c lessonslearned fischer mar151

7/21/2019 C LessonsLearned Fischer Mar151

1/5March 201510

problems and solutionsencountered by practicingstructural engineers

LESSONSLEARNED

Erica C. Fischer, P.E., iscurrently a doctoral candidate

in Civil Engineering at PurdueUniversity investigating therobustness of simple connectionsin fire. Erica can be reached [email protected].

Dr. Judy Liu is an AssociateProfessor in structural engineeringat Purdue Universitys Lyles Schoolof Civil Engineering. Dr. Liu canbe reached [email protected].

Dr. Amit H. Varma is a Professorand University Faculty Scholar in

Purdue Universitys Lyles Schoolof Civil Engineering. He is theChair of the SEI/ACI CompositeConstruction Committee andmember of the AISC Committeeof Specifications. Dr Varmacan be reached [email protected].

By Erica C. Fischer, P.E.,Judy Liu, Ph.D. andAmit H. Varma, Ph.D.

Earthquake Damage toCylindrical Steel Tanks

On August 24, 2014, a magnitude 6.0earthquake occurred northwest ofAmerican Canyon, California. Teearthquake was located between

two faults: the West Napa

Fault and the Carneros-Franklin Fault near thenorth shore of the SanPablo Bay. Structuraldamage was most severein the downtown Napa

region, where a number of unreinforced masonry(URM) buildings were located. Damage to resi-dential building construction was also observedsurrounding the downtown region, and becameless severe farther away from town. Damage tovineyards and wine storage facilities was focusedmainly on damage to stainless steel storage and

fermentation tanks, and damage to the winestorage barrels due to racks collapsing.Tis article focuses on the cylindrical steel tank

damage observed at wineries after the NapaValley earthquake. Te tank damage discussedin this article was not unique to the Napa ValleyEarthquake. Tis type of damage has been docu-mented after previous earthquakes around the

world. Large-scale testing and numerical modelshave been developed to demonstrate the behav-ior of cylindrical steel fluid-filled tanks duringearthquakes. Although damage to cylindricalsteel tanks from earthquakes has been well doc-

umented, and research has demonstrated betteranchorage systems may improve the seismic per-formance, it seems the design and constructionof these tanks used in the wine industry has notadvanced with these known improvements.Discussions with selected wineries in Napa after

the earthquake demonstrated that the perfor-mance objectives of the steel tanks in the wineindustry differs from those in other industriesthat use cylindrical fluid-filled steel tanks (water,oil, chemical). Wineries experienced buckling ofthe tank walls, anchorage failures, and racking ofthe tanks against one another. However, this type

of damage was not unique to the Napa ValleyEarthquake. Tis damage has been exhibited in

previous earthquakes in California and aroundthe world: the 1977 San Juan earthquake,1980 Greenville-Mt. Diablo earthquake, 1984Morgan-Hill earthquake, 1989 Loma Prietaearthquake, 2003 San-Simeon earthquake, 2010Maule earthquake, and the 2013 Marlboroughearthquake. Documentation of damage aftereach of these earthquakes demonstrates thatbuckling of steel tank walls and anchorage failureoccurred in tanks that were full with fluid andanchored to the ground.Te February 2010 Maule Earthquake affected

a region in which 70% of the wine production

of Chile takes place. Te ground motion mea-sured during the earthquake was about 0.35g,and damage fell mostly in three categories: (1)damage to steel fermentation tanks, (2) wine stor-age barrels falling off their racks, and (3) spilledunprocessed wine.Stainless steel tanks can be either leg supported,

or continuously supported with a flat base.Damage was observed to both of tank-types. Telegged supported stainless steel (LSSS) tanks areused often to ferment and store small volumes ofhigh quality wine. Tese tanks are usually between350 to 1765 cubic feet (10 to 50m3) in capac-

ity. Te damage to the LSSS tanks as a result ofthe Maule Earthquake included buckling of thesupporting legs caused by axial resultant forcesfrom the overturning moment, and movementof the tank resulting in the tank falling off of thesupporting concrete base when the LSSS tanks

were not anchored to the concrete base.Buckling of LSSS tanks was not observed after

the Napa Valley Earthquake; however, move-ment of unanchored LSSS tanks was documented.During the Maule Earthquake, LSSS tanks thathad enough room to move and were unanchoredperformed better than the anchored legged tanks.

Figure 1. (a) Movement of tanks, (b) Close upmeasurement of movement.

(a)

(b)


2/5STRUCTURE magazine March 201511

However, when the tank moved, there wasdamage to the piping systems. Movementof the tanks was observed up to 8 inches(20cm), which is consistent with the move-ment of the tanks observed during theNapa Valley Earthquake and shown inFigures 1a &b.Tose tanks that are continuously sup-

ported with a flat bottom are typically

larger storage and fermentation tanks,and are called flat bottom tanks. Te flatbottom tanks are also anchored to con-crete slabs. Damage to these tanks observedafter the 2010 Maule Earthquake includedanchorage failure and buckling of the stain-less steel tank wall. Anchorage failures werecaused by insufficient edge distance, insuf-ficient number of anchors, corrosion of theanchors, insufficient effective anchoragelength, inadequate resistance of the con-crete foundation surrounding the anchor,and lack of proper steel reinforcement sur-

rounding the anchor. Anchorage failuretypically occurred in conjunction with thediamond buckling shape failure of the steeltank walls. Te steel tank walls buckledin two ways: (1) diamond shape buck-ling, and (2) elephant foot failure. Tediamond shape buckling failure was morecommon in tall, slender tanks, whereas theelephant foot failure could be observedin the squat tanks that were full of liquid.Te damage that was observed during theNapa Valley Earthquake was mainly thediamond shape buckling failure of thestainless steel wall in conjunction withanchorage failure of the tanks to the con-crete base and is shown in Figure 2.Another common failure that wasobserved after the 2010 Maule Earthquakewas failure at the connection of the pipingto the tanks. Tis type of failure occurredwhen the tank shifted or rocked duringthe earthquake, and because of the rack-ing of the piping system against the wallof the tanks during the earthquake. Bothconditions were observed after the NapaValley Earthquake as well. Buckling of tank

Figure 2. Damage to fermentation tanks. Examples of buckling of steel tank wall at base.

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walls at the top courses occurred during theMaule earthquake due to the suction effect

when there was rapid loss of liquid inside ofthe tank. Tis did not occur in tanks withouta roof, as no suction effect occurred. Tistype of observed damage could have beenprevented if a relief valve had been present.Te 1977 San Juan earthquake was a mag-

nitude 7.4 earthquake located 50 miles

(80km) north-east of downtown San Juan.Te observed tank damage included bucklingof steel tank walls and anchorage failure at theconnection of the tanks to the concrete base.All of the tanks examined demonstrated ele-phant foot tank wall bulging. Tis damagewas observed at the first course from thebottom of the tank, or just above the jointfrom the first to the second course of the tank.

Anchorage failure was observed in all of thetanks as well. Rehabilitation efforts occurredafter the earthquake for these anchoragefailures, and this included strengthening of

the existing anchorage system in addition toreducing the amount of liquid in each tank.Four of the tanks having severe tank wallbuckling fully collapsed during to the earth-quake. In addition to the tank shell bulging,some of the tanks exhibited weld rupture atthe joint of the bottom course with the angu-lar plate used as part of the anchorage system.Tis caused loss of liquid inside of the tank.Te tank damage observed during the 1977

San Juan earthquake is the same as the damagedocumented through the 2014 Napa Valleyearthquake, as well as the 2010 Maule earth-

quake. Anchorage failure is a common typeof damage observed in all of the previousearthquakes where reconnaissance teamsexamined the tanks. Tis type of failure wasalso documented by the news after the 2013Marlborough earthquake in New Zealand.

Te tanks are bolted to the slabs with earth-quake bolts and the bolts did what they weredesigned to do. Tey stretched, and in somecases broke, but thats what they are designto do they kept the tanks upright.

Te National Business Review NZWinegrowers chief executive Philip Gregan

Te anchor bolts are not meant to dissi-pate the energy from the earthquake, butrather prevent the tank from rocking off thefoundation. Te above quote demonstratesthat the anchor bolts served their purposeduring the earthquake, but with unintendeddamage. Previous research and developedanalytical procedures would allow engineersto design tanks to prevent this behavior.

Anchorage failures occurred after the 2014Napa Valley earthquake, mainly in tanksthat were full. Figure 3demonstrates some

examples of anchorage failures observedafter the Napa Valley earthquake. Figure 3ashows a corroded anchor, Figure 3bshows ananchor that failed due to insufficient anchor-age length and edge distance.In addition to movement of legged tanks,

buckling of the steel tank wall, and anchorage

failures after the 2014 Napa Valley earth-quake, there was also damage to the top ofthe steel tanks. Tis was not due to the suc-tion effect as seen during the 2010 Mauleearthquake, rather due to the pounding ofcatwalk systems against the tank walls. Figure4demonstrates an example of this pounding.Te catwalk in this portion of the warehousehad been removed because it was damaged;however, the denting of the tank at the topis seen.Te base shear and overturning moment

have two components: convective, and impul-

sive. When the liquid inside of the tank movesin unison with the tank, the resulting stressesare the impulsive component. Te sloshingof liquid inside of the tank against the tank

walls causes the convective component. Teimpulsive component controls during theshorter periods, whereas the convective com-ponent controls during the long periods ofseismic excitation. For a majority of tanks(0.3 < H/r< 3, where His the liquid height

within the tank and ris the tank radius), thefirst convective and first impulsive modes ofvibration generally account for 85 to 98%of the total liquid mass in the tank. For talltanks (H/r> 1), the remaining liquid massvibrates at higher impulsive periods, and forsquat tanks (H/r1) the remaining liquid massvibrates at higher convective periods.Te first simplified models developed took

both the impulsive and convective dynamiccontributions into account. However, the firstmodels developed assumed only horizontalground movement. Tis research determinedthere is a portion of the liquid in the tank thatmoves in a long period, while the remainder ofthe liquid moves rigidly with the tank walls.

Te liquid that moves with the tank wall(impulsive) moves with the same accelera-tion as the ground during an earthquake. Teconvective (translatory, sloshing) behavior is aresult of the impulsive pressures. Te impulseperiod is the major contributor to the baseshear and overturning moment of the tank

during an earthquake. However, these firstmodels assumed the tank walls were rigid anddid not deform during their own motion. Tisassumption caused unconservative base shearand overturning moment predictions usingthese simplified models.Simplified models produced based upon the

work performed by Housner and Jacobsendemonstrated the tank walls will deform andcause the impulse motion to be larger thanoriginally determined. Te flexibility of thetank walls can cause the impulsive motionto be greater than the ground acceleration.

Tese models have shown that the liquidinside the tank and the flexibility of the tankwalls can amplify the base shear and over-turning moments during an earthquake. Inaddition, the models have demonstrated thatrigid or flexible foundations can significantlyaffect the dynamic response of these tanks.Tese models have shown the maximumallowable compressive stresses reported incodes should be reevaluated to take intoaccount vertical compressive forces and thecombination of vertical compressive stress,hoop stress, and bending stress to preventyielding of the tank walls.Tese models were compared with experi-

ments performed on cylindrical fluid-filledtanks. Tese tests highlighted the need forfurther investigation into anchorage designfor anchored tanks during an earthquake, andthicker tank walls. Te damage to the tanksobserved during the experiments was consistent

with damage viewed in previous earthquakes.After the 2010 Maule Earthquake, the simpli-fied method presented by Malhotra et al. [16]

was used to determine the allowable stress forthe damage steel tank walls [11]. Te tanks that

Figure 3. Anchorage failure of fermentation tanks (a) corroded anchor used to attach flat bottom tank toconcrete base, (b) anchor failure at edge of concrete base support.

(a) (b)



were examined after the earthquake were usedas examples, and calculations were performedto understand if allowable stress using a sim-plified model would have predicted failure inthe tank walls. Tese results demonstrated thatthose tanks that exhibited tank wall bucklingduring the 2010 Maule Earthquake exceededthe allowable stress in the tank wall using theMalhotra et al. simplified model.

Analytical modeling on base isolationsystems has progressed for the applicationto LNG tanks. Tese systems significantlyreduce the seismic base shear and overturn-ing moment of the tanks by 60 to 80% FEMmodeling was compared with simplifiedmethods with good agreement for prelimi-nary design. Tese research projects highlightthe need for similar projects for liquid-filledcylindrical tanks for the wine industry. LNGtanks are double-walled tanks with the out-side wall typically post-tensioned concrete,as compared to the single-walled steel tanks

used in the wine industry.While the simplified methods of analysis pro-vide tools for engineers to evaluate the baseshear and overturning moments of fluid-filledcylindrical steel tanks during seismic events,these methods of analysis are for the elasticresponse analysis. Many vineyards are locatedin regions of strong ground motion (i.e. Chile,

California, New Zealand). Te forces obtainedfrom the elastic response analysis are very largeand reduced by factors up to 3 to obtain design

forces for the tanks. Previous earthquakes andexperimental research has demonstrated thatfluid-filled cylindrical steel tanks will respond

with non-linear behavior and sustain damageduring a strong seismic event. However, therecurrently are no methods of analysis for non-linear behavior of these tanks. Terefore it isvery difficult to predict and quantify the damage

that will be sustained by these tanks during anearthquake with strong ground shaking. Teengineering community has demonstrated

the benefits of implementing relevant researchresults when applicable to LNG tanks andpetroleum filled tanks. Tere is a great needfor practical non-linear analysis methods forthe design of fluid-filled cylindrical steel tanks.Tis research needs to conform to the expectedperformance objectives of the vineyards.

Figure 4. Denting of steel tank due to pounding of tank against catwalk.

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A. Niwa and R. W. Clough, Buckling of cylindrical liquid-stor-age tanks under earthquake loading, Earthquake Engineering andStructural Dynamics, vol. 10, pp. 107-122, 1982.

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Risk Magement Solutions (RMS), 2003 San Simeon, California,Earthquake, RMS, 2003.

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78/04), Earthquake Engineering Research Center, University ofCalifornia Berkeley, Berkeley, 1978.

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