Technology used to protect building from damaging earthquake effect

Technology used to protect building from damaging earthquake effect are

Base Isolation Devices

Seismic Dampers 

In Base Isolation Device the building is detach from the ground, so that earthquake motions are not transmitted up through the building, or at least it is reduced.

In Seismic dampers are devices introduced in the building to absorb the energy provided by the ground motion to the building.

Why Earthquake Effects are to be reduced?

First question arise why there is need of reducing earthquake effect on buildings. 

Due to Seismic Design structural buildings do not collapse under strong earthquake shaking. But, it may damage to non- structural elements like glass facades.

Seismic designed building are more costlier, but this cost is justified through improved seismic performance.

 

 

 

Structure of Earth

Structure of Earth

Structure of earth

Interior is divided into: 

1. Crust
2. Mantle
3. Core

 

1. Crust  

• Uppermost part
•  Solid
• Thickness extend from 5 to 40 km inside earth
• Consist of rock line Granite, basalt etc.
• Density 2.5 - 3 gm/cc. 

2. Mantle


• Below crust is mantle divide into outer and inner part.
• Outer part extends up to 660 km.
• Inner part extends up to 2900 km. 
• Density 3.38-5.56 gm/cc.
• Semi - Solid state.
• Temperature is around 3000 ⁰C 

mantle is further divided into three parts

       3. Core

• It have two parts Inner core and Outer core.
• Inner core is solid  and composition consist of Nickel and Iron.
• Outer Core is Liquid zone.
• Radius of Inner core - 1290 km.
• Outer core thickness -2300 km.
• Density around 13.5 gm/cc contrast to 25  ⁰C



outer core

• Temperature is estimated around 2500 ⁰C 

inner core

Seismology

Seismology

Introduction 

Seismology or Earthquake engineering is defined as branch of science which deal with study of earthquake.

Seismology include:

Cause and type of Earthquake

1. Study of origin of Seismic waves
2. Propagation of Seismic waves
3. Recording of Seismic waves
4. Analyzing Seismic waves.

Importance of Earthquake engineering:

1. Understanding earthquake nature and effect on our life.
2.  Help in designing earthquake resistant structure.
3. Planning effective disaster management program during earthquake.

Plate Tectonics

Plate Tectonics

According to this theory large amount of material mass join together form earth. Large amount of heat was generated during fusion. As earth cool down, heavier material sank to center and lighter one toward top. The upper part is called crust and inner part is called as core. 

Plate Tectonics

                              

                                             Due to high temperature and pressure difference between crust and core it results in convention current (like flow of water molecule when heated up). These current result in circulation of earth mass from crust to core and vice versa. Hot molten lava comes out and cold rock mass goes into earth. This flow of material cause crust and some part of mantle to slide on outer core. This sliding of earth mass take place in difference part of earth called tectonic plate.

                                              Earth surface has seven tectonic plate. These plate move in different direction and different speed, which result in rising and sinking of continent (formation of mountain and valleys).

Major tectonic plates on earth surface are :

1. Eurasian plate
2. North American plate
3. Indo Australian plate
4. Pacific plate
5. South American plate
6. African plate
7. Antarctic plate

Faults

Faults

Displacement in strata of the same rock take place , either vertically or horizontally. Such offset of geological structure are called faults. Such movement can be either slow slip, which produces no ground shaking or sudden rupture, which result in perceptible vibrations which result in earthquake.

Fault displacement is classified into three types:

Faults

• Normal fault
• Strike - slip fault
• Reverse fault

Normal Fault: 

                       When the rock on that side of the fault hanging over the fracture slips downward, below the other side. The dip of a normal fault vary from 0 to 90. 

Reverse Fault: 

                         The hanging wall of the fault moves upward in relation to the bottom.

Strike Slip Fault: 

                          Faulting that causes only horizontal displacements along the strike of the fault.

Cause of Earthquake

Cause of Earthquake

1.Natural disturbances

2. Artificial disturbances

 1. Natural disturbance which  cause earthquake are:

 a) Volcanic cause

                            Volcanic earthquake is one that occurs in conjunction with volcanic activity. Volcanic cause sudden out burst or explosion. It produce vibration in nearby area. Volcanic eruption don't produce earthquake. Eruptions and earthquakes both result from tectonic forces in the rocks, they need not occur together.

b) Tectonic cause

                              It occur inside earth. According to theory, crust (outer layer) is made of rock & divide into many plates. plate are in motion in different direction and with different speed. These rock or plate have strain energy store in them. Sudden slip at fault result in release of strain energy in form of seismic wave which cause shaking of earth.
                              Earthquake which occur due to sudden release of strain energy as a result of sudden slip of fault is called Tectonic earthquake. Magnitude of earthquake depend on amount of energy release. 90% of earthquake is due to tectonic reason.

Basic terminology of Earthquake

Basic terminology of Earthquake

Hypocenter

Basic terminology of earthquake

                    Point from where earthquake originate.

Epicenter

                    Point on earth surface vertically above hypo center.

Focal depth

                   Depth of focus from epicenter. It is important in determining extent of damage of earthquake.

Epicentral distance

                 Distance from epicenter to point of any interest is called epicentral distance. As distance increase, effect of earthquake become less.  

Fore shock and After shock                      

                Fault movement generate series of earthquake. Smaller size earthquake take place before big earthquake called Fore shock and those after big earthquake is After shock.

 

 

What happens during earthquake?

What happens during earthquake?

In an Earthquake,

-Seismic Waves arise from sudden movements in a rupture zone in the earth's crust.

-Waves of different types and velocities travel different paths before reaching a building's site.

What happens to the buildings?

During earthquake, ground moves rapidly back and forth, which result in foundation movement of foundations.While upper part of the building prefer to remain where it is because of its mass of inertia. 

                                  This result in strong vibrations of the structure with resonance phenomena between the structure and the ground, and thus result in large internal forces. This results in plastic deformation of the structure and substantial damage with local failures and , in extreme cases, collapse..

Determination of effect of earthquake on building

Determination of effect of earthquake on building

The effect of an earthquake on a building is determined by the :

• Time histories of the three ground motion parameters
• Ground acceralation (ag)
• Velocity (vg)
• Displacement(dg)

Ground motion parameters depends on:

• Distance
• Direction
• Soil characteristics (layer thickness, shear wave velocity)
• Depth and
• Mechanism of the fault zone in the earth crust

As for the response of a building  to the ground motion, it depends on structural characteristics (eigenfrequency, type of structure, ductility etc.)

In skeleton structures, separate non- structural masonry walls by joints!

In skeleton structures, separate non- structural masonry walls by joints!

In flexible skeleton structures, it can be beneficial to separate non-strucutral partition walls from the structure by soft joints. This is particularly true for inplane stiff and brittle masonry walls.This way, damage occuring even for weak earthquakes can be prevented. 

Basic Principle for Seismic Design of Building: Match structural and non-structural element

Match structural and non-structural element

If deformation-sensitive non-structural partition walls and facade elements (e.g. of masonry) are incorporated into a horizontally soft structure (e.g. a frame structure) without using joints, substantial damage may develop even for relatively weak earthquakes.

Basic Principle for Seismic Design of Building: Reinforced structural masonry walls to resist earthquake actions

REINFORCED STRUCTURAL MASONRY WALLS

Reinforced structural masonry walls to resist earthquake actions

A possible alternative to basic principle 12 for making masonry structures substantially more suitable for seismic actions is to reinforce some long masonry walls and thus stiffen them in the longitudinal direction.

In this case, for example, vertical and horizontal minimum reinforcement and stronger vertical reinforcement in the boundary zones must be detailed .

Basic Principle for Seismic Design of Building: Brace masonry buildings with reinforced concrete structural walls!

Brace masonry buildings with reinforced concrete structural walls!

Masonry is a good construction material in terms of thermal insulation, storage and vertical loads carrying capacity. For seismic actions however, masonry structures are not well suited. On one hand they are relatively stiff, so they usually have a high natural frequency- within the plateau area of the design response spectrum- and therefore experience large earthquake actions. On the other hand unreinforced masonry walls are rather brittle and generally exhibit relatively little energy dissipation. 

Basic principle for Seismic design of building: Avoid bracing of frames with masonry infills

Avoid bracing of frames with masonry infills

The frame structure is relatively flexible and somewhat ductile, while unreinforced masonry is very stiff and fragile and may explode under the effect of only small deformations. 

        At the beginning of an earthquake the masonry carries most of the earthquake actions but as the shaking intensifies the masonry fails due to shear or sliding. The appearance of diagonal cracks is characteristics of a seismic failure.

There are two cases:

• With stronger columns the masonry is completely destroyed and falls out.
• With weaker columns the masonry can damage and shear the columns, which often lead to collapse.

Basic Principle for the Seismic design of the building: Avoid mixed systems with columns and masonry walls!

Avoid mixed systems with columns and masonry walls!

The columns in combination with the slabs or beams form frames, which have smaller horizontal stiffness than the masonry walls. The earthquake actions are therefore carried to a large extent by the masonry walls. In addition to the inertia forces from their own influence zone, the walls must resist those from the parts of the building with the columns. This result in a seismic resistance considerably less than that of pure masonry construction. When masonry walls fail due to the seismic actions or deflections, they can no longer carry the gravity loads, which usually leads to a total collapse of the building.

Such mixed systems is unfavorable because of their lack of flexibility with regard to increasingly frequent building modifications required by changes in their use.

Basic principle for the Seismic design of building: Two slender reinforced concrete structural walls in each principal direction!

Two slender reinforced concrete structural walls in each principal direction!

Reinforced concrete structural walls of rectangular cross- section constitute the most suitable bracing system against seismic actions for skeleton structures. The walls may be short in horizontal direction e.g. 3 to 6 m- they must extend over entire height of the building. 

To minimise the effects of torsion, the walls should be placed symmetrically with respect to center of mass and as close as possible to the edges of the building.

Basic principle for the Seismic design of building: Discontinuities in stiffness and resistance cause problems !

Discontinuities in stiffness and resistance cause problems !

Some time designer modifies the cross section of bracing systems over the height of a building, which result in discontinuities and lead to variations in the  stiffness and resistance of the building.

This also disturbs the local flow of forces. An increase in the stiffness from bottom up is generally less favorable than the opposite.

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Basic principle for the Seismic design of building: Avoid bracing offsets

Avoid bracing offsets

Avoid bracing offsets

 Horizontal bracing offset, in plane or out of plane, result when the position of the bracing changes from one storey to another. The bending moments and the shear forces are induced by the offset. The offset disturb direct flow of forces, weaken the resistance and reduce the ductility of the bracing. Moreover, they cause large additional forces and deformation in other structural elements (e.g. slabs and columns) during an earthquake. 

Compare to bracings that are continuous over the height of the building, bracings with offsets increase the vulnerability of the construction and usually reduce its seismic resistance.

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Basic principle for the Seismic design of building: Avoid asymmetric bracing!

Avoid asymmetric bracing!

Asymmetric bracing is a frequent cause of building collapses during earthquakes.

Column should also be able to withstand horizontal displacements of the structure without loosing load bearing capacity.

In this sketch only lateral bracing element (walls and trusses) are represented.  The column are not drawn because their frame action to resist horizontal forces and displacements is small. In above sketch M- center of mass, W- center of resistant for horizontal forces, S - center of stiffness. 

W is the center of gravity of the flexural and frame resistance of structural elements along the two major axes. If the center of resistance and the center of mass do not coincide, eccentricity and twisting occur. The building twists in the horizontal plane about the center of stiffness. In particular, this torsion generates significant relative displacements between the top and bottom of the column furthest away from the center of stiffness and these often fail rapidly. 

                             Therefore the center of resistance should coincide with , or be close to, the center of mass, and sufficient torsional resistance should be available.  

This can be achieved by symmetrical arrangement of the lateral bracing elements.These should be placed, along the edges of building, or in any case sufficiently far away from the center of mass.

In this building with flat slabs and small structural columns designed to carry gravity loads, the only bracing against horizontal forces and displacements is a reinforced concrete elevator and stairway shaft, placed very asymmetrically at the corner of the building. Which result in large eccentricity between center of mass and resistance or stiffness. Twisting in the plan will lead to large relative displacements in the columns furthest away from the shaft and the danger of punching shear failure.