When the ground shakes beneath your feet

You can't though avoid the tragedy, but you can avoid the amount of loss by taking precautions


Earthquake is a physical phenomenon characterized by the shaking of the ground with violence of varying intensity. The intensity of earthquake varies from imperceptible motion which can be recorded only with the sensitive instruments to the most violent shaking of ground. Earthquakes are among the most destructive forces that nature unleashes on earth.thy cause loss of life as well as lost of property.
The science which deals with the study of earthquake is called Seismology. The engineers need to know the characteristics and the magnitude of forces related during the earthquake in order to design and construct structures which will resist such forces in order to prevent the loss of life and property.
Nature of Earthquakes.
According to the seismological observations the earthquake starts with an explosion like disturbances somewhere within the outer layer of earth. The centre of earthquake is called Focus of earthquake. The energy of the shock generated at the focus is radiated outwards by means of elastic waves of varying nature. These Shockwaves (elastic waves) travel through and along the crust of earth, changing the pattern of oscillation by refraction, reflection and superposition of one type of wave on other. The intensity of these waves diminishes with distance. The place on the earth’s surface located directly over the focus is called the Epicenter of the earthquake. The distance between the Focus and epicenter is called as Focal Depth.
Movements of ground during earthquakes are recorded on the instruments called seismographs and accelerographs. Which measure three components of ground motion i.e.vertical, horizontal north-south and horizontal east-west.
Classification of earthquakes
Earthquakes are designated as shallow, intermediate and deep earthquakes. If focal depth is less than 70km it is called as shallow earthquake. If focal depth is between 70 to 300km it is called intermediate earthquake and if depth exceeds 300km it is designated as deep earthquake.
Earthquake waves travel through the body and surface of earth. The two basic waves of seismology are body waves called as P and S waves. P waves are longitudinal, compression and rarefaction waves. They are first to arrive and be recorded.
S waves are second to arrive and usually have greater amplitudes and destructive power as compared to p waves.
The majority of earthquakes result from the structural changes in the crust of earth. They are called tectonic earthquake. The Elastic rebound theory explains it in detail. It says that when two tectonic plates strike with each other the elastic energy gets developed in the rock strata. Since the rock is an inelastic material it does not deform but the energy gets stored in the form of Strain energy .when this strain energy exceeds the permissible value of rock strata the strata breaks and releases this energy in the form of rebound (shock) and causes vibration in the earth called as Tectonic Earthquake. 
Magnitude and intensity scales
The Magnitude M of a shallow quake has been defined by Richter as the Logarithm to base 10 of the maximum trace amplitude expressed in thousandths of a millimeter with which the standard short period torsion seismometer (which has natural period 0.8 second,a magnification 2800,damping nearly critical) would register that earthquake at and epicentral distance of 100 km. The magnitude is thus a number that indicates the size of earthquake. The larger the magnitude of a shock, the greater is the energy released by it and larger is the area which suffers damage. Richter suggested the following equation for the energy in ergs released by a shock of Magnitude M.
Log10E = 11.4 +1.5M
Intensity of an earthquake at a place is taken as a measure of the effect of earthquake and is indicated by a number according to the scale of intensities used. The most widely used intensity scales now are the Modified Mercalli scale.
Seismic strengthening provisions
Seismic strengthening provisions in simple masonry structures.
The seismic strengthening of stone buildings follows the pattern of brick buildings i.e we have to use horizontal steel at roof level and lintel level and vertical steel in the corners of structure.
a).Horizontal reinforcement of walls.
The horizontal reinforced concrete bands should be used for brick buildings and random rubble masonry construction. As an alternative wooden planks of rectangular section, effectively spliced longitudinally and held by lateral members in lattices form be used where timber is available and also more economical.
B).Vertical reinforcing of walls.
The vertical steel in masonry walls required to be provided at the corner and T-junction of walls and should be tied with the horizontal reinforcement.
In case of vertical bars in stone masonry, use of a casing pipe is recommended around which the masonry is built to height of 0.6m. The pipe is kept loose by rotating it during masonry construction. The casing pipe is raised and cavity below is filled with 1:2:4 concrete mix and rodded to compact it. The concrete will not only provide the bond between the bar and masonry but will protect the bars from corrosion. The steel at the jambs of opening may be taken from the footing upto the lintel band and anchored into it. The corner steel must be taken from the footing upto the roof slab or roof band and anchored into it.
Seismic strengthening provisions in timber structures.
Two types of wooden wall construction are discussed here which are recommended to be used in the seismic areas. These are:-
1. Stud wall 2.brick nogged timber frames.
Stud wall construction consists of timber studs and corner posts framed into soil, top plates and wall plates. Horizontal struts and diagonal braces are used to stiffen the frame against the lateral loads due to earthquake and wind. The wall covering may consist of matting made from bamboo, reeds and timber boarding or the like. If wooden sheathing boards are used properly nailed to the timber frame, the diagonal bracing may be omitted. The diagonal bracing may be nailed to the surface.
Brick nogged timber frame.
The brick nogged timber frame consists of intermediate vertical, columns, sills, wall plates, horizontal nogging members framed into each other. Diagonal braces may also be framed with the vertical or nailed or bolted on the face. The space between framing members is filled with tight fitting bricks or dressed stone masonry in stretcher bond.Dhajji wall construction is used in Jammu and Kashmir.
Braced wooden frames
The braces in above cases may be arranged around the main rooms only but made sufficiently strong so as to take the inertial forces of the whole building.
Two types of bracing are usually use. These are
1. In one case the lateral strength has been provided near corner or junctions of walls. This is adopted where cross walls are available at short intervals as in the case of residential buildings.
2. In other case the strength is provided by knee bracing. This is adopted in case of long halls, schools and godowns. 
To avoid the deterioration of timber, below the plinth level construction should be done in masonry or concrete and the studs and columns of the superstructure should be securely connected with the footing through the sills and the column directly. Painting with hot tar should be done to protect the timber from termites and aunts. A hallow space between the wooden floor and the ground which permits air ventilation and increases the life of timber.
Strength provisions in Existing reinforced concrete structures
In present age new structures are usually designed by taking seismic forces into consideration however structures already constructed should be made fit to resist the future earthquake.
Two approaches are used for structure-level retrofitting;
1).Conventional method based on increasing the seismic resistance of existing structures.
2).Non-conventional method based on the reduction of seismic demand. Conventional Methods
Adding shear Walls
One of the most common methods to increase the lateral strength of the reinforced concrete building is to make the provision for additional shear walls. The technique of adding new shear walls is often taken as the best and simple solution for improving seismic performance. Therefore; it is frequently used for retrofitting of non-ductile reinforced concrete buildings. The added elements can be either cast-in-place or pre-cast concrete elements. New elements preferably are placed at the exterior of the building; however it may cause alteration in the appearance and window layouts. Placing of shear walls in the interior of the structure is not preferred in order to avoid interior moldings.
Adding Steel bracing.
Another method of strengthening is to use of steel bracing, which also has similar advantages as that of shear wall. The installation of steel bracing members can be an effective solution when large openings are required. The method of use of steel bracing has potential advantage over the other schemes as;
1. Higher strength and stiffness can be achieved.
2. Opening for natural light can be made easily and good aesthetic look can be given to the building.
3. The amount of work is less since foundation cost may be minimized, which in turn minimizes the cost of structure.
4. Bracing system adds much less weight to the existing structure and as such it becomes seismically safe.
Adding Infill Walls.
Strengthening of existing reinforced moment resist frames often involves addition of infill walls. It is an effective and economical method for improving strength and reducing drift of existing frames. But a relative strong masonry infill cause short column effect results in a failure of the columns of existing frame. Proper selection of the infill masonry strength along with the prevention of its premature separation from the column, a more desirable failure mode can be achieved. Anchorage of the masonry to the frame is a critical factor in determining an overall performance. By proper anchoring, it should be possible to force failure in the masonry and prevent a premature shear/flexure column failure.
Non Conventional Methods
In recent years, several alternative approaches are being used in the retrofitting of structures. Among them, seismic base isolation and addition of supplement devices techniques are the most popular. These techniques precede with quiet a different philosophy that these reduce the horizontal seismic forces.
Seismic Base Isolation
This method proceeds with quiet different philosophy in the sense that this method reduces the horizontal seismic force. It is the best and cheaper method of seismic rehabilitation of buildings. Few advent ages of this method are:-
1. It reduces shear force developed due to seismic forces.
2. Superstructure needs little reinforcement.
3. No reinforcement in foundation is required.
Mostly in base isolation buildings rubber bearings located at the base of building are used, mostly below columns or under shear walls.
Supplement Damping Devices
This method of damping is an effective method to resist seismic forces. Mostly in this method supplement dampers to a structure are installed by using viscous-dampers, viscoelastic dampers, frictional dampers etc, to reduce the horizontal drift in structure.
Reinforced Concrete Jacketing
The main objective of jacketing is to increase the seismic capacity of the framed structure. Reinforced concrete jacketing can be employed as repair or strengthening of structure. The most common type of jackets are steel jackets, concrete jackets, fibre reinforced polymers, jackets with high tension material like carbon fibre, glass fiber. In this method of retrofitting additional concrete with reinforcement are added to existing structural element to increase its capacity. The capacity of resisting the forces in a structure is determined by various advanced methods like pushover analysis etc to see which structural element will fail in advance.
The above discussed methods should be adopted to any structure after a comparative analysis like push over analysis etc (using latest software’s like SAP,STAAD,STRUD,SAFE) has to be made depends on effectiveness and cost of structure to see the actual behavior of structure during an earthquake in order prevent their failure.
{The author is a structural engineer and can be mailed at

Lastupdate on : Wed, 20 Mar 2013 21:30:00 Makkah time
Lastupdate on : Wed, 20 Mar 2013 18:30:00 GMT
Lastupdate on : Thu, 21 Mar 2013 00:00:00 IST

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