Paper 04

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Paper Number 04

Revised NZSEE Recommendations for Seismic Design

of Storage Tanks

D. Whittaker & D. Saunders

Beca International Ltd, New Zealand

2008 NZSEE

Conference

ABSTRACT: The 1986 NZSEE document Recommendations for Seismic Design of

Storage Tanks has been updated and will be republished during 2008. The original

publication has been widely used and acknowledged internationally. Since 1986 there

have been substantial changes to legislation and applicable standards in New Zealand and

internationally. The design basis used in the 1986 document assumed no yield or damage

being permitted to tanks under the design earthquake loading, and therefore led to some

conservatism in the design of large steel storage tanks. The 2008 document bases designseismic loads for tanks on the recently issued national Standard for loads for buildings in

New Zealand, NZS 1170.5. Design seismic loads are based on the ductility and damping

applicable to tank behaviour. Modest levels of ductility (or force reduction) are permitted

for steel tanks on grade, which generally reduces the load demands from those given

previously. Benchmarking comparisons of the revised approach against the previous

document and other relevant codes suggest that the proposed approach is reasonable.

NZSEE is aiming to have this document recognised as a code of practice in New Zealand.

The document has been structured in a way that it could also be used with other

international codes.

1 INTRODUCTIONIn 1986 the NZ Society for Earthquake Engineering (NZSEE) published a comprehensive document

entitled Recommendations for Seismic Design of Storage Tanks. The document has been used

extensively in New Zealand, and has been widely referenced internationally, including by Eurocode 8.

The document remains one of the most comprehensive guidelines available for the seismic design of

storage tanks. The NZSEE study group has recently been working to revise the 1986 document to

bring the recommended seismic design loads in to line with the latest NZ Loading Code NZS 1170.

The completed revision of the document is soon to be published.

New Zealand practice for seismic design of steel storage tanks is generally based on US codes, mainly

Appendix E of API 650. During the 1980s a modified version of API 650 adapted for use in New

Zealand (SDPP) was used widely for seismic design of pertrochemical plants. In 1986 the NZSEERecommendations for Seismic Design of Storage Tanks was published.

Since the early 1990s various methods including API, SDPP and the NZSEE have been used for

design of steel storage tanks, and these methods gave significantly different requirements.

New Zealand Standard NZS 3106:1986 covers design of concrete water retaining structures including

seismic effects. The seismic design coefficients specified in that Standard are now out of date.

Recently issued legislative requirements for storage of hazardous products in New Zealand (HSNO)

permit the use of API 650 or NZS 1170.5 for seismic design. The revised NZSEE document is also

intended to be recognised by the HSNO regulations as an approved code of practice.

2

BASIS FOR SEISMIC DESIGN OF TANKS

Procedures for the seismic analysis and design of storage tanks are generally based on the Housner

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multi-component spring/mass analogy. The analogy allows the complex dynamic behaviour of a tank

and its contents to be considered as simple modes of response including a short period impulsive

mode, with a period of around 0.5 seconds or less, and a longer period convective (sloshing) mode

with periods up to several seconds. For most tanks the impulsive mode dominates the loading on the

tank wall and the convective mode dominates the wave height and required freeboard. The mechanical

spring/mass analogy for the assumed modes of vibration is shown in Figure 1.

Figure 1. Mechanical spring/mass analogy for impulsive and convective modes of vibration.

Damping levels for tanks have generally been assumed to be of the order of 2-5% for the impulsive

modes and 0.5% for the convective modes. The additional effects of radiation damping (i.e. energy

lost into the foundation) in reducing earthquake response can be considerable, providing equivalentviscous damping levels of as much as 20-30%.

3 1986 NZSEE PUBLICATIONThe 1986 NZSEE recommendations provided a detailed approach to seismic analysis and design for a

range of types of storage tanks. Practical design examples were included and detailed methods for

analysing the seismic behaviour of anchored or unanchored tanks were included.

The seismic loadings included in the document were based on early seismic hazard assessments for

New Zealand available at that time, however these are now considered to be well out of date. The

design method for steel tanks is an ultimate limit state approach, but it assumed no yielding or ductility

of tank elements is permitted.

Experience with use of this document in designing steel storage tanks has indicated that the

requirements were often significantly more conservative compared with design to the API 650

approach.

4 RECENT NEW ZEALAND SEISMIC DESIGN STANDARDS NZS 4203 AND NZS 1170Since publication of the 1986 document there have been two major revisions to the loading standard

for buildings NZS 4203. In 1992 seismic loads in NZS 4203 were revised based on hazardbased

seismicity maps of New Zealand. Since 2004 a joint Australian and New Zealand Standard AS/NZS

1170 Design Loads for Buildings, has been introduced and is a means of compliance with the

performance-based NZ Building Code. The Standard now supersedes NZS 4203. NZS 1170.5 is thesection of the Standard that deals with seismic loads.

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The design spectra in NZS 1170 are based on a comprehensive seismic hazard analysis using New

Zealand seismicity models and earthquake shaking attenuation criteria. NZS 1170 is a limit-state code

with the ultimate limit state based on strength design and serviceability limit state dealing with

damage and displacement control. Although NZS 1170 is not directly applicable to tanks, it has been

used as the basis for setting seismic loads for design of storage tanks, with a correction factor being

applied to better account for ductility and damping levels in tanks.

5 API 650API 650 Appendix E has been widely used around the world for seismic design of steel storage tanks.

It covers the United States and a wide range of other locations around the world. A simplified analysis

of seismic effects including impulsive and convective modes and a working stress approach is used in

the code for checking stress limits in the tank shell.

Until recently the design loads for the most seismic region of the US were based on an impulsive

mode acceleration of up to a maximum of 0.24 g in the most severe seismic zone in US, together with

a simple overturning stability limit for unanchored tanks.

The 2006 edition of API 650 Appendix E is significantly changed, and is based on the following:

Seismic hazard maps and design spectra for the whole of the US (short period spectralcoefficients of up to 2.0 g are specified for earthquake shaking with a return period of 2,500

years).

The methodology allows use for locations outside the US, based on seismic coefficientsobtained probabilistic design spectra or peak ground acceleration values.

Force reduction factors are used to determine design seismic loads (e.g. 3.5 for unanchoredtanks).

Stress limits are working stresses with maximum stresses of up to 0.9 times yield stress.

An overturning stability limit is given for unanchored tanks.Care is required in applying NZS 1170.5 loadings to API 650.

6 NZSEE 2008 REVISED METHODOLOGYThe NZSEE 2008 method includes the following principal changes to the 1986 document:

Seismic load for tanks is in accordance with NZS 1170.5:2004, the national loading standardfor buildings, including probabilistic hazard based seismic design spectra.

Adjustments are made for the effects of damping levels and ductility levels appropriate fortanks.

Some limited ductility is permitted and is reflected in the force reduction of design loads.The ultimate limit state design methodology is in alignment with other NZ structural design standards

and can also be interfaced with other international codes and design guides.

The horizontal seismic force acting on a tank, associated with a particular mode of response, is

calculated from the expression in Equations 1 to 3.

gmiTiCdVi )(= (1)

where ( ) ( ) ( ) pifiid SkTCTC ,= (2)

( ) ( ) ( )DTNRZTCTC iuihi ,= (3)

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and Vi = horizontal base shear for mode i (impulsive, convective etc).

mi = equivalent mass of tank and contents responding in mode considered.

Cd(Ti) = horizontal design action coefficient for mode i.

C(Ti) = ordinate of the elastic site hazard spectrum for horizontal loading for the site

subsoil type, and the relevant mode. Obtained from NZS 1170.5 for tankimportance level.

kf(,i) = correction factor for NZS 1170.5 elastic site hazard spectrum to account for

ductility and level of damping, refer Table 2.

Sp = structural performance factor, to be taken as 1.0.

Ch (Ti) = spectral shape factor for site subsoil type and relevant mode, from NZS 1170.5.

Z = seismic zone hazard factor, from NZS 1170.5.

Ru = return period factor for the ultimate limit state, from AS/NZS 1170.5 for tank

importance level.

N(Ti,D) = near fault factor, from NZS 1170.5.

= displacement ductility factor for horizontal impulsive modes. Taken as 1.0 for the

convective and vertical modes.

Ti = period of vibration of appropriate mode of response

i = damping level appropriate to mode of response.

This approach replaces the design loading section of the 1986 NZSEE document with loads from the

NZS 1170:2004 design spectra, with an additional correction factor for ductility and damping

appropriate for tanks.

Displacement ductility factors of either 1.25 or 2.0 are permitted for seismic design of tanks, as shown

in Table 1. Although not strictly ductile behaviour, certain modes of inelastic behaviour, such as baseuplift and elephants foot buckling of steel tanks, are assumed to be an equivalent ductile response. The

use of ductility is a departure from the 1986 Red Book approach and results in lower design loads.

Table 1. Displacement ductility factors.

Ductility Factor = 1.25 Ductility Factor = 2.0

Steel tank designed to remain elastic Steel tank unanchored and designed for uplift

Steel tank elastic (diamond) buckling mode Steel tank with ductile holding down bolts

Steel tank with non-ductile holding-down bolts Steel tank with yielding skirt pedestal

Timber tanks Concrete tanks on gradeOther non-ductile materials Other tanks from ductile materials

An example chart giving recommended levels of damping for the impulsive modes of steel tanks is

given in Figure 2. The figure gives the total damping, made up of the tank-liquid system fixed-based

damping plus the foundation radiation damping. Damping for the convective (sloshing) mode is

assumed to be 0.5%.

The NZS 1170.5 design acceleration spectrum coefficient for an elastic 5% damped system is

modified by a spectrum correction factor kf(,i),shown in Figure 3, to account for assumed ductility

and equivalent viscous damping levels. These acceleration spectrum coefficients then generally have a

similar basis as those in NZS 1170.5:2004.

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Steel Tanks, t / R = 0.002

0

5

10

15

20

25

30

35

200 250 300 350 400 450 500 550 600

Shear Wave Velocity, m/s

DampingFactor,%

H/R = 3.0

0.5

2.0

0.3

1.5

1.0

Figure 2. Example damping for horizontal impulsive mode. Steel tanks t/R = 0.002.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0% 5% 10% 15% 20% 25% 30%

Damping

CorrectionFactork

ductility = 1.0

ductility = 1.25

ductility = 2.0

Figure 3. Spectrum Correction Factors kf(,).

The seismic load derivation is based on the same approach used in NZS 1170 using Importance Levels

to reflect the significance of the facility and appropriate Return Period Factors. The Importance Level

is derived from considering the consequences of failure, based on separate consideration of life safety,

environmental risk, community significance and adjacent property value. Recommended

classifications of risk for each of these aspects can be found in the document. The Importance Level

used in determining the design load is then based on the worst of these considerations. ProposedImportance Levels and Return Period Factors are given in Table 2.

Table 2. Importance Level and Return Period Factors to be used in application of AS/NZS 1170.

Consequence of

Failure

Importance Level Design Return Period Return Period Factor

Slight 1 100 0.5

Moderate 2 500 1.0

Serious 3 1000 1.3

Extreme 4 2500 1.8

NZS 1170 incorporates a Structural Performance Factor Sp in the derivation of design seismic loads

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for buildings, with values of between 0.7 and 1.0. The use of Sp = 1.0 is specified for tanks in

recognition of their limited ductility capability.

7 COMPARITIVE EXAMPLESThe seismic loads specified by the NZSEE 2008 method are compared with the requirements of the1986 NZSEE and API 650 (2003 and 2006) methods for several example steel tanks. Comparison is

also made with seismic loads required by the existing water retaining structures code NZS 3106:1986.

Details of the example tanks are summarised in Table 3.

Table 3. Details of example tanks for comparison.

Case Location Tank

Material

Product

Stored

Anchorage Design

Return

Period

(yrs)

Ground

Type

1 Wellington Steel Diesel Unanchored 500 Shallow soil

2 Wellington Steel Petrol Unanchored 2,500 Deep soil

3 Auckland Steel Diesel Unanchored 500 Shallow soil

4 Auckland Steel Petrol Unanchored 2500 Deep soil

5 Wellington Steel Diesel Anchored 500 Deep soil

6 Wellington Steel Petrol Anchored 2500 Deep soil

7 Wellington Concrete Water Unanchored 1000 Rock

8 Auckland Concrete Water Unanchored 1000 Rock

Figure 4 shows the impulsive mode base shear coefficients required by each design method for the

example tanks. The seismic design of tanks is generally governed by the impulsive mode base shear,

so comparison of the impulsive coefficients given by the various methods gives a good indication of

the overall seismic loads.

The API 650 methods are based on working stresses, so the coefficients shown in the figure include

scaling factors (2.0 for the 2003 method and 1.1 for the 2006 method) for the purposes of comparing it

with the NZSEE ultimate limit state approach. For reference, Wellington has similar seismicity levels

to the most active areas in California.

The 2008 NZSEE method generally gives lower impulsive mode base shear coefficients than the 1986

method. The method gives seismic coefficients and wall thickness of a similar order to the API 650

method, although greater in some cases and less in others.

The 2006 revision of API 650 Appendix E alters the seismic loads for many tanks compared to theprevious 2003 document. The geometry limits of tanks that are required to be anchored has been

tightened resulting in a greater number of tanks than before. The NZSEE 2008 document allows more

tanks to remain unanchored than API 650 permits.

Based on these comparisons the NZSEE 2008 method appears reasonable in relation to the other

methods available.

8 SUMMARYThe 2008 NZSEE publication entitled Seismic Design of Storage Tanks is summarised. The document

updates the original 1986 document to be consistent with the current New Zealand structural loading

and materials standards. Several example tanks have been considered to compare the seismic designloads required against other available methods.

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ACKNOWLEDGEMENTS

The authors wish to acknowledge the financial assistance of the following industry sponsor

companies: New Zealand Earthquake Engineering Society, Mobil Oil New Zealand Ltd, Shell Oil

New Zealand Ltd, Caltex Oil New Zealand Ltd, BP Oil New Zealand Ltd, NZ Refining Ltd., Shell

Todd Oil Services Ltd, New Zealand Insurance Ltd.

The NZSEE members that prepared the 2008 document were: David Whittaker (Convenor), Dean

Saunders, Rob Jury, John Wood, Barry Davidson, Graeme McVerry and John Mason.

Case 1

type: unanchored steel petrol tank

location: Wellington

risk: Moderate

0.47

0.74

0.48

0.36

0.0

0.5

1.0

1.5

2.0

NZSEE

2008

NZNSEE

1986

API 650

2003

API 650

2006 &

NZS1170

ImpulsiveCoefficient(g)

Case 2



risk: Extreme

1.08

1.76

0.600.69

0.0

0.5

1.0

1.5

2.0

NZSEE

2008

NZNSEE

1986

API 650

2003

API 650

2006 &

NZS1170


Case 3


location: Auckland

risk: Moderate

0.15

0.40

0.24

0.12

0.0

0.5

1.0

1.5

2.0

NZSEE

2008

NZNSEE

1986

API 650

2003

API 650

2006 &

NZS1170


Case 4


location: Auckland

risk: Extreme

0.35

0.97

0.300.22

0.0

0.5

1.0

1.5

2.0

NZSEE

2008

NZNSEE

1986

API 650

2003

API 650

2006 &

NZS1170


Case 5

type: anchored steel petrol tank


risk: Moderate

0.62

1.40

0.480.40

0.0

1.0

2.0

3.0

4.0

NZSEE

2008

NZNSEE

1986

API 650

2003

API 650

2006 &

NZS1170


Case 6

type: anchored steel petrol tank


risk: Extreme

1.12

3.36

0.60 0.60

0.0

1.0

2.0

3.0

4.0

NZSEE

2008

NZNSEE

1986

API 650

2003

API 650

2006 &

NZS1170


Case 7

type: prestressed concrete water reservoir


risk: Intended Facility (Serious)

0.66

1.731.95

0.0

0.5

1.0

1.5

2.0

2.5

3.0

NZSEE 2008 NZNSEE 1986 NZS 3106


Case 8

type: prestressed concrete water reservoir

location: Auckland

risk: Intended Facility (Serious)

0.230.35

0.86

0.0

0.5

1.0

1.5

2.0

2.5

3.0

NZSEE 2008 NZNSEE 1986 NZS 3106


Figure 4. Comparison of design impulsive mode bas shear coefficients for example tanks to various designstandards.

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REFERENCES:

American Petroleum Institute. 2003.API 650 Welded Steel Tanks for Oil Storage.

American Petroleum Institute. 2006.API 650 Welded Steel Tanks for Oil Storage.

Environmental Risk Management Authority, Hazardous Substances (Dangerous Goods and Scheduled Toxic

Substances) Transfer Notice, (Amended) 08 August 2006.Ministry of Works and Development, 1981.Recommendations for Seismic Design of Petrochemcial Plants.

New Zealand National Society for Earthquake Engineering, 1986. Recommendations for Seismic Design ofStorage Tanks.

New Zealand Society for Earthquake Engineering. 2008. Seismic Design of Storage Tanks (In Preparation).

Standards New Zealand. NZS 1170.5, 2004, Structural Design Actions, Part 5 Earthquake Actions.

Standards New Zealand. NZS 3106, 1986, Code of Practice for Concrete Structures for the Storage of Liquids.

Whittaker, D. and Jury, R.D. 2000. Seismic Design Loads for Storage Tanks. Proceedings 12th WorldConference on Earthquake Engineering, Paper Number 2376.

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