Traditional poor man’s houses in the tropics have mostly wattle and daub walls with thatched roofs - these houses consume little material, and are light by weight. The inherent limitations of the houses do not permit higher storeys. Additional advantage these houses afford is that due to their lightness, the houses, even if they collapse, may not kill people.
Another way of improving earthquake resistance is by way of making the houses rigid, viz, making sure that the houses vibrate together as one unit. This prevents unnecessary absorption of energy by the structural members and improves the quake resistance of buildings. Schematically, in such houses the top portions of the houses are so joined to the bottom that all movements are transferred immediately from lower levels to the entire building and the entire house vibrates as one rigid body. Consequently no disharmonious stresses are set up and the house remains safe.
The bamboo construction in North-East India follows this principle of rigid house. Bamboo being light gives added natural advantage. In these houses rigidity is achieved by means of cross bracing and triangulation. All joints are strengthened by means of cross members that can transmit earthquake forces directly to the remaining portions of the house.
Reinforced concrete ‘frames’ are rigid by design and their rigidity can be improved further by small increases in steel used. The ‘framework’ of such beams and columns can be made to resist earthquake-induced vibrations of considerable magnitudes. Almost all the multi-storied buildings in towns and cities are framed, thus the structure finds great support. It must be noted here that though the RCC frames take care of major portions of earthquake induced forces, the forces generated in the non-load bearing walls could still lead to damages byway of wall collapses. In places like rural India where concrete technology has little reach, masonry in brick and / or in stone takes major share in the form of lead bearing walls
The strength of masonry depends to a great extent on the strength of the mortar joining them to distribute the earthquake-induced forces equally throughout. One modern method ‘Foamed Concrete’ construction combines principles of structural masonry together with the advantages of lightweight foam concrete. Foam concrete is prepared by aerating the concrete profusely as it is mixed to create a highly fluffy, lightweight substance. Building blocks and units made out of this material lead however to centralization and are subject presently to patent laws.
Masonry when properly construct, possesses good resistance to seismic forces. Bad construction, however, means disastrous consequences. Improving the rigidity of load bearing masonry by way of three RCC bands one each at plinth, lintel and roof levels is the most common technique employed in a country’s earthquake resistant housing programmes today. These houses, when constructed under supervision can resist earthquakes of moderate intensities quite efficiently.
