SCIENCE TRIBUNE Thursday, January 30, 2003, Chandigarh, India

Quake-resistant buildings

n India, most non-urban buildings are made in masonry. In the plains, masonry is generally made of burnt clay bricks and cement mortar. However, in hilly areas, stone masonry with mud mortar is more prevalent; but, in recent times, it is being replaced with cement mortar.




Quake-resistant buildings

In India, most non-urban buildings are made in masonry. In the plains, masonry is generally made of burnt clay bricks and cement mortar. However, in hilly areas, stone masonry with mud mortar is more prevalent; but, in recent times, it is being replaced with cement mortar. Masonry can carry loads that cause compression (i.e., pressing together), but can hardly take load that causes tension (i.e., pulling apart) (Figure 1).

Concrete is another material that has been popularly used in building construction particularly over the last four decades. Cement concrete is made of crushed stone pieces (called aggregate), sand, cement and water mixed in appropriate proportions. Concrete is much stronger than masonry under compressive loads, but again its behaviour in tension is poor. The properties of concrete critically depend on the amount of water used in making concrete; too much and too little water, both can cause havoc. In general, both masonry and concrete are brittle, and fail suddenly.

Steel is used in masonry and concrete buildings as reinforcement bars of diameter ranging from 6mm to 40mm. Reinforcing steel can carry both tensile and compressive loads.

Moreover, steel is a ductile material. This important property of ductility enables steel bars to undergo large elongation before breaking.

Concrete is used in buildings along with steel reinforcement bars. This composite material is called reinforced cement concrete or simply reinforced concrete (RC). The amount and location of steel in a member should be such that the failure of the member is by steel reaching its strength in tension before concrete reaches its strength in compression. This type of failure is ductile failure, and hence is preferred over a failure where concrete fails first in compression. Therefore, contrary to common thinking, providing too much steel in RC buildings can be harmful even!!

Capacity Design

Let us take two bars of same length and cross-sectional area — one made of a ductile material and another of a brittle material. Now, pull these two bars until they break!! You will notice that the ductile bar elongates by a large amount before it breaks, while the brittle bar breaks suddenly on reaching its maximum strength at a relatively small elongation (Figure 2). Amongst the materials used in building construction, steel is ductile, while masonry and concrete are brittle.

Now, let us make a chain with links made of brittle and ductile materials (Figure 3). Each of these links will fail just like the bars shown in Figure 2. Now, hold the last link at either end of the chain and apply a force F. Since the same force F is being transferred through all the links, the force in each link is the same, i.e., F. As more and more force is applied, eventually the chain will break when the weakest link in it breaks. If the ductile link is the weak one (i.e., its capacity to take load is less), then the chain will show large final elongation. Instead, if the brittle link is the weak one, then the chain will fail suddenly and show small final elongation. Therefore, if we want to have such a ductile chain, we have to make the ductile link to be the weakest link.

Buildings should be designed like the ductile chain. For example, consider the common urban residential apartment construction — the multi-storey building made of reinforced concrete. It consists of horizontal and vertical members, namely beams and columns. The seismic inertia forces generated at its floor levels are transferred through the various beams and columns to the ground. The correct building components need to be made ductile. The failure of a column can affect the stability of the whole building, but the failure of a beam causes localised 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 (Figure 4).

By using the routine design codes (meant for design against non-earthquake effects), designers may not be able to achieve a ductile structure. Special design provisions are required to help designers improve the ductility of the structure. Such provisions are usually put together in the form of a special seismic design code, e.g., IS:13920-1993 for RC structures. These codes also ensure that adequate ductility is provided in the members where damage is expected.

Quality Control

The capacity design concept in earthquake-resistant design of buildings will fail if the strengths of the brittle links fall below their minimum assured values. The strength of brittle construction materials, like masonry and concrete, is highly sensitive to the quality of construction materials, workmanship, supervision, and construction methods. Similarly, special care is needed in construction to ensure that the elements meant to be ductile are indeed provided with features that give adequate ductility. Thus, strict adherence to prescribed standards of construction materials and construction processes is essential in assuring an earthquake-resistant building. Regular testing of construction materials at qualified laboratories (at site or away), periodic training of workmen at professional training houses, and on-site evaluation of the technical work are elements of good quality control.

Authored by C.V.R.Murty (Indian Institute of Technology, Kanpur) for Building Materials and Technology Promotion Council, New Delhi.



The radius of a grown up man increases or decreases but the height remains the same. Why?

The height of a man and the length of his limbs depend on the dimensions of the bone structure he acquires by the time he is grown up. Our genes have determined that this structure should remain fixed after maturity. This is the steel structure around which the architecture of the body is built. Flesh, sinews and muscles are tied to and are draped over that frame. The draping of the basic structure can be smart and tight or loose and flabby, depending on our ways of living, eating and physical activity.

We are lucky that at least some aspects of architectural dimensions are well controlled. You must know that our brain size also does not change with age. If the skull size also increased or decreased like our girth, life would become a bit hazardous. Lengthening or contraction of noses or jaws would make things unpleasant. We are punished for the follies of our living but a limit has been placed on the severity of this punishment.

You can, of course, ask me why our bone dimensions are not as susceptible to our ways of living. I do not know the answer except to say that while the dimensions may not be susceptible, their health is. The process of evolution must have made this choice as best for survival. The changing radius of the fat around our middle, as you call it, keeps reminding us of the style of our living

Incidentally, some environmental conditions do have an impact on the height of a human. When people spend long periods of time in weightless condition during space flight their height is seen to increase a little. This is believed to be due to expansion of the discs separating different vertebrae in our backbone. These discs are not made of the same material as bone; they provide flexibility and shock absorbing capability to the backbone.





Across :

1. An egg shaped figure.

5. Deforestation may cause this to soil.

6. Symbol for Selenium.

7. A column in periodic table is also known thus.

8. Major Indian Telecom company in public sector.

10. Pure natural form of hydrated aluminium silicate; china clay.

11. A trigonometrical ratio.

14. A non public network supporting a wide range of data devices and computers.

16. Long hollow cylinder for passage of fluids.

18. Respiratory organ in fish.

19. This volume of a gas is the volume occupied by one of its moles.

22. A monobasic corrosive aid used to make explosives, dyes and drugs.

25. SI unit of amount of substance.

26. Prefix for one hundredth.

Down :

1. Part of a circle between a chord and circumference.

2. This scientist proved that strain is proportional to stress under elastic limit.

3. This angle is important for stability of slopes.

4. SI unit of thermodynamic temperature.

6. Symbol for Tin.

8. Used to store aggregates.

9. He gave the second law of refraction.

12. A large jet plane for passenger carriage.

13. Semi solid colloidal solution.

15. Symbol for Aluminium.

17. We do so to soften hard water.

18. Wire network between filament and anode of thermionic valve.

20. Abbr. for American Concrete Institute.

21. Symbol for Fermium.

23. Symbol for Radon.

24. Short for Information Technology.

Solution to last week’s Crossword: