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Earthquake Resistant Structure A PRESENTATION BY M.B.RAJIV GANDHI (04B21) R.RAJA GURU (05LB07) DEPARTMENT OF CIVIL ENGINEERING
THIAGARAJAR COLLEGE OF ENGINEERING, MADURAI-15 (A Govt. Aided ISO 9001:2000 Autonomous Institution affiliated to Anna University)
TO CONTACT: [email protected] [email protected] Ph.No:0452-4367243
Abstract: We have discussed here about the construction of structure with earthquake resistant phenomenon. Causes for earthquake, its effects, design of structure, earthquake design philosophy , Indian seismic codes followed while designing the structure, measurement of earthquake intensity, behaviour of types of buildings towards earthquake, activities to be followed by during an earthquake and after the earthquake, specification of materials, techniques used in overcoming the earthquake disaster, earthquake safety tips , etc. are discussed with sketches.
Introduction: A shaking of the ground caused by sudden movements in the earth's crust is termed as an earthquake. Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter of the earthquake. Earthquakes occur all the time all over the world, both along plate edges and along faults. Earthquakes exert lateral as well as vertical forces. Earthquake-resistant structures absorb and dissipate seismically induced motion Building should be designed to prevent total collapse, preserve life, and minimize damage in case of an earthquake or tremor. Earthquakes, Tsunamis, Landslides, Floods and Fires are natural calamities causing severe damage and sufferings to persons by collapsing the structures, cutting off transport systems, killing or trapping persons, animals etc. Such natural disasters are challenges to the progress of development. However, civil engineers as designers have a major role to play in minimising the damages by proper designing the structures or taking other useful decisions. Because of the vastness of the topic, “Disaster management and mitigation”, this module includes understanding the earthquakes, behaviour of the materials of construction and structures and the extent to which structural engineers make use of the knowledge in taking proper decisions in designing the structures made of reinforced concrete
Types of earthquakes There are many different types of earthquakes: tectonic, volcanic, and explosion. The type of earthquake depends on the region where it occurs and the geological make-up of that region. The most common are tectonic earthquakes. These occur when rocks in the earth's crust break due to geological forces created by movement of tectonic plates. Another type, volcanic earthquakes, occur in conjunction with volcanic activity. Collapse earthquakes are small earthquakes in underground caverns and mines, and explosion earthquakes result from the explosion of nuclear and chemical devices.
Tsunami A tsunami is a large ocean wave usually caused by an underwater earthquake or a volcanic explosion. Tsunamis are not tidal waves. Tidal waves are caused by the forces of the moon, sun, and planets upon the tides, as well as the wind as it moves over the water. With typical waves, water flows in circles, but with a tsunami, water flows straight. This is why tsunamis cause so much damage
Earthquake Design Philosophy The earthquake design philosophy may be summarized as follows: (a) Under minor but frequent shaking, the main members of the building that carry vertical and horizontal forces should not be damaged; however building parts that do not carry load may sustain repairable damage. (b) Under moderate but occasional shaking, the main members may sustain repairable damage, while the other parts of the building may be damaged such that they may even have to be replaced after the earthquake; and (c) Under strong but rare shaking, the main members may sustain severe (even irreparable) damage, but the building should not collapse. Thus, after minor shaking, the building will be fully operational within a short time and the repair costs will be small. And, after moderate shaking, the building will be operational once the repair and strengthening of the damaged main members is completed. But, after a strong
earthquake, the building may become dysfunctional for further use, but will stand so that people can be evacuated and property recovered.
Acceptable Damage: Ductility Buildings have the ability to sway back-and-forth during an earthquake, and to withstand earthquake effects with some damage, but without collapse .Ductility is one of the most important factors affecting the building performance. Thus, earthquake-resistant design strives to predetermine the locations where damage takes place and then to provide good detailing at these locations to ensure ductile behaviour of the building.
Earthquake-Resistant Design of Buildings Buildings should be designed like the ductile chain. The failure of a column can affect the stability of the whole building, but the failure of a beam causes localized effect. Therefore, it is better to make beams to be the ductile weak links than columns. This method of designing RC buildings is called the strong-column weak-beam design method.
Response of building based on soil layer thickness During the 1967 Caracas earthquake in South America, the response of buildings was found to depend on the thickness of soil under the buildings. Figure above shows that for buildings 3-5 storey tall, the damage intensity was higher in areas with underlying soil cover of around 40-60 m thick, but was minimal in areas with larger thickness of soil cover. On the other hand, the damage intensity was just the reverse in the case of 10-14 storey buildings; the damage intensity was more when the soil cover was in the range 150-300 m, and small for lower thickness of soil cover. Here, the soil layer under the building plays the role of a filter, allowing some ground waves to pass through and filtering the rest.
Detection and recording Seismograph
A seismograph is an instrument used for recording the intensity and duration of an earthquake.
Vertical seismometer
Geologists use seismographs to record the surface and body waves.
As waves from
earthquakes reach the seismograph the mass stays in relatively the same place, while the ground and the support move around it. This movement is recorded on magnetic tape by a pen attached to the mass. In a seismograph designed to measure vertical motion, the mass is
connected to a spring, so as the ground and support move up and down, the pen on the mass measures the vertical motion. The metal tape which the motion is recorded on is marked with lines that correspond to one minute intervals. When motion is recorded a seismogram is created, which tells about the waves--how big they were and how long they lasted. P waves are recorded first, followed by S waves and then surface waves. While surface waves are the last to reach the seismograph, they last the longest time
Importance of Seismic Design Codes Seismic codes help to improve the behaviour of structures so that they may withstand the earthquake effects without significant loss of life and property
Indian Seismic Codes IS 1893 (Part I), 2002, Indian Standard Criteria for Earthquake Resistant Design of Structures (5th Revision) IS 4326, 1993, Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Buildings (2nd Revision) IS 13827, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Earthen Buildings IS 13828, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Low Strength Masonry Buildings IS 13920, 1993, Indian Standard Code of Practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces IS 13935, 1993, Indian Standard Guidelines for Repair and Seismic Strengthening of Buildings
GPS in earthquake studies: This is done with the help of the highly accurate measurements made by the GPS system which allow scientists to record millimeter-scale slip on faults that cannot ordinarily be measured.
In the near future, this network will act as a key in the improvement of emergency preparedness and response; determining of aftershock risk areas following major earthquakes; helping prevent destruction of buildings, property and infrastructure; advancing the understanding of the earthquake process; providing better geophysical models; and the opening of new directions in the field of solid earth dynamics.
Reducing earthquake damage Where to build Earth scientists try to identify areas that would likely suffer great damage during an earthquake. They develop maps that show fault zones, flood plains (areas that get flooded), areas subject to landslides or to soil liquefaction, and the sites of past earthquakes. From these maps, land-use planners develop zoning restrictions that can help prevent construction of unsafe structures in earthquake-prone areas.
An earthquake-resistant building includes such structures as shear walls, a shear core, and cross-bracing. Base isolators act as shock absorbers. A moat allows the building to sway.
How to build Engineers have developed a number of ways to build earthquake-resistant structures. Their techniques range from extremely simple to fairly complex. For small- to medium-sized buildings, the simpler reinforcement techniques include bolting buildings to their foundations and providing support walls called shear walls. Shear walls, made of reinforced concrete (concrete with steel rods or bars embedded in it), help strengthen the structure and help resist rocking forces. Shear walls in the center of a building, often around an elevator shaft or stairwell, form what is called a shear core. Walls may also be reinforced with diagonal steel beams in a technique called cross-bracing. Builders also protect medium-sized buildings with devices that act like shock absorbers between the building and its foundation. These devices, called base isolators, are usually bearings made of alternate layers of steel and an elastic material, such as synthetic rubber. Base isolators absorb some of the sideways motion that would otherwise damage a building. Skyscrapers need special construction to make them earthquake-resistant. They must be anchored deeply and securely into the ground. They need a reinforced framework with stronger joints than an ordinary skyscraper has. Such a framework makes the skyscraper strong enough and yet flexible enough to withstand an earthquake. Shake table To study the effects of earthquakes on structures, researchers at the Multidisciplinary Center for Earthquake Engineering Research (MCEER) mechanically recreate these natural disasters in their laboratory at the State University of New York at Buffalo (UB) using an earthquake simulator or shake table.
The shake table is a 12 feet by 12 feet square platform made of concrete poured around a steel frame, with a ferro-cement exterior (concrete and wire reinforcement). It has five degrees of freedom, meaning it can move in five separate directions. The table can move horizontally and vertically, as well as roll back and forth, rock from side to side, and twist on a central axis. A computer system is used to control the motion of the table.
(i) General specification of materials (a) The minimum grade of concrete shall be M 20 for all buildings, which are more than three storeys in height (cl. 5.2 of IS 13920:1993). (b) Steel reinforcing bars of grade Fe 415 or less shall be used. However, steel bars of grades Fe 500 and Fe 550 may be used if they are produced by thermo-mechanical treatment process having elongation more than 14.5 per cent (cl. 5.3 of IS 13920:1993).
How to Check if Your Building is Earthquake Proof ? It is difficult for a non-technical person to decide whether his/her building is earthquake proof or not. It is not possible to see the steel that has been placed inside the concrete. Nor it is possible to see how much anchorage length has been provided, that is, how the steel has been bent or anchored ? Nevertheless, the following simple points may help: •
The plan of the building should be nearly symmetric about the two orthogonal axes.
•
The elevation should be nearly symmetric.
•
There should be no sudden discontinuities either in plan or in elevation.
•
Check the net safe bearing capacity of the soil, depth of the water table and level of loose / in-filled soil.
•
If the net safe bearing capacity is more than about 120 kN/m2, isolated footing may be provided but they should be interconnected through a tie or plinth or foundation beam having proper dimensions.
•
If the net safe bearing capacity is less than about 120 kN/m2, a raft foundation should be provided at a suitable depth. Pile foundation may also be provided. The length of piles depends upon the depth of good soil strata from the ground level. However, it is difficult to ensure good quality of construction of piles since they are buried inside the ground and are not visible.
•
The presence of clayey soil or very loose soil requires special attention.
•
All columns at any floor level must form a uniform rectangular grid. They should be oriented along the principal axes of the building.
•
Each column must start from the foundation level.
•
The first storey must not be supported on stilts or soft storey, that is, on columns alone.
•
The minimum dimension of a column should be 300 mm. The concept of 230 mm (9" wide) columns is likely to pose serious threat.
•
The ratio of width to depth of a column should be more than 0.40.
•
In each direction, certain frames must be identified to transfer the lateral load due to earthquake or severe wind to the foundation.
•
Each frame must have strong column-weak girder proportions.
•
It means, the sum of the moment capacities of the columns for the design axial loads at a given beam-column joint, must be greater than 1.20 times the moment capacities of the beams along each principal plane.
•
Good building materials must be used. The quality of concrete and brick work must be very good.
•
Good construction practices must be employed.
•
It must be designed and supervised by a qualified and experienced structural engineer.
•
Each building must be certified by the owner, architect and a qualified structural engineer that it conforms to the latest Indian Standard Codes : IS:456-2000, IS:875-1987, IS:1893-2002 and IS: 13920-1993 besides other relevant codes
What important Variables need to be considered while designing structure? Variables need to be considered while designing structures are such as. •
Shape of the building. Different shaped buildings behave differently. Geometric shapes such as a square or rectangle usually perform
•
Various materials used to construct the buildings can be used. Each material behaves differently. Ductile materials perform better than brittle ones. Examples of ductile materials include steel and aluminium. Examples of brittle materials include brick, stone
•
Height of the building. Different heights shake at different frequencies
•
Soil beneath the building
•
Regional topography
•
Magnitude/duration of the quake
•
Direction and frequency of shaking
•
The number of earthquakes the building has previously had and the kinds of damage suffered, if any
•
Proximity to other buildings
•
Intended function of the building (e.g. hospital, fire station, office building).
Some care should be taken while constructing a building in earthquake prone areas. Here are some tips for designing safer structures. •
Building should be of regular shapes. Cylindrical structures perform better in high-wind areas.
•
Architect should try to design the building as aerodynamic as possible. This reduces the effect of Wind load on tall structures.
•
There should no odd shapes in elevation and the whole building should be in balance. The center of gravity of building should not move
•
Cantilever projections should be minimum and their length should not be more than 3 to 4 feet.
•
The span between the columns should be as small as possible.
•
Point loads on load-carrying beams should be avoided.
•
The dead loads on the cottage-building should not be increased unnecessarily. For Example, Terrace garden or terrace swimming pools should be avoided, if possible.
•
Building should be an R.C.C. framed structure. It provides better stability and reliability in Earthquake-prone areas.
•
Cottage-building's foundation should be placed on hard and level ground.
•
There should not be very large overhead water tanks than are required. If it has to have larger capacity, then it should be divided into two three smaller tanks and should be kept at different locations to maintain balance of cottage-building.
•
If the column length is more than 12 feet, then bracing beams should be provided in between the column at regular intervals. Bracing beams strengthen a column, and allow construction of multistoried buildings.
•
The columns should be connected at each level.
•
For strengthening the brickwork, a sill or a lintel should be provided at every 3 feet level, and R.C.C. wall should be taken where it is possible.
•
Cottage- building should not contain very large and heavy windows. They are bound to weaken the structure.
•
The cottage- building's electrification should contain a main switch and circuit breakers so as to avoid fire hazards because of short circuit in the earthquake.
•
The glass used any structure should be fiber-reinforced glass or wire glass.
•
Use of new and better materials like Fiber-reinforced Concrete and fiber-glass should be recommended. These new materials decrease dead load and increase the structure's strength.
For best earthquake resistance is necessary that all walls in the house perform together as a box. This is referred to as box action.
Fig. Resisting earthquake loads through box action On the figure above case (a) features a structure that is as a whole unit i.e. like a box. Case (b) shows a damaged structure, lacking box action because wall integrity is not being enforced. The resistance of the house subject to an earthquake is defined by the inter-connectivity between structural components as well as the individual component's strength, stiffness and ductility. Good connection between different members creates continuous load path for the inertia loads. Such load path is called complete load path. In this case (a) complete load path is achieved by:
Providing wall to wall connection through good quality bond at corners and construction of horizontal roof band (bond beam)
Providing walls to foundations connection through construction of plinth band (bond beam at plinth level)
Provide connection between walls and roof band
Horizontal bands: Horizontal bands are the most important earthquake-resistant feature in masonry buildings. The bands are provided to hold a masonry building as a single unit by tying all the walls together, and are similar to a closed belt provided around cardboard boxes. There are four types of bands in a typical masonry building, namely gable band, roof band, lintel band and plinth band, named after their location in the building. The lintel band is the most important of all, and needs to be provided in almost all buildings. The gable band is employed only in buildings with pitched or sloped roofs. Design of Lintel Bands During earthquake shaking, the lintel band undergoes bending and pulling actions. To resist these actions, the construction of lintel band requires special attention. Bands can be made of wood (including bamboo splits) or of reinforced concrete (RC); the RC bands are the best. The straight lengths of the band must be properly connected at the wall corners. This will allow the band to support walls loaded in their weak direction by walls loaded in their strong direction. Small lengths of wood spacers (in wooden bands) or steel links (in RC bands) are used to make the straight lengths of wood runners or steel bars act together. In wooden bands, proper nailing of straight lengths with spacers is important. Likewise, in RC bands, adequate anchoring of steel links with steel bars is necessary.
The Indian Standards IS: 4326-1993 and IS: 13828 (1993) provide sizes and details of the bands.
How Vertical Reinforcement Helps Embedding vertical reinforcement bars in the edges of the wall piers and anchoring them in the foundation at the bottom and in the roof band at the top, forces the slender masonry piers to undergo bending instead of rocking. In wider wall piers, the vertical bars enhance their capability to resist horizontal earthquake forces and delay the X-cracking. Adequate cross-sectional area of these vertical bars prevents the bar from yielding in tension. Further, the vertical bars also help protect the wall from sliding as well as from collapsing in the weak direction.
Protection of Openings in Walls The most common damage, observed after an earthquake, in masonry building is diagonal X-cracking of wall piers, and also inclined cracks at the corners of door and
window openings. When a wall with an opening deforms during earthquake shaking, the shape of the opening distorts and becomes more like a rhombus - two opposite corners move away and the other two come closer. Under this type of deformation, the corners that come closer develop cracks. The cracks are bigger when the opening sizes are larger. Steel bars provided in the wall masonry all around the openings restrict these cracks at the corners. In summary, lintel and sill bands above and below openings, and vertical reinforcement adjacent to vertical edges, provide protection against this type of damage.
What is a Shear Wall Building Shear walls are vertical elements of the horizontal force resisting system. In structural engineering, a shear wall is a wall composed of braced panels (also known as shear panels) to counter the effects of lateral loads acting on a structure
Reinforced concrete (RC) buildings often have vertical plate-like RC walls called Shear Walls in addition to slabs, beams and columns. These walls generally start at foundation level and are continuous throughout the building height. Their thickness can be as low as 150mm, or as high as 400mm in high rise buildings. Shear walls are usually provided along both length and width of buildings. Shear walls are like vertically-oriented wide beams that carry earthquake loads downwards to the foundation.
Advantages of Shear Walls in RC Buildings Properly designed and detailed buildings with shear walls have shown very good performance in past earthquakes. Shear walls in high seismic regions require special detailing. However, in past earthquakes, even buildings with sufficient amount of walls that were not specially detailed for seismic performance (but had enough well-distributed reinforcement) were saved from collapse. Shear wall buildings are a popular choice in many earthquake prone countries, like Chile, New Zealand and USA. Shear walls are easy to construct, because reinforcement detailing of walls is relatively straight-forward and therefore easily
implemented at site. Shear walls are efficient, both in terms of construction cost and effectiveness in minimizing earthquake damage in structural and non-structural elements (like glass windows and building contents).
Architectural Aspects of Shear Walls Most RC buildings with shear walls also have columns; these columns primarily carry gravity loads (i.e., those due to self-weight and contents of building). Shear walls provide large strength and stiffness to buildings in the direction of their orientation, which significantly reduces lateral sway of the building and thereby reduces damage to structure and its contents
Ductile Design of Shear Walls Just like reinforced concrete (RC) beams and columns, RC shear walls also perform much better if designed to be ductile. Overall geometric proportions of the wall, types and amount of reinforcement, and connection with remaining elements in the building help in improving the ductility of walls. The Indian Standard Ductile Detailing Code for RC members (IS: 13920-1993) provides special design guidelines for ductile detailing of shear walls. Two basic technologies are used to protect buildings from damaging earthquake effects. These are Base Isolation Devices and Seismic Dampers. The idea behind base isolation is 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. Seismic dampers are special devices introduced in the building to absorb the energy provided by the ground motion to the building (much like the way shock absorbers in motor vehicles absorb the impacts due to undulations of the road).
Base isolation If the same building is rested on flexible pads that offer resistance against lateral movements, then some effect of the ground shaking will be transferred to the building above. If the flexible pads are properly chosen, the forces induced by ground shaking can be a few times smaller than that experienced by the building built directly on ground, namely a fixed base building. The flexible pads are called base-isolators, whereas the structures protected by means of these devices are called base-isolated buildings. The main feature of the base isolation technology is that it introduces flexibility in the structure. As a result, a robust medium-rise masonry or reinforced concrete building becomes extremely flexible. The isolators are often designed to absorb energy and thus add damping to the system. This helps in further reducing the seismic response of the building.
Seismic Dampers Another approach for controlling seismic damage in buildings and improving their seismic performance is by installing seismic dampers in place of structural elements, such as diagonal braces. When seismic energy is transmitted through them, dampers absorb part of it, and thus damp the motion of the building. Dampers were used since 1960s to protect tall buildings against wind effects. However, it was only since 1990s, that they were used to protect buildings against earthquake effects. Commonly used types of seismic dampers include viscous dampers (energy is absorbed by silicone-based fluid passing between pistoncylinder arrangement), friction dampers (energy is absorbed by surfaces with friction between them rubbing against each other), and yielding dampers (energy is absorbed by metallic components that yield). In India, friction dampers have been provided in a 18storey RC frame structure in Gurgaon
Conclusion: Disaster prevention involves engineering intervention in buildings and structures to make them strong enough to withstand the impact of natural hazard or to impose restrictions on land use so that the exposure of the society to the hazard situation is avoided or minimized. Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. Raise the awareness
of residents, non-governmental organizations, public agencies about
their high risk from earthquakes and their options to mitigate that risk. Train
residents, non-governmental organizations, public agencies and business of these
cities in risk mitigation. Reach out
to support and promote disaster mitigation activities throughout the region
If we manage to construct our buildings this way, we will be capable to fight the Earthquake and preventing the trail of loss of life and property that an Earthquake leaves behind.
THIAGARAJAR COLLEGE OF ENGINEERING, MADURAI-15 (A Govt. Aided ISO 9001:2000 Autonomous Institution affiliated to Anna University)
TO CONTACT: [email protected] [email protected] Ph.No:0452-4367243
Abstract: We have discussed here about the construction of structure with earthquake resistant phenomenon. Causes for earthquake, its effects, design of structure, earthquake design philosophy , Indian seismic codes followed while designing the structure, measurement of earthquake intensity, behaviour of types of buildings towards earthquake, activities to be followed by during an earthquake and after the earthquake, specification of materials, techniques used in overcoming the earthquake disaster, earthquake safety tips , etc. are discussed with sketches.
Introduction: A shaking of the ground caused by sudden movements in the earth's crust is termed as an earthquake. Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter of the earthquake. Earthquakes occur all the time all over the world, both along plate edges and along faults. Earthquakes exert lateral as well as vertical forces. Earthquake-resistant structures absorb and dissipate seismically induced motion Building should be designed to prevent total collapse, preserve life, and minimize damage in case of an earthquake or tremor. Earthquakes, Tsunamis, Landslides, Floods and Fires are natural calamities causing severe damage and sufferings to persons by collapsing the structures, cutting off transport systems, killing or trapping persons, animals etc. Such natural disasters are challenges to the progress of development. However, civil engineers as designers have a major role to play in minimising the damages by proper designing the structures or taking other useful decisions. Because of the vastness of the topic, “Disaster management and mitigation”, this module includes understanding the earthquakes, behaviour of the materials of construction and structures and the extent to which structural engineers make use of the knowledge in taking proper decisions in designing the structures made of reinforced concrete
Types of earthquakes There are many different types of earthquakes: tectonic, volcanic, and explosion. The type of earthquake depends on the region where it occurs and the geological make-up of that region. The most common are tectonic earthquakes. These occur when rocks in the earth's crust break due to geological forces created by movement of tectonic plates. Another type, volcanic earthquakes, occur in conjunction with volcanic activity. Collapse earthquakes are small earthquakes in underground caverns and mines, and explosion earthquakes result from the explosion of nuclear and chemical devices.
Tsunami A tsunami is a large ocean wave usually caused by an underwater earthquake or a volcanic explosion. Tsunamis are not tidal waves. Tidal waves are caused by the forces of the moon, sun, and planets upon the tides, as well as the wind as it moves over the water. With typical waves, water flows in circles, but with a tsunami, water flows straight. This is why tsunamis cause so much damage
Earthquake Design Philosophy The earthquake design philosophy may be summarized as follows: (a) Under minor but frequent shaking, the main members of the building that carry vertical and horizontal forces should not be damaged; however building parts that do not carry load may sustain repairable damage. (b) Under moderate but occasional shaking, the main members may sustain repairable damage, while the other parts of the building may be damaged such that they may even have to be replaced after the earthquake; and (c) Under strong but rare shaking, the main members may sustain severe (even irreparable) damage, but the building should not collapse. Thus, after minor shaking, the building will be fully operational within a short time and the repair costs will be small. And, after moderate shaking, the building will be operational once the repair and strengthening of the damaged main members is completed. But, after a strong
earthquake, the building may become dysfunctional for further use, but will stand so that people can be evacuated and property recovered.
Acceptable Damage: Ductility Buildings have the ability to sway back-and-forth during an earthquake, and to withstand earthquake effects with some damage, but without collapse .Ductility is one of the most important factors affecting the building performance. Thus, earthquake-resistant design strives to predetermine the locations where damage takes place and then to provide good detailing at these locations to ensure ductile behaviour of the building.
Earthquake-Resistant Design of Buildings Buildings should be designed like the ductile chain. The failure of a column can affect the stability of the whole building, but the failure of a beam causes localized effect. Therefore, it is better to make beams to be the ductile weak links than columns. This method of designing RC buildings is called the strong-column weak-beam design method.
Response of building based on soil layer thickness During the 1967 Caracas earthquake in South America, the response of buildings was found to depend on the thickness of soil under the buildings. Figure above shows that for buildings 3-5 storey tall, the damage intensity was higher in areas with underlying soil cover of around 40-60 m thick, but was minimal in areas with larger thickness of soil cover. On the other hand, the damage intensity was just the reverse in the case of 10-14 storey buildings; the damage intensity was more when the soil cover was in the range 150-300 m, and small for lower thickness of soil cover. Here, the soil layer under the building plays the role of a filter, allowing some ground waves to pass through and filtering the rest.
Detection and recording Seismograph
A seismograph is an instrument used for recording the intensity and duration of an earthquake.
Vertical seismometer
Geologists use seismographs to record the surface and body waves.
As waves from
earthquakes reach the seismograph the mass stays in relatively the same place, while the ground and the support move around it. This movement is recorded on magnetic tape by a pen attached to the mass. In a seismograph designed to measure vertical motion, the mass is
connected to a spring, so as the ground and support move up and down, the pen on the mass measures the vertical motion. The metal tape which the motion is recorded on is marked with lines that correspond to one minute intervals. When motion is recorded a seismogram is created, which tells about the waves--how big they were and how long they lasted. P waves are recorded first, followed by S waves and then surface waves. While surface waves are the last to reach the seismograph, they last the longest time
Importance of Seismic Design Codes Seismic codes help to improve the behaviour of structures so that they may withstand the earthquake effects without significant loss of life and property
Indian Seismic Codes IS 1893 (Part I), 2002, Indian Standard Criteria for Earthquake Resistant Design of Structures (5th Revision) IS 4326, 1993, Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Buildings (2nd Revision) IS 13827, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Earthen Buildings IS 13828, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Low Strength Masonry Buildings IS 13920, 1993, Indian Standard Code of Practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces IS 13935, 1993, Indian Standard Guidelines for Repair and Seismic Strengthening of Buildings
GPS in earthquake studies: This is done with the help of the highly accurate measurements made by the GPS system which allow scientists to record millimeter-scale slip on faults that cannot ordinarily be measured.
In the near future, this network will act as a key in the improvement of emergency preparedness and response; determining of aftershock risk areas following major earthquakes; helping prevent destruction of buildings, property and infrastructure; advancing the understanding of the earthquake process; providing better geophysical models; and the opening of new directions in the field of solid earth dynamics.
Reducing earthquake damage Where to build Earth scientists try to identify areas that would likely suffer great damage during an earthquake. They develop maps that show fault zones, flood plains (areas that get flooded), areas subject to landslides or to soil liquefaction, and the sites of past earthquakes. From these maps, land-use planners develop zoning restrictions that can help prevent construction of unsafe structures in earthquake-prone areas.
An earthquake-resistant building includes such structures as shear walls, a shear core, and cross-bracing. Base isolators act as shock absorbers. A moat allows the building to sway.
How to build Engineers have developed a number of ways to build earthquake-resistant structures. Their techniques range from extremely simple to fairly complex. For small- to medium-sized buildings, the simpler reinforcement techniques include bolting buildings to their foundations and providing support walls called shear walls. Shear walls, made of reinforced concrete (concrete with steel rods or bars embedded in it), help strengthen the structure and help resist rocking forces. Shear walls in the center of a building, often around an elevator shaft or stairwell, form what is called a shear core. Walls may also be reinforced with diagonal steel beams in a technique called cross-bracing. Builders also protect medium-sized buildings with devices that act like shock absorbers between the building and its foundation. These devices, called base isolators, are usually bearings made of alternate layers of steel and an elastic material, such as synthetic rubber. Base isolators absorb some of the sideways motion that would otherwise damage a building. Skyscrapers need special construction to make them earthquake-resistant. They must be anchored deeply and securely into the ground. They need a reinforced framework with stronger joints than an ordinary skyscraper has. Such a framework makes the skyscraper strong enough and yet flexible enough to withstand an earthquake. Shake table To study the effects of earthquakes on structures, researchers at the Multidisciplinary Center for Earthquake Engineering Research (MCEER) mechanically recreate these natural disasters in their laboratory at the State University of New York at Buffalo (UB) using an earthquake simulator or shake table.
The shake table is a 12 feet by 12 feet square platform made of concrete poured around a steel frame, with a ferro-cement exterior (concrete and wire reinforcement). It has five degrees of freedom, meaning it can move in five separate directions. The table can move horizontally and vertically, as well as roll back and forth, rock from side to side, and twist on a central axis. A computer system is used to control the motion of the table.
(i) General specification of materials (a) The minimum grade of concrete shall be M 20 for all buildings, which are more than three storeys in height (cl. 5.2 of IS 13920:1993). (b) Steel reinforcing bars of grade Fe 415 or less shall be used. However, steel bars of grades Fe 500 and Fe 550 may be used if they are produced by thermo-mechanical treatment process having elongation more than 14.5 per cent (cl. 5.3 of IS 13920:1993).
How to Check if Your Building is Earthquake Proof ? It is difficult for a non-technical person to decide whether his/her building is earthquake proof or not. It is not possible to see the steel that has been placed inside the concrete. Nor it is possible to see how much anchorage length has been provided, that is, how the steel has been bent or anchored ? Nevertheless, the following simple points may help: •
The plan of the building should be nearly symmetric about the two orthogonal axes.
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The elevation should be nearly symmetric.
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There should be no sudden discontinuities either in plan or in elevation.
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Check the net safe bearing capacity of the soil, depth of the water table and level of loose / in-filled soil.
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If the net safe bearing capacity is more than about 120 kN/m2, isolated footing may be provided but they should be interconnected through a tie or plinth or foundation beam having proper dimensions.
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If the net safe bearing capacity is less than about 120 kN/m2, a raft foundation should be provided at a suitable depth. Pile foundation may also be provided. The length of piles depends upon the depth of good soil strata from the ground level. However, it is difficult to ensure good quality of construction of piles since they are buried inside the ground and are not visible.
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The presence of clayey soil or very loose soil requires special attention.
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All columns at any floor level must form a uniform rectangular grid. They should be oriented along the principal axes of the building.
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Each column must start from the foundation level.
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The first storey must not be supported on stilts or soft storey, that is, on columns alone.
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The minimum dimension of a column should be 300 mm. The concept of 230 mm (9" wide) columns is likely to pose serious threat.
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The ratio of width to depth of a column should be more than 0.40.
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In each direction, certain frames must be identified to transfer the lateral load due to earthquake or severe wind to the foundation.
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Each frame must have strong column-weak girder proportions.
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It means, the sum of the moment capacities of the columns for the design axial loads at a given beam-column joint, must be greater than 1.20 times the moment capacities of the beams along each principal plane.
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Good building materials must be used. The quality of concrete and brick work must be very good.
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Good construction practices must be employed.
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It must be designed and supervised by a qualified and experienced structural engineer.
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Each building must be certified by the owner, architect and a qualified structural engineer that it conforms to the latest Indian Standard Codes : IS:456-2000, IS:875-1987, IS:1893-2002 and IS: 13920-1993 besides other relevant codes
What important Variables need to be considered while designing structure? Variables need to be considered while designing structures are such as. •
Shape of the building. Different shaped buildings behave differently. Geometric shapes such as a square or rectangle usually perform
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Various materials used to construct the buildings can be used. Each material behaves differently. Ductile materials perform better than brittle ones. Examples of ductile materials include steel and aluminium. Examples of brittle materials include brick, stone
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Height of the building. Different heights shake at different frequencies
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Soil beneath the building
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Regional topography
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Magnitude/duration of the quake
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Direction and frequency of shaking
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The number of earthquakes the building has previously had and the kinds of damage suffered, if any
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Proximity to other buildings
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Intended function of the building (e.g. hospital, fire station, office building).
Some care should be taken while constructing a building in earthquake prone areas. Here are some tips for designing safer structures. •
Building should be of regular shapes. Cylindrical structures perform better in high-wind areas.
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Architect should try to design the building as aerodynamic as possible. This reduces the effect of Wind load on tall structures.
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There should no odd shapes in elevation and the whole building should be in balance. The center of gravity of building should not move
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Cantilever projections should be minimum and their length should not be more than 3 to 4 feet.
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The span between the columns should be as small as possible.
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Point loads on load-carrying beams should be avoided.
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The dead loads on the cottage-building should not be increased unnecessarily. For Example, Terrace garden or terrace swimming pools should be avoided, if possible.
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Building should be an R.C.C. framed structure. It provides better stability and reliability in Earthquake-prone areas.
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Cottage-building's foundation should be placed on hard and level ground.
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There should not be very large overhead water tanks than are required. If it has to have larger capacity, then it should be divided into two three smaller tanks and should be kept at different locations to maintain balance of cottage-building.
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If the column length is more than 12 feet, then bracing beams should be provided in between the column at regular intervals. Bracing beams strengthen a column, and allow construction of multistoried buildings.
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The columns should be connected at each level.
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For strengthening the brickwork, a sill or a lintel should be provided at every 3 feet level, and R.C.C. wall should be taken where it is possible.
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Cottage- building should not contain very large and heavy windows. They are bound to weaken the structure.
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The cottage- building's electrification should contain a main switch and circuit breakers so as to avoid fire hazards because of short circuit in the earthquake.
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The glass used any structure should be fiber-reinforced glass or wire glass.
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Use of new and better materials like Fiber-reinforced Concrete and fiber-glass should be recommended. These new materials decrease dead load and increase the structure's strength.
For best earthquake resistance is necessary that all walls in the house perform together as a box. This is referred to as box action.
Fig. Resisting earthquake loads through box action On the figure above case (a) features a structure that is as a whole unit i.e. like a box. Case (b) shows a damaged structure, lacking box action because wall integrity is not being enforced. The resistance of the house subject to an earthquake is defined by the inter-connectivity between structural components as well as the individual component's strength, stiffness and ductility. Good connection between different members creates continuous load path for the inertia loads. Such load path is called complete load path. In this case (a) complete load path is achieved by:
Providing wall to wall connection through good quality bond at corners and construction of horizontal roof band (bond beam)
Providing walls to foundations connection through construction of plinth band (bond beam at plinth level)
Provide connection between walls and roof band
Horizontal bands: Horizontal bands are the most important earthquake-resistant feature in masonry buildings. The bands are provided to hold a masonry building as a single unit by tying all the walls together, and are similar to a closed belt provided around cardboard boxes. There are four types of bands in a typical masonry building, namely gable band, roof band, lintel band and plinth band, named after their location in the building. The lintel band is the most important of all, and needs to be provided in almost all buildings. The gable band is employed only in buildings with pitched or sloped roofs. Design of Lintel Bands During earthquake shaking, the lintel band undergoes bending and pulling actions. To resist these actions, the construction of lintel band requires special attention. Bands can be made of wood (including bamboo splits) or of reinforced concrete (RC); the RC bands are the best. The straight lengths of the band must be properly connected at the wall corners. This will allow the band to support walls loaded in their weak direction by walls loaded in their strong direction. Small lengths of wood spacers (in wooden bands) or steel links (in RC bands) are used to make the straight lengths of wood runners or steel bars act together. In wooden bands, proper nailing of straight lengths with spacers is important. Likewise, in RC bands, adequate anchoring of steel links with steel bars is necessary.
The Indian Standards IS: 4326-1993 and IS: 13828 (1993) provide sizes and details of the bands.
How Vertical Reinforcement Helps Embedding vertical reinforcement bars in the edges of the wall piers and anchoring them in the foundation at the bottom and in the roof band at the top, forces the slender masonry piers to undergo bending instead of rocking. In wider wall piers, the vertical bars enhance their capability to resist horizontal earthquake forces and delay the X-cracking. Adequate cross-sectional area of these vertical bars prevents the bar from yielding in tension. Further, the vertical bars also help protect the wall from sliding as well as from collapsing in the weak direction.
Protection of Openings in Walls The most common damage, observed after an earthquake, in masonry building is diagonal X-cracking of wall piers, and also inclined cracks at the corners of door and
window openings. When a wall with an opening deforms during earthquake shaking, the shape of the opening distorts and becomes more like a rhombus - two opposite corners move away and the other two come closer. Under this type of deformation, the corners that come closer develop cracks. The cracks are bigger when the opening sizes are larger. Steel bars provided in the wall masonry all around the openings restrict these cracks at the corners. In summary, lintel and sill bands above and below openings, and vertical reinforcement adjacent to vertical edges, provide protection against this type of damage.
What is a Shear Wall Building Shear walls are vertical elements of the horizontal force resisting system. In structural engineering, a shear wall is a wall composed of braced panels (also known as shear panels) to counter the effects of lateral loads acting on a structure
Reinforced concrete (RC) buildings often have vertical plate-like RC walls called Shear Walls in addition to slabs, beams and columns. These walls generally start at foundation level and are continuous throughout the building height. Their thickness can be as low as 150mm, or as high as 400mm in high rise buildings. Shear walls are usually provided along both length and width of buildings. Shear walls are like vertically-oriented wide beams that carry earthquake loads downwards to the foundation.
Advantages of Shear Walls in RC Buildings Properly designed and detailed buildings with shear walls have shown very good performance in past earthquakes. Shear walls in high seismic regions require special detailing. However, in past earthquakes, even buildings with sufficient amount of walls that were not specially detailed for seismic performance (but had enough well-distributed reinforcement) were saved from collapse. Shear wall buildings are a popular choice in many earthquake prone countries, like Chile, New Zealand and USA. Shear walls are easy to construct, because reinforcement detailing of walls is relatively straight-forward and therefore easily
implemented at site. Shear walls are efficient, both in terms of construction cost and effectiveness in minimizing earthquake damage in structural and non-structural elements (like glass windows and building contents).
Architectural Aspects of Shear Walls Most RC buildings with shear walls also have columns; these columns primarily carry gravity loads (i.e., those due to self-weight and contents of building). Shear walls provide large strength and stiffness to buildings in the direction of their orientation, which significantly reduces lateral sway of the building and thereby reduces damage to structure and its contents
Ductile Design of Shear Walls Just like reinforced concrete (RC) beams and columns, RC shear walls also perform much better if designed to be ductile. Overall geometric proportions of the wall, types and amount of reinforcement, and connection with remaining elements in the building help in improving the ductility of walls. The Indian Standard Ductile Detailing Code for RC members (IS: 13920-1993) provides special design guidelines for ductile detailing of shear walls. Two basic technologies are used to protect buildings from damaging earthquake effects. These are Base Isolation Devices and Seismic Dampers. The idea behind base isolation is 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. Seismic dampers are special devices introduced in the building to absorb the energy provided by the ground motion to the building (much like the way shock absorbers in motor vehicles absorb the impacts due to undulations of the road).
Base isolation If the same building is rested on flexible pads that offer resistance against lateral movements, then some effect of the ground shaking will be transferred to the building above. If the flexible pads are properly chosen, the forces induced by ground shaking can be a few times smaller than that experienced by the building built directly on ground, namely a fixed base building. The flexible pads are called base-isolators, whereas the structures protected by means of these devices are called base-isolated buildings. The main feature of the base isolation technology is that it introduces flexibility in the structure. As a result, a robust medium-rise masonry or reinforced concrete building becomes extremely flexible. The isolators are often designed to absorb energy and thus add damping to the system. This helps in further reducing the seismic response of the building.
Seismic Dampers Another approach for controlling seismic damage in buildings and improving their seismic performance is by installing seismic dampers in place of structural elements, such as diagonal braces. When seismic energy is transmitted through them, dampers absorb part of it, and thus damp the motion of the building. Dampers were used since 1960s to protect tall buildings against wind effects. However, it was only since 1990s, that they were used to protect buildings against earthquake effects. Commonly used types of seismic dampers include viscous dampers (energy is absorbed by silicone-based fluid passing between pistoncylinder arrangement), friction dampers (energy is absorbed by surfaces with friction between them rubbing against each other), and yielding dampers (energy is absorbed by metallic components that yield). In India, friction dampers have been provided in a 18storey RC frame structure in Gurgaon
Conclusion: Disaster prevention involves engineering intervention in buildings and structures to make them strong enough to withstand the impact of natural hazard or to impose restrictions on land use so that the exposure of the society to the hazard situation is avoided or minimized. Seismic retrofitting is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes. Raise the awareness
of residents, non-governmental organizations, public agencies about
their high risk from earthquakes and their options to mitigate that risk. Train
residents, non-governmental organizations, public agencies and business of these
cities in risk mitigation. Reach out
to support and promote disaster mitigation activities throughout the region
If we manage to construct our buildings this way, we will be capable to fight the Earthquake and preventing the trail of loss of life and property that an Earthquake leaves behind.
