Seismic Performance of Storage Steel Tanks during theMay 2012 Emilia, Italy, Earthquakes

Emanuele Brunesi1; Roberto Nascimbene, Ph.D.2; Marco Pagani, Ph.D.3; Dumitru Beilic4

Abstract: Field observations following the May 2012 Emilia, Italy, earthquakes have revealed the seismic vulnerability of storage steel tankstypical of the past Italian design practice, highlighting structural deficiencies observed during previous events in other areas and mostlyrelated to lack of structural seismic design and detailing, lack of redundancy, and inadequate anchorage design and execution. Damagewas disproportionately high, considering the moderate size of the events (MW ¼ 6.11 and MW ¼ 5.96 on 20th and 29th May, respectively).Failure modes observed are reported and classified. Comparison is provided with the results of high-definition finite element (FE) dynamicsimulations, performed on tanks representative of those inspected. Numerical analyses, accounting for material and geometrical nonlinear-ities, at the element level, as well as for fluid-structure interaction through highly nonlinear methods, were able to capture the stress/strainconcentrations that caused them to collapse. Elephant’s foot and diamond buckling induced by hydrodynamic pressures as a result of inertialforces imparted during the earthquakes were reproduced, as well as shear-buckling failure of leg-supported tanks or sliding of unanchoredsystems. DOI: 10.1061/(ASCE)CF.1943-5509.0000628. © 2014 American Society of Civil Engineers.

Author keywords: Earthquakes; Storage tanks; Thin shell structures; Failure modes; Buckling; Finite element models; Fluid-structureinteraction.

Introduction

On May 20th and 29th, 2012, two earthquakes of magnitude (MW)6.11 and 5.96—according to the European and MediterraneanRegional Centroid Moment Tensor Solution (RCMT 1997)—hitthe Emilia region in the Po Valley, one of the most industrializedzones of Northern Italy. The majority of structures severely dam-aged by the seismic events were industrial facilities: one-story pre-cast reinforced concrete structures (Magliulo et al. 2013; Bournaset al. 2013; Belleri et al. 2014) and nearby storage steel tanks, caus-ing an economic loss of approximately 5 billion Euros, mostly dueto the industrial production interruption. The large amount ofindustrial facilities in the stricken area, in combination with theirintrinsic deficiencies, makes several industries disproportionatelyvulnerable to these events, compared to their moderate seismicintensity.

In the aftermath of the earthquakes, a large reconnaissance effortwas undertaken. Field teams were deployed by the EuropeanCentre for Training and Research in Earthquake Engineering(Eucentre), and a clearinghouse (http://www.eqclearinghouse.org/2012-05-20-italy/) hosted by Eucentre and the Earthquake

Engineering Research Institute (EERI) was prepared. Out of thesignificant number of the structures surveyed, this paper investi-gates the poor seismic performance provided by storage steel tanks,typical of the past Italian design practice, confirming vulnerabilitiescommon for such early designs and already shown by otherdestructive earthquakes in other areas (Manos 1991; Niwa andClough 1982; Swan et al. 1985; Stepp et al. 1990; Zareian et al.2012; González et al. 2013).

The most common types of failures observed were fracture ofanchors and elephant’s foot buckling near the base of the tanks.Generally, elephant’s foot buckling was experienced in squat tanks,while some of the slender tanks surveyed developed diamond-shaped buckling. Total and partial collapse of legged tanks was an-other common occurrence, induced by shear failure and/or bucklingof their legs due to axial forces resulting from tank overturningmoment. In some cases, flat-bottomed steel cylindrical tanks, typ-ically larger than legged tanks, failed in tension at the bottom of thetank wall, in correspondence of the anchor rods or massive concretepads. In the following, these mechanisms will be reported and dis-cussed in comparison with numerical observations, obtained fromdetailed FE analyses.

Seismic Hazard and Ground Motions Observedduring the 2012 Sequence

The area stricken by the Emilia earthquake sequence can be con-sidered an area of intermediate seismicity in the Italian context(Meletti et al. 2012) as highlighted in Fig. 1, which depicts the of-ficial Italian hazard map (PGAwith 10% probability of exceedancein 50 years) as well as the epicentral location of the main events ofthe sequence (data from INGVand ISIDe). The value of hazard—interms of PGAwith a return period of 475 years—computed for theepicentral area corresponds approximately to 0.14 g. According tothe results of Stucchi et al. (2011) about half of the Italian territory(but Sardinia) has values of hazard equal to or larger than theepicentral area.

1Ph.D. Student, ROSE Programme, UME School, IUSS Pavia, Institutefor Advanced Study, Via Ferrata 1, 27100 Pavia, Italy (correspondingauthor). E-mail: emanuele.brunesi@eucentre.it

2EUCENTRE, European Centre for Training and Research in Earth-quake Engineering, Via Ferrata 1, 27100 Pavia, Italy. E-mail: roberto.nascimbene@eucentre.it

3Hazard Team, GEM Foundation, Via Ferrata 1, 27100 Pavia, Italy.E-mail: marco.pagani@globalquakemodel.org

4M.Sc. Student, ROSE Programme, UME School, IUSS Pavia, Institutefor Advanced Study, Via Ferrata 1, 27100 Pavia, Italy. E-mail: dumitru.beilic@umeschool.it

Note. This manuscript was submitted on March 19, 2014; approved onMay 12, 2014; published online on September 12, 2014. Discussion periodopen until February 12, 2015; separate discussions must be submitted forindividual papers. This paper is part of the Journal of Performance ofConstructed Facilities, © ASCE, ISSN 0887-3828/04014137(9)/$25.00.

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The main characteristics of the shaking produced by the 2012Emilia sequence are well summarized by Luzi et al. (2013), Castroet al. (2013) and Massa et al. (2012). At least two are the distin-guishing features of this earthquake sequence as far as strongground motion is concerned. The first one is the rupture mecha-nism, since thrust earthquakes are a less frequent rupture typologyin the Italian strong-ground motion database (Pacor et al. 2011).The second distinctive characteristic of the ground shaking pro-duced by this set of events is the excitation of a vast portion ofthe Po plain, a very large sedimentary basin occupying a vast por-tion of the Northern part of Italy. This produced a spatially variablepattern of shaking (Castro et al. 2013), a general increase in theduration caused by the significant presence of surface waves (Luziet al. 2013; Cultrera et al. 2014) and—more in general—a signifi-cant role of the long period component of ground motion (Koketsuand Miyake 2008). Fig. 2 shows the official Shakemap computed interms of PGA for the main shock that occurred on May 20th (http://shakemap.rm.ingv.it/, last accessed March 2014), which clearlyspots the area exposed to the highest values of shaking. The largestvalues of shaking recorded in the investigated area (MirandolaStation—code MRN; see Figs. 2, 3 and 4) correspond to a PGAof about 0.28 g [on the horizontal component; see Luzi et al.(2013)]. As is more extensively discussed in the following sections,the N-S component of the main shock recorded at the MRN stationhas been used to analyze failure modes of steel tanks close to theepicentral area.

Past and Current Design Practice

These systems, typically designed as cylindrical thin shell elementsto support the lateral pressure demands generated when filled, werebuilt up before the development of internationally recognizedmodern seismic design Codes and Standards [EC8-4, UNI EN14015 (UNI EN 14015 2006), NZSEE (NZSEE 2009), ACI350.3 (ACI 350.3-06 2006), ACI 371.R (ACI 371R-98 1998),API 650 (API 650 2012), API 620 (API 620 2012), ASCE 7 (ASCE7-10 2010), AWWA D100 (AWWA D100-11 2011), AWWA D103(AWWA D103-09 2009), AWWA D110 (AWWA D110-04 2004),AWWA D115 (AWWA D115-06 2006), IITK-GSDMA (IITK-GSDMA 2005), COVENIN 3623, AIJ (AIJ 2010)], categorizedin Table 1. Their design was carried out in compliance with theItalian pre-code (DM 1987) and low-code (DM 1996) classifica-tion and, hence, essentially performed against static loads only(i.e., gravity, fluid, and wind). Inadequate protection against earth-quakes was provided, without any additional requirement and/ordetailing demanded in current Codes. In particular, the allowabletension method was used to size the wall thickness, and the conven-tional tie-down requirements between the base of the container andthe supporting foundation to prevent overturning and translation ofthe tank were only demanded to concepts of “good-design-practice,”available at that time. Standard welds were used to fabricate thecylinder and no flexible piping connection was introduced to allowfor potential movement during seismic events. In some cases, a

Fig. 1. Seismic hazard map representing the PGAwith 10% probability of exceedance in 50 years and location of the epicenters of the main shocksbelonging to the 2012 sequence (data from INGVand ISIDe; base map © OpenStreetMap contributors, www.openstreetmap.org)

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static horizontal force equal to a fraction of the total weight(i.e., 5%) was imposed, even though not mandatorily prescribed.Dynamic buckling mechanisms are recognized to be crucial inthe design of such structures because of the small thicknessesof their walls, but they were accounted only by the simplifiedclosed-form expressions known at that time.

Nowadays, the design process can be performed using aStandard adequate to the characteristics of the tank: type (i.e., aboveor underground rigid tanks, elevated tanks on shaft-type tower,

elevated tanks on frame-type tower, and flexible tanks), shape(i.e., cylindrical or rectangular), material (i.e., steel, reinforcedconcrete, and prestressed concrete), and support (i.e., anchored ormechanically-anchored versus unanchored or self-anchored), incompliance with the largely upgraded capabilities of FE analyses,potentially accounting for fluid-structure and soil-structure interac-tion. However, even if tanks of new construction are potentiallysafe, if properly designed and detailed, those severely damagedduring the 2012 sequence still represent the most common formof storage structure in the Italian context, and their poor perfor-mance appears a vivid example of the disastrous results shownby early design, when put to test by seismic events.

Field Observation of Failures

The prevailing types of failure encountered during the surveys areshown in Fig. 5. In particular, the most common collapse mecha-nism, observed in flat-base steel tanks, is the classical elephant’sfoot buckling [see Fig. 5(a)]. The characteristic outward bulgeabove the base of the tank results from the combination of the largecircumferential tensile stresses induced by the internal hydrostaticand hydrodynamic pressures and the axial compression due to theoverturning moment caused by the horizontal seismic loads. Theresisting mechanism, developed by the axial compressive and hooptensile stresses, cannot survive further vertical load increments,

Fig. 2. Shakemap (in PGA) computed for the main shock (May 20th) of the 2012 sequence. Dots show the position of the recording stations (datafrom ShakeMap Working Group; base map © OpenStreetMap contributors, www.openstreetmap.org)

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since the annular strips at the base yield in tension. Another well-known failure mode, typical for such flat-bottomed cylindricaltanks, is the diamond-shaped buckling of the tank wall near thebase [Fig. 5(b)]. This damage pattern, less common than the pre-vious, occurs at small hoop stress levels and, hence, is particularlysensitive to internal pressure and imperfection amplitude: thebuckling strength decreases, as the former reduces or as the latterincreases.

An even less common collapse mechanism, not yet included inmodern Codes, is provided in Fig. 5(c), where an example of the

earthquake-induced secondary diamond-shaped buckling, ob-served in the May 2012 sequence, is shown. In this case, theelastic mechanism, due to an effective inward acting pressure, isuplifted in correspondence of the circumferential welds, wherethe amplitude of the imperfections are larger and their effects—concomitantly with wall thickness reduction along the height—more visible.

Base-anchorage failures in flat-bottomed systems are collectedin Figs. 5(d), (e), and (f). In particular, these mechanisms are usu-ally associated with the elastic diamond-shaped buckling of thetank wall, as presented in Fig. 5(d). In many cases, excessive in-elastic strain demands took place in the anchor bolts, causing theirfracture or debonding from the concrete pads. Figs. 5(e) and (f)show other two anchoring system–related modes, encountered dur-ing the surveys; the former depicts the spalling of concrete, inducedby insufficient distance between the anchor bolt and the edge of thefoundation and low resistance of the concrete, while the latterpresents a flexural failure occurred in the anchor plates. Hence,these systems, poorly anchored and detailed to sustain earth-quake-induced demand, collapsed because of lack of proper steelreinforcement around the anchor and inadequate resistance of thefoundation concrete. Failures were induced by sliding and rockingof the tank, and they were observed to occur in the weakest link ofthe anchoring system.

The leg-supported tanks, usually smaller than flat-bottomed sys-tems, mostly failed in their legs, if anchored, or they were observedto slide relatively to the ground, when unanchored. An example ofthese two behaviors is shown in Figs. 5(g) and (h), respectively.More in detail, Fig. 5(g) presents a typical case of damage by com-bined shear and buckling of the tapered tank legs. This mechanismis induced by the coupling between heavy static loads and the effectof horizontal and vertical seismic excitation, resulting into promi-nent stress/strain concentrations at the base of the stocky legs,where the area and moment of inertia of the section are lower.Therefore, the leg slides, after failure of the anchorage, and thenvisibly buckles, losing its verticality; the rest of the tank doesnot exhibit any appreciable deformation and, hence, the seismicperformance of this system typology is mainly governed by the re-sponse of its legs and anchorages.

By contrast, unanchored legged systems performed consider-ably better; no damage in the tanks or falling off their foundations

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Table 1. Classification of Tanks in Accordance with Modern Seismic Codes and Standards

Document Year Nation Supporta Materialb Typec Shaped

EC8-4 2006 Europe A, U S, RC, P 1, 2, 3, 4, 5 R, CUNI EN 14015 2006 Europe A, U S 1 CNZSEE 2009 New Zealand A, U S, RC, P 1, 2, 3, 4, 5 R, CACI 350.3 2006 USA A, U RC, P 1, 2, 3 R, CACI 371.R 1998 USA A S 3 CAPI 650 2012 USA A, U S 1 CAPI 620 2012 USA A, U S 1 CASCE 7 2010 USA A, U S, RC, P 1, 2, 3, 4 R, CAWWA D100 2011 USA A, U S 1, 3, 4 CAWWA D103 2009 USA A, U S 1 CAWWA D110 2004 USA A, U P 1, 2 CAWWA D115 2006 USA A, U P 1, 2 CIITK-GSDMA 2005 India A, U S, RC, P 1, 2, 3, 4 R, CCOVENIN 3623 2000 Venezuela A, U S 1 CAIJ 2010 Japan A, U S, RC, P 1, 2, 3, 4 CaA = anchored or mechanically-anchored; U = unanchored or self-anchored.bS = steel, RC = reinforced concrete; P = prestressed concrete.c1 = above ground rigid tanks; 2 = underground rigid tanks; 3 = elevated tanks on shaft-type tower; 4 = elevated tanks on frame-type tower; 5 = flexibletanks.dC = cylindrical; R = rectangular.

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was detected, due to a combination of rocking and sliding. If thelegs were strong enough to rock and the piping system was flexibleenough to accommodate the seismic displacement demand, a sortof safe base-isolation mechanism is provided. In the majority of thecases, displacements of the order of about 10 cm were measured, asshown in Fig. 5(h) and roughly in accordance with the spectraldemand given in Fig. 4(b).

Finally, the deficient performance of the storage steel tanksaffected by the May 2012, Emilia, Italy, earthquakes is assessed,through detailed FE analyses, in order to numerically verify theprevailing seismic vulnerabilities discussed in this section.

FE Simulations of Damaged Tanks

Over the last decade, a significant number of research efforts(Virella et al. 2006; Sezen et al. 2008; Ozdemir et al. 2010;Moslemi and Kianoush 2012; González et al. 2013; Buratti andTavano 2014) have demonstrated the feasibility of detailed FEmodels to accurately simulate the seismic response of liquid storagesteel tanks. Virella et al. (2006) investigated the fundamental im-pulsive modes of vibration of cylindrical tank-liquid systems anch-ored to the foundation. Sezen et al. (2008) assessed the dynamicbehavior of above-ground, liquid-containing tanks damaged duringthe 1999 Kocaeli, Turkey, earthquake. Ozdemir et al. (2010) vali-dated their nonlinear dynamic numerical procedure in comparisonwith the results of shake table tests of past experimental programson both anchored and unanchored systems. Moslemi and Kianoush(2012) carried out a series of parametric analyses on concrete cylin-drical open top ground-supported water tanks, with the main focusof identifying the major parameters that affect the dynamicresponse of such structures, quantifying their interaction as well.Finally, González et al. (2013) reproduced the collapse mechanismof a leg-supported tank, affected by the 2010 Maule, Chile, earth-quake, while Buratti and Tavano (2014) proposed seismic fragilitycurves of an anchored steel tank by the added mass method.

In light of this scenario, FE analyses appear an attractive tool forseismic performance assessment of such structures, being able to

capture stress/strain concentrations, crucial in interpreting damagepatterns and failure modes. Therefore, a series of high-definition FEmodels has been developed to reproduce the seismic response bothof flat-bottomed and leg-supported steel tanks representative ofthose inspected. Geometrically and materially nonlinear dynamicanalyses have been prepared into a general purpose FE package,with fluid-structure interaction capabilities; LS-DYNA (2006) isadopted to play out the numerical procedure. Arbitrary LagrangianEulerian (ALE) method is used to allow for large structural andliquid deformation. In particular, penalty coupling method for shelland solid, within the CONSTRAINED_LAGRANGE_IN_SOLIDframework, is chosen to act in compression only. In the proposedFE approach, compressible Navier-Stokes equations are assumed todescribe the motion of the fluid, represented by a mesh of 8-noded,1-point integration ALE solid elements; 4-noded, 2-points integra-tion, Belytschko et al. (1984) shell elements are selected to materi-alize the wall of the tank. This shell element type represents acomputationally efficient solution, since it’s based on a combinedco-rotational and velocity-strain formulation. Bilinear elastic-plastic constitutive law (MAT_003), with a combination of iso-tropic and kinematic hardening, is used to capture the cyclicpermanent deformations, exhibited by the wall of the tank underseismic excitation. Null material is chosen to simulate the fluid that,in this paper, is conservatively assumed to be water, thus implying abulk modulus of 2.25 × 109 N=m2; in addition, the case studytanks are analyzed as completely filled, which is the normal oper-ation condition. According to different tank types and geometriesstudied, the resulting FE models have a number of nodes rangingfrom 14551 to 28288. An explicit solution strategy is adopted toperform the series of dynamic simulations, with automatic mesh-dependent integration time step of the order of 10−5. MRN_NSrecord is applied as seismic input, being the most severe groundmotion of the two horizontal components recorded in the stationclosest to the epicenter of the May 20th event.

A general nonlinear contact scheme, where master-slave pairsare explicitly recognized, permits to accurately represent rigidpavement-tank legs interaction, when analyzing unanchored leg-supported systems, potentially accounting for sliding and friction,

Fig. 5. Observed failure mechanisms: (a) elephant’s foot buckling; (b) diamond-shaped buckling; (c) secondary diamond-shaped buckling; (d) com-bined diamond buckling and failure of anchors; (e) concrete spalling at the anchorage; (f) bending of anchor plates; (g) shear-buckling of a leg;(h) sliding of an unanchored system

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in a phenomenological sense. In particular, the instantaneous fric-tional coefficient is assumed to exponentially decay with respect tothe relative velocity, in order to fictitiously describe the transitionfrom static to dynamic friction coefficient; values of 0.3 and 0.1 arerespectively chosen, as representative of a smooth contact surface.

Hence, the numerical technique proposed herein is able to ac-count for many sources of nonlinearity, such as large amplitudenonlinear sloshing of free surface liquid, yielding and bucklingmechanisms of steel tank wall, contact, separation and potentialsliding of tank base plates with respect to ground and excessivestrain demands in the anchorage system or in the underlying con-crete pads. Finally, Table 2 summarizes the main characteristics ofthe case study tanks numerically studied, in terms of wall geometry(i.e., height-to-radius (H=R) and radius-to-thickness (R=t) ratios),dimension of legs (if present), and steel grade [i.e., yielding stress(fy)]. A total of five reference structures are considered: one anch-ored leg-supported (ALS), one unanchored leg-supported (ULS),and three anchored flat-bottomed (AFB) tanks. Numerical observa-tions, in comparison with actual collapses, collected just after theearthquakes will be discussed in the following section to show thestrain concentration that caused these failures.

Results and Discussion

AFB tank T1 failed by the classical elastic-plastic elephant’s footbuckling mechanism. In Fig. 6, the effective plastic strain distribu-tions, at ultimate conditions, are shown; peak values of approxi-mately 3.5 × 10−2 have been reached during the nonlinear timehistory (NLTH) analysis just above the base of the wall, thusrevealing a good agreement with the observed damage pattern.Similar consideration can be drawn for tank T2, whose wall was

welded, by a series of gusset plate couples, to the related baseplates, which, in turn, were bolted to the underlying concrete pads,by 30-mm anchor rods. The anchorage system was modeled by acombination of equivalent nonlinear Hughes and Liu (1981) beams,using MAT_174 to simulate steel stress-strain relationships, whileconcrete was represented by nonlinear fracture mechanisms,through a smooth surface cap model (MAT_159). In addition, aCONSTRAINED_LAGRANGE_IN_SOLID algorithm was usedto capture the interaction between concrete and embedded lineelements; reinforcement is introduced as slave element and con-crete as master, using their acceleration and velocity as couplingconstraints. As presented in Fig. 7, this FE approach is effectivein accurately reproducing the severe plastic strain concentrationsthat caused concrete spalling in the anchor zones, due to lack ofproper steel reinforcement around the anchor and inadequate resis-tance of concrete. Peaks of up to 1.5 × 10−3 were high enough tofracture this low-strength concrete in tension.

Furthermore, a comparison between observed and numericallypredicted secondary diamond-shaped buckling mechanism isprovided in Fig. 8, where the principal strain distribution showsprominent concentrations in the top half of the tank wall, in cor-respondence of the circumferential fabrication welds, representedin the numerical simulation through an inward single bulgedisplacement pattern, whose amplitude equals the wall thickness(Rotter et al. 2011).

ALS tank T4 collapsed by a shear-buckling mechanism, devel-oped at the base of its tapered stocky legs, as shown in Fig. 9. Yieldstrain was exceeded at the early stage of the FE analysis, due to thelarge axial forces induced by tank overturning moment, thus indi-cating the formation of an incipient collapse. As a result, thismechanism rapidly evolved, causing huge plastic strain demands,roughly equal to 4 × 10−1, when the imposed seismic excitationreached the peak of its horizontal component. The shear-bucklingfailure zone observed on site consistently matches the strain con-centration zone, obtained from the numerical model.

As previously mentioned, unanchored systems performed muchsafer than anchored. This trend is confirmed by FE analysis, sincethis tank typology was observed to rock and slide relatively to theground, accommodating displacement demands, rather than tryingto make some of their components, not specifically designed anddetailed in accordance with seismic requirements, work for unfea-sible strain and load levels. An example of such behavior is shownfor ULS tank T5 in Fig. 10, where the relative displacement

Table 2. FE Analyses: Characteristics of the Case Study Tanks

Tank number Type H=R R=t E (MPa) fy (MPa) Leg dimension (cm)

T1 AFB 4 800 210,000 275 —T2 AFB 3 600T3 AFB 6 800T4 ALS 4 600 35 × 30 (top) and

10 × 10 (base)T5 ULS 4 600 60 × 20

(height × diameter)

Fig. 6. Numerically predicted elephant’s foot buckling (effective plastic strain distribution)

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demand, experienced during the NLTH analysis, is presented.Peaks of about 8 cm were predicted, in close agreement with thosemeasured in-situ.

Current seismic Codes recognize the resultant shear force andoverturning moment on the structure, imposed by the hydrostaticand hydrodynamic pressure at wall-liquid interface, as the mostcrucial factors for assessing the safety of tanks against shell buck-ling and uplift. Predictive expressions for impulsive and convectivepressure distributions are compared with the results of FE simula-tions. In particular, EC8, NZSEE, and AWWA approaches are con-sidered, using the input motion response spectrum and the

combination rule for impulsive and convective pressure compo-nents, specified in each corresponding Code. The comparison isshown in Fig. 11, in terms of total wall pressure, thus highlightingall the analytical approaches to underestimate FE results; the mis-match ranges from 6–10% and from 63–72% at base and top [i.e.,current position normalized by tank height (z=H) roughly equal to0.9] of the wall, respectively. Even though the small discrepancy atthe base can be easily accounted, for design purposes, the largelyunsafe estimation in the upper portion of the tank, due to the con-vective component, may potentially result into an evidently uncon-servative design, in the case of a large thickness reduction along the

Fig. 7. Failure of the anchorage system: effective plastic strain concentrations in the underlying concrete pad

Fig. 8. Numerically predicted versus observed secondary diamond-shaped buckling

Fig. 9. Shear-buckling of a leg-supported tank (effective plastic strain concentrations)

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height. Finally, this sloshing motion contribution is observed toclearly increase as the radius-to-thickness ratio increases, whileanalytical proposals, at their current stage, are not able to safelyaccount for such behavior.

Conclusions

The work described herein investigates the seismic response ofstorage steel tanks, in light of field observations collected in theaftermath of the May 2012 earthquakes in Northern Italy. Despitethe moderate ground motion intensity [according to the current of-ficial hazard map (Stucchi et al. 2011) half of the Italian Territory isexposed to values of hazard equal or higher than the value com-puted for the epicentral area], the vast majority of them sufferedexcessive damage, which led to substantial direct and indirectlosses. Their poor performance is mainly motivated by a significantdelay in the adoption and implementation of adequate seismic pro-visions, resulting in lack of design and detailing. The prevailingtypes of failure, including elephant’s foot and diamond-shapedbuckling of thin shell walls, as well as base-anchorage and tanksupport-system failures, have been collected and discussed, in com-pliance with a series of detailed FE analyses.

The fully nonlinear dynamic numerical procedure proposed inthis paper revealed its effectiveness in accurately predicting theunfeasible stress/strain concentrations demanded in the weakestlinks of the systems. In particular, ALE method is used to allowfor large structural and fluid deformations, while a nonlinear con-tact algorithm is assumed to describe the rigid pavement-tank legsinteraction, in the case of ULS systems, representing the sort of safebase-isolation mechanism—based on sliding and rocking—thatpermitted them to perform much better than ALS and AFB tanks.

Hence, the modeling and numerical outcome of this research, incompliance with field observations of failures, may be applicable todesign, assessment, and strengthening of such vulnerable struc-tures. The results shown here suggest the need for analysis and pro-posal of efficient retrofit solutions for cost-effective mitigation/prevention of further failures due to earthquake. Recommendationsfor earthquake-resistant strengthening methodologies, acting onstructural and structural components performance, are currentlythe focus of ongoing research.

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