Concept of earth quake
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Concept of earth quake Document Transcript

  • 1. GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGNChapter 3 GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGN3.1 INTRODUCTION reinforcement as required.Experience in past earthquakes has dem- Chapter 2 has provided a good overviewonstrated that many common buildings of structural action, mechanism of damageand typical methods of construction lack and modes of failure of buildings. Frombasic resistance to earthquake forces. In these studies, certain general principlesmost cases this resistance can be achieved have emerged:by following simple, inexpensive princi-ples of good building construction prac- (i) Structures should not be brittle ortice. Adherence to these simple rules will collapse suddenly. Rather, theynot prevent all damage in moderate or large should be tough, able to deflect orearthquakes, but life threatening collapses deform a considerable amount.should be prevented, and damage limitedto repairable proportions. These principles (ii) Resisting elements, such as bracingfall into several broad categories: or shear walls, must be provided evenly throughout the building, in (i) Planning and layout of the building both directions side-to-side, as well involving consideration of the loca- as top to bottom. tion of rooms and walls, openings (iii) All elements, such as walls and the such as doors and windows, the roof, should be tied together so as to number of storeys, etc. At this stage, act as an integrated unit during site and foundation aspects should earthquake shaking, transferring also be considered. forces across connections and pre- (ii) Lay out and general design of the venting separation. structural framing system with spe- (iv) The building must be well connected cial attention to furnishing lateral to a good foundation and the earth. resistance, and Wet, soft soils should be avoided, and (iii) Consideration of highly loaded and the foundation must be well tied to- critical sections with provision of gether, as well as tied to the wall. 1
  • 2. IAEE MANUAL Where soft soils cannot be avoided, special strengthening must be pro- vided. (v) Care must be taken that all materials used are of good quality, and are pro- tected from rain, sun, 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. Gen- erally, they must be suitably rein- forced by steel or wood. These principles will be discussed and illustrated in this Chapter. 3.2 CATEGORIES OF BUILDINGS For categorising the buildings with the purpose of achieving seismic resistance at2
  • 3. GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGNSoft: Those soils, which have allowable many projections Fig 3.2 (b). Tor- bearing capacity less than or equal sional effects of ground motion are to 10 t/m2. pronounced in long narrow rectan- gular blocks. Therefore, it is desirableWeak: Those soils, which are liable to large to restrict the length of a block to differential settlement, or liquefac- three times its width. If longer tion during an earthquake. lengths are required two separate Buildings can be constructed on firm blocks with sufficient separation inand soft soils but it will be dangerous to between should be provided,build them on weak soils. Hence appropri- Fig 3.2 (c).ate soil investigations should be carried out (iii) Separation of Blocks: Separation of ato establish the allowable bearing capacity large building into several blocksand nature of soil. Weak soils must be may be required so as to obtain sym-avoided or compacted to improve them so metry and regularity of each block.as to qualify as firm or soft.3.2.4 Combination ofparametersFor defining the categories of buildings forseismic strengthening purposes, four cat-egories I to IV are defined in Table 3.1. inwhich category I will require maximumstrengthening and category IV the least in-puts. The general planning and designingprinciples are, however, equally applica-ble to them.3.3. GENERAL PLANNING ANDDESIGN ASPECTS3.3.1. Plan of building (i) Symmetry: The building as a whole or its various blocks should be kept symmetrical about both the axes. Asymmetry leads to torsion during earthquakes and is dangerous, Fig 3.1. Symmetry is also desirable in the placing and sizing of door and window openings, as far as possi- ble. (ii) Regularity: Simple rectangular shapes, Fig 3.2 (a) behave better in Fig 3.1 Torsion of unsymmetrical plans an earthquake than shapes with 3
  • 4. IAEE MANUAL For preventing hammering or in larger buildings since it may not pounding damage between blocks a be convenient in small buildings. physical separation of 3 to 4 cm (iv) Simplicity: Ornamentation throughout the height above the invo1ving large cornices, vertical or plinth level will be adequate as well horizontal cantilever projections, fa- as practical for upto 3 storeyed cia stones and the like are danger- buildings, Fig 3.2 (c). ous and undesirable from a seismic The separation section can be treated viewpoint. Simplicity is the best ap- just like expansion joint or it may be proach. filled or covered with a weak mate- Where ornamentation is insisted rial which would easily crush and upon, it must be reinforced with crumble during earthquake shaking. steel, which should be properly em- Such separation may be consideredFig 3.2 Plan of building blocks.4
  • 5. GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGN bedded or tied into the main struc- (i) Stability of Slope: Hillside slopes li- ture of the building. able to slide during an earthquake should be avoided and only stable Note: If designed, a seismic coeffi- slopes should be chosen to locate the cient about 5 times the coefficient building. Also it will be preferable used for designing the main struc- ture should be used for cantilever ornamentation. (v) Enclosed Area: A small building en- closure with properly intercon- nected walls acts like a rigid box since the earthquake strength which long walls derive from transverse walls increases as their length de- creases. Therefore structurally it will be ad- visable to have separately enclosed rooms rather than one long room, Fig 3.3. For unframed walls of thick- ness t and wall spacing of a, a ratio of a/t = 40 should be the upper limit between the cross walls for mortars of cement sand 1:6 or richer, and less for poor mortars. For larger panels or thinner walls, framing elements should be introduced as shown at Fig 3.3(c). (vi) Separate Buildings for Different Functions: In view of the difference in importance of hospitals, schools, assembly halls, residences, commu- nication and security buildings, etc., it may be economical to plan sepa- rate blocks for different functions so as to affect economy in strengthen- ing costs.3.3.2 Choice of siteThe choice of site for a building from theseismic point of view is mainly concernedwith the stability of the ground. The fol-lowing are important: Fig 3.3 Enclosed area forming box units 5
  • 6. IAEE MANUAL to have several blocks on terraces 3.3.4 Fire resistance than have one large block with It is not unusual during earthquakes that footings at very different elevations. due to snapping of electrical fittings short A site subject to the danger of rock circuiting takes place, or gas pipes may falls has to be avoided. develop leaks and catch fire. Fire could also be started due to kerosene lamps and (ii) Very Loose Sands or Sensitive Clays: kitchen fires. The fire hazard sometimes These two types of soils are liable to could even be more serious than the earth- be destroyed by the earthquake so quake damage. The buildings should there- much as to lose their original struc- fore preferably be constructed of fire resist- ture and thereby undergo ant materials. compaction. This would result in large unequal settlements and dam- 3.4 STRUCTURAL FRAMING age the building. If the loose There are basically two types structural cohesionless soils are saturated with framing possible to withstand gravity and water they are apt to lose their shear seismic load, viz. bearing wall construction resistance altogether during shaking and framed construction. The framed con- and become liquefied. struction may again consist of: Although such soils can be compacted, for small buildings the operation may be (i) Light framing members which must too costly and these soils are better avoided. have diagonal bracing such as wood For large building complexes, such as hous- frames (see Chapter 6) or infill walls ing developments, new towns, etc., this fac- for lateral load resistance, Fig 3.3 (c), tor should be thoroughly investigated and or appropriate action taken. (ii) Substantial rigid jointed beams and columns capable of resisting the lat- Therefore a site with sufficient bearing eral loads by themselves. capacity and free from the above defects should be chosen and its drainage condi- The latter will be required for large col- tion improved so that no water accumu- umn free spaces such as assembly halls. lates and saturates the ground close to the footing level. The framed constructions can be used for a greater number of storeys compared to 3.3.3. Structural design bearing wall construction. The strength and Ductility (defined in Section 3.6) is the most ductility can be better controlled in framed desirable quality for good earthquake per- construction through design. The strength formance and can be incorporated to some of the framed construction is not affected extent in otherwise brittle masonry con- by the size and number of openings. Such structions by introduction of steel reinforc- frames fall in the category of engineered ing bars at critical sections as indicated construction, hence outside the scope of the later in Chapters 4 and 5. present book.6
  • 7. GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGN3.5 REQUIREMENTS OF The strengthening measures necessarySTRUCTURAL SAFETY to meet these safety requirements are pre-As a result of the discussion of structural sented in the following Chapters for vari-action and mechanism of failure of Chap- ous building types. In view of the lowter 2, the following main requirements of seismicity of Zone D, no strengtheningstructural safety of buildings can be arrived measures from seismic consideration areat. considered necessary except an emphasis on good quality of construction. The fol- (i) A free standing wall must be de- lowing recommendations are therefore in- signed to be safe as a vertical canti- tended for Zones A, B and C. For this pur- lever. pose certain categories of construction in a number of situations were defined in This requirement will be difficult to Table 3.1. achieve in un-reinforced masonry in Zone A. Therefore all partitions in- 3.6 CONCEPTS OF DUCTILITY, side the buildings must be held on DEFORMABILITY AND the sides as well as top. Parapets of DAMAGEABILITY category I and II buildings must be Desirable properties of earthquake-resist- reinforced and held to the main ant design include ductility, deformability structural slabs or frames. and damageability. Ductility and (ii) Horizontal reinforcement in walls is deformability are interrelated concepts sig- required for transferring their own nifying the ability of a structure to sustain out-of-plane inertia load horizon- large deformations without collapse. tally to the shear walls. Damageability refers to the ability of a struc- (iii) The walls must be effectively tied together to avoid separation at verti- Table 3.1 Categories of buildings for strengthening purposes cal joints due to ground shaking. Category Combination of conditions for the Category I Important building on soft soil in zone A (iv) Shear walls must be present along both axes of the building. II Important building on firm soil in zone A Important building on soft soil in zone B Ordinary building on soft soil in zone A (v) A shear wall must be capable of re- sisting all horizontal forces due to III Important building on firm soil in zone B Important building on soft soil in zone C its own mass and those transmitted Ordinary building on firm soil in zone A to it. Ordinary building on soft soil in zone B IV Important building on firm soil in zone C (vi) Roof or floor elements must be tied Ordinary building on firm soil in zone B together and be capable of exhibit- Ordinary building on firm soil in zone C ing diaphragm action. Notes: (i) Seismic zones A, B and C and important buildings are defined in Section 3.2. (vii) Trusses must be anchored to the sup- (ii) Firm soil refers to those having safe bearing value more than porting walls and have an arrange- 10 t/m2 and soft those less than 10 t/m2. ment for transferring their inertia (iii) Weak soils liable to compaction and liquefaction under earth- quake condition are not covered here. force to the end walls. 7
  • 8. IAEE MANUAL ture to undergo substantial damage, with- together so that excessive stress concentra- out partial or total collapse. This is desir- tions are avoided and forces are capable of able because it means that structures can being transmitted from one component to absorb more damage, and because it per- another even through large deformations. mits the deformations to be observed and repairs or evacuation to proceed, prior to Ductility is a term applied to material collapse. In this sense, a warning is received and structures, while deformability is ap- and lives are saved. plicable only to structures. 3.6.1 Ductility Even when ductile materials are present Formally, ductility refers to the ratio of the in sufficient amounts in structural compo- displacement just prior to ultimate dis- nents such as beams and walls, overall placement or collapse to the displacement structural deformability requires that geo- at first damage or yield. Some materials are metrical and material instability be inherently ductile, such as steel, wrought avoided. That is, components must have iron and wood. Other materials are not proper aspect ratios (that is not be too high), ductile (this is termed brittle), such as cast must be adequately connected to resisting iron, plain masonry, adobe or concrete, that elements (for example sufficient wall ties is, they break suddenly, without warning. for a masonry wall, tying it to floors, roof Brittle materials can be made ductile, usu- and shear walls), and must be well tied to- ally by the addition of modest amounts of gether (for example positive connection at ductile materials, Such as wood elements beam seats, so that deformations do not in adobe construction, or steel reinforcing permit a beam to simply fall off a post) so in masonry and concrete constructions. as to permit large deformations and dy- namic motions to occur without sudden For these ductile materials to achieve a collapse. ductile effect in the overall behaviour of the component, they must be proportioned and 3.6.3 Damageability placed so that they come in tension and are Damageability is also a desirable quality subjected to yielding. Thus, a necessary re- for construction, and refers to the ability of quirement for good earthquake-resistant a structure to undergo substantial damages, design is to have sufficient ductile materi- without partial or total collapse als at points of tensile stresses. A key to good damageability is redun- 3.6.2 Deformability dancy, or provision of several supports for Deformability is a less formal term refer- key structural members, such as ridge ring to the ability of a structure to displace beams, and avoidance of central columns or deform substantial amounts without or walls supporting excessively large por- collapsing. Besides inherently relying on tions of a building. A key to achieving good ductility of materials and components, damageability is to always ask the ques- deformability requires that structures be tion, “if this beam or column, wall connec- well-proportioned, regular and well tied tion, foundation, etc. fails, what is the con- sequence?”. If the consequence is total col-8
  • 9. GENERAL CONCEPTS OF EARTHQUAKE RESISTANT DESIGNlapse of the structure, additional supports high frequency motions. Unfortunately, tra-or alternative structural layouts should be ditional applications of this technique usu-examined, or an additional factor of safety ally do not account for occasional largebe furnished for such critical members or displacements of this pin-connectedconnections. mechanism, due to rare very large earth- quakes or unusually large low-frequency3.7 CONCEPT OF ISOLATION content in the ground motion, so that whenThe foregoing discussion of earthquake- lateral displacements reach a certain point,resistant design has emphasized the tradi- collapse results. A solution to this problemtional approach of resisting the forces an would be provision of a plinth slightly be-earthquake imposes on a structure. An al- low the level of the top of the posts, so thatternative approach which is presently when the posts rock too far, the structure isemerging is to avoid these forces, by isola- only dropped a centimeter or so.tion of the structure from the ground mo-tions which actually impose the forces on 3.8 FOUNDATIONSthe structure. For the purpose of making a building truly earthquake resistant, it will be necessary to This is termed base-isolation. For sim- choose an appropriate foundation type forple buildings, base- friction isolation may it. Since loads from typical low heightbe achieved by reducing the coefficient of buildings will be light, providing the re-friction between the structure and its foun- quired bearing area will not usually be adation, or by placing a flexible connection problem. The depth of footing in the soilbetween the structure and its foundation. should go below the zone of deep freezing in cold countries and below the level of For reduction of the coefficient of fric- shrinkage cracks in clayey soils. For choos-tion between the structure and its founda- ing the type of footing from the earthquaketion, one suggested technique is to place angle, the soils may be grouped as Firm andtwo layers of good quality plastic between Soft (see Section 3.2.3) avoiding the weakthe structure and its foundation, so that the soil unless compacted and brought to Softplastic layers may slide over each other. or Firm condition. Flexible connections between the struc- 3.8.1 Firm soilture and its foundation are also difficult to In firm soil conditions, any type of footingachieve on a permanent basis. One tech- (individual or strip type) can be used. Itnique that has been used for generations should of course have a firm base of lime orhas been to build a house on short posts cement concrete with requisite width overresting on large stones, so that under earth- which the construction of the footing mayquake motions, the posts are effectively pin- start. It will be desirable to connect the in-connected at the top and bottom and the dividual reinforced concrete columnstructure can rock to and fro somewhat. footings in Zone A by means of RC beamsThis has the advantage of substantially re- just below plinth level intersecting at rightducing the lateral forces, effectively isolat- angles.ing the structure from the high amplitude 9
  • 10. IAEE MANUAL 3.8.2 Soft soil footings are presented in Chapters 4 and 9 In soft soil, it will be desirable to use a plinth respectively. band in all walls and where necessary to connect the individual column footings by These should ordinarily be provided means of plinth beams as suggested above. continuously under all the walls. Continu- It may be mentioned that continuous rein- ous footing should be reinforced both in forced concrete footings are considered to the top and bottom faces, width of the foot- be most effective from earthquake consid- ing should be wide enough to make the erations as well as to avoid differential set- contact pressures uniform, and the depth tlements under normal vertical loads. De- of footing should be below the lowest level tails of plinth band and continuous RC of weathering. •••10