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European ProjectFire Resistance of Innovative and Slender Concrete Filled

Tubular Composite Columns (FRISCC)

Prof Leroy Gardner

Dr Finian McCann

Elliptical section members

OUTLINE

FRISCC - Fire Resistance of Innovative and Slender Concrete Filled Tubular Composite Columns

1. STEEL EHS MEMBERS

INTRODUCTION

STRUCTURAL INVESTIGATIONS

DESIGN RULES

2. CONCRETE-FILLED EHS MEMBERS

INTRODUCTION

TESTING AND SIMULATIONS

DESIGN GUIDANCE

DESIGN EXAMPLE

Elliptical section members

FRISCC

Steel EHS members:

• Recently introduced as hot-finished products in

EN 10210

• Combine merits of CHS and RHS

• Elegant aesthetics (CHS)

• Differing rigidities about principal axes

(RHS)� more suitable for applications in

bending

STEEL EHS MEMBERS - INTRODUCTION

a

a

b b

z

y

FRISCC

Applications of steel EHS

STEEL EHS MEMBERS - APPLICATIONS

Heathrow Airport, UK Jarrold store, UK

FRISCC

Applications of steel EHS

STEEL EHS MEMBERS - APPLICATIONS

Madrid Airport, Spain Society Bridge, Scotland

FRISCC

Structural scenarios addressed:

1. Local buckling and cross-section classification

2. Shear resistance

3. Combined bending and shear

4. Flexural buckling of columns

STEEL EHS MEMBERS – STRUCTURAL INVESTIGATIONS

FRISCC

Cross-section classification:

STEEL EHS MEMBERS – CROSS-SECTION CLASSIFICATION

b

b

a a

z

y

In compression or minor

axis bending, equivalent

diameter is:

De

= 2rmax

=2a2/b

Elastic critical local buckling – compression and minor axis bending

Initial aim was to determine an equivalent CHS diameter De

rmax)(r

tE

max

cr2

13 ν−

=σ

FRISCC



rmax

is the maximum local

radius of curvature

a

a

b b

rmax

Maximum

compression

Compression

Tension

z

y

Buckling

initiates

De= 0.8a2/b

Elastic critical local buckling – major axis bending

FRISCC

Cross-section classification – Testing:


Material testing of

tensile coupons

Geometric

measurementsCompression

tests

Minor axis bending tests

FRISCC

Cross-section classification – Finite element modelling:


• FE models developed in ABAQUS

• Models validated against test results

• Full loading history and failure modes well predicted

• Parametric studies conducted, varying:

• Cross-section slenderness

• Aspect ratios (for all tests, a/b = 2)

FRISCC

Cross-section classification – FE validation:


0

600

1200

1800

0 6 12 18 24

End shortening δ (mm)

Lo

ad

N(k

N)

FE

Test

FRISCC



De/tε2

Fu/F

y

0.0

0.5

1.0

1.5

2.0

0 30 60 90 120 150 180 210 240 270

2a

2b

EHS

CHS

FE

Class 1-3 Class 4

De = 2rmax = 2a2/b

ε = (235/fy)0.5

Max. load Fu

normalised by yield load Fy

FRISCC



Minor axis bending – ultimate moment to elastic moment

De/tε2

Mu/M

el

0.0

0.5

1.0

1.5

2.0

2.5

0 20 40 60 80 100 120 140 160 180 200 220 240 260

EHS

CHS

FE

2a

2b

Class 4Class 1-3

De = 2rmax = 2a2/b

ε = (235/fy)0.5

FRISCC

Cross-section classification – summary of measurements of slenderness:


Loading Equivalent diameterCorresponding point on

cross-section

2a

2b

Axial

compressionDe = 2a

2/b

2a

2b

Minor axis

bending (z-z)De = 2a

2/b

2b

2aMajor axis

bending (y-y)

De = 0.8a2/b a/b > 1.36

2b

2a

De = 2b2/a a/b ≤ 1.36

FRISCC

Cross-section classification – summary of slenderness limits:


Type of

compression

loading

Diameter

ratio

Proposed slenderness limits

Class 1 Class 2 Class 3

Axial

compression D

e/t Not applicable 90ε2

Minor axis

bending (z-y)D

e/t

50ε2 70ε2 140ε2

Major axis

bending (y-y)D

e/t

FRISCC

Shear resistance:

STEEL EHS MEMBERS – SHEAR RESISTANCE

• Three-point bending tests (a/b = 2):

• 12 major axis, 12 minor axis

• Varying slenderness and length

L/2 L/2

F

Moment gradient

Uniform shear

Uniform shear

FRISCC


Design plastic shear resistance:

(Av = shear area, fy = yield strength, γM0 = 1.0)

0

,

3/

M

yv

Rdpl

fAV

γ=

b b

a

a

z

y

For shear along z-z:

a a

b

b

z

y

For shear along y-y:

Av = (4b-2t)t A

v = (4a-2t)t

FRISCC


Moment–shear interaction design guidance based on test results:

0.0

0.5

1.0

1.5

0.00 0.25 0.50 0.75 1.00 1.25

Vu/Vpl,Rd

Mu/M

pl,Rd

or M

u/M

el,Rd

Shear along y-y

Shear along z-z

Proposed shear-moment

interaction

FRISCC

Column buckling:

STEEL EHS MEMBERS – COLUMN BUCKLING

• Column tests performed (a/b = 2):

• 12 major axis, 12 minor axis, varying slenderness and length

Knife edge

Load cell

LVDT

Strain gauge

CL

Hydraulic jack

FRISCC

Column buckling – finite element validation:


0

250

500

750

0 15 30 45 60

Lateral deflection at mid-height ω (mm)

Lo

ad

N (

kN

)

Test

FE

0.0

0.5

1.0

1.5

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8

Member slenderness λ

Buckling about z-z

Buckling about y-yN

u/N

yo

r N

u/N

eff

z

y

EC3 – curve ‘a’


Buckling curve ‘a’ can be used for EHS, as for other hot-finished

hollow sections


Design guidance:

• Presented proposals are

reflected in the Blue book

• Also in equivalent US

design guidance

• Expected to be

incorporated in future

revisions of EC3

FRISCC

Steel EHS members - conclusions:

STEEL EHS MEMBERS – SUMMARY

• New addition to hot-rolled range

• Significant testing and FE modelling programmes

• Design rules for primary structural configurations

• Incorporation into structural design codes ongoing

FRISCC

Concrete-filled EHS columns:

• Design guidance currently exists for other concrete-filled tubular

columns (CHS, SHS, RHS)

• No current guidance for emerging CFEHS structural solution

• Among aims of FRISCC project: develop guidance on the design

of CFEHS columns

• At room temperature (Imperial College)

• In fire conditions (UP Valencia)

CONCRETE-FILLED EHS MEMBERS - INTRODUCTION

FRISCC

Current guidance:

• Cross-section classification - Eurocode 4: “composite section classified

according to least favourable class of steel elements in compression” (using

Eurocode 3 limits)

• Resistance of compression members: not available for CFEHS

� adopt rules for CHS / RHS?

Strategy for development of design guidance:

• Experimental programme

• Validation of numerical model against experiments

• Numerical parametric study

• Develop design rules for CFEHS columns and beam-columns based on results

CONCRETE-FILLED EHS MEMBERS - INTRODUCTION

FRISCC

Experimental investigation:

• 27 concrete-filled 150×75×6.3 EHS

specimens tested in compression

• Grade S355 steel, grade C30 concrete

• Loading was either concentric or with various

major / minor axis eccentricities

• Specimens with different global slenderness

(lengths) examined

• Some specimens with steel reinforcement

(4No. T10 bars)

CONCRETE-FILLED EHS MEMBERS - EXPERIMENTS

FRISCC

Cross-sectional geometry of experimental specimens:


a

b

ez

ey

Position of eccentric load

10 mm

18 mm

Specimen buckling about major

axis

Specimen buckling about minor

axis

40 mm

15 mm

T10 reinforcing bar

FRISCC

Testing of columns:


FRISCC

Numerical modelling:

• Finite element model of CFEHS column developed in ABAQUS

• Steel material model based on tensile testing of coupons

• Concrete damage plasticity model used

CONCRETE-FILLED EHS MEMBERS – NUMERICAL MODELLING

concrete core

steel tube

Buckling axis

end-plate

FRISCC

Validation of numerical model – ultimate loads:


0

200

400

600

800

1000

1200

1400

0 200 400 600 800 1000 1200

Nu,exp(kN)

Nu,FEA (kN)

Present study

+10%

Unity

-10%

Nu,exp / Nu,FEA: average = 1.12, STDEV = 0.07

FRISCC

Validation of numerical model – load–deflection behaviour:


0

100

200

300

400

500

600

700

800

0 5 10 15 20 25

Load

(kN

)

Axial displacement (mm)

E20:L2-MA-50-R - test

E20:L2-MA-50-R - FEA

E21:L1-MA-50-R - test

E21:L1-MA-50-R - FEA

E22:L3-MI-25-R - test

E22:L3-MI-25-R - FEA

FRISCC

Validation of numerical model – failure mode:


FRISCC

Numerical parametric study:

• 360 specimens modelled, varying

• cross-section

• slenderness

• reinforcement ratio

• cover to reinforcement

• load eccentricity (also modelled concentric loading)

• buckling axis

• Results used as basis to formulate design rules


FRISCC

Design guidance strategy:

• Apply rules for concrete-filled CHS and RHS to CFEHS columns

• Buckling curve relates to EC3 curve for members in axial compression

• Member imperfection used to determine first-order moments for members

in combined compression and uniaxial bending (i.e. eccentrically-loaded)

CONCRETE-FILLED EHS MEMBERS – DESIGN GUIDANCE

FRISCC

Assessment of use of CHS and RHS rules for CFEHS columns:

CONCRETE-FILLED EHS MEMBERS – DESIGN GUIDANCE

Ratios of FE parametric study results to EC4 predictions

(using design strengths i.e. with partial factors)

Conclusion: CHS and RHS rules are suitable for design of CFEHS columns

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

1.30

Nult,FE

/ N

ult,EC4

Design strengths

SAFE

UNSAFE

FRISCC

Design example: determine capacity of concentrically-loaded CFEHS

Column is 400 × 200 × 12.5 EHS, L = 4 m, B.C. = P-P

2a = 400 mm, 2b = 200 mm, t = 12.5 mm

fcd = 30 MPa, fyd = 355 MPa, Ea = 210 GPa, Ecm = 36 GPa

Cross-sectional properties of concrete element:

Ac =

= 515 cm2

Ic,z=

= 9865 cm4

Cross-sectional properties of steel element:

As = 113 cm2

, Is,z = 5843 cm4 (from Tata section tables)

CONCRETE-FILLED EHS MEMBERS – DESIGN EXAMPLE

400 mm

200 mm

12.5 mm

( ) ( ) ( )( )5.1222005.1224004

22224

×−×−=−−

ππ

tb ta

( ) ( ) ( )( )335.1222005.122400

642222

64×−×−=−−

ππ

tb ta

FRISCC


Plastic resistance to compression:

Npl,Rd = Aa fyd + Ac fyc = (113)(355)+(515)(30) = 5557 kN

Effective minor axis flexural rigidity:

(EI)eff,z = EaIa,z + 0.6 EcmIc,z= (210000)(5843)+(36000)(9865)

= 13790 kN m2

Elastic critical load for buckling about minor axis:

Ncr,z = π2(EI)eff,z / L

2 = π2(13790) / 42 = 8506 kN


400 mm

200 mm

12.5 mm

FRISCC


Nondimensional slenderness:

Reinforcement ratio ρ = 0, therefore use buckling curve a:

� Imperfection factor α = 0.21


82.08508

5676

cr,z

Rdpl,===

N

Nλ

( )( ) ( )( ) 898.082.021.082.021.015.015.02

2

o =+−+=+−+=Φ λλλα

( ) 786.0850.0898.0898.0/1/122

22 =−+=

−Φ+Φ= λχ

kN44616526767.0Rdpl,Rdb, =×== NN χ

FRISCC

Thank you!
