Quake resistant design, construction

C. V. R. Murty     

IIT, Kanpur     

IN INDIA, most non-urban buildings are made of 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 recently 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 - see figure).

Cement concrete is made of crushed stone pieces (called aggregate), sand, cement and water mixed in appropriate proportions.

Concrete is 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 6-40mm. Reinforcing steel can carry tensile and compressive loads. Moreover its 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 concept

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. 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. Each of these links will fail. 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 the weakest link. Sponsored by Building Materials and Technology Promotion Council, New Delhi, India.