British Comparison EC3 and EC4

Steel and Steel and ConcreteComposite Buildings

Companion Document toEN 1993 and EN 1994

On 5th May 2006 the responsibilities of the Office of the Deputy Prime Minister (ODPM)transferred to the Department for Communities and Local Government (DCLG)

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Companion Document to EN 1993 and EN 1994 Steel and Steel and ConcreteComposite Buildings 2005Whilst this document provides practical guidance on the use of Eurocode BS EN 1993 and 1994 Steel and Steel and Concrete Composite Structures. It shall not be used for the design ofactual projects until both the Eurocode and its National Annex are published by the BritishStandards Institution and approved for use by the First Secretary of State for England andWales.It should be noted that the guidance has been based on the latest draft EurocodeBS EN 1993 and 1994 available at the time of writing.

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January 2007

Product Code: 06 BD 04021 (f)

CONTENTS

EXECUTIVE SUMMARY 7

CHAPTER 1 INTRODUCTION 8

1.1 Steel Structures 8

1.2 Composite steel and concrete structures 9

1.3 Aim and scope of this publication 9

CHAPTER 2 EUROCODES SYSTEM 10

2.1 Eurocodes Terminology 12

2.1.1 Types of clause used in the Eurocodes 12

CHAPTER 3 GENERAL DESIGN ISSUES 14

3.1 Convention for member axes 14

3.2 The explicit use of factors 143.2.1 Symbols used in the Eurocodes 15

3.3 Documents required when designing with the Eurocodes 16

CHAPTER 4 EN1993 STEEL STRUCTURES 17

4.1 Part 1-1: General Rules and Rules for Buildings 17

4.1.1 Material Properties 17

4.1.2 Ductility requirements for structural steel 17

4.1.3 Fracture toughness 18

4.1.4 Structural stability of frames 18

4.1.5 Structural imperfections 19

4.1.6 Buckling members in compression 19

4.1.7 Buckling uniform members in bending 20

4.1.8 Buckling uniform members in bending and axial compression 21

4.2 Part 1-2: General rules Structural fire design 21

4.2.1 Material properties 21

4.2.2 Structural fire design 23

4.2.3 Members in compression 23

4.2.4 Combined bending and axial compression 23

4.2.5 Structural connections 23

4.3 Part 1-8: Design of joints 23

4.3.1 Definitions 24

4.3.2 Material properties 24

4.3.3 Groups of fasteners 24

4.3.4 Analysis, classification and modelling 24

4.3.5 Structural joints connecting H or I sections 26

4.4 Part 1-10: Material toughness andthrough-thickness properties 27

4.4.1 Fracture toughness 27

4.4.2 Through-thickness properties 28

4.5 Part 5: Steel Piling 29

CHAPTER 5 EN1994 STEEL AND CONCRETECOMPOSITE STRUCTURES 31

5.1 Part 1-1: General rules and rules for buildings 31

5.1.1 Material Properties 31

5.1.2 Structural stability 32

5.1.3 Structural imperfections 32

5.1.4 Calculation of action (load) effects 32

5.1.5 Beams Ultimate Limit State 33

5.1.6 Beam serviceability limit state 34

5.1.7 Lateral torsional buckling 35

5.1.8 Members in compression 35

5.1.9 Composite joints in frames for buildings 36

5.1.10Composite slabs with profiled metal sheeting 36

5.2 Part 1-2: Structural fire design 37

5.2.1 Fire exposure 37

5.2.2 Material Partial Factors 37

5.2.3 Structural analysis 37

5.2.4 Design procedures 37

5.2.5 Unprotected Composite Slabs 38

CHAPTER 6 EFFECTS ON UK STRUCTURALDESIGN PROCEDURES 40

5CHAPTER 7 DESIGN ROUTE MAPS 41

CHAPTER 8 REFERENCES 54

APPENDICES 56

Appendix A Eurocode clause reference tables 56

WORKED EXAMPLES 61

Anchored Sheet Pile Wall 62

Cantilever 83

Cantilever Sheet Pile Wall 90

Simply Supported Beam with Full Lateral Restraint 104

Simply Supported Beam with Lateral Restraint at the Load Points 111

Steel Driven Pile in Stiff Clay 119

Base Plate without Bending Moment 128

Simply Supported Beam with Full Lateral Restraint Fire Limit State 134

Simply Supported Composite Beam Fire Limit State 148

Partial Depth (flexible) End Plate Connection 157

Connections in Fire 176

Column in Simple Construction Fire Limit State 186

Column with Axial and Bi-Axial Moments (Due to simple connection) 195

Simply Supported Steel and Concrete Composite Beam 210

Concrete Filled CHS Composite Column 223

Continuous Steel and Concrete Composite Beam 235

Companion Document to EN 1993 and EN 1994 Steel and Steel and Concrete Composite Buildings

6

Executive Summary

7

Executive Summary

The aim of this Companion Document is to provide UK designers with an overview of the

Eurocodes system, and with detailed information for the principal parts of Eurocode 3 and

Eurocode 4 namely:

Eurocode 3

Part 1-1 General rules and rules for buildings

Part 1-2 Structural fire design

Part 1-8 Design of joints

Part 1-10 Material toughness and through-thickness properties

Part 5 Piling

Eurocode 4

Part 1-1 General rules and rules for buildings

Part 1-2 Structural fire design

The scope of this document was developed in consultation with industry. It comprises:

An overview of the impact that Eurocodes 3 and 4 will have in the UK

Route maps for the design of building elements to the Eurocodes in the UK

The major technical differences between the Eurocodes and the UK Standards

The document focuses on guidance for buildings. Design guidance relating to bridges and

other civil engineering works is not considered. Where the Eurocode design guidance is the

same as that currently (late 2004) given in British Standards or there is little change between

the Codes no discussion has been included. To keep this document concise detailed design

guidance is not presented.

BRE and Buro Happold have made every effort to ensure the accuracy and quality of all the

information in this document when first published. However, they can take no responsibility for

the subsequent use of this information, nor for any errors or omissions it may contain.

Queen's Printer and Controller of Her Majesty's Stationery Office


8

1 Introduction

The objectives of the Eurocodes are

To establish a common set of design rules for buildings and civil engineering works to be

used across Europe.

To remove the barriers to free movement of products and engineering services between

European countries, by removing the obstacles caused by different nationally codified

practices for the assessment of structural reliability.

The emerging Eurocodes (ENs) have been developed following work undertaken to modify

the European Prestandards (ENVs). The ENVs were published with National Application

Documents in the early 1990s to allow Designers to undertake provisional designs and make

comments on their content. Unlike the Eurocodes the ENVs did not have the status of

European Standards.

Following a period of co-existence the current British Standards will be superseded by the

Eurocodes. These Eurocodes will be denoted as BS EN in the UK.

The Eurocodes can be considered to be divided into codes that provide fundamental

guidance for structural design (Basis of Structural design), guidance that may apply to all

designs (loads, geotechnics and seismic) and detailed guidance for structural materials (steel

concrete etc.).

1.1 Steel Structures

EN 1993 (Eurocode 3) gives structural design rules for steel structures. It is divided into six

main design areas, which are sub-divided into the following parts:

Part 1 General rules and rules for buildings Part 1-1 General rules and rules for buildings Part 1-2 Structural fire design Part 1-3 General rules Supplementary rules for cold-formed members and sheeting Part 1-4 Stainless steels Part 1-5 Plated structural elements Part 1-6 Strength and stability of shell elements Part 1-7 Strength and stability of planar plated structures transversely loaded

1

Part 1-8 Design of joints Part 1-9 Fatigue strength of steel structures Part 1-10 Material toughness and through-thickness properties Part 1-11 Design of structures with tension components made of steel Part 1-12 Additional rules for the extension of EN 1993 to steel grades up to S700

Part 2 Steel Bridges

Part 3 Towers, masts and chimneys

Part 3-1 Towers and masts

Part 3-2 Chimneys

Part 4 Silos, tanks and pipelines

1 It should be noted that while there is an ENV version of part 1-7 there may not be an EN

version of this part of Eurocode 3.

Introduction

9

Part 4-1 SilosPart 4-2 TanksPart 4-3 Pipelines

Part 5 Piling

Part 6 Crane supporting structures

1.2 Composite steel and concrete structures

EN1994 (Eurocode 4) gives structural design rules for composite steel and concretestructures. It is divided into two main design areas, which are sub-divided into the followingparts:

Part 1 General rules and rules for buildingsPart 1-1 General rules and rules for buildingsPart 1-2 Structural fire design

Part 2 Bridges

1.3 Aim and scope of this publication

The aim of this Companion Document is to provide UK designers with an overview of theEurocodes system, and with more detail given for parts of Eurocode 3 and Eurocode 4.

This Companion Document focuses on the guidance given for buildings. Design guidancepresented in the Eurocodes relating to bridges and other civil engineering works is notconsidered.

The main differences between the current British Standards (2004) and the Eurocodes 3 and4 are discussed. Where the design guidance is the same or there is little change between theCodes no discussion has been included.

To keep this document concise detailed design guidance is not presented.

The parts of Eurocode 3 and 4 that are covered by this companion document are:

Eurocode 3Part 1-1 General rules and rules for buildingsPart 1-2 Structural fire designPart 1-8 Design of jointsPart 1-10 Material toughness and through-thickness propertiesPart 5 Piling

Eurocode 4Part 1-1 General rules and rules for buildingsPart 1-2 Structural fire design


10

p g

2 Eurocodes System

The numbering system used by the structural Eurocodes is EN199#-#-#: ####. The 199#number is not the publication date, but the number of the Eurocode. The second and third #denote the part of the Eurocode. The year of publication is given after the Eurocode number(####). Eurocode 3 part 1.1 is used here to illustrate the Eurocodes numbering system thatwill be used in the UK, BS EN 1993-1-1:2004. The letters BS are added to the front of theEurocode number to show that it has been published by BSI2 and contains the UK Nationaltitle page, forward and annex.

The structural Eurocode system will contain the following codes:

BS EN 1990 Basis of Structural DesignBS EN 1991 Actions on StructuresBS EN 1992 Design of Concrete StructuresBS EN 1993 Design of Steel StructuresBS EN 1994 Design of Composite Steel and Concrete StructuresBS EN 1995 Design of Timber StructuresBS EN 1996 Design of Masonry StructuresBS EN 1997 Geotechnical DesignBS EN 1998 Design of Structures for Earthquake ResistanceBS EN 1999 Design of Aluminium Structures

The organisation of design guidance in the Eurocode system is different to the current BritishStandards (BS) system. Safety, serviceability and durability design guidance for differenttypes of structures is presented in BS EN 1990 (Basis of Structural Design), the current BSsystem presents this design guidance within each material code. Therefore a copy of Basisof Structural Design is required for all designs performed using the Eurocodes. For both theEurocodes and current BS systems product standards are used with design codes. The linksbetween the different Eurocodes are shown in Figure 1.

Figure 1. Links between the individual Eurocodes

2 British Standards Institute

EN1990

EN1991

EN1992, EN1993, EN1994

EN1995, EN1996, EN1999

EN1997 EN1998

Structural safety,serviceability & durability

Actions on structures

Design & detailing(material codes)

Geotechnical & Seismicdesign

Product harmonisedtechnical standards

ETAs

Eurocodes System

11

The individual material Eurocodes are divided into parts. Part 1 gives general rules and rules

for buildings, Parts 2, 3 etc. give rules for other applications (bridges etc.). These high level

parts are divided into sub-parts.

In addition to the inter-action between the materials codes and Basis of Structural Design the

parts of each material code may cross-reference each other. This is due to the Eurocodes

presenting guidance in only one place (i.e. rules are not repeated in several parts) and

subsequently referring to that clause in other parts of the Eurocode. In some cases parts of

different material Eurocodes may be referenced e.g. a part of EN 1994 (Composite Steel and

Concrete Structures) may reference a part of EN 1992 (Concrete Structures) or EN 1993

(Steel Structures).

Each part of a Eurocode published by a National Standards Authority will be divided into

distinct sections, these are:

National title page

National forward

EN title page

EN main text

EN Annex(es)

o Normative Annexes contain design rules / methods / values to be used when designing

to the Eurocode.

o Informative Annexes contain recommended design rules / methods or informative

values, e.g. snow densities.

National Annex

The technical content of the EN main text and EN Annex(es) is the same across the whole of

Europe. Those sections and the EN title page make up the 'EN' document published by

CEN3. The National Standards Authority (BSI in the UK) is responsible for developing and

publishing the National title page, National forward and National Annex. The addition of these

National sections in the UK makes the 'EN' document in to a 'BS EN' document.

Each part of a Eurocode will have an accompanying National Annex. These annexes will

contain information that should be referred to when designing a structure to be constructed in

that country. Therefore if a UK designer was designing a building to be constructed in France

they would need to refer to the French National Annexes for all the Eurocodes used during

design and not the UK National Annexes.

The National Annex will contain information on the values / methods that should be used,

where a national choice is allowed in the main text of the Eurocode. The national choices are

collectively referred to as Nationally Determined Parameters (NDPs). NDPs may be given for

coefficient values, loads (both applied and self-weight) and where a choice in design

approach is given. The EN main text specifies recommended values / approaches, the

National Annex can either accept the recommendations given or specify different values /

approaches to be used.

The National Annex will state how / if the content of an Informative EN Annex may be used for

the design of structures to be constructed in that country. Information given in a Normative EN

Annex may only be altered by the National Annex if the EN text allows different rules / values

to be given in the National Annex. References may be given to separate documents that give

guidance to help with the design of a structure. Such guidance is known as Non-Conflicting

Complementary Information (NCCI) and may not be presented in the National Annex itself.

3 European committee for standardization


12

The numbering system used in the Eurocodes follows the International Organisation for

Standardisation (ISO) practice i.e. a comma is used in place of a decimal point.

2.1 Eurocodes Terminology

The Eurocode system uses different terminology to that used in the current BS system. An

important change that will effect every design approach is the change in terminology for

loading. In the Eurocodes the term loads is replaced by the term actions. The Eurocodes

also introduce the terms permanent action, variable action and accidental action.

Permanent actions include the self-weight of the structural and non-structural elements.

These self-weights are combined to form a single value for consideration during design

checks. Loads due to prestressing are also considered as permanent actions.

Variable actions are defined in Basis of Structural Design as actions for which the variation in

magnitude with time is neither negligible nor monotonic. Loads considered as variable

actions include:

Imposed floor & roof loads

Snow loads

Wind loads

Variable actions are sub-divided into two groups:

Leading variable actions

These are variable actions which when acting on a structure cause the most significant

structural effects.

Accompanying variable actions

These are variable actions that act on a structure at the same time as the leading

variable action.

Accidental actions are caused by events that usually have a short duration but have a

significant effect. It is considered that such events have a low probability of occurrence

during the design working life of a structure. Accidental design situations that should be

considered include fire and explosion.

Some variable actions may be classed as accidental actions for design checks. These are,

snow, wind and seismic. The Eurocodes and National Annexes identify when they may be

considered as accidental actions.

Another difference in the terminology used is that the Eurocodes use the term "resistance"

rather than "capacity" when defining the value of the forces that can be resisted by an

element before it fails i.e. moment resistance, shear force resistance etc.

The term "execution" is used in the Eurocodes to define all the processes associated with the

erection of a building or civil engineering works. The term may be applied to both on and off

site processes.

2.1.1 Types of clause used in the Eurocodes

The Eurocodes define two types of clause, Principles and Application rules. These terms will

be new to UK designers as the current BS system does not contain these clause types.

Principles are generally denoted by the letter P following a clause number, e.g. 1.3(2)P.

Principles are general statements and definitions for which there is no alternative, as well as,

Eurocodes System

13

requirements and analytical methods for which no alternative is permitted unless specifically

stated.

Application rules are generally denoted by a clause number without the letter P, e.g. 1.3(2).

Application rules are generally recognised rules which comply with the Principles and satisfy

their requirements. It is permitted to use alternative design rules in place of those given in

Application rules. However, it must be shown that the alternative design rules meet the

requirements of any relevant Principles. It must also be shown that the alternative rules

provide equivalent structural safety, serviceability and durability to that expected from the

Eurocodes. If a design is carried out using an alternative rule to that given in an Application

rule the design cannot be said to be wholly in accordance with the Eurocode. However, it can

be said that the design is in accordance with the Principles of the Eurocode. This may have

implications for CE marking.

The Eurocodes also use different terms to identify when a rule must be used or when an

alternative to that given can be used. When the term shall is used in a clause the rule must

be used (as for a Principle). If a clause contains the word should an alternative to that rule

can be used (as for an Application rule).

The majority of Eurocodes make the distinction between Principle and Application rules using

the notation discussed earlier. However of the Eurocodes considered by this Companion

Document, EN1993-1-1 (General rules), EN1993-1-2 (Fire design) and EN1993-1-10

(material toughness and through-thickness properties), do not currently (November 2004) use

the letter P to denote a Principle, instead only the term shall identifies a rule as a Principle.

EN1993-1-1 does present supplementary guidance for the design of steel buildings, denoted

by the letter B after the clause number e.g. 5.1.1(4)B.


14

3 General Design Issues

3.1 Convention for member axes

The Eurocodes define the member axes differently to BS 5950. The Eurocodes system is in

keeping with the system generally used in computer software for global structural analysis. It

defines the longitudinal axis of the member as x-x, with the major axis of the cross-section as

y-y and the minor axis as z-z. The convention used in BS 5950 defines the major axis of the

cross-section as x-x, the minor axis as y-y and the longitudinal axis of the member as z-z.

The same convention is used for the u-u and v-v axes for angle sections in both Eurocode 3

and BS 5950.

Designers unfamiliar with using the Eurocodes should pay particular attention to the

difference in axes convention. This is particularly important when using section tables that

use the BS 5950 convention. Figure 2 shows the axes convention and notation used for a

universal beam section.

Figure 2. Member axes convention and dimension symbols used in the Eurocodes

3.2 The explicit use of factors

In contrast to the current British Standards the Eurocodes do not hide the material partial

factors ( Mi). This results in expressions appearing more complex, or different property values

compared with those currently used in the UK.

An example of expressions with an increase in the number of terms from the British Standard

to the Eurocodes is the resistance of a cross-section for uniform compression:

0M

yRd,c

AfN

= For Class 1, 2 or 3 cross-sections

Where: NcRd is the resistance of the cross-section for uniform compression (N)

A is the cross-sectional area (mm2)

fy is the yield strength (N/mm2)

M0 is the partial material factor for the resistance of the cross-section

d

tf

r

tw

b

h

z

z

y y

M0

(Mi)

General Design Issues

15

3.2.1 Symbols used in the Eurocodes

The Eurocodes use different symbols for section properties compared with those used in BS5950. The section properties with different symbols used in the Eurocode and BS 5950 aregiven in Table 1 and shown in Figure 2. Section properties not included in Table 1 have thesame symbols in both codes.

Table 1. Section properties with different symbols used in the Eurocodes and BS 5950

Symbol used in design code

Section property BS 5950 Eurocode

Depth of cross-section D hEffective section modulus Zeff Weff

Elastic section modulus Z Wel

Flange thickness T tf

Net area of cross-section An Aeff

Outer diameter of circularsections

D d

Plastic section modulus S Wpl

Radius of gyration r iRadius of root fillet -channel sections*

r r1

Torsional constant J IT

Warping constant H Iw

Web thickness t tw

Width of cross-section B b* Symbol used for radius of root fillet for other sections does not differ between codes

In addition to the section property symbols given in Table 1, symbols for other coefficients andvalues differ between the Eurocodes and British Standards. Table 2 presents some Latinupper case letters used in the Eurocodes to define actions and forces. The letters given inTable 2 define a number of different terms within the British Standards therefore a directcomparison can not be given.

Table 2. Examples of Latin upper case letters used within the Eurocodes to define actionsand forces

Terms Latin upper case letter used

within the Eurocodes system

Actions (General) FPermanent action G

Variable action Q

Moment M

Axial force N

Shear force VResistance of element (used as themain symbol or as a subscript)

R

Effect of an action (used as a subscriptto one of the above)

E


16

The symbols used by the Eurocodes can have long chains of subscripts. This appears

cumbersome at first, however with use this system will be found to help interpretation

because the subscripts result in symbols that are nearly self defining. The multiple subscripts

used in the Eurocodes have been assembled following the guidance given in ISO3898: 1987,

commas are used to separate the multiple subscripts. Examples of the use of multiple

subscripts in the Eurocodes are given in Table 3.

Table 3. Examples of symbols with multiple subscripts used in the Eurocodes

Terms Eurocode symbol

Design bending moment about the y-y axis My.Ed Design resistance to bending moment about the y-y axis My.Rd

Characteristic resistance to bending moment about the y-y axis My,Rk Plastic design shear resistance Vpl,Rd Design resistance to tension forces Nt,Rd Effective cross-sectional area for local buckling when considering plate buckling

Ac,eff,loc

Minimum elastic section modulus Wel.min

3.3 Documents required when designing with the Eurocodes

The Eurocodes present Principles and Application rules for design rather than design

guidance. This approach results in information that is considered to be textbook information

being omitted. Therefore the designer must rely on appropriate textbooks/design guides to

provide this information. Information that is omitted from Eurocodes 3 and 4 includes:

Calculation of buckling lengths for members in compression.

Determining the non-dimensional slenderness parameter for later torsional buckling and

torsional or flexural torsional buckling.

Determining the critical moment for lateral torsional buckling.

Tables giving expressions to determine moments in continuous beams. The above list should not be considered as exhaustive.

The structure and the content of the Eurocodes results in the following documents being

required for design:

Eurocodes

o EN1990 Basis of Structural Design

o EN1991 Actions on Structures

o EN199# - Material codes (normally several parts will be needed)

o EN1997 & EN1998 Geotechnical and Seismic design

Textbooks, design guides or similar sources of information

Product standards / manufacturers information

EN 1993 Steel Structures

17

4 EN1993 Steel Structures

The following sections highlight the main differences between the guidance given in Eurocode3 and BS 5950.

4.1 Part 1-1: General Rules and Rules for Buildings

EN1993-1-1 [1] (hereafter referred to as EC3-1-1) gives general structural design rules forsteel structures and buildings. Steel grades from S235 to S460 are covered by the guidancegiven in EC3-1-1. BS 5950: Part 1 covers steel grades from S275 to S460. Part 1-12 ofEurocode 3 will present guidance that can be used to apply the rules given in part 1-1 to steelgrades up to S700.

4.1.1 Material Properties

Clause 3.2.1(1) of EC3-1-1 allows the National Annex to choose between the nominal valuesfor the yield and ultimate strength of structural steel given in the product standard BS EN10025 and those given in Table 3.1 of EC3-1-1.

The material properties for structural steels given in BS 5950: Part 1 [2] are based on theproperties given in the product standard BS EN 10025 [3].

The main difference between the properties given in the product standard and those given inEC3-1-1 is that the simplified table in EC3 uses a reduced number of thickness steps. Theresult is that for steel thickness between 16mm and 40mm and between 63mm and 80mm thevalues given in Table 3.1 of EC3-1-1 are approximately 4% higher than those values given inboth BS 5950:Part 1 and BS EN 10025. Furthermore, Table 3.1 only gives values up to80mm thick while BS EN 10025 gives values up to 250mm and BS 5950 Part 1 has amaximum thickness of 150mm. The UK National Annex to EC3-1-1 may recommend the useof the nominal values given in BS EN 10025 in place of those given in Table 3.1.

4.1.2 Ductility requirements for structural steel

The ductility requirements given in EC3-1-1 apply to all steels regardless of the method usedfor global analysis. Whilst EC3-1-1 allows the National Annex to define ductility limits, theEurocode recommended limits are:

fu/fy 1.10 Elongation at failure not less than 15% u 15y

Where:fu is the ultimate strengthfy is the yield strengthu is the ultimate strainy is the yield strain (fy / E)

BS 5950: Part 1 has a different approach. It states that the design strength py should not begreater than Us/1.2 where Us is the minimum tensile strength Rm specified in the relevantproduct standard (BS EN 10025). This limit applies to all grades of steel regardless of the


18

method used for global analysis. However when plastic global analysis is used the steel

grades must satisfy the following additional criteria:

fu/fy 1.20

Elongation at failure not less than 15%

u 20 y

A comparison of the above limits shows that the EC3-1-1 limits are less onerous that those

given in BS 5950: Part 1.

The reason for the differences in the two sets of recommendations has been difficult to

establish but the following comments on the development of the limits used in both BS 5950

and EC3-1-1 might be helpful in understanding the code writers' thinking.

The origin of the BS 5950: Part 1 rules was the old BCSA black book 23 or 29 which

extended plastic design from BS15 steels (later grade 43 and now called S275) to BS968

steels (later grade 50 and now called S355). The 1969 amendment extended BS 449 to

grade 50 for elastic analysis. For the early draft of BS 5950 the issue of allowing plastic

design of grade 50 steel in the UK was considered. On the basis of specific tests it

seemed plastic design could be allowed with smaller b/t and d/t limits i.e. for more

compact sections. The use of a general rule to avoid having to test every new grade of

steel was investigated. Professor Horne was consulted and his view was that the only

way to be sure a steel was NOT alright would be if it failed specific tests, but that it was

possible to make an informed judgement about parameters that would help decide if a test

was even necessary. As a result of these discussions a set of rules specific to plastic

global analysis were developed which meant than any steel that satisfied them was

satisfactory. A steel that did not meet these criteria might also be satisfactory but specific

tests were needed to be certain it could be used for plastic global analysis.

The EC3-1-1 drafting panel had a wider definition of plastic analysis than that used in the

UK. Their understanding was that plastic analysis or even plastic design means not only

plastic global analysis but that using the plastic modulus of a class 1 or class 2 cross-

section is also plastic analysis. The wider definition may have contributed to the

difference in values given in EC3-1-1 and BS5950: Part 1 for the plastic analysis limits.

4.1.3 Fracture toughness

EC 3 and BS 5950 use different terminology and different approaches to establish the fracture

toughness of a material to avoid brittle fracture. BS 5950: Part 1 uses the minimum service

temperature, Tmin, to determine fracture toughness. In the UK Tmin is usually taken as -5C

for internal steelwork and -15C for external steelwork. The method used in EC3 is based on

a reference temperature of TEd which is determined from equation 2.2 of EC3-1-10 (see

section 4.4.1 for further details).

4.1.4 Structural stability of frames

In both standards the designer is required to determine if the effects of the deformed

geometry of the structure will significantly affect or modify structural behaviour, for example by

introducing additional (secondary) moments. In EC3-1-1 this is achieved by checking that the

critical load factor, cr, for the structure under consideration satisfies the following limits:

cr 10 for elastic analysis

cr 15 for plastic analysis

fu/fy 1.20

u 20y

cr1

cr

cr


19

If cr is above these limits then the effects of deformed geometry (second order effects) can

be neglected and a first order analysis may be used. If cr is less than 10, or 15, then the

effects of the deformed geometry should be considered. This defines the boundaries, but

unlike BS5950: Part 1 EC3-1-1 does not use the terms non-sway and sway sensitive to

describe the frames.

The limit used for elastic analysis in BS 5950: Part 1 is identical to that used in EC3-1-1. The

only difference is that the limit in BS 5950: Part 1 is for clad structures where the stiffening

effect of the cladding is not explicitly taken into account when calculating the elastic critical

load factor. No such limitation is placed on the method given in EC3. Consequently, bare

steel frames designed using EC3-1-1 may be less stiff than those designed to BS 5950.

Unlike EC3-1-1, BS 5950: Part 1 includes two simplified methods for taking account of

secondary effects for the plastic design of multi-storey rigid frames and a separate method for

the plastic design of portal frames.

4.1.5 Structural imperfections

A feature of EC3-1-1 is its explicit allowance in the calculation procedures for practical

imperfections that have an influence on the resistance of members or structures. A number

of alternative procedures are given in Section 5.3, some with limited scope. Generally they

consider:

System imperfections

An initial-bow imperfection is introduced in the design of braced bays and built up

compression members. In the case of bracing systems any additional deflections due

to the action of the bracing system in resisting externally applied forces also have to

be taken into account.

Frame imperfections

These are introduced into the analysis of all frames in the form of an equivalent initial

sway. For convenience this can be replaced by a closed system of equivalent forces,

except when determining reactions onto foundations. The frame imperfections are

intended to account for the possible effects of other forms of imperfection which may

affect the stability of frames such as lack-of-fit.

Member imperfections

These are introduced in the design of compression members through a series of

imperfection factors which represent an equivalent lack of straightness. The values of

the imperfection factors also account for the effects of typical residual stress patterns.

Local bow imperfections of members, in addition to global sway imperfections, should

be included in the global analysis of frames that are sensitive to second order effects.

While BS 5950: Part 1 does not disallow this method of analysis system, frame and member

imperfections are not explicitly included in the standard. An allowance is made for them

within the buckling curves given in BS 5950: Part 1.

4.1.6 Buckling members in compression

BS 5950: Part 1 uses a modified Perry formula to determine member buckling resistance.

This method is described in Annex C of BS 5950: Part 1. In EC3-1-1 the member buckling

resistance is derived from the resistance of the cross-section by applying a reduction factor,

. Different values of are determined for flexural buckling ( y (y-y axis) or z (z-z axis)),

lateral torsional buckling ( LT), torsional ( T) and torsional-flexural buckling ( TF). The

reduction factor is a function of an imperfection factor ( ) and the non-dimensional

BS 5950: Part 1 uses a modified Perry formula to determine member buckling resistance.This method is described in Annex C of BS 5950: Part 1. In EC3-1-1 the member bucklingresistance is derived from the resistance of the cross-section by applying a reduction factor,. Different values of are determined for flexural buckling (y (y-y axis) or z (z-z axis)),lateral torsional buckling (LT), torsional (T) and torsional-flexural buckling (TF). Thereduction factor is a function of an imperfection factor () and the non-dimensional

crcr


20

slenderness ratio of the compression member. is a function of the slenderness ratio ofthe member Lcr/i, where Lcr is the buckling length in the plane of buckling. The buckling lengthis similar to the effective length used in BS 5950: Part 1. Unfortunately, unlike BS 5950: Part1, EC3-1-1 does not give guidance on the buckling lengths to be used. Consequently,guidance on the buckling lengths (or effective length) must be obtained from either BS 5950:Part 1, design guides or appropriate textbooks.

Another feature of EC3-1-1 is the introduction of two additional checks for members with opencross-sections subject to compression. These checks are for the torsional and torsional-flexural buckling of members in compression. The methods use the same base equations

used for flexural buckling but with the non-dimensional slenderness replaced by either thenon-dimensional slenderness for torsional ( T ) or torsional-flexural buckling ( TF ). Theseparameters can be used to determined either T or TF and either the elastic torsional flexuralbuckling force or the elastic torsional buckling force of the member. EC3-1-1 does not includeguidance on how to calculate these two parameters and the designer must rely on anappropriate textbook.

4.1.7 Buckling uniform members in bending

In EC3-1-1 the lateral torsional buckling of a laterally unrestrained beam is determined fromthe resistance of the cross-section by applying a reduction factor, . The reduction factor, LT,is a function of both the imperfection factor ,LT, and the non-dimensional slenderness ratio,

LT , of the beam. This approach is similar to the method used for calculating the bucklingresistance of a column.

The method used in BS 5950: Part 1 is different and is based on a modified Perry-Robertsonexpression. A full description of this method is given in Annex B of BS 5950: Part 1.

The main difference between these two methods is that while BS 5950 is based on thecalculation of the equivalent slenderness , LT, EC3-1-1 requires the designer to evaluate theelastic critical moment for lateral-torsional buckling (Mcr) as an intermediate step before

calculating the non-dimensional slenderness ratio , LT . This is the traditional way ofevaluating LT but unfortunately EC3-1-1 does not include data for the evaluation of Mcr.Designers must therefore rely on an appropriate textbook.

Furthermore, EC3-1-1 contains two methods for calculating the lateral torsional buckling of amember. These are:

The general case, and A method specifically for rolled sections or equivalent welded sections.

The second method has been calibrated against test data and has been shown to givereasonable results for rolled sections. The calibration also showed the method to beunsatisfactory for welded sections. It is therefore suggested that designers use the generalcase for welded sections and the specific method for rolled sections. However, the UKNational Annex (once published) should be referred to for guidance on which method to use.

The second method includes a correction factor to allow for the shape of the bending momentdiagram. This correction factor is in addition to the equivalent uniform moment factors usedto allow for the differences between a uniform moment and the actual moment distributionalong the beam.


21

4.1.8 Buckling uniform members in bending and axial compression

EC3-1-1 introduces two alternative methods for calculating the buckling resistance of a

member subject to combined bending and axial compression. Both approaches use

interaction equations which have a similar general form to those used in BS 5950: Part 1.

However, this is where the similarity ends. The methods in EC3-1-1 include interaction

factors, k, which account for the shape of the bending moment diagram and the class of the

cross-section. The interaction factors have been derived from two alternative approaches

and expressions for each interaction factor are included in Annex A for Method 1 and Annex

B for Method 2. Both methods require the evaluation of complex expressions in order to

determine the interaction factors. However, Method 2 is a little easier. A comparison

between Methods 1 and 2 and BS 5950 has shown that Method 2 is in better agreement with

BS 5950: Part 1 than Method 1. Furthermore, there is some doubt over the applicability of

Method 1 to asymmetric sections. For these reasons the National Annex may allow both

methods to be used but restrict the scope of Method 1 to bi-symmetrical sections.

4.2 Part 1-2: General rules Structural fire design

The fire part of Eurocode 3 (EN1993-1-2 [4], hereafter referred to as EC3-1-2) is not radically

different from the UK standard for the fire resistant design of steel structures. BS 5950 Part 8

[5] is a performance based code that allows for calculation of fire resistance in addition to the

use of fire test data. The principal difference between the two codes is that the calculation

procedures in BS 5950: Part 8 are limited to a thermal exposure based on the standard fire

curve while EC3-1-2 allows for alternative thermal exposures based on the factors influencing

fire growth and development. The design procedure for EC3-1-2 is illustrated in Figure 3.

Effectively the scope of BS 5950: Part 8 is restricted to the left hand branch of the diagram.

All the fire parts of the structural Eurocodes are designed to be used with the fire part of the

Eurocode for Actions (EN1991-1-2 [6] hereafter referred to as EC1-1-2). The thermal actions

(either nominal or parametric) are taken from this document and the resulting thermal and

mechanical analysis undertaken using the principles and design methods detailed in EC3-1-2.

4.2.1 Material properties

For fire resistant design by calculation the most common method in the Eurocodes is to use a

modified form of the equations for resistance at ambient temperature using reduced material

properties corresponding to the appropriate temperature. For this reason EC3-1-2 contains

detailed guidance on the material properties of carbon and stainless steels. These are

presented as stress-strain relationships and as reduction factors relative to the ambient

temperature strength and elastic modulus. It is important to note that the variation of Youngs

modulus with temperature is different to the variation in steel strength . The information is

presented in the form of strength reduction factors (ky,

) in EC3-1-2 and strength retention

factors in BS 5950: Part 8. The strength reduction factors given in EC3-1-2 correspond to the

2% strain values in Table 1 of BS 5950: Part 8. Elevated temperature properties are also

presented for thermal elongation, specific heat and thermal conductivity. The relationships

given in EC3-1-2 are identical to those in BS 5950: Part 8. The corresponding properties for

stainless steel may be found in Annex C of EC3-1-2. Annex A of EC3-1-2 presents an

alternative stress-strain relationship for carbon steels allowing for strain hardening.

(ky)


22

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23

4.2.2 Structural fire design

In EC3-1-2 fire resistance may be determined either by simple calculation models, advancedcalculation models or testing. The current British Standard is based on fire resistance derivedfrom standard fire tests and fire resistance derived from calculations. The main difference inapproach is that BS 5950: Part 8 includes tabulated data for limiting temperatures and designtemperatures based on the results from standard tests while EC3-1-2 does not includetabulated data for design temperatures.

4.2.3 Members in compression

For compression members with Class 1, Class 2 or Class 3 cross-sections a non-dimensionalslenderness is calculated based on the buckling length in the fire situation. In general thebuckling length should be determined as for ambient temperature design. However, in abraced frame the buckling length may be determined based on continuity at the connectionsprovided that the fire resistance of the building components that separate the firecompartments is not less than the fire resistance of the column. Thus in a braced framewhere each storey comprises a separate fire compartment, intermediate columns areassumed to be fixed in direction at either end and the effective length is half of the systemlength. In the top storey the buckling length may be taken as 0.7 x the system length. This isdifferent to the approach used in BS 5950: Part 8 where the buckling length is determinedfollowing the guidance given for ambient temperature design i.e. current UK practice is moreconservative. It is anticipated that this issue will be addressed in the UK National Annex forEC3-1-2.

4.2.4 Combined bending and axial compression

For members subject to combined bending and axial compression the calculation method inEC3-1-2 is more complex than the corresponding calculation in BS 5950: Part 8 and differsfrom the method in EC3-1-1. The interaction formula for the combination of axial load andminor and major axis bending is based on the procedure in the original draft for thedevelopment of the Eurocode, ENV 1993-1-1 as the new method in EC3-1-1 has not beenverified for the fire situation at the time of writing.

4.2.5 Structural connections

The latest version of BS 5950: Part 8 contains guidance on the calculation of the thickness ofprotection required for structural connections and takes into account the relative load ratio ofthe connection compared to that of the connected members. EC3-1-2 in addition to similarguidance includes a more detailed approach in Annex D where the design resistance of boltsin shear and tension, and the design resistance of welds can be calculated using atemperature profile based on the temperature of the bottom flange of the beam at mid-span.This method is mainly applicable for simple connections although potentially could be appliedto all components of the connection using the approach in EN1993-1-8.

4.3 Part 1-8: Design of joints

EN1993-1-8 [7] (hereafter referred to as EC3-1-8) gives guidance for the design of steel jointssubject to predominantly static loads. Steel grades S235, S275, S355 and S460 are coveredby the guidance given.


24

4.3.1 Definitions

EC3-1-8 starts by defining the different components that constitute a steel joint and makes a

clear distinction between a connection and a joint. This can be confusing for UK designers

who generally use the words joint and connection interchangeably to describe the junction

between two steel members. In EC3-1-8 the word connection is used to define the location

at which two or more elements meet, while the word joint is used to define the zone where

two or more members are interconnected. Therefore a beam-to-column connection is the

interface between the flange (or web) of the column and the end of the beam, and includes all

the components (bolts, welds, end-plate, column flange etc) required to transfer the internal

forces from the beam to the column. The joint however is the assembly of all the basic

components which play a part in the behaviour of the configuration. For example, a single-

sided beam-to-column joint consists of a connection and a column web panel. It is important

that UK designers recognise this distinction as it is used throughout the standard.

4.3.2 Material properties

In EC3-1-8 the nominal values of the yield strength, fyb, and the ultimate tensile strength, fub,

for grade 8.8 and 10.9 bolts are considerably greater than the equivalent values used in BS

5950: Part 1. This is due to the standards taking account of different effects within the quoted

material property values. EC3-1-1 gives ultimate values and BS5950: Part 1 gives

permissible values. The partial material factors are included in the properties given in BS

5950: Part 1 but are defined separately in EC3-1-8. BS5950 Part 1 material properties may

be used to account for prying actions without the direct calculation of the prying force by

applying a factor to the material properties, EC3 gives a separate check for prying action.

4.3.3 Groups of fasteners

The approach used in EC3-1-8 is different to that used in the BCSA/SCI publications on

Joints in Steel Construction [8, 9 & 10]. In EC3-1-8 the design resistance of a group of

fasteners may be taken as the sum of the design bearing resistances of the individual

fasteners provided the design shear resistance of each individual fastener is greater than or

equal to the design bearing resistance. If this condition is not satisfied then the design

resistance of a group of fasteners should be taken as the number of fasteners multiplied by

the smallest design resistance of any of the individual fasteners.

In the BCSA/SCI publications the design resistance of a group of fasteners is taken as the

sum of the design resistances of the individual fasteners.

This difference in approach may cause problems for flexible end-plates. The current

approach in the UK often means that the top bolts are designed for bearing failure and the

remaining bolts for shear. Because the EC3-1-8 rules do not allow mixed modes of failure the

capacity of the bolt group according to the Eurocode philosophy would often be based on the

number of fasters multiplied by the design bearing resistance of the top bolts. Clearly this

may significantly reduce the apparent shear capacity of flexible end-plate connections and in

some cases may result in an increase in the number of bolts needed.

4.3.4 Analysis, classification and modelling

Joint design depends very much on the designers decision regarding the method by which

the structure is to be analysed. Both EC3-1-8 and BS 5950: Part 1 recognise that either

elastic or plastic global analysis may be used, for frames that are simple, semi-continuous or

continuous. When elastic analysis is adopted joint stiffness is relevant, when the analysis is

plastic then strength of the joint is relevant. EC3-1-8 goes a step further than the British

Standard and includes a table that relates the type of framing, method of global analysis and


25

the joint classification. Table 4 gives details (note that some of the terminology used in the

Eurocode has been slightly modified for clarity).

Table 4. Type of framing, analysis used and joint classification/requirements

Method of global

analysis

Classification/requirements of joint

Elastic Nominally pinned Rigid Semi-rigid

Rigid-plastic Nominally pinned Full-strength Partial-strength

Elastic-plastic Nominally pinned Rigid and

full-strength

Semi-rigid and partial strength or

Semi-rigid and full-strength or

Rigid and partial-strength

Type of framing Simple Continuous Semi-continuous

Although the relationship between type of framing, method of global analysis and joint

requirements (represented by their classification) has been known for some time, its inclusion

in a major structural code is new and some explanation of its use is required.

Simple frame design is based on the assumption that the beams are simply supported and

that the beam-to-column joints are sufficiently flexible and weak to restrict the development of

significant beam end-moments. In continuous framing the type of joint used will depend on

the method of global analysis. When elastic analysis is used the joints are classified

according to their stiffness and rigid joints must be used. When plastic analysis is used the

joints are classified according to their strength and full-strength joints must be used. When

elastic-plastic analysis is adopted then the joints are classified according to both their stiffness

and strength and rigid, full-strength joints must be used.

Semi-continuous frame design recognises the fact that most practical joints possess some

degree of both stiffness and moment resistance. When elastic analysis is used the joints are

classified according to their stiffness and semi-rigid joints should be used. If plastic global

analysis is used the joints are classified according to their strength and partial-strength joints

should be used. When elastic-plastic analysis is used the joints are classified according to

their stiffness and strength, and semi-continuity could be achieved in a number of ways (see

Table 4).

The traditional UK approach of classifying a joint only recognises two types (pinned and rigid)

and it is relatively straightforward to use engineering judgement to choose between these.

For an extended system, such as the one used in EC3-1-8, the structural properties of a joint

may need to be quantified in order to classify it. EC3-1-8 includes methods for doing this, and

it is the inclusion of these methods that constitutes the biggest difference between the design

of joints to the Eurocode and the traditional methods used in the UK.

By comparing the quantified stiffness of a joint against the limits given in EC3-1-8 it can be

classified as pinned, rigid or semi-rigid. Similarly a joint can be classified by comparing its

quantified moment resistance with limits for pinned, full-strength or partial strength joints. A

fuller description of a joints behaviour can also be obtained by classifying it using both

stiffness and strength. Such a classification leads to joints which are pinned, rigid/full-

strength, rigid/partial strength and semi-rigid/partial-strength.

One problem that this may cause is that joints which have traditionally been taken as pinned

or rigid may not be pinned or rigid under the new classification system. This situation is

complicated by the fact that the Eurocode not only gives guidance on calculating stiffness and

strength (for some joint types), but clause 5.2.2.1 also allows classification on the basis of

experimental evidence or experience of previous performance. Clearly the results of these

three approaches for a given joint may not always agree. This could prove problematical if


26

checking authorities require designers to demonstrate that a joint is pinned or rigid, and could

lead to increased design time and/or changes to the UKs commonly used joints.

To establish the stiffness boundary between rigid and semi-rigid joints the relationship

between joint stiffness and the Euler buckling load for a single-bay, single-storey frame was

investigated [11]. It was decided that a semi-rigid joint can be considered as rigid provided

the difference between the Euler buckling load for a single-bay, single-storey frame with semi-

rigid joints and the Euler buckling load of a similar frame with rigid joints was less than 5%.

By adopting this approach a classification method based on the rigidity of the connected

beam was developed. While such a system is easy to use it has attracted criticism, some of

which is detailed below:

When compared to the stiffness limits given in some national standards the limits in EC3-

1-8 appear to be conservative.

The classification system given in EC3-1-8 can be applied to any steel structure but as

the limits have been determined on the basis of a single-bay, single-storey frame the

accuracy of its application to multi-bay, multi-storey frames is questionable.

The stiffness boundaries between joint types have been determined on the basis of the

ultimate limit state and on the assumption that a difference of 5% between the

performance of a frame with rigid and semi-rigid joints is small and can be neglected.

However, this does not necessarily mean that the differences at serviceability limit states,

where displacements of the structure are more important, are equally small and can be

neglected. Clearly, when deriving classification criteria both serviceability and ultimate

limit states should be considered.

4.3.5 Structural joints connecting H or I sections

The method described in Chapter 6 of EC3-1-8 for the design of joints between H or I

sections is based on the component approach. Explicit guidance is only given for flush and

extended end plate joints, although a designer might need to calculate stiffness and/or

strength for other types of joint in order to classify them. As well as this limitation it is worth

noting that the procedures for calculating the design moment resistance and rotational

stiffness of a joint are complex and time-consuming and are not suitable for hand calculation.

Computer software is recommended for these complex calculations.

4.3.5.1 Design resistance

In the given method for calculating moment resistance the potential resistance of each

component is calculated. These potential resistances are then converted to actual forces by

balancing the forces in the tension components with those in compression. The moment

capacity of the joint is then calculated by summing the product of the forces in the tension

components and their distances from the centre of compression. This approach is very

similar to the method described in the BCSA/SCI publication Joints in Steel Construction:

Moment connections [9] (which was in fact heavily based on the Eurocode). However, the

Eurocode also includes methods for calculating a joints rotational stiffness and rotation

capacity. Both of these methods will be new to UK designers and are therefore briefly

described below.

4.3.5.2 Rotational stiffness

Calculating the stiffness of any joint can be a difficult process. For this reason Reference 9

takes a pragmatic approach and gives simple rule-of-thumb detailing guidelines which, if

followed, will in most circumstances ensure an appropriate joint stiffness, so that frame design

assumptions are not invalidated.


27

EC3-1-8 incorporates a method for calculating the stiffness of a bare steel joint based on work

initially carried out by Zoetemeijer [12] and more recently by Jaspart [13 & 14]. This method

uses the component approach in which the rotational response of the joint is determined from

the mechanical properties of the different components (end-plate, cleat, column flange, bolts

etc.). The advantage of this approach is that the behaviour of any joint can be calculated by

decomposing it into its components.

The stiffness of each joint component is represented by a linear spring with a force-

displacement relationship. Tables are included in EC3-1-8 which give expressions for

evaluating the stiffness of the different components. The combined effect of the components

is determined by considering each spring, with an appropriate lever arm, to give a rotational

stiffness.

4.3.5.3 Rotation capacity

The rotation capacity of a joint is very important (a pin or plastic hinge must be able to

rotate sufficiently) but this is difficult to calculate accurately. However, numerous researchers

have investigated rotation capacity and have identified many sources of ductility in joints,

some of which are listed below:

Column web panel in shear

Column flange in bending

End plate in bending

Tension flange cleat in bending

EC3-1-8 gives a number of practical rules for checking the rotation capacity of a joint. These

rules are based on the above sources of ductility for bolted joints and entail checking that the

critical mode of failure is based on one of the above components.

4.4 Part 1-10: Material toughness and through-thickness properties

EN1993-1-10 [15] (here after referred to as EC3-1-10), gives design guidance for the

selection of steel for fracture toughness and through-thickness properties.

4.4.1 Fracture toughness

To determine the maximum permissible thickness of a steel element using EC3-1-10 the

reference temperature, steel grade and stress at the reference temperature are required. The

following expression is used to determine the reference temperature:

TEd = Tmd + Tr + T

+ TR + T

+ T cf

Where

Tmd is the minimum service temperature with a specific return period, given in EN 1991-1-5

Tr is an adjustment for radiation loss, obtained from EN 1991-1-5

T

is the adjustment for stress and yield strength of material, crack imperfections and

member shape and dimensions, given in EN 1993-1-10

TR is a safety allowance, if required, to reflect different reliability levels for different

applications, obtained from EN 1993-1-10

T

is the adjustment for a strain rate other than the reference strain rate, obtained from

EN 1993-1-10 T

cf is the adjustment for the degree of cold forming, defined in EN 1993-1-10

Elastic analysis should be used to determine the stress at the reference temperature. The

maximum element thicknesses given in Table 2.1 of EC3-1-10 relate to three levels of stress,

TEd = Tmd + Tr + T + TR + T + TcfWhere

Tmd is the minimum service temperature with a specific return period, given in EN 1991-1-5Tr is an adjustment for radiation loss, obtained from EN 1991-1-5T is the adjustment for stress and yield strength of material, crack imperfections and

member shape and dimensions, given in EN 1993-1-10TR is a safety allowance, if required, to reflect different reliability levels for different

applications, obtained from EN 1993-1-10T is the adjustment for a strain rate other than the reference strain rate, obtained from EN

1993-1-10Tcf is the adjustment for the degree of cold forming, defined in EN 1993-1-10


28

0.25fy(t), 0.5fy(t) and 0.75fy(t). Where fy(t) is the nominal yield strength adjusted for the

thickness of the element.

The current UK guidance gives maximum thickness values for minimum temperatures of

-5C, -15C, -25C, -35C and -45C. The minimum temperature of -5C for internal

steelwork given in BS 5950: Part 1 relates to the temperatures experienced during

construction, when it is vulnerable to brittle fracture. The values given in EC3-1-10 consider a

wider range of temperatures, +10C to -50C in 10C intervals. Interpolation between the

values is allowed, but extrapolation beyond the extreme values given in the table is not

permitted.

The minimum temperature used in BS 5950: Part 1 and the reference temperature (TEd) of

EC3-1-10 are not equivalent to each other. The minimum temperature used in BS 5950: Part

1 is similar to the minimum service temperature with a specific return period (Tmd).

Maximum element thickness values are given for different steel grades in both codes,

although more steel grades/types are considered in BS 5950: Part 1. Table 5 gives the steel

grades/types considered in both standards. Comparing the steel grades covered by BS 5950:

Part 1 and EC3-1-10 it appears that no allowance has been made in Table 2.1 of EC3-1-10

for the steel grades used for hollow sections. EC3-1-10 allows the use of fracture mechanics

for a numerical evaluation. Therefore this method may be used for the steel grades used for

hollow sections.

Table 5. Material property standards for which maximum element thicknesses are given in

BS 5950: Part 1 and EN 1993-1-1

Material property standards for which maximum element thicknesses are given in standards

BS 5950: Part 1 S275 to S460 steel grades

EN 1993-1-1 S275 to S690 steel grades

BS EN 10025 BS EN 10025

BS EN 10113 BS EN 10113

BS EN 10137 BS EN 10137

BS EN 10166

BS EN 10210

BS EN 10219

BS7668

A note to clause 2.2(5) of EC3-1-10 allows the National Annex to give maximum values of the

range between TEd and the test temperature and also the range of Ed, to which the validity of

values for permissible thickness in Table 2.1 may be restricted. A further note to this clause

allows the National Annex to limit the use of Table 2.1 for steel grades up to S460. The UK

National Annex to EC3-1-10 is currently under development and no comment can be made at

this time on the values that may be included in it.

4.4.2 Through-thickness properties

Section 3 of EC3-1-10 gives a method for determining the susceptibility of steel to lamellar

tearing. Lamellar tearing is a weld induced flaw and is usually detected during ultrasonic

inspection of welds. The main risk of lamellar tearing is with cruciform joints, T-joints, corner

joints and when full penetration welds are used.

Ed,


29

To check if lamellar tearing may be ignored EC3-1-10 requires the available design and

required design Z-values4 to be compared. The available design Z-value is given in BS EN

10164 [16]. The required design Z-value is obtained from coefficients given in EC3-1-10

relating to weld depth, shape and position of welds, material thickness, restraint of shrinkage

and influence of preheating. BS 5950: Part 2 [17] states that the material shall be tested for

through-thickness properties to the specified quality class in accordance with BS EN 10164.

The inclusion of the Z-value check in EC3-1-10 may result in designers having to perform this

check for every welded joint in a structure. Currently in the UK only joints identified as being

at risk from lamellar tearing are checked. EC3-1-10 allows the National Annex to limit the

scope of section 3 to certain steel products. This may be used in the UK National Annex to

limit the Z-value checks to specific types of welded joints.

4.5 Part 5: Steel Piling

EN1993-5 [18] (hereafter referred to as EC3-5) gives guidance for the design of all types of

steel piles including hot and cold formed sheet piles, bearing piles and piling systems built up

from component parts. It gives guidance for different shapes, sizes and arrangements of

steel piles. Although some of these are not common in the UK at present they may find future

application.

The fields of application considered by the Eurocode are:

Steel piled foundations of civil engineering works on land and over water

Temporary or permanent structures necessary for the execution of steel piling

Temporary or permanent retaining structures composed of steel sheet piles, including all

kinds of combined walls.

Guidance for steel piles filled with concrete is also included in EC3-5.

EC3-5 contains an annex giving detailed rules for the design of cold formed pile sections and

combined walls. These areas have not previously been dealt with in UK guidance.

Current UK standards do not contain an equivalent code to EC3-5. BS 8002 [19] is basically

a geotechnical code that requires input from BS 5950: Part 1 to allow the design of steel piles.

Current SCI documents cover some aspects of UK steel pile design. However, the guidance

given in these documents does not give the detail required for a full design, and it only

applies to simple structures.

EC3-5 introduces some new concepts to the traditional UK design process, these include:

The use of plastic design for piling

Four classes for sheet piling and the resultant different design approaches

A more formal system for assessing the structural performance of piling structures

The checks on shear in a sheet pile wall, which are perhaps covered by inspection in current

practice need to be formally assessed, as do shear buckling and combined moments, shear

and axial loading. Many of these checks will require section data and it is likely that either

data sheets giving member capacities or the basic geometric information will be provided by

the sheet pile manufacturers. The effects of water pressures on the structural design are also

to be taken into account (which is a new concept for UK designers), and specific rules for the

transfer of shear in the interlocks of piles and its effect on the strength and stiffness of pile

4 Z-value is the transverse reduction of area in a tensile test of the through-thickness ductility

of a specimen, measured as a percentage


30

sections are included. This issue is addressed qualitatively in BS 8002 but is covered in moredepth in EC3-5, as it has a much higher profile outside the UK. Compared with current UKpractice EC3-5 deals more formally with combined-walls and cellular structures, as well ashigh modulus walls.

Conflicting views have emerged within the UK industry on the implementation of EC3-5.These views have emerged because of the significant differences in scope and approachbetween current UK practice and the Eurocode system. One of the major areas for concern isthe effect that a move from lumped factor design to a partial factor approach will have ondesign requirements. This is compounded by changes in the specific calculations that arerequired to satisfy the new code. There are situations where formal calculations are nowrequired which would previously have been dealt with by inspection in the UK. There is alsoconcern that these design changes may make designs less efficient, or effective, comparedwith current UK practice.

One of the most difficult areas to assess is the effect that the plastic design rules will have onthe design process as there is little or no experience with these design rules within the UK.The design calculations need to consider the situation at all stages in the life of the structureand if the proposed section has appropriate parameters, the wall can be designed on thebasis of plastic section properties and moment redistribution. This assumes that the pilesection is capable of sustaining a moment of resistance as the pile rotates plastically and thisability may change with the amount of corrosion that the section has sustained. This may beaccepted practice in structural designs but the response of soil when the system is at orapproaching plastic conditions is not understood.

There is reference made to EN 12063, the standard covering site activities which goes intosignificantly more detail than current British Standards on some aspects of site work (i.e.welding).

One potential area of conflict with current UK methods is the fact that there is no overtdifference between the requirements for temporary and permanent construction. This waspreviously dealt with by allowing increased stress levels in temporary works piling (BS8002:1994 [19]) and not considering corrosion on the section properties. Under the new rules therewill be no change in stress, which may be a retrograde step in some minds.

EN 1994 Steel and Concrete Composite Structures

31

5 EN1994 Steel and Concrete Composite Structures

The main differences between the design guidance given in Eurocode 4 and BS 5950 arediscussed in this section.

5.1 Part 1-1: General rules and rules for buildings

Eurocode 4 applies to the design of composite structures and members for building and civilengineering works. The Eurocode is concerned only with the requirements for the resistance,serviceability and durability of composite steel and concrete structures. EN1994-1-1 [20](hereafter referred to as EC4-1-1) gives a general basis for the design of composite structuresalong with specific rules for buildings.

EC4-1-1 provides design guidance for some types of element not common in the UK, such aspartially encased concrete beams, composite columns in buildings, high strength structuralsteels and composite joints together with various methods of continuous beam design and thedetailing of the continuous joints.

5.1.1 Material Properties

5.1.1.1 Concrete

Unless given in EC4-1-1, concrete material properties must be obtained from EN1992-1-1[21] (hereafter referred to as EC2-1-1) for both normal weight and lightweight concrete.However, EC4-1-1 does not cover the design of composite structures with concrete gradeslower than C20/25 or higher than C60/75. EC4-1-1 therefore extends the range of concretestrengths compared to those available in BS 5950.

The classification for normal weight concrete used in the Eurocodes system (Cx/y) gives thecylinder strength (x) and the cube strength (y) in N/mm2. The design strengths used in theEurocodes are based on the cylinder strengths and not the cube strengths, so care should betaken by designers to use the correct value.

5.1.1.2 Structural steel

Reference should be made to EC3-1-1, clauses 3.1 and 3.2 for the properties of structuralsteel, however, EC4-1-1 does not cover steel grades with a characteristic strength greaterthan 460N/mm2. This is in common with BS 5950: Part 3 where the design strength ofstructural steel is obtained by reference to BS 5950: Part 1. However, a comment is made inBS 5950 that due to a lack of test evidence, the design strength should not be taken asgreater than 355N/mm2. This limit is lower than that given in EC4-1-1.

Research has shown that to prevent premature concrete crushing some design rules shouldbe modified for steels with strength greater than 355N/mm2. Such modifications have beenincorporated into EC4-1-1 so that it can cover steels with strengths up to 460N/mm2.


32

5.1.2 Structural stability

The effects of the deformed geometry of the structure must be considered and an importantdesign Principle outlined states that second-order effects should be considered if theyincrease the action effects significantly or modify significantly the structural behaviour.

It is suggested that this increase in internal forces may be neglected if the increase in forcesdue to second-order effects is less than 10% of the forces determined in first-order analysis.

The Eurocode also states that if second-order effects in individual members and relevantmember imperfections are fully accounted for in the global analysis of the structure, individualstability checks on the members (such as lateral torsional buckling presumably) are notnecessary.

This is in contrast to BS5950 where there is no specific requirement to consider the increasein internal forces due to second-order effects but individual stability checks are required.

An additional Principle stated is that appropriate allowances must be made for creep andcracking of concrete and for the behaviour of joints when determining the stiffness of thestructure.

Part 3, BS5950, uses a slightly different approach where the specific effects of concrete creepdo not have to be considered provided that material values given are used when calculatingthe modular ratio.

The effects of concrete cracking are considered in BS5950, where the cracked sectionmethod is used to determine member stiffness for elastic analysis, although the un-crackedsection is used to calculate deflections.

5.1.3 Structural imperfections

When using EC4-1-1 appropriate allowances must be made to cover the effects ofimperfections, including residual stresses and geometrical imperfections present even in theunloaded structure, such as lack of verticality, out of straightness and the unavoidable minoreccentricities present in joints. The assumed shape of any imperfections must take intoaccount the elastic buckling mode of the structure or member in the most unfavourabledirection and form, in the plane of buckling considered.

Equivalent geometric imperfections should be used unless the effects of local imperfectionsare included in the member resistance design formulae. EC4-1-1 gives values of initial bowimperfections for composite columns and whilst there are no specific imperfectionrequirements for beams, EC4-1-1 incorporates the effects of imperfections within the formulaefor the buckling resistance moment of laterally unrestrained composite beams. A similarapproach is adopted in the current British Standard. The designer should refer to EC3-1-1 forthe effects of global imperfections and for the formulae for buckling resistance of steelmembers, which also incorporate the effects of member imperfections.

No specific requirements for dealing with member or global imperfections are outlined withinBS 5950: Part 3.

5.1.4 Calculation of action (load) effects

Action effects are generally calculated by elastic global analysis even where the capacity of across-section is based on its plastic or non-linear resistance. Elastic global analysis shouldalso be used for serviceability limit states, with, where appropriate, corrections for non-lineareffects.

EN 1994 Steel and Concrete Composite Structures

33

Allowance must also be made for shear lag. This is achieved for continuous beams by using

an effective width of slab. In much the same way as in BS 5950: Part 3, EC4-1-1 outlines a

number of provisions for determining the effective width of concrete slab, with the total

effective width for the sagging portion of a beam (noted as beff1 in EC4-1-1) being the familiar

Le/4 but no greater than the geometric distance between the beam centres.

EC4-1-1 does not give separate values of effective slab width for slabs spanning

perpendicular to and parallel to the supporting beam. A subtle distinction between the two

cases is given in BS 5950: Part 3, where the effective width of a slab spanning parallel to the

beam is limited to 0.8 times the beam spacing.

In contrast to BS 5950: Part 3, EC4-1-1 makes allowance for the shrinkage of concrete, in the

serviceability limit state, as well as cracking of concrete, creep, the sequence of construction

and any pre-stressing.

The effects of creep are dealt with using the modular ratio for short-term loading modified by a

creep coefficient depending upon the age of the concrete at the moment considered, t, and

the age at loading, t0, and a creep multiplier which can be used to account for the effects of

concrete shrinkage. In practice, the effects of curvature due to shrinkage of normal weight

concrete may often be ignored (see clause 7.3.1(8), EC4-1-1 for details). This is a little

different to the approach used in BS 5950: Part 3, where the modular ratio is determined

considering the proportion of long-term to short-term loading.

In common with BS 5950: Part 3, EC4-1-1 considers the effects of cracking on the flexural

stiffness of composite beams in two ways.

Involved Method An initial un-cracked analysis is carried out assuming the un-cracked

stiffness, EaI1, throughout. In areas where the extreme fibre tensile stress

in the concrete is twice the concrete strength, the stiffness of the section is

reduced to the cracked flexural stiffness, EaI2. An updated distribution of

internal forces is then determined by re-analysis, termed the cracked

analysis.

Simple Method The effect of cracking can be modelled by taking a reduced flexural

stiffness over 15% of the span on each side of each internal support, with

the un-cracked flexural stiffness taken elsewhere. This method may be

used for continuous beams where the ratio of the adjacent spans

(shorter/longer) is greater than or equal to 0.6.

The more complicated method given in BS 5950: Part 3, is basically the same as the simple

method given in EC4-1-1, where a cracked section is assumed over 15% of the span on each

side of each internal support, with the un-cracked section assumed elsewhere.

The simplified method given in the BS 5950: Part 3 involves carrying out an elastic analysis,

assuming all members are un-cracked. The resulting negative moments over the supports

and at mid-span can then be re-distributed in accordance with guidance given in Table 4 of

BS 5950: Part 3, which effectively models the reduced stiffness of the member over the

supports. EC4-1-1 also allows some limited redistribution, in accordance with Table 5.1, with

both cracked and uncracked analysis for buildings, for the verification of all limit states other

than fatigue.

5.1.5 Beams Ultimate Limit State

In EC4-1-1 the resistance moment of a composite cross-section with full interaction between

the structural steel, reinforcement and concrete is given by plastic theory. It is assumed that

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