band|
Retrofitting means scientifically preparing a structure or building so that all building elements act as an integral unit.
It is generally the fastest and most economical way to achieve the safety of the building. The following are some of the methods for 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 a sloping belt on the gable wall.
4. Strengthen corners with seismic belts.
5. Anchor floor joints to walls with brackets.
6. Improve story 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.
12. Selection of Materials
As far as possible, highly ductile materials should be prioritised over others.
B. Increasing Earthquake Resistivity of Big Buildings
Tall buildings are subjected to heavy horizontal forces due to inertia during the earthquake.
Hence they need shear walls.
Shear walls should be provided evenly throughout the buildings in both directions and from bottom to top. Apart from providing shear walls, the given following techniques are also used for making tall buildings earthquake-resistant:
This idea behind isolation is to detach (isolate) the building from the ground, so that earthquake motions are not transmitted through the building or at least greatly reduced.
The concept of base isolation is explained through an example of a building resting on a roller.
When the ground shakes, 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 on the building.
The flexible pads are called base-isolator, whereas the structures constructed utilizing these devices are called base-isolated buildings.
2. Using seismic dampers
Another method for controlling building damage is installing seismic dampers in place of structural elements, such as diagonal braces.
Fig: Viscous Damper
When the seismic energy is transmitted through them, dampers absorb part of it and thus dampen the motion of the building.
There are three types of seismic isolation bearings:-
a. High-density rubber bearing
b. Laminated rubber bearings
c. Friction pendulum bearings
|Read: Seismic Dampers|
C. 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 an earthquake, and most structures fail due to the failure of joints.
Thus by increasing the strength of joints, some resistance can be achieved.
Strength of joints can be gained or achieved by using high-strength or fibre-reinforced concrete, increasing sections near joints or providing haunches. This might work as a knot as in bamboo. And thus giving stiffness to the joints.
2. Hollow foundation
As we all know, secondary and love types of waves are the most destructible among other earthquake waves.
And the secondary waves can not pass through water media.
Thus the provision of a hollow type raft foundation filled with water can be used to reduce 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 decreased very much.
|Read Also: Lintel Level|
|Read Also: Parapet Wall|