Buro Happold Specialist Consulting

STRUCTURAL FIRE ENGINEERING ASSESSMENTS OF THE MOKRSKO FIRE TESTS

An Engineering Prediction

Anthony Abu, Berenice Wong, Florian Block and Ian Burgess

The University of Sheffield

and

2

Introduction

• Advanced analysis of structures in fire is only as good as the modelling.

• The programs, “the model”, used should included all relevant physical features and should be validated.

• However, “the modelling” should also be validated as it is at least as important as “the model”.

• Two full scale fire tests were conducted in 2008: the FRACOF fire test and the fire test in Mokrsko.

• A priori modelling of the of both test has been undertaken based on pre-released data and engineering assumptions.

• After the tests more detailed modelling was done.

3

The Mokrsko Fire Test – Czech Technical University of Prague

• Steel and concrete composite office building consisting of four bays with a size of 9m x 6m each.

• Tested three different floor systems:

• “Angelina” composite beams developed by Arcelor-Mittal with elongated web openings,

• Beams with corrugated webs made from thin steel plates,

• Precast hollow-core panels.

Okenní otvor

Dveře

A B C

3

2

1

9 000 9 000

+0,00

Mechanické zatížení pytli se štěrkem

Požární zatížení hranicemi dřeva

+4,00

S

Meteorog.

nad prolamovanými

nad nosníky

Duté

Okenní otvor

6 000staniceMeteorog.

stanice

s vlnitou stojinou

nosníky

předepnuté panely6 000

Sendvičové panelyBetonová stěna

Skládanýplášť

Ocelobetonová deska

Ocelobetonová deska

Sádrovétvárnice

4

Steel Frame

4

5

Beams• Angelina™ Beams are an Arcelor-Mittal product based on a sine wave cut

from an IPE270 with a total new depth of 395mm.

• The beams with the corrugated webs were 500mm with a web thickness of only 4.5mm.

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Connection Details

All beam connections connected only the top flange and a small part of the web of each beam. The bases of the columns were constructed as pinned.

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Composite Slab

120mm composite slab CF60 metal decking using a smooth mesh (196mm2/m) and 10mm bars in each rib.

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View from inside

8

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View from above

9

10

Cladding

10

11

Mechanical Loading

11

Imposed load was 3.0kN/m2 and the self-weight was 2.6kN/m2.

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Fire Load

12

A B C

3

1

9 000 9 000

+0,00

POŽÁRNÍ ZATÍŽENÍ

+4,00

S

6 000

6 000

2

POŽÁRNÍ ZATÍŽENÍ

Timber cribs with a density of

35.5 kg/m2 generated a total fire

load of about 620MJ/m2

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Timber cribs

13

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Ventilation

The two openings were 2.54 m height and 4.00 m wide eachThis resulted in a Opening Coefficient of O = 0.064 m1/2.

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Instrumentation Frame

15

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Thermocouples

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Test predictions – A priori• Only the 3 bays with the composite slab were modelled.

• The Angelina beams and the corrugated-web beams were represented using an effective web thickness approach.

• The beam connections were modelled as rigid.

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Design FiresAs with a normal SFE project a number of parameters were varied in order to test the robustness of the solution. The fire was altered to produce a short-hot fire (1) and a cooler-longer fire (3). The real fire (4) burned cooler than predicted (2).

0

200

400

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0 30 60 90 120 150 180Time [minutes]

Gas

Tem

pera

ture

[°C

]1

2

3

4

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Results of a priori modelling

-1000

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0

0 15 30 45 60 75Time [minutes]

Ver

tical

def

lect

ion

[mm

]

1 2

3

4

• No indication of collapse but the vertical deflections are larger than span/15, which would normally result in an increase of reinforcement to limit the vertical deflections.

• All beams framing into columns would be protected in a robust design for fire.

• Much earlier increase in deflections than the experimental results as the parametric fire curves represent post-flashover fires, and should be moved by about 15min to give a realistic representation of the fire.

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Test day – 18/09/2008

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Test Video 1

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Test Video 2

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1st Structural Collapse of a Large Scale Fire Test

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Crack Pattern of Slab

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Composite Slab Corner

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Compartment Temperatures - Back

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0

200

400

600

800

1000

0 15 30 45 60 75

TG01

TG07

Teplota plynu [°C]

Čas [min]

Průměr změřenýchteplot plynu  

TG07

TG01

Okenní otvory

Termočlánek

Termočlánek

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Compartment Temperatures - Front

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TermočlánekTeplota plynu [°C]

Čas [min]0

200

400

600

800

1000

15 300 45 60 75

TG12

Průměr změřenýchteplot plynu

  TG11 TG12Okenní otvory

TG11

Termočlánek

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Compartment Temperatures - Left

40

Teplota, °C

TG09

TG10

TG07

TG08

0

100

200

300

400

500

600

700

800

900

1000

1100

0 15 30 45 60 75 Čas, min

TG07

TG10

TG08TG09

Průměr zTG07,TG08,TG09,TG10

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Compartment Temperatures - Right

41

0

100

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300

400

500

600

700

800

900

1000

1100

0 15 30 45 60 75 Čas, min

Teplota, °C

TG01

TG04TG05

TG06Průměr z

TG01,TG04,TG05,TG06

TG01

TG04

TG05

TG06

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Steel Temperatures – Angelina Beams

42

0

AS6

15 30 45

100200300400500600700800900

1000

AS2

AS5

AS4

Teplota [°C]

Čas [min]0

AS2

AS4AS5AS6

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Vertical Deflections

43

15 30 45

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0 0

Deformace [mm]

V3

V1

V7

V5

Čas [min]

-100

-300

-500

-700

-900 V1

V3

V5

V7

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Temperatures of Corrugated Web Beams

44

Teplota [°C]

Čas [min]

TC21

0100200300400500600700800900

1000

0 15 30 45 60 75

TC22

TC23TC79

TC80

TC22

TC79

TC21

TC23

TC80

45

Deformed Shape of Corrugated Web Beams

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Connection Temperatures

46

0

100

200

300

400

500

600

700

800

0 15 30 45 60

Teplota [°C]

TC69

TC71TC66

TC67

TC64

TC63

TC70

Čas [min]

TC64

TC63

TC71

TC66

TC67

TC69TC70

47

Block shear failure of beam web

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Concrete Spalling

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Concrete Spalling

50

Concrete Spalling

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Laser mapping of spalling on back wall

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0100200300400500600700800900

1000

0 15 30 45 60 75Time [min]

Tem

pera

ture

[°C

]

TC32TC33TC34TC35TC36TC37TC38

Concrete Slab Temperatures

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Concrete Slab Temperatures

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250

0 15 30 45 60 75Time [min]

Tem

pera

ture

[°C

]

TC33TC34TC35TC37TC38

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Vulcan – Modelling directly after the Test – Deflected Shape

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Vertical deflection comparison

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0

0 15 30 45 60 75Time [minutes]

Ver

tical

def

lect

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[mm

]

V3

V1

V7

V1

V3

V7• The deflection curves

show that when the real temperature data is used the vertical deflections are represented accurately up to about 44 minutes.

• The difference between prediction and reality for the beams with corrugated webs (V7) can be explained by the observed shear buckling of the thin webs.

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Horizontal movement of the mid column

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-15

-10

-5

0

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0 15 30 45 60 75Time [minutes]

Hor

izon

tal d

efle

ctio

n [m

m]

H1

+

H1

• Due to the very flexible beam connections, therefore the connections were modelled as pinned.

• In these cases the Vulcanmodels stops around 43 minutes.

• The horizontal displacement at the top of the edge column connected to an unprotected Angelina beam.

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New modellingExact cause of failure of the structure - unknown

Possibly due to:• Failure of middle column• Compression failure of slab• Failure of connections• Unzipping of slab from edge beams• Buckling of back edge beam• Reversal of crack in Angelina bay

Determine:• Approximate magnitudes of tensile forces in concrete• Approximate magnitudes of compressive stresses in concrete• Connector force distribution along the beam edge• Axial forces in the back edge beam• Effects of spalling on failure• Failure initiation and eventual collapse of the structure

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• 3 bays, excluding hollow core bay

• effective stiffness approach

• include reinforcement in ribs

• test temperatures

• pinned column bases

• pinned (torsion-fixed) beam-to-beam and beam-to-column connections

• shear connectors for all internal beams

• realistic model of Angelina beams

• include cross-bracing

New Vulcan Models

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Angelina Beam - Models

Comparison of models to find equivalent model for use in Vulcan

Initial analyses (comparisons) with bare steel beams – ABAQUS

Angelina Beam

Effective web thickness

Vierendeel girder

Truss

Composite beam comparisons with Vulcan

Effective web thickness

Vierendeel girder

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Modelling of Angelina beams

Beam B2

Beam B

Beam A

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Abacus deflected shape – Beam A

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Abacus deflected shape – Beam B1

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Abacus deflected shape – Beam B2

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Abacus deflected shape – Effective web thickness

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Abacus – Deflection comparison – Ambient Temperature

Angelina beam (ambient)

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20

40

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160

0 50 100 150 200 250

mid-span displacement (mm)

App

lied

load

ing

(kN

)

Beam ABeam BBeam B2Solid beam (eff. thickness)Truss

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Abacus – Deflection comparison – Elevated Temperature

Steel Temp vs Displacement(applied load =70kN)

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00 100 200 300 400 500 600 700

Steel Temp

mid

-spa

n di

spla

cem

ent (

mm

)

Beam ABeam BBeam B2Solid beam (eff. thickness)Truss

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Angelina Beam - VulcanAmbient15kN/m2 applied load

Elevated7.5kN/m2 applied loadStandard FireUniform heating – beamNon-uniform heating - slab

Section A Section BA

A

B

B

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Angelina Beam – Vulcan – Ambient Temperature

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00 2 4 6 8 10 12 14 16

Load [kN/m2]M

idsp

an d

efle

ctio

n [m

m]

EWTBeam BBeam B2

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Angelina Beam – Vulcan – Elevated Temperature

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00 100 200 300 400 500 600 700 800

Temperature [°C]D

efle

ctio

n [m

m]

Beam BBeam B2EWT

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• Beam B2 used for initial analysis – large model – long runtime

• Effective web thickness approach used for most analyses

• Protection material Promatech – H (15mm thick board–870kg/m3, 920J/kgK, 0.21W/mK)

• Reinforcement - S500 (mesh 5mm 100/100 + 10mm bar in ribs)

• Concrete, fcu = 34MPa

• Steel fy = S235 (corrugated beams = S320)

• 19mm diameter shear connectors (3 per 1m)

• Beam-to-beam, and beam-to-column connections = pinned

• Effective stiffness approach for the slab

• Bracings

• Test temperatures – gas temperature

Main Analysis - Assumptions

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New Vulcan Models

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New Vulcan Models

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• Realistic fires should be considered including the cooling phase.

• Integrity failure of the floor slab should be controlled by either deflection/curvature limits or finite cracking modeling.

• Reinforcement mesh in the slab must be sufficiently lapped to form a full tension lap.

• All edge beams should be composite and the slab should be tied to the beams.

• All columns should be tied in by protected beams.

• Connections should be designed to be ductile.

Important points

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Time (minutes)

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Conclusion

• Conservative overall predictions of the response of composite structures to fire using sophisticated FE programs could be made.

• It was not possible to predict the failure mode or time prior to the tests but Vulcan could model the overall behaviour of both fire tests accurately when the correct input data was used.

• The tests showed that failure of structures is often caused by details! Therefore, robust construction details should be used until computer modelling can include these phenomena.

• Everyone who predicts the behaviour of structures in fire using FEA should validate their “modelling” against simple and well documented experimental data as well as full scale tests.

• Further modelling required to find cause of failure.