From BS5950 to EC3

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rom o

Chiew Sing-PingSchool of Civil and Environmental En ineerin

Nanyang Technological University, Singapore

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Basis of design combination of actions

Structural analysis imperfections &-

Member design beam & columnuc ng

Web bearin & bucklin

Shear buckling & plate girder w

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Trend is to use higher grade and better quality

EC3 has additional ductility requirements

compare o . . . n erms o

stress ratio, elongation and strain ratio. It is okay for hot-rolled steel but will be difficult

- -

section.

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uc y equ remen

Normal Strength Steelf < 460 N/mm2

High Strength Steel460 < f < 700 N/mm2

fu/fy 1.10

fu/fy 1.05 ( EC3-1-12) u y . or p as c ana ys s

elongation at failure not

fu/fy 1.10 ( UK NA to EC3-1-12)

elon ation at failure not

u 15 y ( y is the yieldless than 10%

15 s ra n,

y=

y

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-

f

fyUpper

yLower

tress

u Strainy

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-

f

fyUpper

yLower

tress

amount of plastic deformation

represented by shaded area under the

curve

u Strainy

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Problem 1 Steel for Cold ForminSome product standards only have requirements on the nominal

yield and tensile strengths, or their minimum values. The stress

ratio calculated according to these nominal values cannot comply

with the EC3 ductility requirement.

Standard Grade Nominal yieldstrength (MPa)

Nominal tensilestrength (MPa)

Stress ratio

AS1397

G450 450 480 1.07

G500 500 520 1.04

G550 550 550 1.00

AS 1595 CA 500 500 510 1.02

S 550MC 550 600 1.09

EN 10149-2 .

S 650MC 650 700 1.08

S 700MC 700 750 1.07

EN 10326 S550GD 550 560 1.02

.

AS 1397: Steel sheet and strip hot-dip zinc-coated or aluminium/zinc-coated

AS 1595: Cold-rolled, unalloyed, steel sheet and strip

EN 10149-2: Hot-rolled flat products made of high yield strength steel for cold forming

EN 10326: Continuously hot-dip coated strip and sheet of structural steels

ISO 4997: Cold-reduced carbon steel sheet of structural quality

steel for profile metal decks and purlins

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Problem 2 Cold Formed Sections

Most standards only have requirements on the range of tensile. .

face problem with EC3 ductility requirement, for e.g. S355J2H in BS

EN10219: fy>355 MPa and 470 MPa

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Mechanical Pro erties due to Cold Workin

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Typical Curve with no distinct Yield Point

- gain in yield and loss in ductility due to cold working

ss

Rm

Str p

Rt

StrainAgt AtA

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Toughness Requirement

To prevent sudden brittle fracture failure, both adopteda s mp e eeme - o-sa s y es gn approac y

ensuring the maximum permitted thickness value is

adequate notch toughness, taking into account factors

, , ,

member shape and detail, stress level and strain rate.

, - -

for brittle fracture using fracture mechanics in addition

.

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SteelQuality

Grade

(Ed= 0.75 fy) (t)

(Ed = 0.5 fy) (t)

(Ed )> 0.3 fy) (t)

10 (20)C 0C 10 (20)C 0C 20C 0C

S275

JR 55 45 80 70 71 50J0 75 65 115 95 103 71

J2 110 95 155 130 148 103

M,N 135 110 180 155 178 124

ML,NL 185 160 200 200 250 178

JR 40 35 65 55 50 35

S355

J0 60 50 95 80 72 50

J2 90 75 135 110 104 72

M,N 110 90 155 135 124 86

Notes: (1) The units for the thickness values are mm.

ML,NL 155 130 200 180 179 124

e g es re erence empera ure a a owe s .

(3) This comparison is done under generally welded and high stress condition

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load combinations are given in Table 2 of BS5950.

Fundamental combination of actions can be determined

from Eqns. 6.10, 6.10a or 6.10b of EN1990.

+++> 1i

ik,i,iQ,k,1Q,1p1

jk,kG, QQPG (6.10)

Action due to

prestressing

Leading variable

action

Non-leading variable

actions

Permanent

actions

From SS NA of EN1990= .

0= 0.7 for the imposed load;

G = 1.35 for unfavorable permanent action;

Q = . or ea ng or non- ea ng var a e ac on.

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Combination of Actions

Combination Design Action

Dead & imposed 1.4Gk+ 1.6Qk 1.35Gk+ 1.5Qk

Dead & wind 1.4G + 1.4W 1.35G + 1.5W

Dead, imposed & wind 1.2Gk+ 1.2Qk+ 1.2Wk 1.35Gk+ 1.05Qk+ 1.50Wkor

1.35Gk+ 1.50Qk+ 0.75Wk

Gk= permanent action; Qk= imposed variable action; Wk= wind variable action

Example: Gk= 20 kN; Qk= 10 kN; Wk= 8 kN

Design Action

BS5950 EN1990Dead & imposed 44.0 kN 42.0 kN

Dead, imposed & wind 45.6 kN 49.5 kN or 48.0 kN

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-

Global imperfections for frames and bracing systems,

such as global initial sway imperfections =0hm in

Local im erfections for members

The effects of local imperfections in members are

generally incorporated within the formulae given for .

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Frame Im erfection in BS5950

BS5950 uses the notional horizontal force (NHF) concept to

F3 F3

a ow or rame mper ec on suc as ac o rame ver ca y

F2F2 F2

=

F1F1 F1

1 n 200 = 0.5%

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Global Im erfection in EC3

EC3 uses the same concept but called it equivalenthorizontal force (EHF) to allow for initial sway imperfection

(lack of verticality) in frame

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Combination of Actions

CombinationDesign Action

BS5950 EC3

Dead & imposed 1.4Gk + 1.6Qk + NHF 1.35Gk + 1.5Qk + EHF

*ea w n . k . k no . k . k

Dead, imposed & wind 1.2Gk + 1.2Qk+ 1.2Wk* (no NHF) 1.35Gk + 1.05Qk+ 1.50Wk + EHF

or

1.35Gk + 1.50Qk+ 0.75Wk+ EHF

In BS5950, minimum Wk is 1% of factored dead load;

this is to provide a minimum level of robustness but

why no NHF when the wind is blowing?

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Structural Anal sis - Terminolo

First-order analysis: Equilibrium equations are written

in terms of the geometry of the undeformedstructure,

geometrical non-linearity not considered Second-order anal sis: E uilibrium e uations are

written in terms of the geometry of the deformed

structure, geometrical non-linearity considered Elastic analysis: Material properties is assumed to be

elastic and often linear

Inelastic analysis: Inelastic material properties

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T es of tructural Anal sis

First-order, second-order, elastic and inelastic analyses y : x u y,

represents conditions under normal service loads very well

considered. It produces good representation of destabilizing

-

First order inelastic analysis: Geometrical nonlinearity

abruptly (e.g. onset of plastic hinge)

Second order inelastic anal sis: Both eometrical and

material nonlinearity are considered. Enable you to tracethe behavior of the structure up to ultimate state and failure

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Level of Non-Linearit

nd. , , , . . ,John Wiley & Sons, 2000

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BS5950 EC3

.

First-order elastic analysis (initial

geometry and influence of deformation

not considered)

First-order analysis

Elastic global analysis

First-order plastic analysis (small axial

forces & no instability effects)

Plastic global analysis

Using first-order analysis, if:

Second-order elastic analysis (can use

simplified methods such as amplified

cr 10 for elastic analysis

methods for sway sensitive frames

where 4 < cr< 10)

cr= Fcr/Fed, Fcr is the elastic critical

buckling load

Other advanced analyses (generallynot covered)

Second-order analysis (influence ofgeometrical deformation taken into

account)

Allowed more advanced analyses

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-

The frame P-big effect is allowed for in BS5950 using either one of thefollowing 2 methods:

Amplified sway method - the sway moments are multiplied by an,

Effective length method - the actual sway effective lengths from thecharts in Annex E are used.

The column P-small effect is allowed for in BS5950 indirectly through

- .

Note: Although EC3 also allows amplified sway moment or effective

length approach, less guidance is given. Unlike BS5950 which

facilitates hand calculations, EC3 focuses on global second-order

. .

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-

Simplified method:Amplified the sway moments

Exact method:Moments about minor axis only:

-

1++yy

yy

xy

xx

C

C

Zp

m

Zp

m

P

FIn-planebuckling 11

++cy

c

cy

yy

cy

c

PF

M

m

PF In-planebuckling

way e ective length rom Annex E

Lateral-torsional

buckling

15.0 +cy

yyx

cx M

Mm

P

Fc Out-of-planebuckling

1++yyLTLT

MmMmFc

Moment about major axis only:

15.01

++ cxxcFMmF In-plane

bcy

cxcxcx

15.0 + LTLTMm

P

FcOut-of-plane

bucklincy

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Rigorous method:

=

General method:

x

Mb = PbZxMb = PbSx,eff for class 3

1M

y

yLTRd,b WM =

1

b b x,eff

For non-uniform moment or unequal

LT ,2LT

2LTLT

LT

+

2 fWen momen :

Mx Mb/mLT and Mx Mcxwhere LT is imperfection factor

++= LTLTLTLT ,,

cr

LTM

=

where Pb determined by

LT = u v(w)0.5LE/ry

depending on which buckling curves

(Figure 6.4 of EC3)

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Member Desi n Com ressive Resistance

BS5950 EC3

Pc =pcAg for class 1, 2 & 3 for class 1, 2 & 31M

y

Rd,b

AfN

=

Pc =pcsAeff for class 4 for class 41M

yeff

Rd,bN

=

c

based on the strut curve, slenderness

& design strengthpy

buckling mode1

=

= L/r r= (I/A)0.5

but 1,0

+

.

where is imperfection factor( )

++=2

2,015,0cr

yeff

N

f=

depending on buckling curves (EC3 has

5 curves).

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EC3 offers no formulae and gives no guidance on how to calculateNcrand Mcr.

.

2EI5.0

22 Tcrw GIIEI

2

cr

crL

=221

=

zZcr

crEIIL

where Lcr is the effective length and C1 is the correction factor for

non-uniform and unequal end moments. Unlike BS5950, EC3 does

uniform moment factors.

W b B i d B kli

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Web Bearin and Bucklin

BS5950-1 requires two independent checks for web bearing

with these two failure modes for the web subjected to a

transverse force.

However, unlike BS5950, EC3 does not take unrestrainedflan e into account flan e free to swa or rotate .

Flange free to sway

sideways

Flange rotation

relative to the web

W b B i d B kli

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BS5950 EC3e ear ng: bw = 1 + n pyw

Web buckling:

es s ance o we aga ns

transverse force in which their

compression flange is adequately

for unstiffened web &

c> 0.7d restrained in the lateral direction:

bwx Pt

P25

=

weffyw tLf

For unstiffened web & c< 0.7d

n11M

Rd

eff

resistance against transverse force:bwe

x Pdnkbd

P)(4.1

.

1 +=

= 0.15.0 =

Pd

P7.0

=

yeF

wFcr

tEkF

3

9.0= ywwyFftl

=

Ew cr

Web Bearin and B cklin

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EC3 has no equations to calculate stiff bearing length except saying the

over which the applied load is effectively distributed at a slope of 1:1,

and Ss should not be larger than hw. (EC3-1-5, clause 6.3.1)

BS5950-1 EC3-1-5

The Straits Times, 3 Augus t 2004The Straits Times, 3 Augus t 2004

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Shear Bucklin and Plate Girder

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BS 5950-1 Web with depth-to-thickness ratio d/t > 62 is susceptible

to shear buckling.

Shear buckling resistance Vb of the thin web is taken as w

Vb

= Vw

= d t qw

where d is the depth of the web;

t is the web thickness

qw is the shear buckling strength of the web


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EC3-1-5 Web with hw/tgreater than should be checked for

resistance to shear buckling.

The design resistance for shear buckling is taken as:

(5.1),,,3

wywRdbfRdbwRdb

thfVVV +=

in which the contribution from the web and flanges are:

(5.2)1

, 3 M

wyww

Rdbw V =

(5.3)

=

22

, 1Edyfff

Rdbf

MftbV

,1 RdfM

Shear Bucklin Resistance

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Shear Bucklin Resistance

Stiffener spacing ratio, a/d (a/hw) = 1, fy = 275 N/mm2

160

180

)0.6275=165 EC3 rigid end post TFA

140

(N/mm qw (BS5950-2000)TFA

EC3 non-rigid end post TFA

100gthqw

qcr(BS5950-1990) No TFA

60arstre

61

71

55.7

20

40sh

28

0

0 50 100 150 200 250 300

.

we ep - o- c ness ra o w

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the applied shear forcesTension Field Action TFA

when elastic critical

shear buckling

resistance is

exceeded

Tension field action is mobilized in both BS5950 and EC3 torealize much hi her shear bucklin resistance of the thin web

Wh the fuss over Hollow Section Joint?

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Wh the fuss over Hollow Section Joint?

Hollow section joints can be very flexible because they are unstiffened!

Designing unstiffened welded hollow section joints is a skilled task and must

be done at member design stage.

Potential Failure Modes

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Mode A: Plastic failure of

Mode B: Punching shear

failure of the chord facethe chord face

Mode D: Local buckling of

the web member


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Mode E: Overall shear failureof the chord

Mode F: Local buckling ofthe chord walls

Mode G: Local bucklin ofthe chord face

Verification

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Verification

Topic BS5950 EC3

Material

uc y Toughness

Basis - combination of actions Im erfections

analysis

Second-order effects

em er uc ng eam co umn Web bearing & buckling Shear buckling & Plate girder Hollow section oints

Final oncludin Remarks

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Final oncludin Remarks

-adequate and safe for use in the short term.

.

EC3 is more comprehensive but its terminology,

sym o s an va ues are very eren an mu p e

documents are needed. Re-training is absolutely necessary in view of time

frame and ma nitude of chan e involvin man

design codes and documents. .

Documents

From BS5950 to EC3