PERFORMANCE OF WINE
STORAGE TANKS: LESSONS
FROM THE EARTHQUAKES
NEAR MARLBOROUGH
James Rosewitz1, Christopher Kahanek2
ABSTRACT: Stainless steel wine storage tanks suffered
significant and varied damage as a result of the recent
MW6.6 earthquakes near Marlborough, the largest wine
producing region in New Zealand. The wine storage
tanks are a critical production component at the
wineries, providing for the storage of each season's stock
prior to transfer and bottling. The response of the tanks
to the earthquakes, and the associated damage, varied
greatly depending on proximity to the epicentres, volume
of product within the tank, and the tank construction.
This paper discusses observations from damage
assessment of the wine tanks at 20 wineries throughout
the Marlborough region. Different types of observed
damage are discussed along with an evaluation of the
causes and recommendations for future design
improvements.
4,000 stainless steel tanks, to assess damage on behalf of
insurers. While seismic damage to buildings at the
wineries was minor or non-existent, almost half of the
wine tanks had visible deformations or other forms of
minor distress and more than 70 tanks ruptured, resulting
in loss of contents. Many of the catwalks directly
connected to the tanks also suffered damage.
As of April 2014, damage investigations continue at
several of the wineries. The intent of this paper is to
provide a timely summary of the extent, magnitude, and
types of damage to the observed wine tanks in
Marlborough. Much can be learned from the damage
caused by the recent seismic events because they were
large enough to cause deformations indicating the early
modes of failure, but small enough to avoid catastrophic
failures that complicate or prevent forensic investigation.
Conclusions in this paper are preliminary in nature,
provided as data for the on-going discourse about the
design and repair of wine tanks in seismic regions.
2 EARTHQUAKE OVERVIEWS
A series of significant earthquake events occurred in the
Cook Strait in July and August of 2013. The Cook Strait
separates the North and South Islands of New Zealand,
and the Marlborough region is located near its southwest
shore on the South Island. The sequence began on 19
July 2013 (NZT) with a MW5.5 foreshock 30 km east of
Seddon, New Zealand. This was followed by two
significant events, the Cook Strait Earthquake on 21 July
and the Lake Grasmere Earthquake on 16 August 2013.
KEYWORDS: Stainless steel liquid storage tanks, wine
tanks, Lake Grassmere Earthquake, seismic design
1 INTRODUCTION
New Zealand wine exports were valued at $1.2 Billion
(NZD) in 2012, and the industry continues to grow.
Marlborough is the largest wine producing region in
New Zealand, producing approximately 70% of the
industry’s wine by weight. Furthermore, wine production
is approximately 20% of the Marlborough region’s
economy [1]. The industry is relatively new to the
region; most of the exponential growth and associated
construction has occurred in the last two decades.
The Marlborough region experienced seismic events on
21 July 2013 (MW6.6 Cook Strait Earthquake) and 26
August 2013 (MW6.6 Lake Grassmere Earthquake).
Subsequent to the events, the authors visited over 20
wineries in the region, which collectively own more than
1
James Rosewitz, P.E. (USA), Project Engineer - Thornton
Tomasetti, Christchurch, NZ
Email: [email protected]
Figure 1: Earthquake epicentres and Marlborough region
seismograph locations (in yellow)
The Cook Strait Earthquake measured 6.6 on the
moment magnitude scale (MW) and was centred 20 km
east of Seddon. The earthquake struck at 5:09 pm NZT
on 21 July 2013 at a depth of 13 km. The Lake
Grassmere Earthquake measured MW6.6 and was centred
10 km southeast of Seddon. It occurred at 2:31 pm NZT
on 16 August 2013 at a depth of 8 km. The earthquakes
occurred on a previously unknown offshore extension of
the London Hills Fault. The epicentres of these events
are shown in Figure 1.
2
Christopher Kahanek, CPEng, IntPE(NZ), MIPENZ, Senior
Associate - Thornton Tomasetti, Christchurch, NZ Email:
[email protected]
Damage in the Marlborough region from both of these
events was limited. Notable damage occurred in the
region surrounding Seddon, including partial collapses
Figure 2: Leg Supported Tank Diagram
and land damage. Blenheim experienced less severe
ground motions and less damage. Commercial and
residential structures in Blenheim generally performed
well [2].
3 MARLBOROUGH WINERIES
The wineries in Marlborough vary in configuration, but
in general they are light industrial campuses ranging in
size from 1 to 10 hectares located on relatively level
land. They consist of a number of low-rise buildings and
rows of wine tanks commonly referred to as “tank
farms.” The buildings accommodate a variety of uses
including wine tasting rooms, restaurants, administrative
offices, equipment storage, and wine barrel storage.
Wine tanks are generally located outside, but in some
cases, tanks are inside buildings to provide better
temperature control. The tank farms are usually arranged
in parallel rows with similarly sized tanks placed
adjacent to each other. Since the tanks need to be
accessed from above, catwalks are usually installed in a
double-loaded corridor fashion to permit efficient access
to the top of every tank. Piping, usually containing
water and refrigerants, also runs throughout the tank
farms, and is often installed directly under the catwalks.
The tank farms, both indoor and outdoor, are paved with
a continuous concrete slab sloped to drain to
underground stormwater pipes.
The site plan and functional layout of a winery can
evolve over time. Wine tanks are portable structures, and
tank farms are often rearranged to meet operational
needs. Some of the large wineries have multiple
locations, and they occasionally move tanks and large
quantities of wine between sites.
3.1
Although stainless steel has been used to construct wine
storage tanks for more than half a century, tank design
procedures and construction methods can vary widely by
manufacturer and region. The basic components of a
tank are the cone, barrel (or shell), base, skirt, and
support (either plinth or leg supported). The components
are welded together with either a full penetration butt
weld or a fillet weld, depending on the tank’s
configuration. Different thicknesses of stainless steel are
used for different components.
Figure 3: Plinth Supported Tank Diagram
Tank supports in the Marlborough region typically
consist of either steel legs that support a steel framework
(Figure 2), or a concrete “plinth” that directly supports
the base of the tank (Figure 3). Plinths are used to
support tanks of all sizes. Leg supports are commonly
used for the tanks that hold 60,000 L or less. Although it
is possible, tanks larger than 60,000 L on legs were not
observed.
WINE STORAGE TANK CONSTRUCTION
Wine storage tanks in Marlborough are constructed
typically of Type 304 stainless steel sheet metal.
Stainless steel is ideal for storing wine because hygienic
and does not corrode when correctly maintained.
The smallest tanks observed hold 1,200 L, and the
largest tanks hold up to 580,000 L. They can be as tall as
20 m and up to 10 m in diameter, although 6 m is a
common maximum diameter due to road transportation
restrictions.
3.1.1 Cone
The tank cone is the top cover of the tank barrel. It is
shaped to allow for exterior drainage and precise filling
of the tank. It is usually 2.5 - 3 mm thick. Tank cones in
the region are either asymmetric with the turret offset to
one side as shown in Figure 2, or symmetric with the
turret at the centre.
3.1.2 Barrel (Shell)
The tank barrel is constructed with multiple “strakes” of
sheet metal, up to 1,500 mm wide, butt welded together
either with a continuous helical or parallel horizontal
pattern. Barrel thicknesses vary based on the tank size.
The bottom strake in a larger tank can be up to 5 mm
thick and decrease up the height of the tank to 2 mm
thick. Smaller tanks (45,000 L or less) commonly have a
uniform barrel thickness of 2 mm.
The barrels are typically surrounded by refrigerant lines.
Older tanks are surrounded by a refrigerant layer created
by a continuous, often dimpled, second skin of sheet
metal. Newer tanks are surrounded by a helix of stainless
steel parallel flange channels welded continuously to the
side of the barrel walls (Figure 2). The barrels are also
often surrounded by a layer of insulation (polystyrene)
and a “second skin” of stainless steel. These tanks can be
identified because the refrigerant lines are hidden behind
the insulation layer. However, refrigerant lines are
apparent from the tank interior.
drainage. The plinth is typically partially visible under
the tank skirt. In most cases, the plinths are not designed
by the tank manufacturers, but instead are regarded as
site work separate from the tanks.
The plinth diameter is usually 50 mm less than the
diameter of the tank to accommodate placement
tolerances. A gap of varying width exists between the
bottom of the skirt and the ground slab. Materials such as
polystyrene or asphalt are usually sandwiched between
the plinth and the tank base.
3.1.6 Legs and Frame
Leg frame structures have a variety of configurations.
The horizontal frame that supports the tank is composed
of square or rectangular hollow tube sections arranged as
spokes with thinner rectangular hollow tube sections
around the exterior perimeter. The frame supports a
polystyrene or rigid chipboard layer, upon which the
tank is placed. Cylindrical legs, typically made of
structural pipe, are welded to the frame at the centre and
ends of the spokes. Newer leg-supported tanks have a
centre leg for each spoke, but older tanks commonly
only have a single leg at the spoke intersection. The legs
have shims or telescoping bases to adjust to the sloped
floors. A series of welded tabs connect the perimeter
tube of the frame to the bottom strake of the barrel
(Figure 4).
3.1.3 Base
The typical tank base consists of rows of sheet metal laid
flat and sloping toward the drain. Base sheet metal
thickness is uniform and typically varies between 2 mm
and 3 mm based on tank size. Many bases have a
downward indention at the drain described as a “gullet”
to allow for complete removal of the contents.
The base is typically connected to the barrel wall by cold
rolling its edge upward and then either butt welding or
fillet welding it to the bottom strake of the barrel. This
important detail, known as the “knuckle,” has a constant
radius that varies based on the thickness of the base sheet
metal.
3.1.4 Skirt
Most tanks have a stainless steel “skirt” that creates a
level ring around the base. Its top edge is fillet welded to
the side of the bottom strake of the barrel. The skirt can
be used as a structural component where anchor rod
brackets are attached (Figure 3), or as a non-structural
component used to cover framing under the tank (Figure
2). When the skirt is structural, it can be up to 6 mm
thick, and is seldom thicker than 1.5 mm thick when
non-structural.
3.1.5 Plinth
Concrete plinths are used to directly support the base of
larger tanks (capacity larger than 60,000 L). They are
constructed with typically unreinforced concrete cast
directly on top of the slab-on-grade. In some cases, the
plinths are tied into the slab below. They are sloped to
accommodate the slope of the tank base to allow
Figure 4: Typical welded tab connection
Most leg support structures consist of relatively short
legs welded to a tube steel frame sloped to match the
slope of the tank base (Figure 2). Taller leg support
structures, which can be up to 1 m tall, typically have
cross-bracing between the legs.
3.1.7 Anchor Rods
Most plinth-supported tanks are connected to the slabon-grade or their foundations with anchor rods. All
observed anchors were post-installed with epoxy. In
some cases, necked anchor rods are used to induce
ductile behaviour in a controlled region. The anchor rods
are engaged by anchor chairs welded to the tank skirt.
Anchor rods are typically designed to resist uplift
(tension) forces only, but in some cases also resist
compressive forces.
3.2
CATWALKS
The catwalks are typically constructed around the tanks
or after they are in place. They are lightweight
galvanised or painted steel structures meant to support
light foot traffic. They are entirely self-supporting at
some wineries, and at others they rely partially or
entirely on the tanks for support.
The tank-supported catwalks use a variety of details to
allow for differential movement. Some horizontal plate
connections allow for sliding movement, whereas certain
tab connections allow for essentially no differential
movement. Figure 2 shows a condition where the
catwalk is connected to the sides of the tanks with tabs
that allow very little horizontal movement.
4 HISTORY OF TANK DESIGN
GUIDANCE IN NEW ZEALAND
Significant damage to storage tanks was documented in
earthquakes as early as the 1964 Alaska earthquake [3].
The seismic behaviour of tanks has been the subject of
much research in the last five decades. Tanks observed
in the Marlborough region were constructed as early as
the 1960s, and as recently as 2013. The most common
tanks were constructed between 2001 and 2013.
Definitive standards for the seismic design of wine
storage tanks in New Zealand (i.e., directly referenced by
the New Zealand Building Code) do not exist [4]. The
seismic loading standard for structures (NZS1170.5)
specifically excludes liquid storage tanks from its scope
[5]. Different guidelines have been used for tank design
in New Zealand over the last several decades, and all of
these standards were likely used for tanks in the region
[4]. A brief history of pertinent tank design guidance
documents that were likely used for design in the region
is presented below.
4.1
THEORY OF TANK SEISMIC LOADING
The basis for seismic design of liquid storage tanks is the
Housner multi-component spring/mass analogy [6]. The
Housner procedure separates the lateral forces produced
by seismic shaking into impulsive and convective forces
on the tank walls. Impulsive forces include the inertial
movement of the liquid, and it is primarily a function of
tank wall stiffness. Convective forces are a function of
fluid hydrodynamic movement (i.e., sloshing). The
natural periods for these components vary. Impulsive
periods for common wine storage tank geometries are
generally around 0.4 seconds or less, and convective
periods may be as long as several seconds. When tanks
are full and sealed, the convective mode is minimized
due to the liquid being confined, and the impulsive mode
governs response [2].
4.2
API 650 – APPENDIX E
The American Petroleum Institute (API) first published a
specification for welded steel storage tanks in July 1936
entitled API Standard 12C, Welded Oil Storage Tanks.
This specification was replaced by API 650, Welded
Steel Tanks for Oil Storage in 1961. API 650 is currently
in its Eleventh Edition. Seismic loading was introduced
to API 650 in the early 1980s with the addition of
Appendix E – Seismic Design of Storage Tanks. API650-E utilizes the Housner method with modifications by
Wozniak and Mitchell [7] that account for tank
flexibility [3]. This document was used to design
petroleum storage tanks in New Zealand; however, its
seismicity recommendations were for the United States,
and its direct application to New Zealand was limited
[6].
4.3
NZSEE “RED BOOK” (1986)
NZSEE first published Recommendations for Seismic
Design of Storage Tanks in 1986 (the “Red Book”) [8].
Since its publication, the document has been widely used
for storage tank design in New Zealand, and was likely
used in the design of 70 – 80% of the tanks in the region.
The design results from this document tend to be more
conservative than API 650-E [9].
The performance criteria for this guidelines states that a
tank should be serviceable following a design earthquake
(i.e. able to store and move contents). The tank should
also be designed with a hierarchy of failure which
protects the contents in events larger than the design
earthquake. This guideline generally does not allow
yielding of tank elements, or a reduction of design forces
for ductility [9], except for elevated tanks. Design details
are mentioned briefly only to state that they must comply
with the performance objectives, but no specific
guidance is given.
4.4
STANDARD SEISMIC RESISTANT DETAILS
FOR INDUSTRIAL TANKS AND SILOS
(1990)
The New Zealand Earthquake Research Foundation was
commissioned by the Earthquake and War Damage
Commission (EQC) to investigate and provide standard
details for seismic resistance for industrial tanks and
silos [4]. The report reviewed relevant guidelines for
tank seismic design including the Red Book and API
650. Recommendations were made for several aspects of
tank design, including appropriate ductility factors for
different anchorage types and details to resist seismic
overturning. Ductility recommendations were based on
the allowance for ductility for elevated tanks in the Red
Book and only applied to the anchorage, not the tank
itself. The details provided by this document were
commonly observed throughout the region, even in the
most recent tank designs.
4.5
NZSEE “BLUE BOOK” (2009)
The NZSEE guidelines were updated in 2009 (the “Blue
Book”) [10], however the same general performance
criteria were maintained from the Red Book. The
guidelines were updated to be consistent with the current
seismic design philosophy used by the seismic loading
standard NZS1170.5. This included implementation of
updated local seismicity, soil conditions, seismic risk
factors, and damping. The update also allows for the
limited use of ductility for design of the tank
components, as well as hold down anchors [4].
Approximately 10-15% of the tanks observed in the
region were designed using this guideline.
5 SHAKING MAGNITUDE/SEISMIC
INTENSITY AT THE INVESTIGATION
SITES
The Cook Strait and Lake Grassmere earthquakes were
both strongly felt in the Marlborough region. Measured
peak ground accelerations (PGAs) exceeded 0.2g in the
Cook Strait Earthquake, and 0.6g in the Lake Grassmere
Earthquake. Table 1 presents an estimated breakdown of
PGAs experienced at the sites that were visited. PGA
calculations at each site are based on data provided by
GNS Science (www.geonet.org.nz) and linearly
interpolated between recording stations.
Table 1: Estimated Peak Ground Accelerations at
Marlborough sites
21 July 2013
PGA (g)
Quantity
< 0.10
18
0.10 – 0.20
0
0.20 – 0.30
2
0.30 – 0.40
0
> 0.40
0
26 August 2013
PGA (g)
Quantity
< 0.10
7
0.10 – 0.20
4
0.20 – 0.30
7
0.30 – 0.40
0
> 0.40
2
Wine tanks are most often either full or empty. When
they are full, the convective modes with longer periods
typically do not participate in their response, and their
response is governed by the impulsive mode. Figure 5
shows the spectral response from different seismographs
(also obtained through GNS Science) in the Marlborough
region, with the range of expected tank impulsive mode
periods shaded in grey. The seismograph locations are
shown in Figure 1. Note that RCS1 and RCS2 were
installed after the Cook Strait Earthquake, and only
recordings from the Lake Grassmere Earthquake are
available for these two seismographs.
Two design elastic response spectra from the NZSEE
Blue Book for Blenheim (Z = 0.33, 5% damping) with
two different importance levels (IL) are also shown in
Figure 5. The solid line is the response spectrum for
wine tanks (IL1 and a 50-year design life correlating
with a return period of 1 in 100 years). The dashed line is
the response spectrum for a typical building (IL2 and a
50-year design life correlating with a return period of 1
in 500 years).
The Cook Strait Earthquake was relatively minor with
the exception of one recording station near the epicentre
that exceeded the wine tank (100-year) design spectrum.
The earthquake did not exceed the 500-year design
spectrum. This is consistent with reported damage from
this earthquake.
Figure 5: Spectral Accelerations at Various
Seismographs vs. Typical Design Spectrum
The Lake Grassmere Earthquake was more significant.
Two recording stations (RCS2 and WDFS) for the short
period range exceeded both the 100- and 500-year design
spectrums. These recording stations were closest to the
epicentre in the nearby towns of Seddon and Ward. Only
a few of the observed wineries were located near these
stations. Stations farther from the epicentre recorded
similar spectral accelerations to the Cook Strait
Earthquake. MGCS was closest to several wineries, and
was well below both design spectra in the impulsive
period range. Comparing the recorded ground motions to
the design spectra, both events were relatively minor
(well below the design spectra) for the majority of the
observed sites. The event was significant at a limited
number of sites near the Lake Grassmere Earthquake
epicentre. These results are dependent on an assumption
of similar soil types, and the results may vary with more
detailed investigations.
6 EVALUATION OF OBSERVATIONS
The authors observed wine tanks at 20 sites throughout
the Marlborough region. The first site visit occurred
between the Cook Strait and Lake Grassmere
Earthquakes. Additional site visits began within two
days of the Lake Grassmere Earthquake and continue
through the first half of 2014. At some sites, observation
was limited to a thorough walk-through to provide a
high-level estimate of damage. To date, 1900 tanks have
been individually assessed for damage. Of these, 1200
were investigated in detail at specific sites including
component measurements, exact quantification of
damage, and entering the tank for interior inspections.
Percentages discussed in the observations below are
based on the 1200 tanks observed in detail, which consist
of approximately 500 plinth-supported and 700 legsupported tanks.
Most of the observed earthquake-related damage to the
tanks can be organized into typical patterns. The most
convenient way to separate the tanks is by their support
system (i.e. plinth or leg supported). Tanks with differing
support conditions may have similar patterns of damage,
but their response and hierarchy of failure (components
that failed prior to or instead of other components) varies
significantly. The observations described in this section
are the most current understanding of the conditions.
Precise quantification of the patterns of damage (i.e.,
exact numbers of tanks that experienced different types
of damage) is in progress as of April 2014.
6.1
Shell buckling was not common at most sites, but both
types were observed at tanks closer to the epicentre. In
total, only 35 tanks experienced some form of shell
buckling. Shell buckling was observed most commonly
in the bottom strake of the tank, but in several cases
elephant foot buckling was observed in the second and
third strakes between glycol channels (Figure 7),
corresponding with a change in the steel sheet thickness.
This behaviour was limited to a specific group of
100,000 L tanks at one site.
PLINTH-SUPPORTED TANKS
6.1.1 Barrel and Cone
Cone creasing is a common form of damage caused by
suction within the tank, usually by displaced contents, or
sloshing of the tank contents (Figure 6A). In extreme
cases, tanks collapse inward as contents are rapidly
emptied after a rupture in the tank. Collapses due to
suction did not occur during the recent events; however,
cone creasing was observed in approximately 30% of the
plinth-supported tanks. Cone creasing can occur during
the tank’s normal service life when contents are emptied
without opening the turret causing internal suction.
Determining whether cone damage was caused in this
manner or during an earthquake is difficult. Cone
creasing related to earthquakes typically occurs in
conjuncture with other significant damage or in cases
when the tank was partially full and sloshing occurred.
Figure 7: Observed Elephant Foot Buckling Between
Refrigerant Channels
6.1.2 Barrel-to-base knuckle connection
Downward deformation of the knuckle connection
between the edge of the plinth and the barrel wall was a
common condition observed throughout the region,
occurring at about 55% of all plinth-supported tanks
(Figure 8). The deformation occurred when the tank
barrel was not directly supported by the ground via the
skirt, and barrel wall compression forces were
transferred to the plinth support through the knuckle. As
described previously, the plinth diameter is typically up
to 50 mm smaller than the barrel diameter, and the
overhang had high bending stresses that resulted in the
permanent deformation. When viewed from the exterior
only, the deformed knuckle creates a condition that could
be misinterpreted as stretched anchor rods or a plinth that
has settled relative to the anchor rods (Figure 9B).
Figure 6: Typical plinth-supported tank cone and barrel
damage
Damage to the tank barrels includes shell buckling,
indentations related to pounding, and deformations at
catwalk tab connections (Figure 6B). Shell buckling
includes both diamond buckling and elephant foot
buckling. Diamond buckling is membrane compression
buckling (Figure 6C), and elephant foot buckling is
elastic-plastic collapse of the tank wall (Figure 6D).
Deformation of the knuckle can change the response of
the tank in several ways during an earthquake. (1)
Rotation of the knuckle can cause the corner of the plinth
to spall, lengthening the cantilever of the base, thereby
exacerbating the condition. (2) The anchor rods meant to
resist tension forces are disengaged as the tank settles
downward causing the tank to rock, and consequently
drift further than anticipated. (3) In extreme cases, the
knuckle may deform enough to allow the skirt to rest on
the ground increasing compressive stresses in the barrel
wall. Based on our observations, this increases the
potential for shell buckling. (4) When the tank is full, the
settlement can reduce the capacity of the tank and cause
the contents to push on the cone and swell upwards.
deformation, then they often experienced less anchorage
damage because the anchors were no longer engaged.
Anchor rod failures observed included rupture (Figure
9A), concrete pull-out (Figure 9B), epoxy pull-out,
anchor rod buckling (Figure 9C), and anchor rod thread
shearing. In some instances, anchor bolts were installed
horizontally into the concrete plinth. Anchor bolts tended
to shear off when they were installed in this
configuration (Figure 9D).
Figure 9: Typical anchorage failures
6.2
Figure 8: Knuckle Deformation in a Plinth Supported
Tank
Leaks caused solely by knuckle deformation were rare in
spite of its pervasiveness. Only one site had tanks with
leaks caused by this condition, and fewer than 5% of the
tanks with knuckle deformation had leaks. No leaks
resulted from cone swelling.
6.1.3 Skirt
Damage isolated to the skirt was relatively rare for
plinth-supported tanks, but in some cases, elephant foot
buckling of the tank barrel also caused the skirt to
buckle. This occurred rarely, likely because the skirt
increased buckling resistance when acting with the barrel
wall. As a result, in many cases, elephant foot buckling
occurred in the first strake immediately above the skirt.
Skirt buckling also occurred in cases where the tank
anchorage was detailed to resist compressive forces. In
most cases where the anchorage resisted compression,
the anchor would buckle prior to the skirt. However, in
some rare cases involving retrofit anchorage on older
tanks, the anchor bolts were encased in concrete, causing
them to be significantly stiffer and resulting in failure of
the skirt.
6.1.4 Anchorage
A wide variety of anchorage configurations and failures
were observed throughout the region. Anchor rod failure
was common, occurring at approximately 40% of all
plinth-supported tanks and up to 50% at some specific
sites. If plinth-supported tanks settled due to knuckle
LEG-SUPPORTED TANKS
6.2.1 Barrel and Cone
Barrel and cone damage was rare for leg-supported tanks
in the region. Damage to the barrel was typically limited
by the capacity of the legs, which failed first. These
tanks also have a smaller height-to-diameter ratio than
plinth-supported tanks, and therefore they typically had
lower overturning demands relative to the plinthsupported tanks. Less than 1% of the leg-supported tanks
observed in the region sustained cone or barrel damage
that was not related to an adjacent tank or structure
colliding with the tank.
One exception was isolated elephant foot buckling that
occurred directly over the legs at the bottom section of
the barrel in specific groups of tanks. The observed
buckling was different from that in the plinth-supported
tanks because it was in a small concentrated region
rather than extending around a large section of the tank
circumference. These failures occurred due to a
concentration of compression force through the tank’s
barrel directly over the legs. This may have been coupled
with deformation of the tab connecting the tank barrel to
the leg framing. This mode of failure is discussed further
in Section 6.2.3.
6.2.2 Legs and Frames
Approximately 30% of leg-supported tanks had damage
to the legs or frame. Damage was more common to tanks
30,000 L and larger. Damage to the legs included
buckling of the main leg section (Figure 10A), buckling
of the telescopic lower leg section (Figure 10B), and
tilting of the entire leg (Figure 10C). The hollow tube
frame section attached to the leg often deformed inward
when the leg tilted (Figure 10D).
Damage to the tab connections is not obvious when
inspecting a tank because they are usually hidden by the
skirt. The tank skirt must be inspected for subtle
evidence in the form of a square-shaped punched section
or a small localised buckle (Figure 11D). Since the
underside of the tank base cannot be observed because it
is covered by insulation or fibreboard, the tank must be
emptied to assess the extent of base deformation and
rupture. The skirt must be removed in order to directly
view and repair the tabs.
There is an inverse correlation between the tab
connection failures and the leg failures. Tanks with
damaged legs are less likely to have tab damage. This
suggests that either the tab connection or the legs act as
an unintentional fuse that dissipates energy.
Figure 10: Typical leg-supported tank leg damage
Leg-supported tanks were both anchored and unanchored
throughout the region. Most sites were either entirely
anchored or unanchored. No correlation between the
amount of observed damage and tank anchorage was
immediately obvious. However, a site closer to the
epicentre with all tanks anchored experienced less leg
damage than other sites that had lower accelerations.
This may be due to a number of factors other than
anchorage, including tank age and dimensions.
Red wine tanks are different (and often smaller) than the
typical leg-supported tanks in the region, but they are
also higher off the ground. They have taller legs that are
typically diagonally braced by tube sections. Leg
damage was less common in red wine tanks, but in many
cases the diagonal brace buckled, and in rare cases
connections between the brace and the leg ruptured.
6.2.3 Frame-to-Barrel Tabs
Damage related to the steel tab that connects the tank
barrel and the frame occurred at approximately 40% of
observed leg-supported tanks in the region. Damage
associated with the tab includes rupture of the tab
(Figure 11A), buckling of the tab, which tends to fold
under the tank bottom, and subsequent punching of the
tab into the base of the tank (Figure 11B). Localised
elephant foot buckling, as described previously, appears
to be triggered by the tab deforming or punching the tank
at the knuckle (Figure 11C). Significant damage (large
indents in the tank base) was most common for tanks
30,000 L and larger, although less severe damage was
common in smaller tanks.
In several instances, tank bases were ruptured by the tab
pulling away due to overturning forces. Rupture was
most common when only the tip of the tab was welded to
the tank base, or when the tab was welded to the
knuckle. This mechanism was the most common cause
of leaks in the region for both earthquakes. Many leaks
were not immediately noticed because sediment at the
tank bottom plugged the rupture point. These leaks were
not discovered until after the tank had been cleaned, and
in some cases refilled with liquid.
Figure 11: Leg-supported tank tab related damage
6.2.4 Leg Anchors
Many large sites do not anchor the leg-supported tanks.
In locations where they were installed, the leg anchors
had minimal to no damage. Leg anchor failure was rare
because failure of other components such as the legs and
the tab connections limited the force transferred to the
anchors. This contrasts with the widespread anchor bolt
damage at the plinth tanks.
6.3
TRANSPLANTED TANKS
Wine storage tanks are unique because they can be
transported from one region to another relatively easily.
A number of observed tanks were imported from other
regions in New Zealand, and some were previously
affected by the 2007 Gisborne earthquake. The tanks
transplanted from other regions had relatively more
significant damage, including diamond buckling and
severe skirt deformation.
6.4
TANK FARM LAYOUT
Tank farms are arranged to use space as efficiently as
possible. The possibility of tanks and catwalks pounding
is likely given minimal consideration when designing the
site’s layout. Tank pounding was observed at only one
site, and there the damage occurred at approximately
20% of the plinth-supported tanks. Although pounding
caused dents in the tank walls and cones, it did not result
in any leaks.
Damage at the pipe-to-tank connections was reported,
but it was relatively easy to repair. Many repairs were
completed within days. Damage was minimised in most
cases by the typical flexible pipe connections used in the
region.
6.5
that allowed horizontal movement on one side with a
rigid connection similar to the ones describe above
(Figure 12C). Another example is catwalk connections
with bearing pads on both sides, allowing the catwalk to
slide on the supports as the tanks move during an
earthquake (Figure 12D).
CATWALKS
The catwalks have a wide variety of support details,
which had varied degrees of performance. Complete loss
of support was rare, only occurring at two locations.
Collapse of entire catwalks was avoided in some cases
due to inadvertent support provided by the piping and the
handrails wrapping around the tank turrets.
6.5.1 Self-supported catwalks
Self-supported catwalks generally performed very well
with no known instances of damage or collapse. In some
cases, the configuration allowed pounding to occur
between the tank turret and the catwalk. This caused
minor localised damage at most.
6.5.2 Tank-supported catwalks
Catwalks supported by adjacent tanks experienced a
significant amount of damage throughout the region. The
connections between the catwalks and tanks were simple
side-mounted vertical plates that allowed minimal
vertical and horizontal movement in many cases (Figure
12A). They were unable to accommodate the differential
movement between the two tanks they connected and
frequently ruptured (Figure 12B). This may have been
exacerbated by several factors. (1) Similar tanks may
have had different content volumes during the
earthquakes, resulting in substantially different
movement. (2) Different anchorage conditions
(especially at larger leg-supported tanks) may have
caused excessive movement of the tanks. (3) Tank
failures (e.g., knuckle deformation, tab ruptures)
changed the expected tank response during the
earthquake, causing additional tank movement.
7 DISCUSSION
The Cook Strait and Lake Grassmere Earthquakes both
caused damage to wine storage tanks in the Marlborough
region. For a limited number of sites, they were both
notable events, exceeding the typical design response
spectrum for wine storage tanks. For most of the region,
however, both events were below the design forces from
the Blue Book. Damage was generally worse at sites
closer to the epicentres; however, in some cases, damage
was more severe than expected due to specific tank
details. The following discussion interprets the available
information to date, and offers considerations to improve
tank design in the region.
7.1
THE HIERARCHY OF FAILURE AND
REPAIRS
Both the NZSEE Red and Blue Books state that a design
hierarchy of failure should be established to minimise
damage and content loss in a severe earthquake. The
guides however do not attempt to minimise the repairs
that are required due to the anticipated damage that is
established by the hierarchy of failure. Repairs to the less
critical elements of wine tanks can be costly and time
consuming. The repair for shell buckling, for instance,
requires either local or full removal of the strake. Tank
downtime can greatly impact a winery, especially if it is
near vintage when the winery expects to require its full
capacity. The hierarchy of failure in design should
account for the ease of repair to minimise potential
downtime and loss of business. This is already done in
buildings where easy to replace “fuse” elements are
installed in lateral force resisting systems. Easy to repair
wine tank components that could be used as a fuse
include the anchorage and skirt.
7.2
TANK DETAILS
The most common damage observed was not the result
of overturning or an anticipated yielding mechanism, but
instead the result of several key details. Their poor
performance resulted in deformations and ruptures that
had a disproportionate impact on the functionality and
repair of the tanks. The key details are discussed below.
Figure 12: Typical catwalk conditions and damage
Other connections observed around the region designed
to allow differential movement between the tanks
performed well. One example is catwalk connections
7.2.1 Non-Uniform Plinth Support
The typical plinth-supported tank detail with a gap
between the plinth and the skirt, as well as the skirt not
directly resting on the ground, causes a condition where
a varied length of the tank base is unsupported. The
unsupported length is typically up to 50 mm, but
unsupported lengths over 100 mm were observed in
specific cases. Compressive overturning forces must be
transferred through the knuckle into the plinth, causing
downward deformation of the unsupported portion. The
deformation tends to prevent proper drainage of the tank.
Although knuckle deformation rarely led to content
leaks, it is difficult to repair, and often requires replacing
the tank base. This deformation also modifies the tank
behaviour.
Future designs of plinth supports should include a welldefined load-path for compressive forces that avoids
unsupported sections of the tank base. This can be
accomplished in several ways. (1) The gap could be
reduced with tighter construction tolerances between the
tank and plinth. The base should be designed for the
remaining gap to resist the demands without significant
deformation, and packing could be used to prevent the
tank from shifting and increasing the gap on one side. (2)
The anchorage could be designed to resist compressive
forces, most likely by extending the anchor chair to the
ground. Care should be taken to prevent failure of the
skirt due to the concentrated compressive stresses. (3)
The skirt could be placed directly on the ground. One
method to achieve this is by relying completely on the
skirt without a plinth, similar to tanks in the dairy
industry. Alternatively, newer tanks in the region are
accomplishing this by placing the tanks on the ground
without a pre-poured plinth and pumping concrete into
the void under the tank. These tanks generally performed
well in the recent earthquakes.
7.2.2 Leg-Supported Tank Tabs
Specific guidance for design and detailing tanks
supported by frame structures is not available, and not
provided in any of the mentioned design guides. The
frame structure should, however, be considered part of
the tank, and should be designed to meet or exceed the
performance criteria of the tank. Failure of legs, the
frame, and the tab connection of the frame to the tank
have all caused significant damage and a
disproportionate amount of content loss. Damage to the
tab, however, was the most prevalent (and avoidable) in
the region. These elements should be protected by a
hierarchy of failure.
The typical leg-supported tank detail utilises a steel tab
to connect the frame to the tank. The unsupported length
of the tab between the frame and the tank impacted the
performance of the connection. When the tab was
relatively long, it tended to buckle or fracture causing
minimal damage to the tank itself. When the tab was
short and consequently stiffer, it could fold under the
tank and punch into the base. The exact location of the
weld connecting the tab to the tank also impacted
performance. Tabs performed especially poorly when
they were only welded at the end of the tab (rather than
at the top and along the sides), and when they were
welded directly to the knuckle. Tabs welded to the
knuckle also resulted in the most tank ruptures and loss
of contents in both events. Tabs welded to the side of the
tank barrel performed better, but were also prone to
buckling or rupture.
The tab connection is not inherently flawed, but care
must be taken to protect the tank from tab deformation.
Tab ruptures were common and could be avoided with
wider or longer steel plates. The weld length should be
designed to ensure the tab does not rupture the tank.
Longer tabs that cover more of the tank circumference
may also be beneficial to avoid concentrating loads. The
use of capacity design to identify a desirable failure point
and appropriate overstrength factors to protect the tab is
recommended for all alternative designs.
7.2.3 Frame Leg Connections
Legs are welded to the steel hollow tube sections that
compose the frame. In some configurations, the width of
the leg is less than the width of the hollow tube section,
and the leg is not aligned with the wall of the frame
section. As a result, during an earthquake, the leg
punches into the flange of the section (Figure 10D). This
damage was observed throughout the region. This
condition is easily avoided by either matching the
diameter of the leg to the width of the tube section, or by
reinforcing the tube wall where the leg connects with an
additional steel plate.
7.2.4 Glycol Channels
Elephant foot buckling at the second strake of certain
100,000 L tanks was observed. The elephant foot
buckling began near the horizontal strake joint, but
followed above the glycol spiral. The design guide
assumes a uniform tank wall, and does not account for
variations in stiffness that could change the tank’s
behaviour. In the case of the 100,000 L tanks, which
buckled on the second strake, the increased stiffness
from the glycol lines may have altered the tank’s
response in a manner not predicted by the design guide,
thus causing buckling to occur at a different location. We
recommend further assessment into the influence of
stiffness changes caused by ostensible non-structural
tank components (such as the glycol lines and skirts on
leg-supported tanks) to ensure they are not invalidating
guideline assumptions and altering the tank’s response.
7.3
CATWALK DETAILS
Tank-supported catwalk connections often failed when
the catwalk was rigidly connected to the tanks at both
sides. This detail does not allow for relative movement
of the tanks, which can happen for a variety of reasons.
Damage to the catwalk connections is significant
because catwalk collapse can damage the tanks, pipes,
and is a threat to life safety.
Catwalk connection details that allow some horizontal
movement performed better than rigid connections in the
region. A slip one-sided connection can appropriately
allow for movement between tanks. The amount of
movement may be unrealistic when a rocking
mechanism is anticipated. Horizontal seating pads
(similar to precast stair details) that allow the tank to
slide in both directions during an event may be more
appropriate depending on the design assumptions.
Notably, catwalks that are self-supported performed very
well in the earthquakes. Where practical, the best option
is to provide self-supported catwalks that are not subject
to the relative movement of adjacent tanks.
7.4
TANK DESIGN CRITERIA
The NZSEE Blue Book guidelines align tank design
demands with the New Zealand seismic loading
standard, NZS1170.5. Importance level (IL) determines
the return period factor (R) in NZS1170.5 for code-level
earthquake loading. Based on conversations with
manufacturers, wine tanks in the region are typically
designed to IL1 with a 50-year design life, or a 1 in 100
year return period (R = 0.5) [11]. For comparison,
typical buildings are designed to IL2 with a 50-year
design life, or a 1 in 500 year return period (R = 1.0).
Also for comparison, tanks in the dairy industry are often
designed for IL2 with a 25-year design life, or a 1 in 250
year return period (R = 0.75). Wine and dairy tanks that
support catwalks are not designed with a higher return
period factor.
The significant amount of irreparable damage to wine
tanks in relatively moderate earthquakes has led to
discussion about the appropriate earthquake return
period for design of these tanks. A higher return period
factor would increase the design forces, and result in a
more robust tank design. However, it would not prevent
the majority of significant damage that was observed
throughout the region. Knuckle deformation on plinthsupported tanks and base punching due to the tabs on
leg-supported tanks were both caused by detailing rather
than the magnitude of demand. This is exemplified by
empty tanks that exhibited minor indications of these
types of damage, although their demands would have
been substantially lower than design. Also, the newer
plinth-supported tanks with a fully supported base
(designed with R = 0.5) did not exhibit knuckle
deformation, while other nearby similar tanks had severe
knuckle damage. Careful attention to tank details and
consideration of an appropriate hierarchy of failure is
recommended over increasing the design earthquake
return period to achieve better tank performance in
future events.
7.5
RELOCATING TANKS
As discussed previously, wine tanks are unique
structures because they can be transported large
distances relatively easily. However, they are still
designed for site-specific earthquake loading. When
transporting older tanks to a new site, performance
expectations for the tanks must be adjusted to reflect
their age, imperfect condition, and the possibility that
they were designed for the lower seismic demands of
another region. Anchorage installed for these tanks after
transport should be carefully designed to account for the
capacity of the tank itself rather than the site-specific
parameters to prevent overstressing the tank
unintentionally. For new design, it may be practical to
design tanks to the highest anticipated site-specific
parameters (i.e., highest Z factor corresponding to a wine
production region in NZS1170.5) to prevent future issues
with tanks that have been transported during their lifetime. Cost, robust design, and site flexibility need to be
considered with winery owners to determine the
appropriate design parameters.
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