By
NAVNEET KUMAR,B.Tech(CE)
2nd year, ITM UNIVERSITY, GWALIOR
Experience in past earthquakes has demonstrated that many common buildings and typical methods of construction lack basic resistance to earthquake forces in most cases this resistance can be achieved by following simple inexpensive principles of good building construction practices . Adherence to these simple rules will not prevent all damage in moderate or large earthquakes, but life threatening collapses should be prevented, and damage limited to repairable proportions. These principles fall into several broad categories:-
(i) Planning and layout of the building involving considerations of the location of rooms and walls, openings such as doors and windows, the number of storeys etc. At this stage, site and foundation aspects should also be considered.
(ii) Lay out and general design of the structural framing system with special attention to furnishing lateral resistance, and
(iii) Consideration of highly loaded and critical sections with provision of reinforcement as required.
From my general study, certain general principles have emerged:-
(i) Structures should not be brittle or collapse suddenly rather they should not be tough, able to deflect or deform a considerable amount
(ii) Resisting elements such as bracing on shear walls , must be provided evenly throughout the buildings in both directions side to side ,as well as top to bottom.
(iii) All elements such as walls and the roof should be tied together so as to act as an integrated unit during earthquake shaking transferring forces across connections and preventing separation.
(iv) The building must be connected to a good foundation and the earth wet soil should be avoided and the foundation must be well tied together as well tied to the wall where the soft soils strengthening must be provided.
(v) Care must be taken that all materials used are of good quality and are protected from sun , rain, insects and other weakening actions so that their strength lasts.
(vi) Unreinforced earth and masonry have no reliable strength in tension, and are brittle in compression. Generally they must be suitably reinforced by steel or wood.
These principles will be discussed and illustrated in this chapter.
3.2 CATEGORIES OF BUILDINGS
For categorizing the buildings with the purpose of achieving seismic resistance at economical cost three parameters turns out to be significant:
(i) Seismic intensity zone where the building is located.
(ii) How important the building is and
(iii) How stiff is the foundation soil
A combination of these parameters will determine the extent of appropriate seismic strengthening of the building.
SEISMIC ZONES
In most countries, the macro level seismic zones are defined on the basis of seismic intensity scales in this guide, we shall refer to seismic zones:-
ZONE A:- Risk of widespread collapse and destruction
ZONE B:-Risk of collapse and heavy damage
ZONE C:-Risk of minor damage
IMPORTANCE OF BUILDING
The importance of the building should be a factor in grading it for strengthening purposes and the following buildings are suggested as specially important.
IMPORTANT-Hospitals,clinics,communication buildings, fire and police stations, water supply facilities,cinemas,meeting halls,schools,cultural treasures such as museums, monuments and temples etc.
ORDINARY:-Housing, hostels, offices,warehouses,factories etc.
BEARING CAPACITY OF FOUNDATION SOIL
Three soil types are considered here:-
Firm: Those soils which have an allowable bearing capacity of more than 10 t/m2
SOFT: those soils which have an allowable bearing capacity less than or equal to 10 t/m2
WEAK: Those soils which are liable to large differential settlement or liquefaction during earthquake
Buildings can not be constructed on firm and soft soils but it will be dangerous to build them on weak soils. Hence appropriate soil investigations should be carried out to establish the allowable bearing capacity and nature of soil.
CHOICE OF SITE
The choice of site for building from the seismic point of view is mainly concerned with the stability of the ground. The following are important:
1) Stability of slope: hill side slopes liable to slide during an earthquake should be avoided and only stable slopes should be chosen to locate the building. Also it will be preferable to have several blocks on terraces than have one large block with footings at very different elevations. A site subject to the danger of rock falls has to be avoided.
2) Very loose sand or sensitive clays:-These two types of soils are liable to be destroyed by the earthquake so much as to lose their original structure and thereby undergo compaction. This would result in Large unequal settlement and damage the building. If the loose cohesions soils are saturated with water they are apt to lose their shear resistance altogether during shaking and become liquefied.
REQUIREMENTS OF STRUCTURAL SAFETY
(i) A pre standing wall must be difficult to achieve in un-reinforced masonry in zone A. Therefore all partitions inside the buildings must be held on the sides as well as top. Parapets must be reinforced and held to the main structural slabs of frames.
(ii) Horizontal reinforcement in walls is required for transferring their own out of plane inertia load horizontally to shear walls.
(iii) The walls must be effectively tied together to avoid separation at vertical joints due to ground shaking.
(iv) Shear walls must be present along both axes of building.
(v) A shear wall must be capable of resisting all horizontal forces due to its own mass and those transmitted to it.
(vi) Roof or floor elements must be tied together and be capable of exhibiting diaphragm action.
(vii) Trusses must be anchored to the supporting walls and have an arrangement for transferring their inertia force to end walls.
CONCEPTS OF DUCTILITY, DEFORMABILITY AND DAMAGEABILITY
1. DUCTILITY-Formally, ductility refers to the ratio of the displacement just prior to ultimate displacement or collapse to the displacement at first damage or yield . Some materials are inherently ductile such as steel, wrought iron and wood. Other materials are not ductile such as cast iron.
2. DEFORMABILITY:-Ability of a structure to displace or deform substantial amounts without collapsing. Besides inherently relying on ductility of materials and components, deformability requires that structures be well proportioned, regular and well tied together so that excessive stress concentration are avoided and avoided and forces are capable of being transmitted from one component to another even through large deformations.
3.DAMAGEABILITY:-Damageability is also a desirable quality for construction and refers to the ability of a structure to undergo substantial damages, without partial or total collapses.
A key to good damageability is redundancy or provision of several supports for key structural members, such as ridge beams and avoidance of central columns or walls supporting excessively large portions of a building.
CONCEPT OF ISOLATION
For reduction of coefficient of friction between the structures and its foundation, one suggested technique is to place two layers of good quality plastic between the structures and its foundation so that the plastic layers may slide over each other.
FOUNDATIONS
For the purpose of making a building truly earthquake resistant, it will be necessary to chose an appropriate foundation type for it .since loads from typical low height buildings will be light, providing the required bearing area will not usually be a problem. The depth of footing in the soil should go below the zone of deep freezing in cold countries and below the level of shrinkage cracks in clayey soils.
FIRM SOIL
In firm soil conditions, any type of footing can be used. It should be of course have a firm base of lime or cement concrete with requisite width over which the construction of the footing may start. It will be desirable to connect the individual reinforced concrete column footings in zone A by means of RC beams just below plinth level intersecting at right angles.
SOFT SOIL-
In soft soil, it will be desirable to use a plinth band in all walls and where necessary to connect the individual column footings by means of plinth beams suggested above. It may be mentioned that continuous reinforced concrete footings are considered to be most effective from earthquake considerations as well as to avoid differential settlements under normal vertical loads. These should ordinarily be provided continuously under all the walls. Continuous footings should be reinforced both in the top and bottom faces, width of the footing should be wide enough to make the contact pressures uniform and the depth of footing should be below lowest level of weathering.
References
1) National Information Centre of Earthquake Engineering at IIT Kanpur, INDIA
We at engineeringcivil.com are thankful to Mr Navneet Kumar for submitting this very interesting and helpful guide on “General Concepts Earthquake Resistant Design” to us. We are hopeful that this will be of great help to all who wish to understand earthquake resistant designs.
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