Seismic Design for Racks - v 1 01 Oct 2010

8/10/2019 Seismic Design for Racks - v 1 01 Oct 2010

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FEM RACKING AND SHELVING PRODUCT GROUP(European Racking Federation)

FEM 10.2.08

RECOMMENDATIONS FOR THEDESIGN OF STATIC

STEEL PALLET RACKS

IN SEISMIC CONDITIONS

October 2010 - Version 1.01


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FEM 10.2.08RECOMMENDATIONS FOR THE DESIGN OF STATIC STEEL PALLET RACKS IN SEISMIC CONDITIONS

October 2010 Version 1.01

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CONTENTS

1 GENERAL ............................................................................................................. 5

1.1 Introduction ................................................................................................................ 5 1.2 Scope ........................................................................................................................ 7 1.3 Applicability ............................................................................................................... 7 1.4 Normative references ................................................................................................ 7 1.5 Symbols .................................................................................................................... 8 1.6 Terms and Definitions .............................................................................................. 10

2 METHODS OF SEISMIC ANALYSIS .................................................................. 11 2.1 Fundamental requirements and compliance criteria ................................................. 11 2.2 Description of the seismic action ............................................................................. 12

2.2.1 General ............................................................................................................................ 12 2.2.2 Definition of the intensity of the seismic action ................................................................ 12

2.2.3

Earthquake design return period and importance factor I .............................................. 13

2.2.4 Horizontal design spectrum for elastic analysis ............................................................... 16 2.2.5 Vertical component of the seismic action ........................................................................ 18 2.2.6 Structural regularity criteria .............................................................................................. 19 2.2.7 Design ground displacement ........................................................................................... 19

2.3 Design parameters for seismic analysis ................................................................... 20 2.3.1 General ............................................................................................................................ 20 2.3.2 Design spectrum modification factors .............................................................................. 20 2.3.3 Pallet-beam friction coefficients ....................................................................................... 21 2.3.4 Design seismic pallet weight ............................................................................................ 22 2.3.5 Pallet weight modification factor ...................................................................................... 22 2.3.6 Other seismic weights ...................................................................................................... 23 2.3.7 Position of the centre of gravity of the pallet .................................................................... 23

2.3.8 Placement eccentricity ..................................................................................................... 24 2.4 Methods of analysis ................................................................................................. 25 2.4.1 Lateral force method of analysis (LFMA) ......................................................................... 27

2.4.1.1 General ........................................................................................................................ 27 2.4.1.2 Base shear force .......................................................................................................... 27 2.4.1.3 Vertical distribution of the horizontal seismic forces .................................................... 27

2.4.2 Modal response spectrum analysis (MRSA) ................................................................... 28 2.4.2.1 General ........................................................................................................................ 28 2.4.2.2 Number of modes for the analysis ............................................................................... 28 2.4.2.3 Combination of modal responses ................................................................................ 28

2.4.3 Large Displacement method of analysis (LDMA) ............................................................ 29 2.4.4 Combination of the horizontal components of the seismic action ................................... 29 2.4.5 Combination of the vertical component of the seismic action ......................................... 29

2.4.6 Displacements calculation ............................................................................................... 30 3 SPECIFIC RULES FOR THE SEISMIC DESIGN OF RACKS ............................ 31

3.1 Design concepts ...................................................................................................... 31 3.1.1 General ............................................................................................................................ 31 3.1.2 Materials .......................................................................................................................... 31 3.1.3 Structural types and behaviour factors ............................................................................ 32

3.1.3.1 General ........................................................................................................................ 32 3.1.3.2 Unbraced racks ............................................................................................................ 33

3.1.4 Structural regularity criteria .............................................................................................. 33 3.1.4.1 General ........................................................................................................................ 33 3.1.4.2 Cross aisle direction..................................................................................................... 33 3.1.4.3 Down aisle direction ..................................................................................................... 34

3.1.5 Layout regularity .............................................................................................................. 35 3.1.6 Rules for the design of low dissipative structures ............................................................ 35 3.1.7 Rules for the design of dissipative structures .................................................................. 36 3.1.8 Anchoring conditions ....................................................................................................... 36


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3.2 Structural systems withstanding the seismic action ................................................. 37 3.3 Structural analysis ................................................................................................... 38

3.3.1 Sub-modelling .................................................................................................................. 38 3.3.2 Arrangement of the masses ............................................................................................. 38 3.3.3 Specific modelling requirements for the analysis ............................................................ 38

3.4 Structural types and behaviour factors ..................................................................... 40 3.4.1 Upright frames ................................................................................................................. 40 3.4.2 Moment resisting frames ................................................................................................. 41 3.4.3 Vertical bracings .............................................................................................................. 42

3.4.3.1 Low dissipative design concept ................................................................................... 43 3.4.3.2 Dissipative design concept .......................................................................................... 44

3.4.4 Horizontal bracings .......................................................................................................... 44

4 SEISMIC ANALYSIS AND DESIGN ................................................................... 45 4.1 Actions .................................................................................................................... 45

4.1.1 Actions to be considered acting simultaneously with earthquake ................................... 45 4.1.2 Actions not to be considered acting simultaneously with earthquake ............................. 45

4.2 Safety Verifications .................................................................................................. 46 4.2.1 Ultimate limit states .......................................................................................................... 46

4.2.1.1 Combination rules ........................................................................................................ 46 4.2.1.2 Resistance condition .................................................................................................... 46 4.2.1.3 Material partial safety factor M .................................................................................... 46 4.2.1.4 Ductility condition ......................................................................................................... 46 4.2.1.5 Equilibrium condition .................................................................................................... 46 4.2.1.6 Resistance of horizontal bracings ................................................................................ 47 4.2.1.7 Seismic interface condition .......................................................................................... 47

4.2.2 Serviceability limit states .................................................................................................. 47 4.2.2.1 Damage limitation requirement: assessment of the damage after an earthquake ...... 47 4.2.2.2 Unit load sliding ............................................................................................................ 47

4.2.3

Unit load falling ................................................................................................................ 47

4.2.3.1 Unit loads not fixed on the rack ................................................................................... 47 4.2.3.2 Unit loads fixed on the rack ......................................................................................... 48 4.2.3.3 Unit load rocking and overturning ................................................................................ 48

4.2.4 Pallet beams .................................................................................................................... 48 4.2.4.1 Internal actions ............................................................................................................. 48 4.2.4.2 Buckling length horizontal plane ............................................................................... 49 4.2.4.3 Correction coefficient horizontal bending .................................................................... 49 4.2.4.4 Buckling length factor - vertical plane .......................................................................... 49 4.2.4.5 Beam design check...................................................................................................... 50 4.2.4.6 Post-seismic assessment ............................................................................................ 50

5 ADDITIONAL DETAILING RULES FOR DISSIPATIVE ELEMENTS -(CONCEPT B) ........................................................................................................... 51

5.1 Connections ............................................................................................................ 51 5.1.1 Connections of dissipative members ............................................................................... 51

5.1.1.1 Bolted connections....................................................................................................... 51 5.1.2 Connections participating in the energy dissipation ........................................................ 51

5.2 Detailing rules for concentric bracings ..................................................................... 52 5.2.1 Design criteria .................................................................................................................. 52 5.2.2 Consideration of diagonals .............................................................................................. 52 5.2.3 Design of diagonal members ........................................................................................... 52

5.2.3.1 Frames with X-braced tension diagonals .................................................................... 52 5.2.3.2 Frames with V-braced diagonals ................................................................................. 53 5.2.3.3 Resistance of the elements ......................................................................................... 53 5.2.3.4 Ductility requirement of the element ............................................................................ 53

5.2.3.5 Requirement for dissipative homogeneous behaviour ................................................ 53 5.2.3.6 Dissipative connections ............................................................................................... 54 5.2.4 Design of beams, horizontals and columns ..................................................................... 54

5.3 Detailing rules for moment resisting frames ............................................................. 55


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5.3.1 Design criteria .................................................................................................................. 55 5.3.2 Energy dissipation in beam-to-column connections ........................................................ 55

5.4 Requirements for horizontal bracings ...................................................................... 56 5.5 Requirements for base plates and floor anchors ...................................................... 57

5.5.1 Design criteria .................................................................................................................. 57 5.5.2 Energy dissipation in floor connections ........................................................................... 57

6 BIBLIOGRAPHY ................................................................................................. 87

Annexes

Annex A Structural types and maximum behaviour factors (Extract from Chapter 6.3.1 of EN 1998-1:2005)

Annex B Design data to be provided by the Specifier/End User

(Addendum to EN 15629 for racking installations in seismic areas)

Annex C1 Determination of the pallet-beam friction coefficient

Annex C2 Test procedure to determine the pallet-beam friction coefficient

Annex D Pallet rocking assessment criteria (FEMA 460)

Annex E Background on sliding problems

Annex F Design method to reduce the risk of pallet falling

Annex G Testing procedure for beam-upright connection under cyclic loads


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1) The seismic response could be significantly different in the down-aisle and in the cross-aisledirections, and could be considerably affected by the size and the distribution of the masses in theheight. Reliable statistical evaluations are necessary to define the most probable mass distributionwhen earthquake occurs, depending on the racking system typology and dimensions. The mostlikely approach is given in this standard.

2) The natural damping of the steel structure is very small. But the actual damping, measured in realconditions, could be significantly more than expected, because of micro-movements in storedgoods and products and/or sliding effects between pallets (or other unit-loads) and the supportingbeams.

3) Cyclic forces due to earthquake can progressively damage connections and/or other componentsof a racking system. These changes could considerably affect the response of the structure and itsway to resist seismic actions. A reliable modelling of the actual strength and stiffness is offundamental importance in order to predict the behaviour of the structure.

4) In the case of seismic isolation, the effectiveness of the isolation devices must be guaranteed forall the loading conditions and during the specified life of the racking system

This Code describes many of the particular features that affect the seismic response of a rackingstructure. However, not all of the local and global effects can be predicted by the mathematical modelsand design methods in daily use. In particular the phenomena involved in energy dissipation is difficultto predict.Observations of the effects of earthquakes on racks which have suffered seismic events or that havebeen tested on shaking tables show that racking structures have a capability to come through anearthquake better than predicted by a purely theoretical approach and by the analytical modelconsidered in the present document.For this reason, factors modifying the seismic action have been introduced (coefficient E D1, E D2 andED3). These factors are based on experimental research, observations and engineering judgment. It isintended to adjust them in the future on the basis of further theoretical and experimentalinvestigations.Nevertheless, calibration studies show that the requirements of this code are more stringent than othercodes for racking structures.


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1.2 Scope

The design procedures given in this Code of Practice apply to all types of static steel pallet racks thatare supported by ground bearing and pile supported slabs subject to seismic actions.Provisions are also given for racks supported by suspended floors.

The approach to the seismic design is based upon the philosophy of EN 1998-1 (Eurocode 8). Thepeculiar dynamic behaviour of racking structures and of the stored unit loads is included in thisdocument.

In case of clad racks this Code gives relevant information in addition to the National BuildingRegulation, specifically concerning: The interaction between pallets and racking structure, described by the coefficients E D1 , E D2 and

ED3 The figures for the behaviour factors q when appropriate

For the purpose of this Code only self-supporting racks are considered.

Special analysis is required for racks supported by other structures to take into account the effects ofthe structural amplification of the seismic action.

The principles of this code may be applied to other types of storage system with suitable modification.

1.3 Applicability

Non-seismic design shall comply with EN 15512.

The reference to the tests and quality control of components and materials is based on EN 15512.

If the National regulations or risk analysis prepared by the Client determines that the rack shall bedesigned for seismic actions this code gives the necessary guidance.

Seismic actions may be neglected if the product Ia gRS is less than or equal to 0.05g or to the valueprescribed in the National Annexes to EN1998-1 or other National Regulation.

For clad racks in very low seismic zones refer to EN1998-1:2004, Art. 3.2.1, (5)P

1.4 Normative references

EN 1993 - Eurocode 3 Design of steel structuresPart 1-1: General rules and rules for buildings EN 1993-1-1:2005Part 1-3: Cold formed steel sheeting and members EN 1993-1-3:2006Part 1-8: Design of joints EN 1993-1-8:2005

EN 1998-1:2004 - Eurocode 8 - Design of structures for earthquake resistancePart 1: General rules, seismic action and rules for buildings

EN 15512:2009 Steel static storage systems Adjustable pallet racking systems Principlesfor structural design

EN 15629:2008 Steel static storage systems Specifications of storage equipment

EN 15620:2008 Steel static storage systems Adjustable pallet racking systems Tolerances,deformations and clearances


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EN 15635:2008 Steel static storage systems Application and maintenance of storageequipment

EN 15878 Steel static storage systems Terms and Definitions

1.5 Symbols

Ratio of the design ground acceleration a g to the acceleration of gravity g v Ratio of the design vertical ground acceleration a vg to the acceleration of gravity g Lower bound factor for the design spectrumA Section class coefficient Inter-storey drift sensitivity coefficientp Rotational capacity parameter of the plastic hinge region I Importance factor ov Design overstrength factor pb Post buckling residual resistance coefficient GA Partial safety factor for permanent actions L Partial safety factor for loads M Materials safety factor M0 Materials partial safety coefficient M2 Connection partial safety coefficient QA Partial safety factor for variable actions damping spectrum correction factorS Pallet to supporting beam friction coefficient Slenderness ratio Non dimensional slenderness 2,i Partial reduction coefficient for variable actions Viscous damping ratio, expressed as percentage of critical damping Overstrength coefficient

a g Design horizontal ground accelerationa gR Design ground acceleration (PGA) for the reference return period of 475 yearsa v Design vertical ground accelerationde Displacement of a point of the structural system induced by the design seismic action,

determined by a linear analysis based on the design response spectrumdg Design ground displacementd r Design inter-storey driftds Displacement of a point of the structural system induced by the design seismic actionfk Characteristic strength of the material

fu Ultimate tensile strength of the materialfy Nominal yield strength specified for the materialfy,max Actual maximum yield strength of dissipative zonesfy,act Actual yield strength of dissipative zonesg Gravity accelerationh Inter-storey heightm i , m j Masses in the response spectrum analysisq Behaviour factorq Corrected behaviour factorqd Displacement behaviour factors i,, s j Displacement of the masses m i , m j in the fundamental modal shapezi , z j Heights of the masses m i , m j above the level of application of the seismic action

A Cross section gross areaAnet Net area of the member near the connection


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1.6 Terms and DefinitionsFor general terms and definitions refer to EN15878 and EN1998-1

rack filling grade reduction factor R F.A statistical reduction factor to take into account the probability that not all of the pallets will be presentand at their maximum weight at the time of the design earthquake

seismic weight the reduced value of weight of a mass allowed in seismic design for the calculation of the seismicforces.

warehouse safety managerthe person responsible for storage equipment safety (PRSES) as given in EN15635


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2 METHODS OF SEISMIC ANALYSISThe seismic action is evaluated according to the methods of EN 1998-1 - Eurocode 8, as specifiedhereafter.

2.1 Fundamental requirements and compliance criteria(Refer to EN1998-1:2004, Chapter 2.1)Racking structures in seismic regions shall be designed and constructed in such a way that thefollowing requirements are met, each with an adequate degree of reliability.

1) No collapse requirementThe racking structure shall be designed and constructed to withstand the design seismic actionwithout local or general collapse, retaining its structural integrity and a residual load bearingcapacity after the seismic event.Ultimate limit states are those associated with the collapse, or with other forms of structural failure,that may endanger the safety of people.The structural system shall be verified as having the specified resistance and ductility.

2) Damage limitation requirementNo specific design requirement is prescribed in this Code.After a seismic event, with ground acceleration greater than 0.30 a gRS, the Warehouse operatorshall perform a complete check of the integrity of the racking structure. The assessment of thelevel of damage to the structural elements is mandatory before returning the rack to use.The movement of stored unit loads does not constitute damage.

NOTE 1 The seismic intensity can be obtained from publicly available information.NOTE 2 The value of 0.30 a gRS should be provided by the rack supplier in the User manual.

3) Movement of Unit loadsSeismic accelerations can cause sliding of the pallets on the supporting beams, when the inertialhorizontal forces on the pallet exceed the static friction force between pallet and beam.

NOTE: This effect has also been demonstrated by full scale tests to occur for small values of groundaccelerations (low intensity earthquakes) with wooden or plastic pallets on painted or zinc coated steelbeams, because of the structural amplification of the seismic forces at the highest storage levels.

The consequences of these phenomena are: the reduction of the seismic action on the rack, due to the energy dissipation and the limitation

of the horizontal action that can be transferred from the pallet to the rack structure the risk of unit loads falling, can cause local or global collapse of the rack, or injury to people

and damage to nearby equipment.

The modification of the seismic response of the structure is considered in this Code by means of threecoefficients that estimate the effects on the structure of typical phenomena of racking structures, suchas energy dissipation due to the pallet-beam friction, damping due to the movement of the storedproducts, pallet flexibility and others:

ED1 and E D3 = design spectrum modification factorsED2 = mass modification factor

The Client shall assess the risks related to unit loads sliding and possibly falling from the rack.The guidelines for this are given in Chapters 4.2.2.2 and 4.2.3


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2.2 Description of the seismic action

2.2.1 General

The earthquake motion at a given point on the earth surface is described by an elastic responsespectrum (refer to figure 2.1).

The seismic action is described by two orthogonal horizontal components which are assumed to beindependent and represented by the same response spectrum, and by a vertical component.

2.2.2 Definition of the intensity of the seismic action

In most of the seismic design Codes the seismic action is conventionally described by means of anelastic response spectrum with 10% probability to be exceeded in 50 years, corresponding to a returnperiod of 475 years.

This probability is adopted as the reference for ordinary buildings.

The seismic hazard is described by a single parameter a gR , which is the reference Peak GroundAcceleration (PGA).

The earthquake motion is described in EN 1998-1 by two elastic spectra (called Type 1 and Type 2);refer to the National Annexes to EN1998-1. Guidance for the choice of the spectrum is also given inEN 1998-1 Chapter 3.2.2.2 Clause (2)P.

Design spectra based on the EN1998-1 approach defined in National Regulations may also be used.

The spectra of EN1998-1 are given below;

Where:S e(T) elastic response spectrum;T vibration period of a linear single-degree-of-freedom system; = a g /g ratio of the design ground acceleration a g to the acceleration of gravity ga g = I a gR design ground accelerationa gR design Peak Ground Acceleration (PGA) for the reference return period of 475 years I importance factor, defined in Chapter 2.2.3TB lower limit of the constant spectral acceleration branch;TC upper limit of the constant spectral acceleration branch;TD value of period defining the beginning of the constant displacement response range of the

spectrum;S soil factor; damping correction factor with a reference value of = 1 for 5% viscous damping

S, T B, T C, TD are defined in Tables 2.2 and 2.3


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The value of the damping correction factor may be determined from the expression:

where is the viscous damping ratio of the structure, expressed as percentage of the critical damping. = 3%,NOTE: = 3%, resulting in =1.118 has been found a reliable figure for racking structures (refer toSEISRACK report).

Figure 2.1 Shape of the Elastic Response Spectrum

2.2.3 Earthquake design return period and importance factor I

The return period of the design earthquake is governed by the coefficient I. For the reference returnperiod of 475 years the Importance factor I=1.0 is assigned.

Unless otherwise specified in the contract documents, the importance factors in Table 2.1 shall beused. The Client shall specify the Importance Class and design life for the rack. In the case of seismicdesign a design life of at least 30 years shall be considered.

NOTE: this minimum 30 year design life is solely in relation to the seismic design and differs from thenormal static design life of minimum 10 years as given in EN15512

NOTE: For economic or strategic reasons the Client may specify a higher importance factor. Normallyhowever, the importance factor for the rack should not be greater than the importance factor specifiedfor the part of the building in which the racks are located.


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Table 2.1 Importance factors for racks

ImportanceClass Description

Importance factor

50 yeardesign life

30 yeardesign life[See (2)]

IWarehouses with fully automated storageoperationsLow warehouse occupancy [See (1)]

0.8 0.67

II Standard warehouse conditions, including pickingareas 1.0 0.84

III Retail areas with public access 1.2 See (2)IV Hazardous product storage 1.4 See (2)

NOTE: Due to the shorter design life of racking structures (see 2) the importance factors are 0.84 fornormal warehouse conditions and 0.67 for fully automated warehouses and low warehouse

occupancy, unless otherwise stated in the Contract between the Client and the Supplier.(1) Warehouse conditions.

In general only authorized and trained workers are permitted to access the storage area(s) withina warehouse.Low warehouse occupancy for a certain storage area is defined as an operation condition whereno more than 5 authorized and trained workers can operate at one time within that storage area.In relation to the determination of the importance class for the storage area concerned, a storagearea is defined as follows:

Width x Length = (A + h) x (B +h)

Where,A = width of plan view of rack blockB = length of plan view of rack blockh = maximum height of rack with unit loads

In case the rack is closer to a warehouse wall than h, the storage area border at that position isthe warehouse wall.

In case the storage area concerned is adjacent to another warehouse compartment and the rackwith unit loads is at least 2 times higher than the adjacent compartment, the workers present inthis lower warehouse compartment shall be included when determining the number ofsimultaneously present workers.For instance in case of a high bay racking separated by an inner wall or a rack clad building

adjacent to an order pick area (see e.g. figure 2.2).

The Warehouse Safety Manager, considering the risk associated with the working conditions ofthe warehouse, may prescribe a more severe importance class.

(2) For importance classes III and IV a reduction of the importance factor for racks is not permitted.


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1: Order Pick area 2: High bay racking

Figure 2.2


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2.2.4 Horizontal design spectrum for elastic analysis

The energy dissipation capacity of the structure and the interaction between unit loads and rackingallows the use of a reduced design spectrum. This reduced design spectrum is obtained from thedesign spectrum defined in EN1998-1 corrected by the modification factors E D1 and E D3.

The design spectrum, defined in EN 1998-1, is derived from the elastic spectrum by scaling with thebehaviour factor q, which accounts for the ductility and damping of the racking structure.

The design spectrum modification factors E D1 and E D3 are defined in chapter 2.3.2

The seismic design of racks installed on suspended floors shall be performed using the floor responsespectrum; the Client shall provide this information.

The design spectrum S d(T) for the horizontal components of the seismic action, normalized by the

acceleration of gravity g, is defined by the following expressions:

where:

S d(T) ordinate of the design spectrum, normalized by ga g = I a gR design ground accelerationa gR design Peak Ground Acceleration (PGA) for the reference return period of 475 years I importance factor = a g /g ratio of the design ground acceleration a g to the acceleration of gravity gT vibration period of a linear single degree of freedom systemS soil parameterTB lower limit of the constant spectral acceleration branch;TC upper limit of the constant spectral acceleration branch;TD value of period defining the beginning of the constant displacement response range of

the spectrum;q behaviour factor lower bound factor for the spectrum; the recommended value for is =0.2; the

National Annexes to EN 1998-1 may choose for other values of

BT T 0

+=

325.2

32

)(qT

T S T S

Bd

C B T T T < qS T S d

5.2)( =

DC T T T < = T T

qS T S C d

5.2)(

T T D <

= 25.2

)( T T T

qS T S DC

d


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Table 2.2: Values of the parameters describing the Type 1 elastic response spectrum(ref. Table 3.2 of EN 1998-1:2004)

Groundtype S

TB[sec]

TC[sec]

TD[sec]

A 1.0 0.15 0.4 2.0B 1.2 0.15 0.5 2.0C 1.15 0.20 0.6 2.0D 1.35 0.20 0.8 2.0E 1.4 0.15 0.5 2.0

Table 2.3: Values of the parameters describing the Type 2 elastic response spectrum(ref. Table 3.3 of EN 1998-1:2004)

Groundtype S

TB[sec]

TC[sec]

TD[sec]

A 1.0 0.05 0.25 1.2B 1.35 0.05 0.25 1.2C 1.5 0.10 0.25 1.2D 1.8 0.10 0.30 1.2E 1.6 0.05 0.25 1.2

For sites with ground conditions matching either one of the two special ground types S1 or S2 (seeTable 2.4), special studies for the definition of the seismic action are required; refer to EN 1998-1Chapter 3.

When soil properties are not known in sufficient detail to determine the site soil conditions, soil class Dshall be used.

Table 2.4: Subsoil classes (taken from Table 3.1 EN1998-1: 2004)

Subsoilclass Description of stratigraphic profile

Parameters

V s ,30 (m/s) N SPT (blows/30cm) c u (kPa)

ARock or other rock-like geological formation,including at most 5 m of weaker material at thesurface

> 800 - -

B

Deposits of very dense sand, gravel, or verystiff clay, at least several tens of metres inthickness, characterized by a gradual increaseof mechanical properties with depth

360 800 > 50 > 250

C Deep deposits of dense or medium-densesand, gravel or stiff clay with thickness fromseveral tens to many hundreds of m

180 360 15 - 50 70 - 250

DDeposits of loose-to-medium cohesionless soil(with or without some soft cohesive layers), orof predominantly soft-to-firm cohesive soil

< 180 < 15 < 70

E

A soil profile consisting of a surface alluviumlayer with V s ,30 values of type C or D andthickness varying between about 5 m and 20 m,underlain by stiffer material with V s ,30 > 800 m/s

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S 1 Deposits consisting or containing a layer atleast 10 m thick of soft clays/silts with highplasticity index (PI > 40) and high water content

< 100(indicative) - 10 - 20

S 2

Deposits of liquefiable soils, of sensitive clays,

or any other soil profile not included in classesA E or S 1 - - -


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The average shear wave velocity V s ,30 is computed according to the following expression:

==

N j j

jS

V

hV

,1

30,30

where h i and V i denote the thickness and shear-wave velocity (at low strain level) of the i -th formationor layer, in a total of N , existing in the top 30 metres.The site will be classified according to the value of V s ,30 if this is available, otherwise the value of N SPT will be used.

2.2.5 Vertical component of the seismic action

The design spectrum for the vertical component of the seismic action S vd (T) is given by the sameexpressions as the horizontal design spectrum S d(T) with v replacing , S = 1.0, and usingparameters whose values are given in Table 2.5.

where:

a v design vertical ground acceleration (specified in the National Annex to EN1998-1 orNational Regulations)

v ratio of the design ground acceleration a v to the acceleration of gravity g

The behaviour factor q shall be assumed equal to 1.5.

The design spectrum modification factors E D1, E D2 and E D3 cant be applied to the vertical componentof the seismic action

Table 2.5: Values of the parameters for the vertical response spectrum

Spectrum v TB

[sec]TC

[sec]TD

[sec]Type 1 0.90 0.05 0.15 1.0Type 2 0.45 0.05 0.15 1.0

The vertical component of the seismic action shall only be taken into account in the following relevantcases:

1) Cantilever components

2) Beams supporting columns (forexample in order picking tunnels)

3) Their directly associated supportingelements or substructures.

Figure 2.3

1

23 33


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2.2.6 Structural regularity criteria

Structures are classified as regular or non-regular for seismic design.The concept of regularity is related to both mass and stiffness distribution, in plan and in elevation.The criteria for pallet racks are given in chapter 3.1.4.

This distinction has implications in seismic design, as described in the following: the structural analysis that can be carried out using either a simplified planar or a spatial numerical

model the method of analysis that can be either a simplified response spectrum analysis (lateral force

procedure) or modal one the value of the behaviour factor q, which shall be decreased for non regularity in elevation the decreased values of the behaviour factors for pallet racks are given in chapter 3.1.3

Table 2.6: Consequences of structural regularity on seismic analysis and design

Regularity Allowed simplification (1)Behaviourfactor q

Plan Elevation Model Method of linear analysis

Yes Yes PlanarLateral force analysis

(see limitations given in 2.4.1)Reference

value

Yes No Planar Modal response spectrumanalysis Decreased (3)

No Yes Spatial

Modal response spectrumanalysis

(lateral force analysis may beused only when T


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2.3 Design parameters for seismic analysis

2.3.1 General

The seismic design of racks can be performed using the following reduced design spectrum:

where

S d(T) is the reference design spectrum given in 2.2.4,and

ED1 and E D3 are the design spectrum modification factors, with E D1 ED3 0.4

2.3.2 Design spectrum modification factors

The design spectrum modification factors E D1 and E D3 take into account the curtailment andmodification to the ordinate of the design spectrum.

(1) E D1 is affected by the following parameters: intensity of the seismic action number of load levels, total mass and flexibility of the racking structure, expressed by the

period of vibration (dominant period in the direction considered) maximum horizontal force that can be transmitted by the pallet to the pallet beams, expressed

in terms of friction coefficient

ED1 = max [ 0.4 ; S /S e(T1) +0.2 ] 1.0

whereS is the friction coefficient given in 2.3.3T1 is the fundamental period of vibration of the structure in the considered directionS e(T1) is the ordinate of the elastic spectrum defined in 2.2.2 with 3% viscous damping

Background information on the correlation between E D1 and sliding of pallets on beams is providedin Annex E.

When pallets are restrained on the pallet beams by means of any special system (for examplematerials increasing the friction between pallet and beam), E D1 = 1.0.

(2) E D3 is a reduction coefficient of the seismic action

ED3 = 1/1.5 = 0.667

NOTE: E D3 is introduced to account for other phenomena typical of the dynamic behaviour of rackingstructures under seismic actions that are not included in the mathematical approach presented in thisCode, but that are observed on racks that have suffered earthquakes, and from tests performed onshaking tables.


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2.3.3 Pallet-beam friction coefficients

Reference pallet-beam friction coefficients are given in the following table.Table 2.7 Recommended reference values for the pallet-beam friction coefficient

Materials in contact EnvironmentReference Friction

coefficient S Steel beams all coatingsWooden pallet Warehouse conditions 0.37

Steel beams all coatingsPlastic and steel pallet Warehouse conditions 0.15

Steel beams all coatingsWooden pallet Cold store 0.30

Steel beams all coatingsPlastic and steel pallet Cold store 0.10Steel beams all coatingsWooden pallet

Chill storeWet pallets 0.10

NOTE: The friction coefficient is strongly affected by the nature of the materials in contact and by thetype of coating of the beams.

NOTE: Research has demonstrated that static and dynamic friction coefficients are very similar.

NOTE: The reference values of the friction coefficients in Table 2.7 are a safe approximation of theaverage values obtained from tests in literature.

ED1 shall be evaluated using the reference value of the friction coefficient.

The assessment of the occurrence of pallet sliding (4.2.2.2) shall be performed considering theacceleration at each level, using the reduced value of friction coefficient C L S , where C L = 0.67, onthe basis of the elastic response calculated using the elastic spectrum S e(T) defined in chapter 2.2.2with 3% viscous damping.

The local checks for the effects of the seismic actions on pallet beams in the horizontal plane (4.2.4)shall be performed considering the horizontal action not greater than C H S , times the weight of thedesign pallet weight, on the basis of the elastic response calculated using the elastic spectrum S e(T)defined in chapter 2.2.2 with 3% viscous damping. Where C H = 1.5

CL is a factor to give a lower bound value for the friction coefficient

CH is a factor to give an upper bound value for the friction coefficient

Values of S other than the ones given in Table 2.7 and of C L and C H other than the ones previouslydefined may be used if determined by tests performed in accordance to Annex C.


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2.3.4 Design seismic pallet weight

The design pallet weight W E to be considered in the evaluation of the horizontal seismic action, isdetermined by:

WE = R FED2 Q P;rated

where:

RF = rack filling grade reduction factor to be defined by the Client.ED2 = pallet weight modification factor (see 2.3.4)QP;rated = specified value of the weight of unit loads for the compartment, upright frame or global downaisle design (see EN 15512) as given in the Contract documents.

NOTE: E D2 modifies the period and the horizontal action

For analysis in down aisle direction R F0.8.

For analysis in cross aisle direction R F=1.0.

Unless otherwise specified in the Contract documents the following values of the coefficients shouldbe considered:RF = 1.0QP;rated = maximum weight of the unit load

2.3.5 Pallet weight modification factor

(1) The pallet weight modification factor E D2 represents the effects of the interaction between palletand racking structure. This affects the response to earthquake in terms of participating mass andmodification of the period of vibration.

(2) Unless otherwise specified, the values of E D2 in Table 2.8 shall be considered, depending on thetype of pallets and stored goods.

Table 2.8 Pallet weight modification factors

ED2 Stored good classes Example

1.0 COMPACT

CONSTRAINED

Frozen goods (cold storage)Steel sheet packageCoils and paper rolls

0.8 WEAKBig number of pieces stored on the pallet whose size issmall in comparison to the pallet size, including goodsstabilized by stretch wrapping

0.7 LOOSE ANDUNCONSTRAINEDGoods that can easily move around inside the containere.g. granulated materials

1.0 LIQUID Unit load containing liquid that can slosh in the container


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2.3.6 Other seismic weights

All the permanent and live loads other than the unit load weight shall be considered in the seismicanalysis.

Refer to EN 15512 chapter 6 for the definition of:

1) Dead Loads: weights of materials and constructions weights of fixed service equipment

2) Minimum floor and walkway loads (actual values shall be specified by the Client): 1.0 kN/m 2 on walkways and access floors not for storage, intended as global load 3.5 kN/m 2 on floors for storage Walkways for maintenance can be considered not to be loaded at the time of an earthquake

NOTE 1 National regulations may require different values for floor and walkway loads.

If not otherwise specified by the Specifier/User, the following occupancy factors should beconsidered for the evaluation of the horizontal action:

0.8 floors for storage0.3 walkways and access floors

2.3.7 Position of the centre of gravity of the pallet

1) Cross aisle direction:The elevation of the pallets centre of gravity with respect to the support beams (verticaleccentricity) shall be considered.

Pallet masses shall be placed over the support beam (at the position of the centre of gravity) forthe evaluation of the period of vibration and of the seismic action, and for the check of the palletbeams and their connections.

Figure 2.4


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NOTE: In some circumstances the computational effort involved with this requirement can beconsiderable. A practical solution that will reduce computation effort is to place the mass at the beamlevel. However, this can only be done if the result is corrected by deriving a correction factor from acomparison of a frame with the masses placed at the centre of gravity and with the masses placed atthe beam levels. This correction factor should be applied to all components involved.

2) Down aisle direction:For racks not exceeding 5 bays in length the elevation of the pallets centre of gravity with respectto the support beams (vertical eccentricity) shall be considered.For racks exceeding 5 bays in length the eccentricity of the masses can be neglected.

Figure 2.5

2.3.8 Placement eccentricityThe eccentricity due to the placement tolerances of unit loads can be neglected in the seismic design.


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2.4 Methods of analysis

The reference method for the evaluation of the seismic effects is the modal response spectrumanalysis. This shall be performed using a linear-elastic model of the structure and the reduced designspectrum S d,red (T) defined in Chapter 2.3, and when applicable, the design spectrum defined in 2.2.4for the vertical component.

In all cases when Ia gRS 0.1g (or other value prescribed in the National Annex to EN1998-1 orNational Regulations) the following limitation shall be fulfilled:

P tot / P cr,E 0.5

Where P cr,E may be approximated according to EN 15512 (Annex B, C and G)

The requirement to account for second order effects is related to the maximum value of the inter-storey drift sensitivity coefficient defined as:

= (P tot d r) / (V tot h)

where:

= Inter-storey drift sensitivity coefficient for the fundamental modeP tot = total gravity load at and above the considered storey, in the seismic design situationd r = design inter-storey drift, evaluated as the difference of the average lateral

displacements at the top and bottom of the storey under consideration and calculatedaccording to 2.4.6 by means of linear elastic 1 st order analysis

Vtot = total seismic storey shear

h = inter-storey height

NOTE: Care is needed in the selection and use of commercial software packages as some areincapable of dealing with the second order amplification in conjunction with modal analysis

An alternative definition of the inter-storey drift sensitivity coefficient is:

= q P tot / P cr,E

where:

P tot = total gravity load, in the seismic design situationP cr,E = Euler critical loadq = S e(T1)/ S d,red (T1)S e(T1) = ordinate of the elastic spectrum with 3% dampingS d,red (T1) = ordinate of the modified design spectrumT1 = fundamental period of vibration

The following procedure shall be followed (see Table 2.9).

(a) When 0.1, second order effects can be neglected

(b) When 0.1 < 0.3 second order effects may approximately be taken into account by multiplyingthe relevant seismic action by a magnification factor 1/(1- )

If response spectrum analysis is performed, the stiffness matrix of the model may include theterms that reduce the stiffness of the system due to the vertical loads (geometric matrix).


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(c) When > 0.3 second order effects shall be explicitly considered in the analysis.

If q>2 pushover analysis according to EN 1998-1 Annex B or the large displacement analysispresented in chapter 2.4.3 (geometrical nonlinear equivalent analysis) shall be performed.

If q2 and modal response spectrum analysis is performed; where required (see Table 2.9)amplification of modal response due to 2 nd order effects shall be included in the analysis.

(d) If is greater than 0.5 and q>2, a time history analysis including large displacements andnonlinear behaviour of materials and connections is required.

(e) If time-history analysis is used, the recommendations of EN 1998-1 about the number of groundmotions apply and a minimum of 3 different ground motions shall be used. If the response isobtained from at least 7 nonlinear time-history analyses, the design actions for the relevant checksare given by the average value of the effects from these analyses. Otherwise, the mostunfavourable value of the response quantities among the analyses should be used as designvalue.

Other methods of analysis can be used according to EN 1998-1.

The description of the ground motion shall be according to the guidelines of EN 1998-1.

Table 2.9 Summary of methods of analysis

q2 q>2

Method ofanalysis Second order effects

Method ofanalysis Second order effects

0.1

LFMA

or

MRSA

negligible

LFMAor

MRSA

negligible

0.3Shall be considered

eitherdirectly in the analysis

(geometrically nonlinearanalysis)

orindirectly (amplification of

the effects of the horizontalaction by 1/(1- ))

(Note 1)

Shall be consideredeither

directly in the analysis(geometrically nonlinear

analysis)or

indirectly (amplification ofthe effects of the horizontal

action by 1/(1- ))

0.5

Pushover analysis accordingto EN 1998-1 Annex B

or LDMA

according to 2.4.3 > 0.5 Time history analysis including geometricaland material nonlinearity

LFMA Lateral Force Method of Analysis (Section 2.4.1)MRSA Modal Response Spectrum Analysis (Section 2.4.2)LDMA Large displacement method of analysis (Section 2.4.3)

NOTE (1) Amplification of 2 nd order effects by factor 1/(1- ) not recommended if >0.3 as resultstend to be unduly conservative


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2.4.1 Lateral force method of analysis (LFMA)

2.4.1.1 GeneralThis method of analysis can be applied to regular structures that can be analysed by means of twoindependent planar models in two orthogonal directions and whose response is not significantlyaffected by contribution of higher modes of vibration.

The fundamental period of vibration T 1 in the two main orthogonal directions shall be less than thefollowing values:T1 4TC T1 2 secwhere T C is given in Tables 2.2 and 2.3.

2.4.1.2 Base shear forceThe seismic base shear force F b for each main direction is determined as follows:Fb = S d,red (T1)WE,totwhere:S d,red (T1) ordinate of the modified design spectrumT1 fundamental period of vibration for translational motion in the direction under considerationWE,tot total seismic mass

The fundamental period of vibration shall be evaluated by means of modal analysis. The simplifiedformulas proposed by various Codes and Standards for the evaluation of T 1, which are typical forbuildings, are not allowed for racks.

If the fundamental period is not evaluated, the maximum value of the design spectrum shall beassumed.

2.4.1.3 Vertical distribution of the horizontal seismic forcesThe effects of seismic action shall be determined by applying to all masses m i a set of horizontalforces F i.

(a) The forces shall be determined by assuming the entire mass as substitute mass of thefundamental mode of vibration, hence:

= j j

iibi Ws

WsFF

whereF i horizontal force at level iFb seismic base shears i , s j displacement of the masses m i , m j in the fundamental modal shapeW i , W j weight of masses m i , m j (seismic weights)

(b) With a simplified approach, the fundamental mode shape can be approximated by horizontaldisplacements increasing linearly in the height; hence the horizontal forces F i are given by:

= j j

iibi Wz

WzFF

wherez i , z j heights of the masses m i , m j above the level of application of the seismic action

The reference level for application of the seismic action is usually the floor level.


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2.4.2 Modal response spectrum analysis (MRSA)

2.4.2.1 General

For structures complying with the criteria for regularity in plan the analysis can be performed using twoplanar models, one for each principal direction.

Structures not complying with these criteria shall be analyzed by means of a spatial model.

Whenever a spatial model is used, the design seismic action shall be applied along all the relevanthorizontal directions simultaneously.

2.4.2.2 Number of modes for the analysis

The response of all significant modes of vibration shall be considered.

At least one torsional mode shall be taken into account for structures that are non-regular in plan.

The number of modes to be considered for the analysis in each direction is such that:

1) the total modal mass for the considered modes amounts at least 90% of the total mass carried bythe structure

and

2) all the modes with effective modal masses greater than 5% of the total mass are considered.

When using a spatial model the above conditions shall be fulfilled for each relevant direction.

If one of the above conditions is fulfilled (for example when the global torsional behaviour is relevant,such as in case of racks with spine bracing), the minimum number (k) of modes to be considered inthe spatial analysis shall meet the following conditions:

k 3nandTk 0.20 s

wherek = number of modes consideredn = number of load levelsTk = period of vibration of the mode k

2.4.2.3 Combination of modal responses

(1) Modes i and j (including both translational and torsional modes) may be taken as independent i.e.not coupled, if their periods T i and T j (with T j Ti) meet the following condition:T j 0.9 T i

(2) Whenever all relevant modal responses may be regarded as independent of each other, themaximum value E E of a seismic action effect may be taken as (Square root of the sum of the squares:SRSS):EE = [(EEi2)]1/2where:EE = effect of the seismic action under consideration (force, displacement, etc)EEi = value of the effect of the seismic action due to vibration mode i


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(3) If (1) is not met, more accurate procedures for the combination of the modal maxima, such as the"Complete Quadratic Combination" (CQC) shall be applied.

2.4.3 Large Displacement method of analysis (LDMA)

When q>2 large displacements analysis shall be performed with a load history defined as follows:

a) horizontal actions are determined according to the lateral forces method of analysis as given inchapter 2.4.1.3 (a) with increasing load steps using the load multiplier K L .The value of K L changes with time and varies monotonically in a range from zero to a maximumvalue not less than q

b) constant vertical loads, equal to the design values in the seismic load condition

The nonlinear behaviour of materials and connections shall be taken into account.

Combinations of the seismic action according to 2.4.4 shall be considered in the same analysis whenthe structure is non regular in plan (i.e. superimposition of the effects not permitted).

NOTE: This requires a 3D analysis with the load histories applied in two directions

2.4.4 Combination of the horizontal components of the seismic action

The horizontal components of the seismic action shall be considered acting simultaneously. The twoorthogonal components shall be combined as follows.

a) EEdx

+ 0.30 EEdy

b) 0.30 EEdx + EEdy

where

+ implies to be combined with

EEdx = action effects due to the application of the seismic action along the horizontal x-axis of thestructureEEdy = action effects due to the application of the same seismic action along the orthogonal horizontaly-axis of the structure

The sign of each component in the above combinations shall be taken as the most unfavourable for

the effect under consideration.

2.4.5 Combination of the vertical component of the seismic action

In case the horizontal components of the seismic action are also relevant for those elements that areaffected by the vertical seismic action, the following three combinations shall be used for thecomputation of the action effects:

a) 0.30 EEdx + 0.30 EEdy + EEdz b) E Edx + 0.30 EEdy + 0.30 EEdz c) 0.30 EEdx + EEdy + 0.30 EEdz

where:

+, EEdx and E Edy are as defined in 2.4.4


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EEdz = action effects due to the application of the vertical component of the design seismic action

2.4.6 Displacements calculationDisplacements induced by the design seismic action shall be calculated on the basis of the elasticdeformation obtained from the analysis by means of the following simplified expression:

ds = q d d e

where

ds = displacement of a point of the structural system induced by the design seismic actionqd = displacement behaviour factor, assumed equal to qde = displacement of the same point of the structural system, determined by a linear analysis based

on the reduced design response spectrum


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3 SPECIFIC RULES FOR THE SEISMIC DESIGN OF RACKS

3.1 Design concepts

3.1.1 GeneralEarthquake resistant racks shall be designed according to one of the following concepts:

a) Low dissipative structural behaviourThe effects of seismic action are calculated by means of elastic global analysis without taking intoaccount relevant non-linear material behaviour.

b) Dissipative structural behaviourZones of the structure can undergo plastic deformation (dissipative zones). This capacity isdefined by the behaviour factor q.The value of q depends on the structural type and on the classification of the members crosssection (refer to EN 1993-1 for classification scheme).

Table 3.1. Design concepts and upper limit reference values of the behaviour factor

Design Concept Range of reference values ofthe behaviour factor q

Concept A)Low dissipative structure

q 2

Concept B)Dissipative structural behaviour

q > 2according to chapter 3.4

3.1.2 Materials

1) Structural steel shall comply with EN 15512 chapter 8.

2) In bolted connections of earthquake resisting structure, bolt grade 8.8 or 10.9 shall be used.

3) When dissipative structural behaviour concept is used, the distribution of material properties, suchas yield strength and toughness, shall be such that dissipative zones form where they areintended to in the design; yielding is expected to develop in dissipative zones before other zonesleave the elastic range during an earthquake.Such requirements may be met if the yield strength of the steel in dissipative zones and the designof the structure conform to the conditions given in EN 1998-1 Chapter 6.2.EN1998-1 gives 3 conditions (a), (b) and (c). For industrial racks conditions (b) is unsuitable andso conditions (a) and (c) are recommended as given in the following.

Condition a)the actual maximum yield strength f y,max of the steel of dissipative zones meets the followingrequirements:

fy,max 1,1 ov fy where:


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ov design overstrength factorfy nominal yield strength specified for the steel grade.If not otherwise specified ov = 1.25

Condition c)For each dissipative zone, the actual yield strength f y,act of the steel is determined from tensiletesting, and the overstrength factor is computed as ov= factor fy,act /fy,Where

fy is the nominal yield strengththe factor is 1.0 for hot rolled profile and 1.1 for cold-formed profiles.

NOTE: The tensile test piece is normally taken from the beginning of the coil prior to fabrication.The factor 1.1 allows for scatter of test results.

In condition a), the quality control of materials used for construction of dissipative componentsshall ensure that the maximum yield strength f y,max is less than or equal to 1.25 times the specifiednominal yield strength f y.

In condition c), f y,act is the weighted mean yield strength taken from tensile tests of materials usedfor each type of dissipative component. The weighting depends upon the number of componentspresent.The tensile tests are either:- the ones given in the material certificates of the batch or coil,- the quality control certificates performed according to EN 15512 on the batch or coil

4) In the project specification the designer shall specify the required fracture toughness of steel andwelds and the lowest service temperature adopted in combination with earthquake action. Refer toEN 1998-1 or National Regulations.

5) In those areas where the designer requires plasticity the components shall exhibit the necessarystrength and ductility.

3.1.3 Structural types and behaviour factors

3.1.3.1 GeneralFor the purpose of this Code, the following structural systems are considered, according to theirseismic behaviour.

a) Moment resisting frames: horizontal seismic forces are resisted by the flexural behaviour ofmembers and connections.In these structures, dissipative zones are mainly located in plastic hinges near, or in, the beam-upright joints, and energy is dissipated by means of cyclic bending.

b) Frames with concentric bracings, in which members subject to axial forces withstand thehorizontal seismic action. In these structures, dissipative zones are mainly located in the tensiondiagonals.

Other mechanisms for energy dissipation may be considered as described in EN 1998-1.

The behaviour factor q accounts for the energy dissipation capacity of the structure.

The reference values of behaviour factors q for racks are given in the following sections.

For non-regular assemblies in elevation the value of q is reduced by 20% (see Chapter 3.1.4 forregularity criteria); the lower bound value is q = 1.5 (unless specified otherwise).


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Seismic resisting structures connected to the rack (such as independent bracings, frames or shearwalls) shall be designed according to EN 1998-1.

3.1.3.2 Unbraced racks

Unbraced racks shall have plan bracing in seismic design

For racks with 5 beam levels or less it is sufficient to provide one set of horizontal bracing at the top-most beam level at least every 10 bays.For racks with more than 5 beam levels additional horizontal bracing is required at a frequency of oneset per 5 levels at least every 10 bays. The plan bracing shall be evenly distributed in the height of therack with one set at the top-most level.

Figure 3.1. Unbraced rack showing plan bracing

3.1.4 Structural regularity criteria

Regularity is required for both the layout and structural configuration

3.1.4.1 General

Regularity criteria for racks relate to both stiffness and mass distribution, in plan and in elevation. Fora regular configuration all the following criteria shall be met.

In storage racks the seismic weight is mostly due to the stored unit loads and this means that it isimpossible to control the mass regularity for all the possible pallet configurations. When the beams arein a regular pattern it is permissible to assume the mass regularity condition in plan and in elevationfor the relevant conditions for seismic design.

3.1.4.2 Cross aisle directionRegularity in plan Upright frames of an un-braced rack are stiffness regular In a spine braced rack the upright frames that are not connected to the spine bracing are stiffness

regular. However, the upright frames that are connected to the spine bracing and/or plan bracingare not stiffness regular

Upright frames in a braced rack with vertical bracing in each line of uprights are stiffness regular


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Regularity in elevation The upright frame can be regarded as regular if:

diagonal bracings are without interruptions from the floor to the top load level (stiffnessregularity)

ANDthe ratio of the maximum and the minimum distance in elevation between the pallet beams,and between floor and 1 st level pallet beam, is less than 2. When the first beam level is lessthan 1000 mm from the floor it may be excluded from this criterion.

3.1.4.3 Down aisle direction

Regularity in plan Racks not braced in the down aisle direction, or with symmetric bracings in the front and rear lines

of uprights, are stiffness regular in plan Racks braced in one plane only in the down aisle direction are not stiffness regular in plan; the

design rules are specified in 3.4.3 (2)

Regularity in elevation Position of pallet beams

The rack can be regarded as mass regular if:pallet beams are at the same level for the entire length of the run

ANDthe ratio between the maximum and the minimum height between the pallet beams, andbetween the floor and 1 st beam level, is less than 2. When the first beam level is less than1000 mm from the floor it may be excluded from this criterion

Type of vertical bracingRacks with continuous vertical bracing from the floor to the top load level are stiffness regular inelevation.Partially braced racks are not stiffness regular in elevation.

Figure 3.2. Examples of regularity in the down aisle direction


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Figure 3.3. Partially braced rack not regular in the down aisle direction

Figure 3.4a Braced racks not regular in planand not regular in elevation (vertical bracinginterrupted)

Figure 3.4b Braced racks regular in planand not regular in elevation (verticalbracing interrupted)

3.1.5 Layout regularity

Regularity is required for both the layout and structural configuration

Racks are normally provided in long runs. Parallel runs that are structurally independent or that arecontinuously connected to each other along the length and at the top of the uprights can beconsidered as regular. All other cases are not regular.

3.1.6 Rules for the design of low dissipative structures

1) For members that are part of the earthquake resisting structure, the rules on materials given in3.1.2 (1) and (2) apply.

2) The strength of members and connections shall be evaluated according to the rules for elastic orplastic resistance in EN1993-1 and EN 15512.

3) Nuts shall be snug tight and shall incorporate some form of locking device. A tooth flanged nut isan example of a suitable device.

4) Unless otherwise specified in the following, when members that contribute to the seismicresistance of the structure in compression or bending have a section classification 1, 2 or 3 thebehaviour factor q > 1.5 may be used.

5) For racking it is permissible to use K , D, Z bracings, and X bracings without horizontal members,in which the resistance to the horizontal actions is provided by diagonals in compression withq=1.5 provided that a safety factor 1.5 is applied to all bracing members and their connections.Higher values of q may be used if demonstrated by test.

6) For bolted shear connections the shear strength of bolts shall be more than 1.20 times the bearingresistance.This requirement need not to be applied when the bearing strength of the bolted connection isgreater than q times the calculated bolt shear due to seismic action.

7) The strength of a connection does not need to be larger than the internal connection forces as aresult of q times the design seismic action


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3.1.7 Rules for the design of dissipative structures

Dissipative structures shall be designed so that plasticity develops in those parts of the structurewhere yielding or local buckling or other hysteretic phenomena do not affect the stability of the

structure or its members.

The strength of structural members and their connections in dissipative zones shall be evaluatedaccording to the rules for elastic or plastic resistance in EN1993-1 and EN 15512.

Non-dissipative parts of dissipative structures and the connections of the dissipative parts to the restof the structure shall have overstrength to allow the development of cyclic yielding of the dissipativeparts.

For general design criteria of dissipative structural elements and connections reference shall be madeto the relevant parts of EN1998-1; specific rules applicable to racking structures are given in Chapter5.

3.1.8 Anchoring conditions

The designer of the slab shall specify the cracked or uncracked conditions for anchor bolts in theconcrete.Under seismic loading the concrete shall be considered as cracked when:

2.5 S 0.33or when

2.5 S T C 0.133.

Reference shall be made to the following:

ETAG No 001 - Edition 1997 GUIDELINE FOR EUROPEAN TECHNICAL APPROVAL OF METALANCHORS FOR USE IN CONCRETE Annex C: DESIGN METHODS FOR ANCHORAGES Chapter 4.1


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3.2 Structural systems withstanding the seismic action

When regularity layout conditions are met the seismic action can be studied separately in the down

aisle and cross aisle directions, and the lateral force resisting systems can also be consideredseparately.

Unless specified otherwise the critical condition for seismic design is a fully loaded rack because thehorizontal seismic action is maximised.If a structural system other than the rack is provided to withstand the seismic action, it should bedesigned with the criteria of EN 1998-1 or National Regulations.

The racks structural systems withstanding seismic actions are:

1) upright frames, in the cross aisle direction

2) one of the following systems, in the down aisle direction,

a) Unbraced framesThe stability is provided by beam to upright joints, and no vertical bracing; horizontalbracing shall be provided connecting the front and the rear frames.

b) A single line of vertical bracingThe bracing system consists of the following elements: a spine bracing placed behind the rear frame, which can be an independent structure

connected to the rack, or bracing elements connected directly to the rack. horizontal bracing connecting the front unbraced line of uprights to the rear braced

uprightsThe vertical bracing withstands the horizontal seismic action.The horizontal bracings and the upright frames connected to the horizontal bracings carry theinduced torsional effects due to the eccentricity of the seismic action with respect to thevertical bracing.

c) Symmetrical vertical bracing (each line of uprights is braced).Vertical bracings withstanding seismic action are present in a limited number of bays, in theline of the front and rear uprights.Horizontal bracings connecting the front and the rear uprights are also provided according to3.1.3.


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3.3 Structural analysis

3.3.1 Sub-modelling

It is permissible to consider the rack as a number of separate sub-structures provided that thefollowing conditions are met.

1) Each lateral resisting system shall be analysed individually, subject to the pertinent seismic action,by means of sub-models (i.e. upright frames, vertical bracing)

2) Each subsystem shall be analysed considering all the seismic resistant elements with the massesthat affect the behaviour of the sub-structure (i.e. rear bracing, horizontal bracing and connectedupright frames).

3.3.2 Arrangement of the masses

The most unfavourable loading configuration shall be considered for the seismic analysis.

The following could be considered to find the relevant ones:

1) The analysis in the cross aisle direction must be regarded as a local analysis, and the most severelocal loading configuration must be found for the elements of the upright frame.

In principle it is necessary to consider all possible load configurations that lead to the worst caseconditions in the frame. However, it is permitted to consider only the following configurations

fully loaded top level only loaded

NOTE: The probability that an intermediate condition exists at the same time as the designearthquake occurs is small and therefore it is not necessary to consider further conditions.

The upright frame may always be considered as mass regular in elevation

2) Analysis in the down aisle direction is a global analysis, and the most relevant action is obtainedwhen the rack is fully loaded. Nevertheless, the mass distributions that maximize the internalforces in each element should also be considered for checking the uplift at the base of the uprightin braced racks.It is permissible when checking the uplift to consider that the uprights involved in the verticalbracing system are subjected only to 30%. However, the seismic action shall be determined withthe rack being fully loaded.

3.3.3 Specific modelling requirements for the analysis

1) Rules for the global analysis of the racks are given in EN 15512 Chapter 10.

2) In the down aisle direction, the stiffness of the beam-to-column connection and of the floorconnections (baseplates) shall be the value of the stiffness obtained from static loads testsaccording to EN 15512 Chapter A.2.4 and A.2.7 respectively; the beam-end connector stiffnessshall be determined in accordance with EN 15512.

NOTE: may be chosen


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=1.0 will give lower stiffness, lower seismic forces but a lower plastic reserve. This justifies thatq=2.0 may be considered in any case.

3) In the cross aisle direction, the shear stiffness of the upright frame shall be equal to the design

value obtained according to EN 15512 Chapter A.2.84) When bracings with tension only diagonals are used, only the active elements in tension shall be

considered in the model.The elements in the model shall be consistent with the load path of the horizontal forces.

5) Bracing

a) vertical bracing shall be modelled with the appropriate eccentricity to the rack elements andshall include the elements connecting the vertical bracing to the rack.

b) horizontal bracing shall be modelled to include the effect of its connections.


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3.4 Structural types and behaviour factors

3.4.1 Upright frames

Possible frame bracing arrangements are given in figure 3.5. Table 3.2 gives a review of theprocedure to follow.

(a) (b) (c) (d) (e) (f) (g) (d 1)

X -

b r a c e

d f r a m e w

i t h

h o r i z o n

t a l e l e m e n

t s

b a

t t e n e

d

( V i e r e n d e e

l )

f r a m e

p a r t

i a l l y

b r a c e

d f r a m e

Z -

b r a c e d

f r a m e

D

b r a c e d

f r a m e

K

b r a c e d

f r a m e

X

b r a c e d

f r a m e

Z -

d i s s i p a

t i v e

b r a c e

d f r a m e

Figure 3.5. Frame bracing arrangements

In case of frames with tension and compression diagonals and low dissipative concept (as given inbracing scheme a.3), the horizontal bracing members shall be designed for 50% of the horizontalshear force in the frame.


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Table 3.2. Seismic Design procedure for upright frames

Frame

typeStructural type Detailing rules for

dissipative elements

Reference

behaviour factor

Safety factor and /ordesign rules for

bracings elementsand connections

a

(a.1) Diagonal bracingwith active tension

diagonals

Tension diagonal5.2

Diagonal connections5.1.1

2 or 4(Note 1) See Chapters5.1 and 5.2

(a.2) Diagonal bracingwith active tension

diagonalsLow dissipative 2.0 1.0

(a.3) Diagonal bracing

with tension andcompression diagonals Low dissipative 1.5 1.0

b dissipative battened frame can be used, provided that the requirements of moment resistingframes are met; otherwise q = 1.0c 1.0 1.0

d-e-f-g Low dissipative 1.0 or 1.5(Note 2) 1.5

d 1Eccentric braced frame with

energy dissipation in the horizontalsDesign according to EN 1998-1 Chapter 6.8

4

NOTE (1) q=4 can be taken for X bracings with active diagonal elements in tension when the ductilityrequirements of Chapter 5.2 are met.The requirement for dissipative homogeneous behaviour described in Chapter 5.2.3 for theupright frame is necessary to permit this structure to behave as bracing.When this requirement is not met, plasticity is concentrated in a limited part of the frame,generally at the base, and the behaviour of the upright frame can be regarded as aninverted pendulum cantilever, and q=2 is taken according to EN1998-1.Type a frame can also be designed according to the low dissipative concept (a.2 and a.3)

NOTE (2) If q=1.5, a safety factor of 1.5 is applied to all bracing members and their connections (SeePoint 5) of 3.1.5)

NOTE (3) Eccentricities of the connections must be consistent with the rules of EN 15512

NOTE (4) It is not permitted to use X-bracing in which both the tension and compression diagonalsare active for concept B (dissipative design). The displacement induced in the compressiondiagonal that is necessary to cause the tension diagonal to yield is so great that theresidual resistance of the compression diagonal becomes negligible.

3.4.2 Moment resisting framesIn moment resisting frames dissipative zones shall be located in beams, or beam to columnconnections, or column bases.

For low dissipative concept a minimum value of q=1.5 is a conservative approach.

The behaviour factor q=2 may be used when the following conditions are met


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Seismic Design for Racks - v 1 01 Oct 2010