11

Component Studies for Steelwork Connections in Fire

Component Studies for Steelwork Connections in Fire

Spyros Spyrou, Buick Davison,Ian Burgess, Roger Plank

University of Sheffield, UK

22

Semi-rigid behaviour of connections

At ambient temperature

• Interest based on using connection stiffness and strength to enhance the performance of structural frames.

• Initial approaches based on cantilever and cruciform tests, to create database of M-φcurves. Some semi-empirical models to rationalise results.

• Large range of joint types, arrangements, dimensions implied that database for general design purposes would be very large.

• Finite element models of joints need to be complex in order to produce accurate results.

33

The “Component” method

• Feasible approach based on modelling the zones of fundamental behaviour (“components”) within a connection then constructing a model of the connection.

FcKc

F2

F1K1

K2

Ft

M

44

Component zones in end-plate joint

Column web in tension

Column web in compression

Beam flange incompression

Beam web in tension and compression

Column flange andend plate in bending

Bolts in tension

Column web in shear

M

Beam web in shear

ShearTension Compression

55

Modelling connection behaviour in fire

• Late 1980sSCI sponsored 11 tests at Warrington on different cruciform connection arrangements. One load level applied to each. Concluded that reduced plastic capacity of joint would reduce the midspan bending moments.

• 1990Simple beam fire resistance enhancement design method published in SCI-P-086.

• 1996Included in widely-distributed British Steel design examples.

66

Cardington beam-column joint after fire test

Buckling due to high axial force on heating

77

Component zones in end-plate joint with axial thrust

Reduced tension

Highercompression in web

Beam flange in highercompression

Beam web mainly incompression

Reduced force on column flange

Lower tension

Column web in shear

M

F

Beam web in shear

ShearTension Compression

88

The “Component” method with axial force

• In fire axial compression acts together with moment due to restraint to thermal expansion. Component model would deal with this automatically, though M-φcurves change due to thrust.

FcKc

F2

F1K1

K2

Ft

M

FcKc

F2

F1K1

K2

Ft

M

F

99

Objectives of this project

Tension Zone

• Do experiments on T-stubs at high temperatures.

• Develop simplified/semi-empirical model of tension component behaviour for end-plate joints.

• Check both against finite element modelling.

Compression Zone

• Examine experimentally the effect of elevated temperatures on column web buckling.

• Develop simplified/semi-empirical model of column web compression component behaviour for end-plate joints.

• Check both against finite element modelling.

Generally

• Check moment-rotation predictions against previous cruciform furnace tests.

1010

Furnace and control apparatus

Loadingdevice and

Control panel

View portsfor cameras

Loading actuator

1111

Furnace test setup

1212

T-stub Type B/C tension component specimen

1313

High-temperature T-stub through viewport

1414

Failure modes for tension T-stubs

Failure Mode 1 Failure Mode 2 Failure Mode 3

1515

Forces on a T-stub assembly

kn m

Le Q

δep

F

w wv

x

QSimplified Model

• Uses this arrangement with normal elasto-plastic structural mechanics to track the failure modes.

At High Temperatures

• Uses EC3 strength & stiffness reduction factors for steel sections/plate. Kirby strength reduction for bolts. EC3 stiffness reduction factors verified by separate tests.

1616

Failure mode 1

0

50

100

150

200

250

300

350

400

450

500

0 1 2 3 4 5 6 7 8Displacement (mm)

Force (kN)

505°C

700°C

740°CA

C D

First plastic hinge

Yield & fracture of bolts

1717

Failure mode 2

00

50

100

200

250

300

350

400

450

4 8 12 16 20 24 28 32 36

D

0

50

100

150

200

250

300

350

400

450

500

Displacement (mm)

Force (kN)

505°C

540°C

705°C

650°C

415°C

A

C

B

First plastic hinge

Second plastic

Yield & fracture of bolts

hi

1818

Failure mode 3

Force (kN)

350Ambient

410°C

505°C

510°C

610°CC

D

AC

D300

250

200

150

100

50

0-1 0 1 7 82 3 4 5 6

Displacement (mm)

1919

Von-Mises stresses from ANSYS modelling of T-stubs

High temperatureAmbient temperature

2020

Comparison between simple model and finite element analysis

Force kN Force (kN)300 300

660C

ANSYS

0

50

100

150

200

250

0 2 4 6 8 10 12 14Displacement (mm)

ANSYS

410C

Simple250

Simple200

150

100

50

00 1 2 3 4 5

Displacement (mm)

2121

Compression zone: column web

M M

2222

Compression zone test arrangement

ActuatorReaction Frame

Specimen

Furnace

2323

Column specimen inside the furnace

2424

Formation of plastic hinges in column flange

Second hinge

Second hinge

First hinge

2525

Simplified model of compression zone

Roberts’ model for ultimate strength of stocky webs

Uniform stress σyw

ββ c

Pu1 2 3 4

2626

Principles of simple compression zone model

2 3

β

γ'

w

γ'e'

βc

650tfb

w1

Lt

Yielding

2. Yield of column flange (first plastic hinges)

γ/10

β

γ

w

γe

βc

300

tfb

w1

γ/10

Leff=e+(γ/5)

1.Yield of column web

β ∆w′

Le=β+c/4

c/2Mpfc

1

3. Yield of column flange (final plastic hinges)

2727

Ultimate strength of compression zoneUltimate strength is seen from tests to be higher than the load at which the final hinges are created

For Plate Girders

• Markovic tested 11 semi-emprical ultimate strength formulae.

• Drdacky formula is the only one which applies to relatively stocky (h/t~75) webs. Compares with h/t~20 to 30 for rolled H-sections.

+=Ρ

wcwc

fbwcwcwcu d

ctt

Et 5.19.055.0 2 σ

• New Formula developed from Drdacky, because original gave unsafe predictions compared with FE analyses with varying contact area.

+

+=Ρ

cdc

tt

Etwcwc

fbwcwcwcu β

βσ2

26.165.02

2828

Comparison between tests and various web capacity formulae at high temperatures

100

150

Forc

e(

350

TestsEC3

UC152x152x30

DrdackyModified Drdacky

300

kN) 250

200

50

0400 500 600

Temperature (ºC)00 100 200 300 700 800

2929

Comparison of modified Drdacky formulawith tests

0

100

200

300

200 400 600 800Temperature (°C)

Force (kN)

0 200 400 600 800Temperature (°C)

Force (kN)

UC152x152x30 100

200

300

400

UC203x203x46

200 400 600 800Temperature (°C)

Force (kN)

200

400

600

800

UC203x203x71

0 200 400 600 800Temperature (°C)

Force (kN)

UC203x203x86400

800

1200

0

3030

3D finite element analysis: web buckling

Out-of-plane deflection contours

3131

Typical test and analysis results for compression zone

3D ANSYS

0

20

40

60

80

100

120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17Displacement (mm)

Force (kN)

Test

2D ANSYS

UC203x203x46 at 670°C

Simplified model

3232

Component model of beam-column joint

Kt

Kc

P

M

3333

High-temperature M-φ curves: test and component model

100

200

300

400

500

600

700

800

0 10 20 30 40 50 60 70 80 90

FB11 (4 kNm)

FB13 (13 kNm)

Beam Flange Temperature (°C)

FB14 (17 kNm)

FB12 (8 kNm)

100Rotation (Millirads)

3434

Observations

• Connection behaviour is important during fires. For safety the connections must stay connected, even when beams are highly distorted.

• The traditional approach in ambient-temperature design is to use M-φ data for beam-column joints.

• Extended in fire to M-φ-θ to include degradation of steel properties with temperature.

• With restraint this database would become M-φ-N-δ-θ.Clearly unfeasible.

• For flush/extended end-plate joints the project has produced workable simplified models for the tension zone and the column web component of the compression zone.

• Next step: beam flange compression zone.

• Later: shear panels in column & beam webs.