Fire Resistance of Innovative and Slender Concrete Filled...

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!


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