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EUROPEAN STANDARD
NORME EUROPÉENNE
EUROPÄISCHE NORM

EN 1993-1-5

October 2006

ICS 91.010.30; 91.080.10

Supersedes ENV 1993-1-5:1997
Incorporating corrigendum April 2009

English Version

Eurocode 3 - Design of steel structures - Part 1-5: Plated structural elements

Eurocode 3 - Calcul des structures en acier - Partie 1-5: Plaques planes Eurocode 3 - Bemessung und konstruktion von Stahlbauten - Teil 1-5: Plattenbeulen

This European Standard was approved by CEN on 13 January 2006.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the Central Secretariat or to any CEN member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by translation under the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

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© 2006 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Members.

Ref. No. EN 1993-1-5:2006: E

1

Content

Page
1 Introduction 5
  1.1 Scope 5
  1.2 Normative references 5
  1.3 Terms and definitions 5
  1.4 Symbols 6
2 Basis of design and modelling 7
  2.1 General 7
  2.2 Effective width models for global analysis 7
  2.3 Plate buckling effects on uniform members 7
  2.4 Reduced stress method 8
  2.5 Non uniform members 8
  2.6 Members with corrugated webs 8
3 Shear lag in member design 9
  3.1 General 9
  3.2 Effectives width for elastic shear lag 9
  3.3 Shear lag at the ultimate limit state 12
4 Plate buckling effects due to direct stresses at the ultimate limit state 13
  4.1 General 13
  4.2 Resistance to direct stresses 13
  4.3 Effective cross section 13
  4.4 Plate elements without longitudinal stiffeners 15
  4.5 Stiffened plate elements with longitudinal stiffeners 18
  4.6 Verification 21
5 Resistance to shear 21
  5.1 Basis 21
  5.2 Design resistance 22
  5.3 Contribution from the web 22
  5.4 Contribution from flanges 25
  5.5 Verification 25
6 Resistance to transverse forces 25
  6.1 Basis 25
  6.2 Design resistance 26
  6.3 Length of stiff bearing 26
  6.4 Reduction factor χF for effective length for resistance 27
  6.5 Effective loaded length 27
  6.6 Verification 28
7 Interaction 28
  7.1 Interaction between shear force, bending moment and axial force 28
  7.2 Interaction between transverse force, bending moment and axial force 29
8 Flange induced buckling 29
9 Stiffeners and detailing 30
  9.1 General 30
  9.2 Direct stresses 30
  9.3 Shear 34
  9.4 Transverse loads 35
10 Reduced stress method 36
Annex A (informative) Calculation of critical stresses for stiffened plates 38 2
Annex B (informative) Non uniform members 43
Annex C (informative) Finite Element Methods of Analysis (FEM) 45
Annex D (informative) Plate girders with corrugated web 50
Annex E (normative) Alternative methods for determining effective cross sections 53
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Foreword

This European Standard EN 1993-1-5,, Eurocode 3: Design of steel structures Part 1.5: Plated structural elements, has been prepared by Technical Committee CEN/TC250 « Structural Eurocodes », the Secretariat of which is held by BSI. CEN/TC250 is responsible for all Structural Eurocodes.

This European Standard shall be given the status of a National Standard, either by publication of an identical text or by endorsement, at the latest by April 2007 and conflicting National Standards shall be withdrawn at latest by March 2010.

This Eurocode supersedes ENV 1993-1-5.

According to the CEN-CENELEC Internal Regulations, the National Standard Organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

National annex for EN 1993-1-5

This standard gives alternative procedures, values and recommendations with notes indicating where national choices may have to be made. The National Standard implementing EN 1993-1-5 should have a National Annex containing all Nationally Determined Parameters to be used for the design of steel structures to be constructed in the relevant country.

National choice is allowed in EN 1993-1-5 through:

4

1 Introduction

1.1 Scope

  1. EN 1993-1-5 gives design requirements of stiffened and unstiffened plates which are subject to in-plane forces.
  2. Effects due to shear lag, in-plane load introduction and plate buckling for I-section girders and box girders are covered. Also covered are plated structural components subject to in-plane loads as in tanks and silos. The effects of out-of-plane loading are outside the scope of this document.

    NOTE 1: The rules in this part complement the rules for class 1,2,3 and 4 sections, see EN 1993-1 -1.

    NOTE 2: For the design of slender plates which are subject to repeated direct stress and/or shear and also fatigue due to out-of-plane bending of plate elements (breathing) see EN 1993-2 and EN 1993-6.

    NOTE 3: For the effects of out-of-plane loading and for the combination of in-plane effects and out-of-plane loading effects see EN 1993-2 and EN 1993-1-7.

    NOTE 4: Single plate elements may be considered as flat where the curvature radius r satisfies:

    Image

    where

    a is the panel width
    t is the plate thickness

1.2 Normative references

  1. This European Standard incorporates, by dated or undated reference, provisions from other publications. These normative references are cited at the appropriate places in the text and the publications are listed hereafter. For dated references, subsequent amendments to or revisions of any of these publications apply to this European Standard only when incorporated in it by amendment or revision. For undated references the latest edition of the publication referred to applies.
EN 1993-1-1 Eurocode 3: Design of steel structures: Part 1-1: General rules and rules for buildings

1.3 Terms and definitions

For the purpose of this standard, the following terms and definitions apply:

1.3.1
elastic critical stress

stress in a component at which the component becomes unstable when using small deflection elastic theory of a perfect structure

1.3.2
membrane stress

stress at mid-plane of the plate

1.3.3
gross cross-section

the total cross-sectional area of a member but excluding discontinuous longitudinal stiffeners

1.3.4
effective cross-section and effective width

the gross cross-section or width reduced for the effects of plate buckling or shear lag or both; to distinguish between their effects the word “effective” is clarified as follows:

“effectiveP“ denotes effects of plate buckling

5

“effectives“ denotes effects of shear lag

“effective“ denotes effects of plate buckling and shear lag

1.3.5
plated structure

a structure built up from nominally flat plates which are connected together; the plates may be stiffened or un stiffened

1.3.6
stiffener

a plate or section attached to a plate to resist buckling or to strengthen the plate; a stiffener is denoted:

1.3.7
stiffened plate

plate with transverse or longitudinal stiffeners or both

1.3.8
subpanel

unstiffened plate portion surrounded by flanges and/or stiffeners

1.3.9
hybrid girder

girder with flanges and web made of different steel grades; this standard assumes higher steel grade in flanges compared to webs

1.3.10
sign convention

unless otherwise stated compression is taken as positive

1.4 Symbols

  1. In addition to those given in EN 1990 and EN 1993-1 -1, the following symbols are used:
    Asℓ total area of all the longitudinal stiffeners of a stiffened plate;
    Ast gross cross sectional area of one transverse stiffener;
    Aeff effective cross sectional area;
    Ac,eff effectiveP cross sectional area;
    Ac,eff,loc effectiveP cross sectional area for local buckling;
    a length of a stiffened or unstiffened plate;
    b width of a stiffened or unstiffened plate;
    bw Image clear width between welds for welded sections or between ends of radii for rolled sections; Image
    beff effectives width for elastic shear lag;
    FEd design transverse force;
    hw clear web depth between flanges;
    Leff effective length for resistance to transverse forces, see 6;
    Mf.Rd design plastic moment of resistance of a cross-section consisting of the flanges only;
    Mpl.Rd design plastic moment of resistance of the cross-section (irrespective of cross-section class);
    MEd design bending moment;
    NEd design axial force;
    t thickness of the plate; 6
    VEd design shear force including shear from torque;
    Weff effective elastic section modulus;
    β effectives width factor for elastic shear lag;
  2. Additional symbols are defined where they first occur.

2 Basis of design and modelling

2.1 General

  1. P The effects of shear lag and plate buckling shall be taken into account at the ultimate, serviceability or fatigue limit states.

    NOTE: Partial factors γM0 and γM1 used in this part are defined for different applications in the National Annexes of EN 1993-1 to EN 1993-6.

2.2 Effective width models for global analysis

  1. P The effects of shear lag and of plate buckling on the stiffness of members and joints shall be taken into account in the global analysis.
  2. The effects of shear lag of flanges in global analysis may be taken into account by the use of an effectives width. For simplicity this effectives width may be assumed to be uniform over the length of the span.
  3. For each span of a member the effectives width of flanges should be taken as the lesser of the full width and L/8 per side of the web, where L is the span or twice the distance from the support to the end of a cantilever.
  4. The effects of plate buckling in elastic global analysis may be taken into account by effectives cross sectional areas of the elements in compression, see 4.3.
  5. For global analysis the effect of plate buckling on the stiffness may be ignored when the effectives cross-sectional area of an element in compression is larger than ρlim times the gross cross-sectional area of the same element.

    NOTE 1: The parameter ρlim may be given in the National Annex. The value ρlim = 0,5 is recommended.

    NOTE 2: For determining the stiffness when (5) is not fulfilled, see Annex E.

2.3 Plate buckling effects on uniform members

  1. EffectiveP width models for direct stresses, resistance models for shear buckling and buckling due to transverse loads as well as interactions between these models for determining the resistance of uniform members at the ultimate limit state may be used when the following conditions apply:

    NOTE: The rules may apply to non rectangular panels provided the angle αlimit (see Figure 2.1) is not greater than 10 degrees. If αlimit exceeds 10, panels may be assessed assuming it to be a rectangular panel based on the larger of b1 and b2 of the panel.

    7

    Figure 2.1: Definition of angle α

    Figure 2.1: Definition of angle α

  2. For the calculation of stresses at the serviceability and fatigue limit state the effectives area may be used Image if the condition in 2.2(5) is fulfilled Image. For ultimate limit states the effective area according to 3.3 should be used β replaced by βult.

2.4 Reduced stress method

  1. As an alternative to the use of the effectiveP width models for direct stresses given in sections 4 to 7, the cross sections may be assumed to be class 3 sections provided that the stresses in each panel do not exceed the limits specified in section 10.

    NOTE: The reduced stress method is analogous to the effectiveP width method (see 2.3) for single plated elements. However, in verifying the stress limitations no load shedding has been assumed between the plated elements of the cross section.

2.5 Non uniform members

  1. Non uniform members (e.g. haunched members, non rectangular panels) or members with regular or irregular large openings may be analysed using Finite Element (FE) methods.

    NOTE 1: See Annex B for non uniform members.

    NOTE 2: For FE-calculations see Annex C.

2.6 Members with corrugated webs

  1. For members with corrugated webs, the bending stiffness should be based on the flanges only and webs should be considered to transfer shear and transverse loads.

    NOTE: For Image text deleted Image buckling resistance of flanges in compression and the shear resistance of webs see Annex D.

8

3 Shear lag in member design

3.1 General

  1. Shear lag in flanges may be neglected if b0 < Le/50 where b0 is taken as the flange outstand or half the width of an internal element and Le is the length between points of zero bending moment, see 3.2.1(2).
  2. Where the above limit for b0 is exceeded the effects due to shear lag in flanges should be considered at serviceability and fatigue limit state verifications by the use of an effectives width according to 3.2.1 and a stress distribution according to 3.2.2. For the ultimate limit state verification an effective area according to 3.3 may be used.
  3. Stresses due to patch loading in the web applied at the flange level should be determined from 3.2.3.

3.2 Effectives width for elastic shear lag

3.2.1 Image Effectives width Image

  1. The effectives width beff for shear lag under elastic conditions should be determined from:

    beff = β b0       (3.1)

    where the effectives factor β is given in Table 3.1.

    Image This effectives width may Image be relevant for serviceability and fatigue limit states.

  2. Provided adjacent spans do not differ more than 50% and any cantilever span is not larger than half the adjacent span the effective lengths Le may be determined from Figure 3.1. For all other cases Le should be taken as the distance between adjacent points of zero bending moment.

Figure 3.1 : Effective length Le for continuous beam and distribution of effectives width

Figure 3.1 : Effective length Le for continuous beam and distribution of effectives width

9

Figure 3.2: Notations for shear lag

Figure 3.2: Notations for shear lag

Table 3.1: Effectives width factor β
k Verification β - value
k ≤ 0,02   β = 1,0
0,02 < k ≤ 0,70 sagging bending Image
hogging bending Image
> 0,70 sagging bending Image
hogging bending Image
all k end support β0 = (0,55 + 0,025 /k) β1 but β0 < β1
all k Cantilever β = β2 at support and at the end
Image

in which Asℓ is the area of all longitudinal stiffeners within the width b0 and other symbols are as defined in Figure 3.1 and Figure 3.2.

10

3.2.2 Stress distribution due to shear lag

  1. The distribution of longitudinal stresses across the flange plate due to shear lag should be obtained

    Figure 3.3: Distribution of stresses due to shear lag

    Figure 3.3: Distribution of stresses due to shear lag

3.2.3 In-plane load effects

  1. The elastic stress distribution in a stiffened or unstiffened plate due to the local introduction of in-plane forces (patch loads), see Figure 3.4, should be determined from:

    Image

    where

    ast,1 is the gross cross-sectional Image area of the directly loaded stiffeners divided Image over the length se.
    Image This may be taken as the area of a stiffener smeared over the length of the spacing sst; Image
    tw is the web thickness;
    z is the distance to flange.
    Image Se is the length of the stiff bearing;
    Sst is the spacing of stiffeners; Image

    NOTE: The equation (3.2) is valid when sst/se ≤ 0,5; otherwise the contribution of stiffeners should be neglected.

11

Figure 3.4: In-plane load introduction

Figure 3.4: In-plane load introduction

NOTE: The above stress distribution may also be used for the fatigue verification.

3.3 Shear lag at the ultimate limit state

  1. At the ultimate limit state shear lag effects may be determined as follows:
    1. elastic shear lag effects as determined for serviceability and fatigue limit states,
    2. combined effects of shear lag and of plate buckling,
    3. elastic-plastic shear lag effects allowing for limited plastic strains.

    NOTE 1: The National Annex may choose the method to be applied. Unless specified otherwise in EN 1993-2 to EN 1993-6, the method in NOTE 3 is recommended.

    NOTE 2: The combined effects of plate buckling and shear lag may be taken into account by using Aeff as given by:

    Aeff = Ac,eff βult       (3.3)

    where

    Ac,eff is the effectiveP area of the compression flange due to plate buckling (see 4.4 and 4.5);
    βult is the effectives width factor for the effect of shear lag at the ultimate limit state, which may be taken as β determined from Table 3.1 with α0 replaced by

    Image

    tf is the flange thickness.
    12

    NOTE 3: Elastic-plastic shear lag effects allowing for limited plastic strains may be taken into account using Aeff as follows:

    Aeff = Ac,eff βκAc,eff β       (3.5)

    where    β and κ are taken from Table 3.1.

    The expressions in NOTE 2 and NOTE 3 may also be applied for flanges in tension in which case Ac,eff should be replaced by the gross area of the tension flange.

4 Plate buckling effects due to direct stresses at the ultimate limit state

4.1 General

  1. This section gives rules to account for plate buckling effects from direct stresses at the ultimate limit state when the following criteria are met:
    1. The panels are rectangular and flanges are parallel or nearly parallel (see 2.3);
    2. Stiffeners, if any, are provided in the longitudinal or transverse direction or both;
    3. Open holes and cut outs are small (see 2.3);
    4. Members are of uniform cross section;
    5. No flange induced web buckling occurs.

    NOTE 1: For compression flange buckling in the plane of the web see section 8.

    NOTE 2: For stiffeners and detailing of plated members subject to plate buckling see section 9.

4.2 Resistance to direct stresses

  1. The resistance of plated members may be determined Image using the effectiveP areas Image of plate elements in compression for class 4 sections using cross sectional data (Aeff, leff, Weff for cross sectional verifications and member verifications for column buckling and lateral torsional buckling according to EN 1993-1 -1.
  2. EffectiveP areas should be determined on the basis of the linear strain distributions with the attainment of yield strain in the mid plane of the compression plate.

4.3 Effective cross section

  1. In calculating longitudinal stresses, account should be taken of the combined effect of shear lag and plate buckling using the effective areas given in 3.3.
  2. The effective cross sectional properties of members should be based on the effective areas of the compression elements and on the effectives area of the tension elements due to shear lag.
  3. The effective area Aeff should be determined assuming that the cross section is subject only to stresses due to uniform axial compression. For non-symmetrical cross sections the possible shift eN of the centroid of the effective area Aeff relative to the centre of gravity of the gross cross-section, see Figure 4.1, gives an additional moment which should be taken into account in the cross section verification using 4.6.
  4. The effective section modulus Weff should be determined assuming the cross section is subject only to bending stresses, see Figure 4.2. For biaxial bending effective section moduli should be determined about both main axes.

    NOTE: As an alternative to 4.3(3) and (4) a single effective section may be determined from NEd and MEd acting simultaneously. The effects of eN should be taken into account as in 4.3(3). This requires an iterative procedure.

    13
  5. The stress in a flange should be calculated using the elastic section modulus with reference to the mid-plane of the flange.
  6. Hybrid girders may have flange material with yield strength fyf up to ϕh × fyw provided that:
    1. the increase of flange stresses caused by yielding of the web is taken into account by limiting the stresses in the web to fyw ;
    2. fyf Image text deleted Image is used in determining the effective area of the web.

    NOTE: The National Annex may specify the value ϕh. A value of ϕh = 2,0 is recommended.

  7. The increase of deformations and of stresses at serviceability and fatigue limit states may be ignored for hybrid girders complying with 4.3(6) including the NOTE.
  8. For hybrid girders complying with 4.3(6) the stress range limit in EN 1993-1-9 may be taken as 1,5 fyf.

Figure 4.1: Class 4 cross-sections - axial force

Figure 4.1: Class 4 cross-sections - axial force

Figure 4.2: Class 4 cross-sections - bending moment

Figure 4.2: Class 4 cross-sections - bending moment

14

4.4 Plate elements without longitudinal stiffeners

  1. The effectiveP areas of flat compression elements should be obtained using Table 4.1 for internal elements and Table 4.2 for outstand elements. The effectiveP area of the compression zone of a plate with the gross cross-sectional area Ac should be obtained from:

    Ac,eff = ρ Ac       (4.1)

    where ρ is the reduction factor for plate buckling.

  2. The reduction factor ρ may be taken as follows:

    Image

    ψ is the stress ratio determined in accordance with 4.4(3) and 4.4(4)
    Image is the appropriate width to be taken as follows (for definitions, see Table 5.2 of EN 1993-1 -1)
    bw for webs;
    b for internal flange elements (except RHS);
    b - 3 t for flanges of RHS;
    c for outstand flanges;
    h for equal-leg angles;
    h for unequal-leg angles;
    kσ is the buckling factor corresponding to the stress ratio ψ and boundary conditions. For long plates kσ is given in Table 4.1 or Table 4.2 as appropriate;
    t is the thickness;
    σcr is the elastic critical plate buckling stress see equation (A.1) in Annex A.1(2) and Table 4.1 and Table 4.2;

    Image

  3. For flange elements of I-sections and box girders the stress ratio ψ used in Table 4.1 and Table 4.2 should be based on the properties of the gross cross-sectional area, due allowance being made for shear lag in the flanges if relevant. For web elements the stress ratio ψ used in Table 4.1 should be obtained using a stress distribution based on the effective area of the compression flange and the gross area of the web.

    NOTE: If the stress distribution results from different stages of construction (as e.g. in a composite bridge) the stresses from the various stages may first be calculated with a cross section consisting of effective flanges and

    15

    gross web and these stresses are added together. This resulting stress distribution determines an effective web section that can be used for all stages to calculate the final stress distribution for stress analysis.

  4. Except as given in 4.4(5), the plate slenderness Image of an element may be replaced by:

    Image

    where

    σcom,Ed is the maximum design compressive stress in the element determined using the effectiveP area of the section caused by all simultaneous actions.

    NOTE 1: The above procedure is conservative and requires an iterative calculation in which the stress ratio ψ (see Table 4.1 and Table 4.2) is determined at each step from the stresses calculated on the effectiveP cross-section defined at the end of the previous step.

    NOTE 2: See also alternative procedure in Annex E.

  5. For the verification of the design buckling resistance of a class 4 member using 6.3.1, 6.3.2 or 6.3.4 of EN 1993-1-1, either the plate slenderness Image or Image with σcom,Ed based on second order analysis with global imperfections should be used.
  6. For aspect ratios a/b < 1 a column type of buckling may occur and the check should be performed according to 4.5.4 using the reduction factor ρc.

    NOTE: This applies e.g. for flat elements between transverse stiffeners where plate buckling could be column-like and require a reduction factor ρc close to χc as for column buckling, see Figure 4.3 a) and b). For plates with longitudinal stiffeners column type buckling may also occur for a/b ≥ 1, see Figure 4.3 c).

    Figure 4.3: Column-like behaviour

    Figure 4.3: Column-like behaviour

16
Table 4.1: Internal compression elements
Stress distribution (compression positive) EffectiveP width beff
Image ψ = 1 :

beff = ρ Image

be1 = 0,5 beff      be2 = 0,5 beff

Image 1 > ψ ≥ 0:
beff = ρ Image

Image      be2 = beff - be1

Image ψ < 0:

beff = ρ bc = ρ Image (1-ψ)

be1 = 0,4 beff      be2 = 0,6 beff

ψ = σ2/σ1 1 1 > ψ > 0 0 0 > ψ > -1 -1 Image -1 > ψ ≥ -3 Image
Buckling factor kσ 4,0 8,2/ (1,05 + ψ 7,81 7,81 - 6,29ψ + 9,78ψ2 23,9 5,98 (1 - ψ)2
Table 4.2: Outstand compression elements
Stress distribution (compression positive) EffectiveP width beff

Image

1 > ψ ≥ 0:

beff = ρ c

Image

ψ < 0:

beff = ρ bc = ρ c / (1-ψ)

ψ = σ2/σ1 1 0 -1 1 ≥ ψ ≥ -3
Buckling factor kσ 0,43 0,57 0,85 0.57 - 0,21 ψ + 0,07ψ2
Image 1 > ψ ≥ 0:

beff = ρ c

Image ψ < 0:

beff = ρ bc = ρ c / (1-ψ)

ψ = σ2/σ1 1 1 > ψ > 0 0 0 > ψ > -1 -1
Buckling factor kσ 0,43 0,578 / (ψ + 0,34) 1,70 1,7 - 5ψ + 17,1ψ2 23,8
17

4.5 Stiffened plate elements with longitudinal stiffeners

4.5.1 General

  1. For plates with longitudinal stiffeners the effectiveP areas from local buckling of the various subpanels between the stiffeners and the effectiveP areas from the global buckling of the stiffened panel should be accounted for.
  2. effectiveP section area of each subpanel should be determined by a reduction factor in accordance with 4.4 to account for local plate buckling. The stiffened plate with effectiveP section areas for the stiffeners should be checked for global plate buckling (by modelling it as an equivalent orthotropic plate) and a reduction factor Image ρc Image should be determined for overall plate buckling.
  3. effectiveP area of the compression zone of the stiffened plate should be taken as:

    Image

    where Ac,eff,loc is effectiveP Image section area Image of all the stiffeners and subpanels that are fully or partially in the compression zone except the effective parts supported by an adjacent plate element with the width bedge,eir, see example in Figure 4.4.

  4. The area Ac,eff,loc should be obtained from:

    Image

    where

    Image applies to the part of the stiffened panel width that is in compression except the parts bedge,eff, see Figure 4.4;
    Asℓ,eff is the sum of effectiveP sections according to 4.4 of all longitudinal stiffeners with gross area Asℓ located in the compression zone;
    bc,loc is the width of the compressed part of each subpanel;
    ρloC is the reduction factor from 4.4(2) for each subpanel.

    Figure 4.4: Stiffened plate under uniform compression

    Figure 4.4: Stiffened plate under uniform compression

    NOTE: For non-uniform compression see Figure A. 1.

    18
  5. In determining the reduction factor ρc for overall buckling, the reduction factor for column-type buckling, which is more severe than the reduction factor than for plate buckling, should be considered.
  6. Interpolation should be carried out in accordance with 4.5.4(1) between the reduction factor ρ for plate buckling and the reduction factor χc for column buckling to determine ρc see 4.5.4.
  7. The reduction of the compressed area Ac,eff,loc through ρc may be taken as a uniform reduction across the whole cross section.
  8. If shear lag is relevant (see 3.3), the effective cross-sectional area Ac.eff of the compression zone of the stiffened plate should then be taken as Image accounting not only for local plate buckling effects but also for shear lag effects.
  9. The effective cross-sectional area of the tension zone of the stiffened plate should be taken as the gross area of the tension zone reduced for shear lag if relevant, see 3.3.
  10. The effective section modulus Weff should be taken as the second moment of area of the effective cross section divided by the distance from its centroid to the mid depth of the flange plate.

4.5.2 Plate type behaviour

  1. The relative plate slenderness Image of the equivalent plate is defined as:

    Image

    with Image

    where

    Ac is the gross area of the compression zone of the stiffened plate except the parts of subpanels supported by an adjacent plate, see Figure 4.4 (to be multiplied by the shear lag factor if shear lag is relevant, see 3.3);
    Ac.eff.loc is the effective area of the same part of the plate (including shear lag effect, if relevant) with due allowance made for possible plate buckling of subpanels and/or stiffeners.
  2. The reduction factor ρ for the equivalent orthotropic plate is obtained from 4.4(2) provided Image is calculated from equation (4.7).

    NOTE: For calculation of σcr,p see Annex A.

4.5.3 Column type buckling behaviour

  1. The elastic critical column buckling stress σcr,c of an unstiffened (see 4.4) or stiffened (see 4.5) plate should be taken as the buckling stress with the supports along the longitudinal edges removed.
  2. For an unstiffened plate the elastic critical column buckling stress σcr,c may be obtained from

    Image

  3. For a stiffened plate σcr,c may be determined from the elastic critical column buckling stress σcr,sl of the stiffener closest to the panel edge with the highest compressive stress as follows:

    Image

    19

    where

    Isℓ,1 is the second moment of area of the gross cross section of the stiffener and the adjacent parts of the plate, relative to the out-of-plane bending of the plate;
    Asℓ,1 is the gross cross-sectional area of the stiffener and the adjacent parts of the plate according to Figure A. 1.

    NOTE: σcr,c may be obtained from Image where σcr,c is related to the compressed edge of the plate, and, Image bsℓ,1 Image and bc are geometric values from the stress distribution used for the extrapolation, see Figure A. 1.

  4. The relative column slenderness Image is defined as follows:

    Image

    Image

    where

    Image
    Asℓ,1 is defined in 4.5.3(3);
    Asℓ,1,eff is the effective cross-sectional area of the stiffener and the adjacent parts of the plate with due allowance for plate buckling, see Figure A.1.
  5. The reduction factor χc should be obtained from 6.3.1.2 of EN 1993-1-1. For unstiffened plates α = 0,21 corresponding to buckling curve a should be used. For stiffened plates its value should be increased to:

    Image

    with

    Image
    e = max (e1, e2) is the largest distance from the respective centroids of the plating and the one-sided stiffener (or of the centroids of either set of stiffeners when present on both sides) to the neutral axis of the effective column, see Figure A.1;
    α = 0,34 (curve b) for closed section stiffeners;

    = 0,49 (curve c) for open section stiffeners.

4.5.4 Interaction between plate and column buckling

  1. The final reduction factor ρc should be obtained by interpolation between χc and ρ as follows:

    ρc = (ρcχc) ξ (2 – ξ) + χc       (4.13)

    where Image but 0 ≤ ξ ≤ 1

    σcr,p is the elastic critical plate buckling stress, see Annex A. 1(2);
    σcr,c is the elastic critical column buckling stress according to 4.5.3(2) and (3), respectively; 20
    χc is the reduction factor due to column buckling.
    ρ is the reduction factor due to plate buckling, see 4.4(1).

4.6 Verification

  1. Member verification Image for compression and uniaxial bending Image should be performed as follows:

    Image

    where

    Aeff is the effective cross-section area in accordance with 4.3(3);
    eN is the shift in the position of neutral axis, see 4.3(3);
    MEd is the design bending moment;
    NEd is the design axial force;
    Weff is the effective elastic section modulus, see 4.3(4);
    γM0 is the partial factor, see application parts EN 1993-2 to 6.

    NOTE: For members subject to compression and biaxial bending the above equation (4.14) may be modified as follows:

    Image

    My,Ed, Mz,Ed are the design bending moments with respect to y-y and z-z axes respectively;
    Image ey,N, ez,N Image are the eccentricities with respect to the neutral axis.
  2. Action effects MEd and NEd should include global second order effects where relevant.
  3. The plate buckling verification of the panel should be carried out for the stress resultants at a distance 0,4a or 0,5b, whichever is the smallest, from the panel end where the stresses are the greater. In this case the gross sectional resistance needs to be checked at the end of the panel.

5 Resistance to shear

5.1 Basis

  1. This section gives rules for shear resistance of plates considering shear buckling at the ultimate limit state where the following criteria are met:
    1. the panels are rectangular within the angle limit stated in 2.3;
    2. stiffeners, if any, are provided in the longitudinal or transverse direction or both;
    3. all holes and cut outs are small (see 2.3);
    4. members are of uniform cross section.
  2. Plates with hw/t greater than Image for an unstiffened web, or Image for a stiffened web, should be checked for resistance to shear buckling and should be provided with transverse stiffeners at the supports, where Image 21

    NOTE 1: hw see Figure 5.1 and for Kτ see 5.3(3).

    NOTE 2: The National Annex will define η. The value η = 1,20 is recommended for steel grades up to and including S460. For higher steel grades η = 1,00 is recommended.

5.2 Design resistance

  1. For unstiffened or stiffened webs the design resistance for shear should be taken as:

    Image

    in which the contribution from the web is given by:

    Image

    and the contribution from the flanges Vbf,Rd is according to 5.4.

  2. Stiffeners should comply with the requirements in 9.3 and welds should fulfil the requirement given in 9.3.5.

Figure 5.1 : End supports

Figure 5.1 : End supports

5.3 Contribution from the web

  1. For webs with transverse stiffeners at supports only and for webs with either intermediate transverse stiffeners or longitudinal stiffeners or both, the factor χw, for the contribution of the web to the shear buckling resistance should be obtained from Table 5.1 or Figure 5.2.
    Table 5.1 : Contribution from the web χw to shear buckling resistance
      Rigid end post Non-rigid end post
    Image < 0,83/η η η
    0,83/ηImage < 1,08 0,83/Image 0,83/Image
    Image ≥ 1,08 1,37/(0,7+Image 0,83/Image

    NOTE: See 6.2.6 in EN 1993-1-1.

    22
  2. Figure 5.1 shows various end supports for girders:
    1. No end post, see 6.1 (2), type c);
    2. Rigid end posts, see 9.3.1; this case is also applicable for panels at an intermediate support of a continuous girder;
    3. Non rigid end posts see 9.3.2.
  3. The Image modified slenderness Image Image in Table 5.1 and Figure 5.2 should be taken as:

    Image

    where τcr = kτ σE       (5.4)

    NOTE 1: Values for σE and kτ may be taken from Annex A.

    NOTE 2: The Image modified slenderness Image Image may be taken as follows:

    1. transverse stiffeners at supports only:

      Image

    2. transverse stiffeners at supports and intermediate transverse or longitudinal stiffeners or both:

      Image

      in which kτ is the minimum shear buckling coefficient for the web panel.

      NOTE 3: Where non-rigid transverse stiffeners are also used in addition to rigid transverse stiffeners, kτ is taken as the minimum of the values from the web panels between any two transverse stiffeners (e.g. a2 × hw and a3 × hw) and that between two rigid stiffeners containing non-rigid transverse stiffeners (e.g. a4 × hw).

      NOTE 4: Rigid boundaries may be assumed for panels bordered by flanges and rigid transverse stiffeners. The web buckling analysis can then be based on the panels between two adjacent transverse stiffeners (e.g. a1 × hw in Figure 5.3).

      NOTE 5: For non-rigid transverse stiffeners the minimum value kτ may be obtained from the buckling analysis of the following:

    1. a combination of two adjacent web panels with one flexible transverse stiffener
    2. a combination of three adjacent web panels with two flexible transverse stiffeners

      For procedure to determine kτ see Annex A.3.

  4. The second moment of area of a longitudinal stiffener should be reduced to 1/3 of its actual value when calculating kτ. Formulae for kτ taking this reduction into account in A.3 may be used. 23

    Figure 5.2: Shear buckling factor χw

    Figure 5.2: Shear buckling factor χw

  5. For webs with longitudinal stiffeners the Image modified slenderness Image Image in (3) should not be taken as less than

    Image

    where hwi and kτi, refer to the subpanel with the largest Image modified slenderness Image Image of all subpanels within the web panel under consideration.

    NOTE: To calculate kτi the expression given in A.3 may be used with kτst = 0.

    Figure 5.3: Web with transverse and longitudinal stiffeners

    24

    Figure 5.3: Web with transverse and longitudinal stiffeners

5.4 Contribution from flanges

  1. When the flange resistance is not completely utilized in resisting the bending moment (MEd < Mf,Rd) the contribution from the fiances should be obtained as follows:

    Image

    bf and tf are taken for the flange which provides the least axial resistance,
    bf being taken as not larger than 15εtf on each side of the web,
    Image is the moment of resistance of the cross section consisting of the effective area of the flanges only,

    Image

  2. When an axial force NEd is present, the value of Mf,Rd should be reduced by multiplying it by the

    Image

    where Af1 and Af2 are the areas of the top and bottom flanges respectively.

5.5 Verification

  1. The verification should be performed as follows:

    Image

    where VEd is the design shear force including shear from torque.

6 Resistance to transverse forces

6.1 Basis

  1. The design resistance of the webs of rolled beams and welded girders should be determined in accordance with 6.2, provided that the compression flange is adequately restrained in the lateral direction.
  2. The load is applied as follows:
    1. through the flange and resisted by shear forces in the web, see Figure 6.1 (a);
    2. through one flange and transferred through the web directly to the other flange, see Figure 6.1 (b).
    3. through one flange adjacent to an unstiffened end, see Figure 6.1 (c)
    25
  3. For box girders with inclined webs the resistance of both the web and flange should be checked. The internal forces to be taken into account are the components of the external load in the plane of the web and flange respectively.
  4. The interaction of the transverse force, bending moment and axial force should be verified using 7.2.

Figure 6.1: Buckling coefficients for different types of load application

Figure 6.1: Buckling coefficients for different types of load application

6.2 Design resistance

  1. For unstiffened or stiffened webs the design resistance to local buckling under transverse forces should be taken as

    Image

    where

    tw is the thickness of the web;
    fyw is the yield strength of the web;
    Leff is the effective length for resistance to transverse forces, which should be determined from
    Leff = χFy

    where

    y is the effective loaded length, see 6.5, appropriate to the length of stiff bearing ss, see 6.3;
    χF is the reduction factor due to local buckling, see 6.4(1).

6.3 Length of stiff bearing

  1. The length of stiff bearing ss on the flange should be taken as the distance over which the applied load is effectively distributed at a slope of 1:1, see Figure 6.2. However, ss should not be taken as larger than hw.
  2. If several concentrated forces are closely spaced, the resistance should be checked for each individual force as well as for the total load with ss as the centre-to-centre distance between the outer loads.

    Figure 6.2: Length of stiff bearing

    Figure 6.2: Length of stiff bearing

    26
  3. If the bearing surface of the applied load rests at an angle to the flange surface, see Figure 6.2, ss should be taken as zero.

6.4 Reduction factor χF for effective length for resistance

  1. The reduction factor χF should be obtained from:

    Image

    where

    Image

    Image

  2. For webs without longitudinal stiffeners kF should be obtained from Figure 6.

    NOTE: For webs with longitudinal stiffeners information may be given in the National Annex. The following rules are recommended:

    For webs with longitudinal stiffeners kF may be taken as

    Image

    where b1 is the depth of the loaded subpanel taken as the clear distance between the loaded flange and the stiffener

    Image

    where Isℓ,1 is the second moment of area of the stiffener closest to the loaded flange including contributing parts of the web according to Figure 9.1.

    Equation (6.6) is valid for Image and Image and loading according to type a) in Figure 6.1.

  3. y should be obtained from 6.5.

6.5 Effective loaded length

  1. The effective loaded length y should be calculated as follows:

    Image

    Image

    For box girders, bf in equation (6.8) should be limited to 15εtf on each side of the web.

  2. For types a) and b) in Figure 6.1, y should be obtained using:

    Image but ℓy ≤ distance between adjacent transverse stiffeners     (6.10)

    27
  3. For type c) ℓy should be taken as the smallest value obtained from the Image equations (6.11) and (6.12). Image

    image

    image

    Image Where Image image

6.6 Verification

  1. The verification should be performed as follows:

    Image

    where

    FEd is the design transverse force;
    Leff is the effective length for resistance to transverse forces, see Image 6.2(1); Image
    tw is the thickness of the plate.

7 Interaction

7.1 Interaction between shear force, bending moment and axial force

  1. Provided that Image (see below) does not exceed 0,5, the design resistance to bending moment and axial force need not be reduced to allow for the shear force. If Image is more than 0,5 the combined effects of bending and shear in the web of an I or box girder should satisfy:

    Image

    where

    Mf,Rd is the design plastic moment of resistance of the section consisting of the effective area of the flanges;
    Mpl,Rd is the design plastic resistance of the cross section consisting of the effective area of the flanges and the fully effective web irrespective of its section class.
    Image
    Image Image for Vbw,Rd see expression (5.2). Image

    In addition the requirements in sections 4.6 and 5.5 should be met.

    Action effects should include global second order effects of members where relevant.

  2. The criterion given in (1) should be verified at all sections other than those located at a distance less than hw/2 from a support with vertical stiffeners. 28
  3. The plastic moment of resistance Mf,Rd may be taken as the product of the yield strength, the effective area of the flange with the smallest value of Affy/γM0 and the distance between the centroids of the flanges.
  4. If an axial force NEd is present, Mpl,Rd and Mf,Rd should be reduced in accordance with 6.2.9 of EN 1993-1-1 and 5.4(2) respectively. When the axial force is so large that the whole web is in compression 7.1(5) should be applied.
  5. A flange in a box girder should be verified using 7.1(1) taking Mf,Rd = 0 and τEd taken as the average shear stress in the flange which should not be less than half the maximum shear stress in the flange and Image is taken as η1 according to 4.6(1). In addition the subpanels should be checked using the average shear stress within the subpanel and χw determined for shear buckling of the subpanel according to 5.3, assuming the longitudinal stiffeners to be rigid.

7.2 Interaction between transverse force, bending moment and axial force

  1. If the girder is subjected to a concentrated transverse force acting on the compression flange in conjunction with bending and axial force, the resistance should be verified using 4.6, 6.6 and the following interaction expression:

    η2 + 0,8 η1 ≥ 1,4       (7.2)

  2. If the concentrated load is acting on the tension flange the resistance should be verified according to section 6. Additionally 6.2.1(5) of EN ^993-1-1 should be met.

8 Flange induced buckling

  1. To prevent the compression flange buckling in the plane of the web, the following criterion should be met:

    Image

    Aw is the cross section area of the web;
    Afc is the effective cross section area of the compression flange;
    hw is the depth of the web;
    tw is the thickness of the web.

    The value of the factor k should be taken as follows:

    plastic rotation utilized k = 0,3
    plastic moment resistance utilized k = 0,4
    elastic moment resistance utilized k = 0,55
  2. When the girder is curved in elevation, with the compression flange on the concave face, the following criterion should be met:

    Image

    r is the radius of curvature of the compression flange.

    NOTE: The National Annex may give further information on flange induced buckling.

29

9 Stiffeners and detailing

9.1 General

  1. This section gives design rules for stiffeners in plated structures which supplement the plate buckling rules specified in sections 4 to 7.

    NOTE: The National Annex may give further requirements on stiffeners for specific applications.

  2. When checking the buckling resistance, the section of a stiffener may be taken as the gross area comprising the stiffener plus a width of plate equal to 15εt but not more than the actual dimension available, on each side of the stiffener avoiding any overlap of contributing parts to adjacent stiffeners, see Figure 9.1.
  3. The axial force in a transverse stiffener should be taken as the sum of the force resulting from shear (see 9.3.3(3)) and any external loads.

    Figure 9.1: Effective cross-section of stiffener

    Figure 9.1: Effective cross-section of stiffener

9.2 Direct stresses

9.2.1 Minimum requirements for transverse stiffeners

  1. In order to provide a rigid support for a plate with or without longitudinal stiffeners, intermediate transverse stiffeners should satisfy the criteria given below.
  2. The transverse stiffener should be treated as a simply supported member subject to lateral loading with an initial sinusoidal imperfection w0 equal to s/300, where s is the smallest of al, a2 or b, see Figure 9.2, where a1 and a2 are the lengths of the panels adjacent to the transverse stiffener under consideration and b is the height between the centroids of the flanges or span of the transverse stiffener. Eccentricities should be accounted for.

    Figure 9.2: Transverse stiffener

    Figure 9.2: Transverse stiffener

  3. The transverse stiffener should carry the deviation forces from the adjacent compressed panels under the assumption that both adjacent transverse stiffeners are rigid and straight together with any external load 30

    and axial force according to the NOTE to 9.3.3(3). The compressed panels and the longitudinal stiffeners are considered to be simply supported at the transverse stiffeners.

  4. It should be verified that using a second order elastic method analysis both the following criteria are satisfied at the ultimate limit state:
  5. In the absence of an axial force in the transverse stiffener both the criteria in (4) above may be assumed to be satisfied provided that the second moment of area Ist of the transverse stiffeners is not less than:

    Image

    Image

    emax is the maximum distance from the extreme fibre of the stiffener to the centroid of the stiffener;
    NEd is the maximum compressive force of the adjacent panels but not less than the maximum compressive stress times half the effectiveP compression area of the panel including stiffeners;
    σcr,c, σcr,p are defined in 4.5.3 and Annex A.

    NOTE: Where out of plane loading is applied to the transverse stiffeners reference should be made to EN 1993-2 and EN 1993-1-7.

  6. If the stiffener carries axial compression this should be increased by ΔNst = σmb2 / π2 in order to account for deviation forces. The criteria in (4) apply but ΔNst need not be considered when calculating the uniform stresses from axial load in the stiffener.
  7. As a simplification the requirement of (4) may, in the absence of axial forces, be verified using a first order elastic analysis taking account of the following additional equivalent uniformly distributed lateral load q acting on the length b:

    Image

    where

    σm is defined in (5) above;
    w0 is defined in Figure 9.2;
    wel is the elastic deformation, that may be either determined iteratively or be taken as the maximum additional deflection b/300.
  8. Unless a more advanced method of analysis is carried out in order to prevent torsional buckling of stiffeners with open cross-sections, the following criterion should be satisfied:

    Image

    where

    Ip is the polar second moment of area of the stiffener alone around the edge fixed to the plate;
    IT is the St. Venant torsional constant for the stiffener alone.
    31
  9. Where warping stiffness is considered stiffeners should either fulfil (8) or the criterion

    σcrθ fy       (9.4)

    where

    σcr is the elastic critical stress for torsional buckling not considering rotational restraint from the plate;
    θ is a parameter to ensure class 3 behaviour.

    NOTE: The parameter θ may be given in the National Annex. The value θ = 6 is recommended.

9.2.2 Minimum requirements for longitudinal stiffeners

  1. The requirements concerning torsional buckling in 9.2.1(8) and (9) also apply to longitudinal stiffeners.
  2. Discontinuous longitudinal stiffeners that do not pass through openings made in the transverse stiffeners or are not connected to either side of the transverse stiffeners should be:

9.2.3 Welded plates

  1. Plates with changes in plate thickness should be welded adjacent to the transverse stiffener, see Figure 9.3. The effects of eccentricity need not be taken into account unless the distance to the stiffener from the welded junction exceeds b0/2 or 200 mm whichever is the smallest, where b0 is the width of the plate between longitudinal stiffeners.

    Figure 9.3: Welded plates

    Figure 9.3: Welded plates

32

9.2.4 Cut outs in stiffeners

  1. The dimensions of cut outs in longitudinal stiffeners should be as shown in Figure 9.4.

    Figure 9.4: Cut outs in longitudinal stiffeners

    Figure 9.4: Cut outs in longitudinal stiffeners

  2. The length should not exceed:
    ≤ 6 tmin for flat stiffeners in compression
    ≤ 8 tmin for other stiffeners in compression
    ≤ 15 tmin for stiffeners without compression
    where tmin is the lesser of the plate thicknesses
  3. The limiting values in (2) for stiffeners in compression may be increased by Image when

    σx,Edσ x,Rd and ≤ 15tmin.

    σx,Ed is the compression stress at the location of the cut-out

  4. The dimensions of cut outs in transverse stiffeners should be as shown in Figure 9.5.

    Figure 9.5: Cut outs in transverse stiffeners

    Figure 9.5: Cut outs in transverse stiffeners

  5. The gross web adjacent to the cut out should resist a shear force VEd, where

    Image

    Inet is the second moment of area for the net section of the transverse stiffener;
    e is the maximum distance from the underside of the flange plate to the neutral axis of net section, see Figure 9.5;
    bG is the length of the transverse stiffener between the flanges.
33

9.3 Shear

9.3.1 Rigid end post

  1. The rigid end post (see Figure 5.1) should act as a bearing stiffener resisting the reaction from the support (see 9.4), and should be designed as a short beam resisting the longitudinal membrane stresses in the plane of the web.

    NOTE: For the effects of eccentricity due to movements of bearings, see EN 1993-2.

  2. A rigid end post should comprise of two double-sided transverse stiffeners that form the flanges of a short beam of length hw, see Figure 5.1 (b). The strip of web plate between the stiffeners forms the web of the short beam. Alternatively, a rigid end post may be in the form of a rolled section, connected to the end of the web plate as shown in Figure 9.6.

    Figure 9.6: Rolled section forming an end-post

    Figure 9.6: Rolled section forming an end-post

  3. Each double sided stiffener consisting of flats should have a cross sectional area of at least 4hwt2 / e, where e is the centre to centre distance between the stiffeners and e > 0,1 hw, see Figure 5.1 (b). Where a rolled section other than flats is used for the end-post its section modulus should be not less than 4hwt2 for bending around a horizontal axis perpendicular to the web.
  4. As an alternative the girder end may be provided with a single double-sided stiffener and a vertical stiffener adjacent to the support so that the subpanel resists the maximum shear when designed with a non-rigid end post.

9.3.2 Stiffeners acting as non-rigid end post

  1. A non-rigid end post may be a single double sided stiffener as shown in Figure 5.1 (c). It may act as a bearing stiffener resisting the reaction at the girder support (see 9.4).

9.3.3 Intermediate transverse stiffeners

  1. Intermediate stiffeners that act as rigid supports to interior panels of the web should be designed for strength and stiffness.
  2. When flexible intermediate transverse stiffeners are used, their stiffness should be considered in the calculation of kτ in 5.3(5). 34
  3. The effective section of intermediate stiffeners acting as rigid supports for web panels should have a minimum second moment of area Ist:

    Image

    NOTE: Intermediate rigid stiffeners may be designed for an axial force equal to Image according to 9.2.1(3). In the case of variable shear forces the check is performed for the shear force at the distance 0,5hw from the edge of the panel with the largest shear force.

9.3.4 Longitudinal stiffeners

  1. If longitudinal stiffeners are taken into account in the stress analysis they should be checked for direct stresses for the cross sectional resistance.

9.3.5 Welds

  1. The web to flange welds may be designed for the nominal shear flow VEd / hw if VEd does not exceed Image For larger values VEd the weld between flanges and webs should be designed for the Image
  2. In all other cases welds should be designed to transfer forces along and across welds making up sections taking into account analysis method (elastic/plastic) and second order effects.

9.4 Transverse loads

  1. If the design resistance of an unstiffened web is insufficient, transverse stiffeners should be provided.
  2. The out-of-plane buckling resistance of the transverse stiffener under transverse loads and shear force (see 9.3.3(3)) should be determined from 6.3.3 or 6.3.4 of EN 1993-1-1, using buckling curve c. When both ends are assumed to be fixed laterally a buckling length of not less than 0,75hw should be used. A larger value of should be used for conditions that provide less end restraint. If the stiffeners have cut outs at the loaded end, the cross sectional resistance should be checked at this end.
  3. Where single sided or other asymmetric stiffeners are used, the resulting eccentricity should be allowed for using 6.3.3 or 6.3.4 of EN 1993-1-1. If the stiffeners are assumed to provide lateral restraint to the compression flange they should comply with the stiffness and strength criteria in the design for lateral torsional buckling.
35

10 Reduced stress method

  1. The reduced stress method may be used to determine the stress limits for stiffened or unstiffened plates.

    NOTE 1: This method is an alternative to the effective width method specified in section 4 to 7 in respect of the following:

    NOTE 2: The stress limits may also be used to determine equivalent effective areas. The National Annex may give limits of application for the methods.

  2. For unstiffened or stiffened panels subjected to combined stresses σx,Ed, σz,Ed and τEd class 3 section properties may be assumed, where

    Image

    where

    αult,k is the minimum load amplifier for the design loads to reach the characteristic value of resistance of the most critical point of the plate, see (4);
    ρ is the reduction factor depending on the plate slenderness Image to take account of plate buckling, see (5);
    γM1 is the partial factor applied to this method.
  3. The Image modified plate slenderness Image Image should be taken from

    Image

    where

    αcr is the minimum load amplifier for the design loads to reach the elastic critical load of the plate under the complete stress field, see (6)

    NOTE 1: For calculating σcr for the complete stress field, the stiffened plate may be modelled using the rules in section 4 and 5 without reduction of the second moment of area of longitudinal stiffeners as specified in 5.3(4).

    NOTE 2: When αcr cannot be determined for the panel and its subpanels as a whole, separate checks for the subpanel and the full panel may be applied.

  4. In determining αult,k the yield criterion may be used for resistance:

    Image

    where σEd, σz,Ed and τEd are the components of the stress field in the ultimate limit state.

    NOTE: By using the equation (10.3) it is assumed that the resistance is reached when yielding occurs without plate buckling.

  5. The reduction factor ρ may be determined using either of the following methods:
    1. the minimum value of the following reduction factors:
      ρx for longitudinal stresses from 4.5.4(1) taking into account column-like behaviour where relevant;
      ρz for transverse stresses from 4.5.4(1) taking into account column-like behaviour where relevant;
      χw for shear stresses from Image 5.3(1) Image;
      36

      each calculated for the Image modified plate slenderness Image Image according to equation (10.2).

      NOTE: This method leads to the verification formula:

      Image

      NOTE: For determining ρz for transverse stresses the rules in section 4 for direct stresses σx should be applied to σz in the z-direction. For consistency section 6 should not be applied.

    2. a value interpolated between the values of ρx, ρz and χw as determined in a) by using the formula for αult,k as interpolation function

      NOTE: This method leads to the verification format:

      Image

      NOTE 1: Since verification formulae (10.3), (10.4) and (10.5) include an interaction between shear force, bending moment, axial force and transverse force, section 7 should not be applied.

      NOTE 2: The National Annex may give further information on the use of equations (10.4) and (10.5). In case of panels with tension and compression it is recommended to apply equations (10.4) and (10.5) only for the compressive parts.

  6. Where αcr values for the complete stress field are not available and only αcr,i values for the various components of the stress field σx,Ed, σZ,Ed and τEd can be used, the αcr value may be determined from:

    Image

    Image

    and αcr,x, αcr,z, τcr, ψx and ψz are determined from sections 4 to 6.

  7. Stiffeners and detailing of plate panels should be designed according to section 9.
37

Annex A – Calculation of critical stresses for stiffened plates

[informative]

A.1 Equivalent orthotropic plate

  1. Plates with at least three longitudinal stiffeners may be treated as equivalent orthotropic plates.
  2. The elastic critical plate buckling stress of the equivalent orthotropic plate may be taken as:

    αcr,p = kσ,p σE       (A.1)

    where

    Image
    kσ,p is the buckling coefficient according to orthotropic plate theory with the stiffeners smeared over the plate;
    b is defined in Figure A. 1;
    t is the thickness of the plate.

    NOTE 1: The buckling coefficient kαp is obtained either from appropriate charts for smeared stiffeners or relevant computer simulations; alternatively charts for discretely located stiffeners may be used provided local buckling in the subpanels can be ignored and treated separately.

    NOTE 2: σcr,p is the elastic critical plate buckling stress at the edge of the panel where the maximum compression stress occurs, see Figure A. 1.

    NOTE 3: Where a web is of concern, Image the width h in Image equations (A.1) and (A.2) should be replaced by hw

    NOTE 4: For stiffened plates with at least three equally spaced longitudinal stiffeners the plate buckling coefficient kσ,p (global buckling of the stiffened panel) may be approximated by:

    Image

    with

    Image

    where:

    Ist is the second moment of area of the whole stiffened plate;
    Ip is the second moment of area for bending of the plate Image
    Image Ast Image is the sum of the gross areas of the individual longitudinal stiffeners;
    Ap is the gross area of the plate = bt;
    σ1 is the larger edge stress;
    σ2 is the smaller edge stress;
38

Figure A.1 : Notations for longitudinally stiffened plates

Figure A.1 : Notations for longitudinally stiffened plates

39

A.2 Critical plate buckling stress for plates with one or two stiffeners in the compression zone

A.2.1 General procedure

  1. If the stiffened plate has only one longitudinal stiffener in the compression zone the procedure in A.1 may be simplified by a fictitious isolated strut supported on an elastic foundation reflecting the plate effect in the direction perpendicular to this strut. The elastic critical stress of the strut may be obtained from A.2.2.
  2. For calculation of Ast,1, and Ist,1 the gross cross-section of the column should be taken as the gross area of the stiffener and adjacent parts of the plate described as follows. If the subpanel is fully in compression, a portion (3 – ψ)/(5 – ψ) of its width b1 should be taken at the edge of the panel and 2/(5 – ψ) at the edge with the highest stress. If the stress changes from compression to tension within the subpanel, a portion 0,4 of the width bc of the compressed part of this subpanel should be taken as part of the column, see Figure A.2 and also Table 4.1. ψ is the stress ratio relative to the subpanel in consideration.
  3. The effectiveP cross-sectional area Ast,eff of the column should be taken as the effectiveP cross-section of the stiffener and the adjacent effectiveP parts of the plate, see Figure A.1. The slenderness of the plate elements in the column may be determined according to 4.4(4), with σcom,Ed calculated for the gross cross-section of the plate.
  4. If ρcfy/γM1, with ρc determined according to 4.5.4(1), is greater than the average stress in the column σcom.Ed no further reduction of the effectiveP area of the column should be made. Otherwise the effective area in (4.6) should be modified as follows:

    Image

  5. The reduction mentioned in A.2.1(4) should be applied only to the area of the column. No reduction need be applied to other compressed parts of the plate, except for checking buckling of subpanels.
  6. As an alternative to using an effectivep area according to A.2.1 (4), the resistance of the column may be determined from A.2.1 (5) to (7) and checked to ensure that it exceeds the average stress σcom,Ed.

    NOTE: The method outlined in (6) may be used in the case of multiple stiffeners in which the restraining effect from the plate is neglected, that is the fictitious column is considered free to buckle out of the plane of the web.

    Figure A.2: Notations for a web plate with single stiffener in the compression zone

    Figure A.2: Notations for a web plate with single stiffener in the compression zone

  7. If the stiffened plate has two longitudinal stiffeners in the compression zone, the one stiffener procedure described in A.2.1(1) may be applied, see Figure A.3. First, it is assumed that one of the stiffeners 40 buckles while the other one acts as a rigid support. Buckling of both the stiffeners simultaneously is accounted for by considering a single lumped stiffener that is substituted for both individual ones such that:
    1. its cross-sectional area and its second moment of area Image Isℓ Image are respectively the sum of that for the individual stiffeners
    2. it is positioned at the location of the resultant of the respective forces in the individual stiffeners

    For each of these situations illustrated in Figure A.3 a relevant value of σcr,p is computed, see A.2.2(1), with Image and Image and Image, see Figure A.3.

    Figure A.3: Notations for plate with two stiffeners in the compression zone

    Figure A.3: Notations for plate with two stiffeners in the compression zone

A.2.2 Simplified model using a column restrained by the plate

  1. In the case of a stiffened plate with one longitudinal stiffener located in the compression zone, the elastic critical buckling stress of the stiffener can be calculated as follows ignoring stiffeners in the tension zone:

    Image

    with

    Image

    where

    Asℓ,1 is the gross area of the column obtained from A.2.1 (2)
    Isℓ,1 is the second moment of area of the gross cross-section of the column defined in A.2.1(2)
    about an axis through its centroid and parallel to the plane of the plate;
    b1, b2 are the distances from the longitudinal edges of the web to the stiffener (b1+b2 = b).
    Image note deleted Image
  2. In the case of a stiffened plate with two longitudinal stiffeners located in the compression zone the elastic critical plate buckling stress should be taken as the lowest of those computed for the three cases using 41 equation (A.4) with Image, Image and b = B*. The stiffeners in the tension zone should be ignored in the calculation.

A.3 Shear buckling coefficients

  1. For plates with rigid transverse stiffeners and without longitudinal stiffeners or with more than two longitudinal stiffeners, the shear buckling coefficient kτ can be obtained as follows:

    Image

    where

    Image but not less than Image

    a is the distance between transverse stiffeners (see Figure 5.3);
    Isℓ is the second moment of area of the longitudinal stiffener about the z-z axis, see Figure 5.3 (b).
       Image For webs with Image longitudinal stiffeners, not necessarily equally spaced, Isℓ is the sum of the stiffness of the individual stiffeners.

    NOTE: No intermediate non-rigid transverse stiffeners are allowed for in equation (A.5).

  2. The equation (A.5) also applies to plates with one or two longitudinal stiffeners, if the aspect ratio Image satisfies α ≥ 3. For plates with one or two longitudinal stiffeners and an aspect ratio α < 3 the shear buckling coefficient should be taken from:

    Image

42

Annex B – Non-uniform members

[informative]

B.1 General

  1. The rules in section 10 are applicable to webs of members with non parallel flanges as in haunched beams and to webs with regular or irregular openings and non orthogonal stiffeners.
  2. αult and αcrit may be obtained from FE-methods, see Annex C.
  3. The reduction factors ρx, ρz and χw forImage may be obtained from the appropriate plate buckling curves, see sections 4 and 5.

    NOTE: The reduction factor ρ may be obtained as follows:

    Image

    Image

    This procedure applies to ρx, ρz and χw. The values of Image and αp are given in Table B.1. These values have been calibrated against the plate buckling curves in sections 4 and 5 and give a direct correlation to the equivalent geometric imperfection, by :

    Image

    Table B.1: Values for Image and αp
    Product predominant buckling mode αP Image
    hot rolled direct stress for ψ ≥ 0 0,13 0,70
    direct stress for ψ < 0 shear
    transverse stress
    0,80
    welded or
    cold formed
    direct stress for ψ ≥ 0 0,34 0,70
    direct stress for ψ < 0 shear
    transverse stress
    0,80
43

B.2 Interaction of plate buckling and lateral torsional buckling

  1. The method given in B.1 may be extended to the verification of combined plate buckling and lateral torsional buckling of members by calculating αult and Image αcr Image as follows:
    αult is the minimum load amplifier for the design loads to reach the characteristic value of resistance of the most critical cross section, neglecting any plate buckling and lateral torsional buckling;
    αcr is the minimum load amplifier for the design loads to reach the Image elastic critical loading Image of the member including plate buckling and lateral torsional buckling modes.
  2. When αcr contains lateral torsional buckling modes, the reduction factor ρ used should be the minimum of the reduction factor according to B.1(3) and the χLT – value for lateral torsional buckling according to 6.3.3 of EN 1993-1-1.
44

Annex C – Finite Element Methods of analysis (FEM)

[informative]

C.1 General

  1. Annex C gives guidance on the use of FE-methods for ultimate limit state, serviceability limit state or fatigue verifications of plated structures.

    NOTE 1: For FE-calculation of shell structures see EN 1993-1-6.

    NOTE 2: This guidance is intended for engineers who are experienced in the use of Finite Element methods.

  2. The choice of the FE-method depends on the problem to be analysed and based on the following assumptions:
Table C.1: Assumptions for FE-methods
No Material behaviour Geometric behaviour Imperfections, see section C.5 Example of use
1 linear linear no elastic shear lag effect, elastic resistance
2 non linear linear no plastic resistance in ULS
3 linear non linear no critical plate buckling load
4 linear non linear yes elastic plate buckling resistance
5 non linear non linear yes elastic-plastic resistance in ULS

C.2 Use

  1. In using FEM for design special care should be taken to

NOTE: The National Annex may define the conditions for the use of FEM analysis in design.

C.3 Modelling

  1. The choice of FE-models (shell models or volume models) and the size of mesh determine the accuracy of results. For validation sensitivity checks with successive refinement may be carried out.
  2. The FE-modelling may be carried out either for:

    NOTE: An example for a component could be the web and/or the bottom plate of continuous box girders in the region of an intermediate support where the bottom plate is in compression. An example for a substructure could be a subpanel of a bottom plate subject to biaxial stresses.

  3. The boundary conditions for supports, interfaces and applied loads should be chosen such that results obtained are conservative. 45
  4. Geometric properties should be taken as nominal.
  5. Ail imperfections should be based on the shapes and amplitudes as given in section C.5.
  6. Material properties should conform to C.6(2).

C.4 Choice of software and documentation

  1. The software should be suitable for the task and be proven reliable.

    NOTE: Reliability can be proven by appropriate bench mark tests.

  2. The mesh size, loading, boundary conditions and other input data as well as the output should be documented in a way that they can be reproduced by third parties.

C.5 Use of imperfections

  1. Where imperfections need to be included in the FE-model these imperfections should include both geometric and structural imperfections.
  2. Unless a more refined analysis of the geometric imperfections and the structural imperfections is carried out, equivalent geometric imperfections may be used.

    NOTE 1: Geometric imperfections may be based on the shape of the critical plate buckling modes with amplitudes given in the National Annex. 80 % of the geometric fabrication tolerances is recommended.

    NOTE 2: Structural imperfections in terms of residual stresses may be represented by a stress pattern from the fabrication process with amplitudes equivalent to the mean (expected) values.

  3. The direction of the applied imperfection should be such that the lowest resistance is obtained.
  4. For applying equivalent geometric imperfections Table C.2 and Figure C.1 may be used.
    Table C.2: Equivalent geometric imperfections
    Type of imperfection Component Shape Magnitude
    global member with length bow see EN 1993-1-1, Table 5.1
    global longitudinal stiffener with length a bow min (a/400, b/400)
    local panel or subpanel with short span a or b buckling shape min (a/200, b/200)
    local stiffener or flange subject to twist bow twist 1/50
    46

    Figure C.1 : Modelling of equivalent geometric imperfections

    Figure C.1 : Modelling of equivalent geometric imperfections

    47
  5. In combining imperfections a leading imperfection should be chosen and the accompanying imperfections may have their values reduced to 70%.

    NOTE 1: Any type of imperfection should be taken as the leading imperfection and the others may be taken as the accompanying imperfections.

    NOTE 2: Equivalent geometric imperfections may be substituted by the appropriate fictitious forces acting on the member.

C.6 Material properties

  1. Material properties should be taken as characteristic values.
  2. Depending on the accuracy and the allowable strain required for the analysis the following assumptions for the material behaviour may be used, see Figure C.2:
    1. elastic-plastic without strain hardening;
    2. elastic-plastic with a nominal plateau slope;
    3. elastic-plastic with linear strain hardening;
    4. true stress-strain curve modified from the test results as follows:

      Image

      Figure C.2: Modelling of material behaviour

    Figure C.2: Modelling of material behaviour

    48

    NOTE: For the elastic modulus E the nominal value is relevant.

C.7 Loads

  1. The loads applied to the structures should include relevant load factors and load combination factors. For simplicity a single load multiplier α may be used.

C.8 Limit state criteria

  1. The ultimate limit state criteria should be used as follows:
    1. for structures susceptible to buckling:
      attainment of the maximum load.
    2. for regions subjected to tensile stresses:
      attainment of a limiting value of the principal membrane strain.

    NOTE 1: The National Annex may specify the limiting of principal strain. A value of 5% is recommended.

    NOTE 2: Other criteria may be used, e.g. attainment of the yielding criterion or limitation of the yielding zone.

C.9 Partial factors

  1. The load magnification factor αu to the ultimate limit state should be sufficient to achieve the required reliability.
  2. The magnification factor αu should consist of two factors as follows:
    1. α1 to cover the model uncertainty of the FE-modelling used. It should be obtained from evaluations of test calibrations, see Annex D to EN 1990;
    2. α2 to cover the scatter of the loading and resistance models. It may be taken as γM1 if instability governs and γM2 if fracture governs.
  3. It should be verified that:

    αu > α1 α2       (C.2)

    NOTE: The National Annex may give information on γM1 and γM2. The use of γM1 and γM2 as specified in the relevant parts of EN 1993 is recommended.

49

Annex D – Plate girders with corrugated webs

[informative]

D.1 General

  1. Annex D covers design rules for I-girders with trapezoidal or sinusoidal corrugated webs, see Figure D.1.

    Figure D.1: Geometric notations

    Figure D.1: Geometric notations

D.2 Ultimate limit state

D.2.1 Moment of resistance

  1. The moment of resistance Image My,Rd Image due to bending should be taken as the minimum of the following:

    Image

    where

    fyf, r is the value of yield stress reduced due to transverse moments in the flanges
    fyf,r = fyf fT
    Image
    σx(Mz) is the stress due to the moment in the flange
    χ is the reduction factor for out of plane buckling according to 6.3 of EN 1993-1-1

    NOTE 1: The transverse moment Mz results from the shear flow in flanges as indicated in Figure D.2.

    NOTE 2: For sinusoidally corrugated webs fT is 1,0.

    50

    Figure D.2: Transverse actions due to shear flow introduction into the flange

    Figure D.2: Transverse actions due to shear flow introduction into the flange

  2. The effectivep area of the compression flange should be determined from 4.4(1) using the larger value of the slenderness parameter Image defined in 4.4(2). Image The buckling factor kσ should be taken as the larger of a) and b): Image
    1. Image

      where b       is the maximum width of the outstand from the toe of the weld to the free edge

               a = a1 + 2a4

    2. kσ = 0,60       (D.3)

      Image text deleted Image

D.2.2 Shear resistance

  1. The shear resistance Image Vbw,Rd Image should be taken as:

    Image

    where

    χc is the lesser of the values of reduction factors for local buckling χc,ℓ global buckling χc,g obtained from (2) and (3)
  2. The reduction factor χc,ℓ for local buckling should be calculated from:

    Image

    Image

    Image

    amax should be taken as the greater of a1 and a2.

    51

    NOTE: For sinusoidally corrugated webs the National Annex may give information on the calculation of τcr,ℓ and χc,ℓ.

    The use of the following equation is recommended:

    Image

    where

    w is the length of one half wave, see Figure D.1,

    s is the unfolded length of one half wave, see Figure D.1

  3. The reduction factor χc,g for global buckling should be taken as

    Image

    Image

    Image

    Image

    Iz       second moment of area of one corrugation of length w, see Figure D.1

    NOTE 1: s and Iz are related to the actual shape of the corrugation.

    NOTE 2: Equation (D.10) is valid for plates that are assumed to be hinged at the edges.

D.2.3 Requirements for end stiffeners

  1. Bearing stiffeners should be designed according to section 9. 52

Annex E – Alternative methods for determining effective cross sections

[normative]

E.1 Effective areas for stress levels below the yield strength

  1. As an alternative to the method given in 4.4(2) the following formulae may be applied to determine effective areas at stress levels lower than the yield strength:
    1. for internal compression elements:

      Image

    2. for outstand compression elements:

      Image

      For notations see 4.4(2) and 4.4(4). For calculation of resistance to global buckling 4.4(5) applies.

E.2 Effective areas for stiffness

  1. For the calculation of effective areas for stiffness the serviceability limit state slenderness Image may be calculated from:

    Image

    where

    σcom,Ed,ser       is defined as the maximum compressive stress (calculated on the basis of the effective cross section) in the relevant element under loads at serviceability limit state.

  2. The second moment of area may be calculated by an interpolation of the gross cross section and the effective cross section for the relevant load combination using the expression:

    Image

    where

    Igr is the second moment of area of the gross cross section
    σgr is the maximum bending stress at serviceability limit states based on the gross cross section
    Image is the second moment of area of the effective cross section with allowance for local buckling according to E.1 calculated for the maximum stress σcom,Ed,serσgr within the span length considered.
  3. The effective second moment of area Ieff may be taken as variable along the span according to the most severe locations. Alternatively a uniform value may be used based on the maximum absolute sagging moment under serviceability loading.
  4. The calculations require iterations, but as a conservative approximation they may be carried out as a single calculation at a stress level equal to or higher than σcom,Ed,ser.
53