SCIENCE TRIBUNE Thursday, February 8, 2001, Chandigarh, India
 


Earthquake-resistant buildings
by Pravin Kumar
I
T is not earthquakes that kill people, it is said. In Bhuj, near the epicentre of the recent Gujarat earthquake on January 26, it is reported that nearly 50 per cent of the buildings have been flattened. In Ahmedabad, Surat and Rajkot, high-rise buildings designed by reputed architects and built by good builders have collapsed.

Coping with the calamity
by Jagvir Goyal
H
eart-rending scenes of death and devastation in Bhuj and Ahmedabad have left everybody gasping for breath. Cries of a child sitting under a slab and calling her mother, her hopes diminishing with every call returning unanswered, have pierced the hearts, the bones and reached the bone-marrow. 

Tests your IQ
bY J.P. GARG


 
Top





 

Earthquake-resistant buildings
by Pravin Kumar

IT is not earthquakes that kill people, it is said. In Bhuj, near the epicentre of the recent Gujarat earthquake on January 26, it is reported that nearly 50 per cent of the buildings have been flattened. In Ahmedabad, Surat and Rajkot, high-rise buildings designed by reputed architects and built by good builders have collapsed.

Earthquakes are hazards with a low probability of occurrence, but with major consequences. Hence, our response to them is a case of "being wise after the event". Most constructions in India are non-engineered types, consisting of walls of clay, stone, brick, and the like, built without hazard provisions. Earthquake damage can be mitigated by using earthquake-resistant designs for new buildings and retrofitting existing ones for acceptable levels of risk.

The great majority of earthquake casualties are due to the collapse of buildings and the interruption of transport systems and food supplies. Traditionally, people in rural areas in India attach roof rafters directly to the walls of their houses; when the walls crack, the roof comes down. In houses in Lature and Osmanabad districts (Maharashtra) which were hit by a severe earthquake in September, 1993, the weight of the roof was carried by the walls and no alternate vertical load-carrying system was used. Field stones of greatly varying shapes and sizes, and held together by mud mortar, were used for the metrethick walls. Mud mortar has very poor shear and tensile strength. During earthquakes, the walls have poor resistance to lateral forces. The roofs made of heavy stone slabs had a thick layer of mud for providing insulation but which had a significant mass; during earthquakes they generated a very large inertia force which led to a partial or complete collapse of buildings where the weight of the roof was supported on walls. The buildings are being put up. Indian standard codes and guidelines for earthquake-resistant design and construction of buildings have been developed and revised every few years until 1993, but the recent Kutch earthquake, like the ones at Jabalpur (1997) and Chamoli, UP (1999), show that the guidelines have not been implemented, except in some Central Government departments.

The earthquake intensity hazard depends on the location. A macro-level map has been prepared which divides India, with its total area of 3.3 million square kilometres, into five hazard zones on a scale of decreasing probable intensity. About one-half of the country falls in the "seismic" or earthquake prone zone. Zoning should help by indicating the need to strengthen weaker structures, so that they do not suffer in small-size, local temblors. Building codes have been provided by the Indian Bureau of Standards for the five hazard zones, according to Dr Harsh K. Gupta, Director of the National Geophysical Research Laboratory, Hyderabad, builders should be required to abide by these codes: this is the only way to prevent deaths from earthquakes.


Fig.1

An earthquake is marked by two types of simultaneous motion: a lateral, push-pull motion and an undulating or pitching motion which moves surface structures up and down. The damage to a building depends on how much and on how long the ground motion lasts óthat is, how the shock waves travel through the soil, on the nature of the soil and on the nature of the structure itself. How the structure responds depends in turn on its properties as well as on its damping characteristics, that is, its capacity to absorb the forces acting upon it. Concrete has better damping properties than steel. Brick buildings suffer during earthquakes because the roofs tend to separate from the supports due to the shaking; the walls tend to tear apart, moving diagonally (see Fig. 1 A). These effects can be minimised by tying all the various parts of the buildings together. A horizontal reinforced band can be used at roof, lintel or plinth level to bind walls together (Fig 1 C).

New structural systems: Lately, new systems like "base-isolated systems" have been used to reduce the earthquake forces acting on a structure or to absorb a part of the energy. Unlike conventional, fixed systems (Fig 2 A) which absorb seismic energy in specially designed regions ó such as in beams near beam-column joints ó in the "base-isolated system" the superstructure is isolated from the foundation and damaging motions by "isolators" which de-link structures from damaging motions and also add damping effects. (Fig 2 B) The design of the Century Tower Building in Tokyo uses the "eccentrically braced frame". This comprises steel braces stiff enough to withstand a moderate earthquake, combined with a Ďsacrificialí ductile shear link between the braces; the shear link is designed to give way during an intense shock to prevent buckling of the braces ó almost like a fuse in an electrical circuit.

Energy-dissipating systems: These are "add-ons" to conventional fixed-base systems, sharing the seismic energy with the primary structural members. A number of these use metal hysteresis (lagging of effect), viscous damping and friction.

"Smart" or active control systems control the seismic response through appropriate adjustments within the structure. These involve adjusting the lateral strength, stiffness and dynamic properties of the structure during the earthquake to reduce the structural response.

New materials: Of late, materials like rubber, lead, stainless steel, fibre-reinforced plastic and shape-memory alloys have been used to confer earthquake resistance on buildings. Researchers are working to develop cost-effective hybrids and semi-active classes of systems which combine the robustness of passive systems with the adaptability of active systems.

Perhaps the least understood area of earthquake engineering is the soil-structure interaction. Most design codes ignore this effect for the vast majority of structures. However, the response of structures can be predicted when the properties of the soil are modelled numerically. In the recent Gujarat earthquake, old buildings in Ahmedabad remained firm; new, multi-storied buildings collapsed, due, it is said, to their being constructed on filled-up land, not on natural soil strata. Also, Ahmedabad was not considered a seismic zone when new buildings were constructed.                                                                                

A. Fixed-base system: Conventional structures absorb seismic energy through inelastic deformations in structural members. Large inter-storey drifts cause structural and nonstructural damage. However, loss of life and collapse is prevented.
A. Fixed-base system: Conventional structures absorb seismic energy through inelastic deformations in structural members. Large inter-storey drifts cause structural and nonstructural damage. However, loss of life and collapse is prevented.
B. Seismic isolation systems: Structures are supported on isolators which decouple structures from damaging earthquake components and absorb seismic energy adding substantial damping.
B. Seismic isolation systems: Structures are supported on isolators which decouple structures from damaging earthquake components and absorb seismic energy adding substantial damping.
C. Energy Dissipation Devices (EDDs) absorb seismic energy thereby reducing the demand on primary structural members. Structural and nonstructural damage is significantly reduced.

C. Energy Dissipation Devices (EDDs) absorb seismic energy thereby reducing the demand on primary structural members. Structural and nonstructural damage is significantly reduced.

D. Lateral strength, stiffness and dynamic properties of a structure are adjusted during the earthquake to control its response. Complex control mechanism and elaborate hardware is required.
D. Lateral strength, stiffness and dynamic properties of a structure are adjusted during the earthquake to control its response. Complex control mechanism and elaborate hardware is required.

  Fig.2


Aseismic designs

With the growing ability to predict ground motion and site response, it is now possible to design structures that withstand earthquakes. The "aseismic design" takes into account such factors are the probable sites of future earthquakes, their magnitudes and frequencies and the maximum ground motion. The need for safe nuclear power plants has given an impetus to earthquake engineering research. For locating a nuclear plant, all tectonic features within a radius of 300 km around the chosen site are studied with modern techniques like satellite imagery. Data from all recorded earthquakes are plotted. Sites having a potential for developing surface faulting are not considered for nuclear plants. The vibratory effects of earthquakes can be effectively reduced by appropriate designing. In India, the nuclear power plants are located in very mild to moderate seismic regions, whereas the regions with maximum hydroelectric power potential are the highly seismic regions. The fact that the Kakrapar nuclear power plant, 80 km outside Surat, continued to function even in the aftermath of the recent quake is considered evidence of their in-built quake-proof technology.

For large dams, the International Commission on Large Dams has made recommendations for evaluation of seismic parameters. The nuclear plants are designed for much larger seismic forces than the dams in those regions.

Newer technologies in design as well as noval building materials may eventually lead to the "perfect" structure of the engineerís dreams.


Top

 

Coping with the calamity
by Jagvir Goyal

Heart-rending scenes of death and devastation in Bhuj and Ahmedabad have left everybody gasping for breath. Cries of a child sitting under a slab and calling her mother, her hopes diminishing with every call returning unanswered, have pierced the hearts, the bones and reached the bone-marrow. While watching the impeccable coverage of the grievous disaster by the news channels and grasping the magnitude of tragedy, one question repeatedly raises its head in our minds: Why didnít we have a plan ready to cope with such a disastrous situation? There is no answer! The question leaves us cursing ourselves, bringing forth the stark truth: We act only when it hits our heads hard! We donít learn from the past!

Letís learn this time. Letís now allow the shock and the memory to fade with time. Letís gear ourselves to cope with such disasters ó whenever, wherever they occur in future, howsoever long the recurrence interval may be. If we can maintain a huge army, spend billions of rupees on its maintenance and upgradation every year without having a battle for decades together, why canít we spend something equivalent to a small percentage of defence budget on disaster-management even after knowing very well that disasters and calamities can leave trails of destruction no lesser than those left by the wars?

The question arises: Is it possible to evolve a plan to cope with disastrous earthquakes? The answer is in affirmative. Examples are there before us. When a country like Japan, virtually sitting over an earthquake zone, can cope with it, why canít we when India too is having most of its area prone to earthquakes?

Letís look into the steps to be taken to meet with such an eventuality in future:

Revision of seismic zoning map of India: The first step to be taken by us is the revision of seismic zoning map of India. At present, the Meteorological Department of India has divided the country into five zones from seismic activity point of view. Zone I is declared least prone to earthquakes and Zone V is most prone. This map has not proved realistic at all. Even the cities shown in Zone I have experienced severe earthquakes. This map carries high importance as the design of all structures in a city is to be done by keeping in view the seismic zone in which that city falls. We can very well imagine the fate of structures built in Zone I when an area shown under this zone faces a severe earthquake!

Instead of five, the country needs to be divided into six zones with respect to the intensity of the earthquakes that different areas may experience. The areas that may experience Class X, XI and XII of earthquake on Modified Mercalli Intensity Scale should be kept under Zone IV. Kutch in Gujarat, the whole of Assam, Andaman islands and some parts of Himachal Pradesh and J&K should be kept in Zone VI and the structures built there should be designed accordingly.

Jabalpur, considered to be in least earthquake prone Zone I has experienced major earthquakes in the past. It needs to be placed in Zone III. Similarly Latur needs to be shifted to Zone V, Khandwa region in MP to Zone IV and Mumbai to Zone IV. Bangalore, now kept in Zone I is a classic example of incorrect seismic zoning of India. This city should be assigned Zone IV in view of the intensity of earthquake it has experienced recently.

Avoiding collapse of buildings: It is not the earthquake but the collapse of buildings that causes destruction and loss of human lives. Earthquakes will do least harm to us if the collapse of buildings is avoided. It must be borne in mind by us that the main criteria in design of buildings should be to prevent loss of human lives. The buildings may deform, sway, crack or distort but must not collapse! This requires taking into account the seismic zone in which the area where the building is to be erected falls while designing the building.

A few factors as suggested below should prove quite helpful in design of earthquake resistant buildings to be built in earthquake prone areas:

1. For buildings taller than 30 metres, model analysis should be carried out to study the behaviour of the building under random motion of ground.

2. For buildings taller than 75 metres dynamic behaviour of the structures should be studied to evolve their design.

3. For all buildings taller than 30 metres, effect of positive torsion should be taken into account.

4. Natural frequency of buildings should be mistuned with that of earthquakes. There are a number of empirical formulae available to find the natural frequency of buildings. The vibration period of earthquakes is normally 1.5 to 2.5 seconds.

5. Higher factor of safety should be used in design of tall structures such as chimneys, towers and tanks.

6. Intensity of shocks due to earthquakes may vary with variation in soil conditions. Soil conditions should, therefore, be studied to work out seismic coefficients for hard, medium or soft soils.

7. Stiff buildings are more prone to earthquakes. In earthquake prone areas, buildings should be ductile and flexible.

8. Circular buildings are least affected by earthquakes as compared to other buildings. Buildings should be symmetrical and having minimum corners.

9. Slenderness ratio of buildings should be limited.

10. A large damage occurs due to breakout of fires on occurrence of an earthquake. No time may be available to operate fire fighting equipment when an earthquake occurs. The structures should, therefore, be built by designing them to be safe against fire.

11. The vibrations due to earthquake rise in buildings from foundation upwards. The buildings should therefore be provided with bearings that are able to absorb shock waves. Many kinds of effective and useful bearings have been developed by engineers for use in other structures bearing dynamic loads such as bridges. So there is no dearth on this account.

12. A multi-storeyed building having a tube structure proves highly effective in resisting seismic loads. Such a structure was first suggested by Owings and Merril, Chicago. In such a structure, the core of the building is kept stiff while columns are provided along its periphery and are connected by spandrel beams.

13. Wherever possible, a flat slab design should be preferred for ceilings as it does away with the beams.

14. Mass of the buildings should be kept as low as possible. Light weight materials should be brought under use. Lesser is the self weight of buildings, lesser will be the earthquake force affecting it.

15. Joints of a building should be so designed that cumulative effect of deflection does not occur in any member. Deflection diagram of the buildings should be well studied.

16. The structural framework should be kept highly redundant or indeterminate during design. Such a structure will provide better resistance to earthquake.
Top

 

Tests your IQ
BY J.P. GARG

1. This titan of science of the 20th century is mainly known for his work on evolution and authored the book "The Cause of Evolution". He was the first to derive a formula for mapping of genes along the chromosome. Name this Britisher who spent the last seven years of his life in India and published successfully in India the "Journal of Genetics", besides writing numerous popular articles.

2. Cold and cough medications which are frequently used by the people and are normally available without prescription contain an ingredient called PPA which can cause brain haemorrhage in some people. What is the full name of PPA?

3. In computer technology, what is the process of converting data that is transmitted in one format by the sender into the data format that can be accepted by the receiver called?

4. This animal has front feet like those of human beings and uses these feet to clean its food before eating. It lives in forests near sources of water like lakes, streams and waterfalls. Which is this animal that sleeps during day time and wakes at night and is found in South Canada and Panama?

5. These radiations comprise charged particles such as electrons, protons, alpha particles and nuclei of certain atoms that strike the earth from all directions in space. What are we talking about? What are the two types of these radiations?

6. Name the device that plays a central role in transmission of electrical power. On which basic principle is the working of this device based?

7. "Invisible ink" consists of a dilute solution of some salt which becomes blue when dried on paper and heated. Salts of which element are most commonly used for this purpose?

8. If a gas under high pressure expands suddenly, there occurs a fall in the temperature of the gas. What is this effect called which is of great significance for the liquefaction of gases?

9. Living organisms are naturally radioactive. Their tissues contain traces of two radioactive isotopes which are absorbed by them from the atmosphere. Which are these two isotopes?

10. Some scientists believe that this gas can be the fuel of the 21st century. But the problem is that it is hardly found on earth, although there is abundance of this gas on moon. It is estimated that there are about 1 million tons of this gas on moon, enough to power the world for thousands of years. Which is this gas?

Answers
1. J.B.H. Haldane 2. Phenylpr-opanolamine 3. Mapping 4. Racoon 5. Cosmic rays; Primary and secondary cosmic rays 6. Transformer; Mutual (electromagnetic) induction 7. Cobalt 8. Joule-Thomson effect 9. Potassium-40 and carbon-14 10. Helium-3.

Home Top