Construction Materials for Earthquake ResistanceIn 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 now a day we are very familiar with R.C.C. buildings, and a variety of new composite constructions materials.
CONSTRUCTION MATERIALS
I. MASONRYMasonry is made up of burnt clay bricks and cement or mud mortar. Masonry can carry loads that cause compression (i.e. pressing together) but can hardly take load that causes tension (i.e. pulling apart). Masonry is a brittle material, these walls develop cracks once their ability to carry horizontal load is exceeded. Thus infill walls act like sacrificial fuses in buildings: they develop cracks under severe ground shaking but they share the load of the beams and columns until cracking. II. CONCRETEConcrete 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 behavior 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. III. STEELSteel 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 with steel reinforcement bars. This composite material is called as reinforced cement concrete. 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 is preferred over a failure where concrete fails first in compression. Therefore, Providing more steel in R.C. buildings can be harmful even!!
EARTHQUAKE RESISTANT DESIGN CONCEPT
If two bars of same length and same cross-sectional area - one made of ductile material and another of a brittle material. And a pull is applied on both bars until they break, then we 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 relative small elongation. Amongst the materials used in building construction, steel is ductile, while masonry and concrete are brittle. The correct building components need to be made ductile. The failure of columns can affect the stability of building, but failure of a beam causes localized effect. Therefore, it is better to make beams to be ductile weak links then columns. This method of designing RC buildings is called the strong-column weak-beam design method. Special design provisions from IS: 13920-1993 for RC structures ensures that adequate ductility is provided in the members where damage is expected.
QUALITY CONTROL IN CONSTRUCTIONThe 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 processes is essential in assuring an earthquake resistant building. Regular testing of materials to laboratories, periodic training of workmen at professional training houses, and on-site evaluation of the technical work are elements of good quality control.
BASICS OF EARTHQUAKE RESISTANCEConventional seismic design attempts to make buildings that do not collapse under strong earthquake shaking, but may sustain damage to non-structural elements (like glass facades) and to some structural members in the building. This may render the building non-functional after the earthquake, which may be problematic in some structures, like hospitals, which need to remain functional in the aftermath of earthquake. Special techniques are required to design buildings such that they remain practically undamaged even in a severe earthquake. Buildings with such improved seismic performance usually cost more than the normal buildings do. Two basic technologies are used to protect buildings from damaging earthquake effects. These are Base Isolation Devices and Seismic Dampers. The idea behind base isolation is to detach (isolate) the building from the ground in such a way that earthquake motions are not transmitted up through the building or at least greatly reduced. Seismic dampers are special devices introduced in the buildings to absorb the energy provided by the ground motion to the building (much like the way shock absorbers in motor vehicles absorb due to undulations of the road)
Next: Base Isolation Techniques for Earthquake Resistance
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