 |
|
| SCIENCE TRIBUNE | Thursday, January 30, 2003, Chandigarh, India |
|
Quake-resistant buildings |
|
||
|
Quake-resistant buildings 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 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. 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.
|
||
|
UNDERSTANDING
THE UNIVERSE The radius of a grown up man increases or decreases but the height remains the same. Why?
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.
|
||
 |
SCIENCE & TECHNOLOGY CROSSWORD Clues Across : 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:
|
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).
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.
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.
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.
1.
An egg shaped figure.