Sismos tanques

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Seismic retrofitting of RC shaft support of elevated tanksDurgesh C. Rai

The circular, RC shaft type support forelevated tanks lacks redundancy,damping and additional strength typicallypresent in building framing systems and,therefore, should be designed for largerseismic resistance. However, the Indianseismic code IS 1893:1984 prescribesthe same basic seismic force as that forthe most ductile building framing systemfor which the design force is the least.Furthermore, the code specified one-mass idealization of elevated water tanksis not appropriate for large (large width-to-depth ratio) and partially filled tanks.The low design forces lead to a weak andslender support - a very unfavorablefeature in high seismic arras, asevidenced in the failure of two watertanks in the 1997 Jabalpur earthquakeand a great many in the 2001 Bhujearthquake. It is rather difficult toenhance the ductility and energydissipation capacity of thin-walled, RCshaft supports. Concrete jacketing isused as a retrofit measure to enhancethe lateral strength and ductility bychanging the failure mode ofconcrete crushing to a moreductile tension yielding. Thisscheme requires substantialstrengthening of the existingfoundation.

The stimulus of this paper

wasthe failed support structure oftwo 10-to-12 year old, 0.5-million gallon (2270 m 3)capacity, elevated water tanksin the Jabalpur earthquake of22 May 1997. The cylindricalshaft-type staging developed

circumferential flexural-tension cracksnear the base. Similar damages tosupport structures had been observed inpast earthquakes and recently in the Bhujearthquake of 26 January 2001 as shownin Fig 1(a) which is typical of the damagesustained to a large number of watertanks of capacities ranging from 80 m 3 to1,000 m 3 and as far away as 125 kmfrom the epicentre. 1 Fig 1(b) shows acollapsed water tank in the epicentraltract of the Bhuj earthquake. Such aperformance from essential facilities likewater tanks is not acceptable, as they areexpected to remain functional and safe tooperate even after the occurrence of adesign level earthquake (that is, thelargest conceivable earthquake). Thesupply of safe water is required toprevent outbreaks of disease and to keeptires under control after an earthquake.The performance of existing elevatedtanks during severe earthquakes isquestionable, especially of those locatedin high seismic regions, as evidenced inthe 2001 Bhuj earthquake.

This study identifies the seismicdeficiencies of the shaft supports andhow they can be retrofitted or upgradedfor future earthquakes. It also highlightsthe weaknesses of the current Indian

Selected for reader interest and republishedwith due permission from EarthquakeSpectra, The Professional journal of theEarthquake Engineering Research Institute,Vol 18, No 4, pp. 745-760, November 2002.


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practice of seismic design and analysis ofsuch structures and factors that affect theductility of RC hollow cylindrical shellsections that are also used for bridgepiers besides shafts of elevated tanks.

Damage observed in Jabalpur watertanksTwo RC water tanks supported on 20-mtall shafts developed cracks near thebase. There were five such tanks in thecity, and those damaged were located inareas that suffered heavy damage in the

earthquake.2

The tank containers wereIntze type, (that is, below the cylindricalcontainer is a conical shell with a domeshaped tank floor that provides aneconomical substitute for otherwise thickfloor slabs in elevated tanks).The dimensions of the conicalwalls and the spherical bottomdomes are such that theoutward thrust from thespherical dome is balanced bythe inward thrust from the

conical shell. Because of itsoptimal load balancing shape,the Intze type containers arewidely used.

The Gulaotal water tank, Fig 2,was more seriously affectedbecause it was nearly full whenthe earthquake struck, whereas

the other one was only 60 percent full.The Gulaotal tank developed flexural-tension cracks along half its perimeter asshown in Fig 3, on diametrically oppositesides. Some diagonal cracks of shear-flexure origin and some around comers

of the window openings werealso observed. The flexure-tension cracks in shaftsappeared at the level of the first"lift," a plane of weakness, at1.4 m above the ground level.These tanks were founded onthe compaction-type boredunder-reamed piles and novisible distress to surroundingsoil or foundation was noted.

Jabalpur elevated water tanksare inverted pendulum-typestructures, which resist lateralforces by the flexural strengthand stiffness of their circular,

hollow shafts. The section close to theground is subjected to the maximumflexural demand for a uniform shaft. Anydamage to the shaft at this critical sectionshould be considered serious as it cansignificantly undermine its lateral loadcarrying capacity. In fact, the water tank

was taken out of the city waterdistribution system, causing severehardship to neighboring residentsparticularly in the summer months. Theobserved damage pattern is consistent


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with the expected response of thesestructures under lateral loads.

Dynamic behavior of elevated tanksThe basic dynamics of elevated tanks issomewhat complex, especially thoserelated to the movement of fluids in thetank. However, the estimation of designforces for the supports is relativelysimpler. Under lateral accelerations, thefluids in the upper regions of the tank donot move with the tank wall, thusgenerating seismic waves or sloshingmotion of fluids (convective behavior). Onthe contrary, fluids nearer the base of thetank move with the tank structure and,therefore, add to the inertial mass of thetank structure (impulsive behavior). The

portion of the tank fluid that acts in theimpulsive mode depends largely on theaspect ratio (height/ diameter) of thetank. For tanks of very low aspect ratio,very little tank fluid acts in the impulsivemode. The period of sloshing motions aretypically long (up to 10 s) and areinfluenced by the ground displacementrather than the ground acceleration whichtypically affects impulsive modes ofvibration. 3

Several mechanical analogues involvingspring-mass systems have beenproposed to simulate the dynamicresponse of elevated tanks. 4,5 The inertialmasses are connected to tank walls by

rigid links, whereas the convectivemasses are connected by springs. Theflexibility of these springs and attachedmasses represents various antisymmetricsloshing frequencies of fluids in the tank.Recently Malhotra et al 6 has developed asimplified procedure for seismic analysisof tanks taking account of impulsive andconvective actions of the fluid, which hasbeen adopted in Eurocode 8 7 Theprocedure is developed for cylindricalground supported tanks but can be easilyadapted to elevated tanks.

Housner's two-mass mechanicalanalogue

A satisfactory spring-mass analogue, Fig4(a), to characterize the basic dynamics

of elevated tanks was suggested byHousner after the 1960 Chileanearthquake, which saw damage to alarge number of tank supports. 5 Thissimple two-mass model is moreappropriate for elevated tanks than theone-mass model. The two-mass modeladequately represents the impulsive andconvective modes of vibration, asobserved in many experimental studiesconducted by Boyce 8, Sheperd 9,Gracia 10 , Haroun and Ellaithy. 11 Fig 4(b)

shows the variation in theratio of impulsive andconvective masses to totalmass, with respect to theheight-to-radius ratio of thetank as given by Housner.For the Jabalpur tanks, only20.1 percent of the totalwater in the tankparticipated in the impulsivemode. The modelparameters were obtained

for the Intze-type tankcontainer by considering itan equivalent cylindrical

tank having radius and volume equal tothe Intze tank. Joshi 12 has shown thaterrors associated with such anapproximation are small and modelparameters corresponding to equivalentcylindrical tank for Intze tanks can be


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used for design purposes. Although theflexibility of the tank shell is usuallyneglected for the design of support, itshould be accounted for in designing thetank walls. The damping value for theimpulsive mode of RC structures isusually taken as 5 percent; for convectivemode, a value of 0.5 percent isrecommended.

Inelastic behavior of support shaftsConventional earthquake-resistantdesign is based on the premise thatstructures can undergo large plasticdeformations without collapse. Thisconcept allows the structure to bedesigned for significantly less seismicforces than those required if the structure

had to remain elastic. The seismicperformance of such structures restsheavily on the ductility, the energyabsorbing capacity of the detailedstructural components, and theredundancy due to alternative load paths.The factor used to reduce the elasticseismic forces to arrive at design forcesis, therefore, a function of theseproperties. The design forces for lessductile systemswould be larger than

those for moreductile systems. It isexpected that thesupporting structureof elevated tankswould experienceinelasticdeformations and; asa consequence, theaccelerationresponse can bereduced by using an

appropriate ductility factor. However, thisreduction is applied to only impulsiveforces, and no reduction is permissiblefor convective forces as a result ofductility.

The shaft support of the Jabalpur tanks isa shell structure with a thickness of 150mm and a diameter of 17.4 m. The shell

has reinforcement of about 0.2 percent ineach direction. Studies by Zahn et al 13 and Rao 14 have shown that thin, hollow,circular, RC sections with high axial loadbehave in a brittle manner at the flexuralstrength. The available curvature ductilityof hollow circular sections is largelycontrolled by the level of axial stress, thethickness of the wall, and the longitudinalsteel ratio, as shown in Fig 5. Zahn et al 13 have shown that an appreciable ductilitycan be achieved with low axial load,small longitudinal ratio, and a wallthickness not less than 15 percent of theoverall section diameter. Comparingthese with the section properties of theJabalpur tank shaft, which has a largeaxial load ratio (P/f' c Ag) of 0.26 and a wall

thickness of 0.86 percent of overalldiameter, it is obvious that the shaftsection of the Jabalpur tanks cannotwithstand even moderate inelasticdeformations and, therefore, cannot beconsidered ductile. As a result, designforces cannot be reduced significantly onaccount of ductility and energydissipation capacity of the shaft.

Code provisions for supportsModeling for analysisThe Indian seismic code, IS 1893:1984requires elevated tanks to be analyzedas a single degree-of-freedom system,that is, a one-mass system, suggestingthat all fluid mass participates in theimpulsive mode of vibration and moves


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with the tank wall. 15 This can be arealistic assumption for very long,slender, tank containers with a height-to-radius ratio exceeding 4 (that is, a standpipe), as shown in Fig 4(b). Although the2000 International Building Code (IBC)does not specify how elevated tanksshould be analyzed, a multi-mass modelis generally used in practice. 16 The ACI371R-98 allows one mass approximationin cases where the water weight is about80 percent of total seismic weight of thetank structure. 17 A two-mass model wasrecommended for the analysis ofelevated tanks by a study group of theNew Zealand National Society ofEarthquake Engineering. 3 An estimate ofdesign base shears, using IS 1893:1984

with the one-mass and the two-massmodel approach for the Jabalpur tanks,indicates that by using the two-massmodel approach, the design base shearand the base overturning moment are0.56 and 0.74 times,respectively, of thoseobtained by using theone-mass model. 18 Itshould be noted that thisdiscrepancy will bedifferent for tanks with

different geometricproportions.

Design force levelsIS 1893:1984 prescribesdesign forces for elevated tanks at 1.5times of the most ductile building framesystem. This increase is due to theimportance factor and is not due to thestructure performance factor K, which isassumed to be 1 (a value specified forthe most ductile building framing system).

The factor K represents the acceptablelevel of inelastic deformation demand fora given material and system. Incomparison, the 2000 IBC prescribes animportance factor of 1.25 for water tanksbut specifies different values for thestructure performance factor R(equivalent to K) from building systems toindicate their lack of redundancy, lesser

damping, and strength due to theabsence of non-structural and non-considered resisting elements typicallypresent in building systems. On anaverage, building systems have a valueof R=6, which is reduced to one-third forelevated tank systems, that is, R=2.Similarly, ACI 371R-98 also specifies avalue of 2 for R for the support ofelevated tanks for 1997 Uniform BuildingCode (UBC) 19 formulae for design baseshear, which is not significantly differentfrom the 2000 IBC. This means thedesign forces for elevated tank systemsare about three times as large for abuilding system with similar dynamicproperties. Though these R values are

judgmental, larger R values are assigned

to systems with excellent energydissipation capacity and stability, asensured by very specific design anddetailing procedures.

In conclusion, IS 1893:1984 prescribesforces which are too small for elevatedtanks, compared to those prescribed byadvanced standards, such as 2000 IBC.The damage observed in the Jabalpurand Bhuj earthquake illustrates that the

design forces are currently beingunderestimated. The slender and weaksupport that results from the low designforces is a highly unfavorable feature forhigh seismic areas. For seismic zone III,Fig 6 compares the IS 1893:1984 designspectrum curves for a building system(for example, moment-resisting frame)and a non-building system (for example,


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a water tank) to those of 2000 IBC in asimilar seismic environment (that is, theshort period and 1 s spectral responseacceleration for maximum consideredearthquake ground motion were taken as0.75 g and 0.3 g, respectively). It isapparent that the IS 1893:1984 designforce levels are very low and unrealistic.These spectra, if used with the two-massmodel of elevated tanks, will result infurther lowering of the design forces.

Clearly, the provisions of IS 1893:1984do not truly reflect the state-of-knowledgeand result in questionable designparameters for elevated tanks.

Ductile detailing of shaft supportsThere are no provisions in the IS Codesfor ductile detailing of shaft supports,though they are expected toundergo large inelasticdeformations during amaximum credible

earthquake. The Eurocode8, Part 4, 2000 IBC and1997 UBC expect a ductilityvalue ranging from 1.7 to2.5, which can only beachieved by properproportions of the shaft andreinforcing details. Incontrast, framed supports of

elevated tanks can be detailed inaccordance with IS 13920:1993 20 and IS11682:1985 21 , which refer to the ductilityrequirements of IS 4326:1976. 22 There isvery limited literature available on theductile detailing of thin shell sections, andthey are generally considered non-ductile.

Seismic deficiencies of Jabalpur watertanksEstimate of seismic demand:Response spectrum analysisIn view of the above-mentioneddeficiencies of IS 1893:1984, an estimateof seismic demand is obtained by analternative procedure for the Jabalpurtank structure. A response spectrum

analysis is carried out using Housner'stwo-mass idealization for the Jabalpurtank with spectral values from a responsespectra developed from the procedureoutlined by Newmark and Hall. 23 Thebasic input parameter in this method isthe probable peak ground acceleration(PGA) at the site. A value of 0.16 g formean horizontal PGA is consideredadequate. The proposed draft code ofIS:1893 24 also specifies a value of 0.16gfor the city of Jabalpur. The values for

peak ground velocity (PGV) and peakground displacement (PGD) wereobtained from the typical relations:PGV/PGA = 1.22 m/s/g andPGAxPGD/PGV 20 = 6, recommended forfirm ground. For PGA = 0.16 g, theserelations give, PGV = 195 mm/s andPGD = 146 mm.


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The spectral amplification factorscorrespond to the 84th percentile, whichrepresents the mean plus one standard

deviation of the scattered data points.These values near the upper bound ofthe scatter are desirable, considering thecritical importance of the water tankstructure. A 5 percent damped elasticspectra was developed for the impulsivemode, whereas a 0.5 percent dampedelastic spectra was computed for theconvective mode of vibration, as shownin Fig 7(a). The derived design responsespectra for impulsive mode is comparedwith IS 1893:1984 design spectra in Fig

7(b). It should be noted that in Fig 7(b),the IS 1893:1984 spectra that was basedon a performance factor K=3 assuggested by Jain & Sameer 25 for watertank structures, has smaller spectralordinates at all periods; this means that asignificant level of inelastic deformationcapacity (ductility) is expected from theshaft support. The 0.5 percent dampedspectra for convective modes is notavailable in the IS 1893:1984. Table 1shows the seismic demand as obtained

from the response spectrum analysis interms of unfactored base shear, baseoverturning moment, and axial load usingtwo-mass models of Housner andMalhotra et al 6 It is clear that similarvalues were obtained from bothapproaches; however, the method byMalhotra et al 6 is easier to use andespecially suitable for design offices.

Ultimate strength (capacity) ofshaft sectionThe ultimate strength analysis ofthe shaft section involves thecalculation of the ultimate directforce, P u, and the ultimatebending moment, M u, that can beresisted by the resulting stressenvelope. The calculations areessentially the same as outlinedby Pinfold 26 , with differentmaterial properties of concreteand steel as given in IS

456:2000 27 without partial safety factors,Fig 8. For the shaft section, the envelopeof ultimate resistance is presented in theform of an interaction plot M u as the

abscissa and P u as the ordinate, asshown in Fig 9.

Assessment of seismic vulnerability:demand capacity ratiosTo assess the seismic vulnerability of atank's support, the available ultimatecapacity at the critical section iscompared to the probable demandsshown in Table 1. The criticalcombinations of demands, the factoredaxial load P, and the base moment M,


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The largest demand capacity ratio (DCR)is 1. 2, which is obtained by dividing thefactored loads (in this case, basemoment M) by the expected capacity(moment capacity M u at thecorresponding level of axial load P. DCRlarger than unity represents unsafebehavior of the structure in design levelearthquakes. Moreover, the abovecomputation of capacity assumes that thesection was not damaged and that thematerials were in "new" condition. Ifsuitable allowances are made forreduction in the material strength, thesection properties, and other aspects ofcapacity affected by sustained damage,then it is highly likely that withoutretrofitting, the DCR will increase further

for the damaged structure, indicating itsvulnerability in future earthquakes.

are plotted on the capacity plot(interaction diagram) as shown in Fig 9. Itis obvious that the structure is not safesince demands lie outside the interactioncurve, implying compression or tensionfailure of the section depending on thelevel. of axial compression. The failure bycrushing of the concrete is not regardedas a ductile mode of failure and,therefore, is not preferred.

Seismic strengthening for deficientshaft supportThe preceding sections demonstrated theneed for retrofitting the support, given thedeficiency in the flexural strength of theshaft section even for design levelearthquakes. A number of retrofittingtechniques have been developed forincreasing flexural ductility and strength

of RC members, such as confining bypre-stressing and jacketing with steel,RC, and other composite materials. Steel

jacketing, which enhances thedeformability of the section throughpassive confinement, has been themethod of choice for circular bridgecolumns. Alternatively, a thick layer ofRC concrete can be used to increase theflexural strength, ductility, and shearstrength of RC columns. Longitudinalbars in the jacket should be well

anchored into the footing so thatreinforcement strength can bedeveloped. Moreover, increase in flexuralstrength generally requires retrofitting thefooting to avoid flexural and shearcracking in the footing.

Retrofitting of shaft supportFor the shaft of the Jabalpur tanks, an


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RC jacketing was found to be aconvenient retrofit method because of itsease of construction. Since the ductility ofthe section depends heavily on thethickness of the shell wall, a thick layer ofreinforced concrete would enhance itsseismic response. The thickness of theRC jacket and the amount of longitudinalreinforcement was determined so that thetension yielding occurs in the case ofseismic overload for an extremeearthquake event. As shown in Fig 11, anRC jacket of 150 mm thickness with 0.7percent longitudinal steel resulted in asection with an over strength factor of1.8. This relatively large over strength is

justified for a system which does nothave the advantage of redundancy, and

damping. The RC jacket was providedover a height of 5 m above the top of theretrofitted pile cap, with the exception ofthe top 2.5 m where the thickness of theRC layer was reduced to 100 mm andthe amount of steel was also reduced to0.35 percent.

A cross-section of the retrofitted shaft isshown in Fig 11. ACI 371R-98 specifiesthat cross-ties are required in walls atlocations where concentrated plastichinge or inelastic actions are expectedduring seismic loading. The size andspacing of ties should conform to ACI318 28 requirements for seismic areas.The existing shell had no cross-ties andthe horizontal and vertical steel ratioswere 0.0022 against the minimumrequirement of 0.0025. The shellthickness of 150 mm is less than theminimum requirement of 200 mm. In thenew concrete jacket, cross ties wereprovided for a height of 2.5 m above thefooting top which covers the bottom one-

fourth of the shaft where inelasticbehavior is likely to occur, Fig 12. Alsothe horizontal and vertical steel ratios inthe jacket were increased to 0.007. Thevertical reinforcing bars were spliced atstaggered locations and not more thanone-third of the bars were spliced at a

particular level. In the newly added jacket closely spaced stirrups wereprovided over the length of splice.

Retrofitting of footing

Retrofitting of the footing as a result ofthe shaft's increase in lateral strengthis necessary for the success of theoverall retrofit scheme. Ideally, thefooting should be able to sustainforces large enough to cause plastichinging in the shaft without anyflexural or shear cracking of thefooting. However, this criterion for thefooting retrofit can be very expensiveand even infeasible. There have beenfew incidents of footing failures in

earthquakes, where liquefaction orfailure of the ground occurred.Considering the high cost ofretrofitting the footing and the relativeabsence of footing failure, a lessconservative approach was adoptedfor the design of the footing retrofit.

In case of the Jabalpur tanks, the


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footing should be ideally retrofitted toprovide a flexural capacity of 605 MN m,which means an extremely extensiveretrofitting and, if feasible, it will be veryexpensive. The flexural capacity of theexisting footing of Jabalpur tanks is about75 MN m. In a less conservativeapproach, the footing retrofit design ofthe Jabalpur tanks was based on theflexural strength of the retrofitted sectionignoring the increase due to axial loads

from the water stored, that is, 265 MN mcorresponding to an empty tank.

A schematic of the complete retrofitscheme is shown in Fig 12. The existingfooting was extended to accommodate60 new 300-mm-diameter under reamedpiles, and the flexural strength of thefooting was enhanced by increasing thedepth of the footing as well as byproviding three layers of reinforcement inthe new footing overlay. Longitudinal

reinforcements in the bottom layers werewelded to the bottom reinforcements ofthe old pile cap across the existingfooting boundary. The continuity of thetension reinforcing bars was essential todevelop the full flexural capacity of thecombined footing. The dowels wereprovided to transfer the interfacial shear,which was calculated by assuming the

coefficient of friction equal to unity inaddition to the shear transfer expectedfrom the roughening of the existingsurface. The closed shear stirrupsprovided in the footing extension wouldhelp in transmitting the shear force fromthe outer piles to new piles.29 Fig 13 (a)and (b) show the reinforcement layout inthe footing overlay and in the shell jacket,respectively, and Fig 13 (e) shows thetank shaft after the retrofitting.

ConclusionsThin-walled, circular, shaft supports forelevated tanks behave in a brittle mannerat the flexural strength. And the availableductility is also very small for thin-walledsections. The Indian seismic code, IS1893:1984, specifies design forcesequivalent to building framing systemsand ignores the fact that shaft supportslack the redundancy, damping, andadditional strength of building framing

systems. Housner's two-massidealization is a more accuraterepresentation than the IS code specifiedone-mass model. Failure of shaftsupports of water tanks in the recent2001 Bhuj and the 1997 Jabalpurearthquake illustrate the above-mentioned deficiencies of the currentpractice.


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An appreciable ductility for hollow circularsections can be achieved with low axialload, small longitudinal steel ratio, and athick wall. Concrete jacketing waspractical to enhancing lateral strengthand ductility of the sections by changingthe failure mode from the concretecrushing to a more ductile tensionyielding. The necessary upgrading of thefooting was achieved by extending thepile cap to accommodate new piles, andan overlay of concrete enhanced theflexural strength. It is necessary that thereinforcing bars at the boundaries of theexisting footing be properly joined sincetheir continuity is a must for the section todevelop the desired strength.

AcknowledgmentsThe Department of Science andTechnology, Government of India, isacknowledged for its financial supportthrough Young Scientist ResearchScheme (Grant No. 2804). Also,cooperation and help from variousofficials of Department of Public HealthEngineering, Jabalpur, in providingvarious information about the water tanksis thankfully appreciated. Further, Isincerely thank anonymous reviewers

and Steven R. Close, PE, for valuablesuggestions offered to improve theoriginal manuscript.

References1. RAI, D. C. Performance of elevated

tanks in Mw 7.7 Bhuj earthquake ofJanuary 26,2001, Abstracts,1nternational Conference on SeismicHazard with Particular Reference toBhuj Earthquake of January 26,2001,October, 3-5, 2001, New Delhi. .

2. RAI, D. C., NARAYAN, J. P.,PANKAJANDKUMAR, A. JabalpurEarthquake of May 22, 1997:Reconnaissance Report, Department ofEarthquake Engineering, University ofRoorkee, Roorkee, 1997, pp. 101.

3. PRlESTLEY, M.J.N., DAVIDSON,B.J., HONEY, G. D., HOPKINS, D.C.,

MARTIN, R.J., RAMSEY, G.,VESSEY, J. V. and WOOD, J. H.Seismic design of storage tanks:Recommendations of a study group ofthe New Zealand National Society ofEarthquake Engineering, December,1986.

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6. MALHOTRA, P. K., WENK, T., andWEILAND, M. Simple procedure ofseismic analysis of liquid-storage

tanks, Journal of Structural EngineersInternational, IABSE, 2000, 10(3),pp.197-201.

7. Design Provisions of EarthquakeResistance of Structures, Part 4: Silos,tanks and pipelines, Eurocode 8,European Committee forStandardization, Brussels, 1998.

8. BOYCE, W. H. Vibration tests on asimple water tower, Third WorldConference on EarthquakeEngineering, Rome, Italy,

Proceedings, 1973, 1, pp. 220-225.9. SHEPHERD, R. Two mass

representation of a water towerstructure, Journal Sound & Vibration,1972,23(3), pp. 391-196.

10.GRACIA, S. M. Earthquake responseanalysis and seismic design ofcylindrical tanks, Fourth WorldConference on EarthquakeEngineering., Chile, Proceeding,1969, pp. 169-182.

11. HAROUN, M. A and ELLIATHY,

H. M. Seismically induced fluid forcedon elevated tanks, Journal ofTechnical Topics in Civil Engineering,

ASCE, 1985, pp. 1-15.12. JOSHI, S.P. Equivalent

mechanical model for horizontalvibration of rigid Intze tanks, ISETJournal Earthquake Technology,2000, 37(1-3), pp. 39-47.


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13. ZAHN, F.A, PARK, R. andPRIESTLEY, M.J.N. Flexural strengthand ductility of circular hollowreinforced concrete columns withoutconfinement on inside face, ACIStructural Journal, 1990,87(2), pp.156-166.

14. RAO, M.L.N. Effect ofconfinement on ductility of RC hollowcircular columns, Masters' thesissubmitted to Department ofEarthquake Engineering, University ofRoorkee, Roorkee, India, 2000.

15. Criteria for earthquake resistantdesign structures, IS 1893: 1984,Bureau of Indian Standards, NewDelhi, India.

16. International Code Council (ICC)

International Building Code, FallsChurch, VA,2000.

17. Guide for the analysis, design,and construction of concrete-pedestalwater towers (ACI 371R-98),

American Concrete Institute (ACI).,Farrnington Hills, MI, 1998.

18. RAI, D. C. Seismic design andstrengthening of shaft type staging ofelevated tanks, Eleventh Symposiumon Earthquake Engineering, Roorkee,Proceeding, 1998, pp. 771-782.

19. International Conference ofBuilding Officials (ICBO), UniformBuilding Code, Whittier, CA, 1997.

20. Ductile detailing of reinforcedconcrete structures subjected toseismic forces - Code of practice, IS13920 : 1993, Bureau of IndianStandards, New Delhi, India.

21. __ Criteria for design of RCC stagingfor overhead water tanks, IS 11682:1985, Bureau of Indian Standards,New Delhi, India.

22. Code of practice for earthquakeresistant design and construction ofbuildings, IS 4326:1976, Bureau ofIndian Standards, New Delhi, India.

23. NEWMARK, N.M. and HALL, W.J.Earthquake Spectra and Design,Earthquake Engineering ResearchInstitute, Oakland, CA, 1982.

24. Criteria for earthquake resistant

design structures, IS:1893, Bureau ofIndian Standards, Draft, New Delhi,India, 1998.

25. JAIN, S.K. and SAMEER, S.U. Areview of requirements of Indiancodes for aseismic design of elevatedwater tanks, Bridge & StructuralEngineer, 1993, XXTII(l).

26. PINFOLD, G.M. ReinforcedConcrete Chimneys and Towers,Viewpoint Publications, Cement andConcrete Association, London, 1989.

27. Indian standard for plain andreinforced concrete - Code of practice,IS 456:2000, Bureau of IndianStandards, New Delhi, India.

28. Building code requirements farstructural concrete, ACI 318-99,

American Concrete Institute,Farmington Hills, MI, 1999.

29. PRIESTLEY, M.J.N., SEIBLE, F.and CALVI, G.M. Seismic Design andRetrofit of Bridges, John Wiley, NewYork, 1996.

Dr Durgesh C. Rai is an assistantprofessor in the department of civilengineering at the Indian Institute ofTechnology (lIT) Kanpur since 2002.Previously he has been on the Faculty

of department of earthquakeengineering at lIT Roorkee. Hereceived his Ph.D. from the Universityof Michigan, Ann Arbor, USA, in 1996.His research interests are in designand behavior of structures underearthquake loads, experimentalinvestigations, supplemental damping,seismic rehabilitation and seismicdesign codes. He has been awarded2000 Shah Family Innovation Prize ofEarthquake Engineering Institute, USA

and 1999 Young Engineer Award ofIndian National Academy ofEngineering.

FUENTE: The Indian Concrete journalVol. 77, Nov. 2003, No. 11

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Sismos tanques