Design at the interfaceA structural fire engineering strategy for an expressed Cor-Ten frame

Dr. Danny Hopkin – Associate Director

Scope

• What interfaces? And why?• Our project;• The challenges we faced; • The lessons we learned; &• Convention.

Fire safety engineering

Structural engineering

Structural fire engineering

The interfaces

What?

Who? How?

An interface between disciplines

The interfaces between facets of competence

Regulations

Responsibility Skill & Care

Structural engineers understood they were responsible for ensuring “stability for a reasonable period” in fire

Those responsible for construction were engaged at an early stage and became familiar with the requirements

Design team understood that the fire performance demands were beyond their competency & delegated

Competence – A prerequisite for success

4 Pancras SquareFor an industrial building

An industrial site

The building• A 10 storey office – 46 m in height;• Predominantly a concrete frame – cast

insitu & PT;• Architectural feature – external Cor-Ten

frame;• A huge Cor-Ten transfer structure;• Tricky interfaces.

A successful solutionA melange of competing goals, obligations & constraints, of varying intelligibility

The life safety goal

• "Stability for a reasonable period";• Consistency of risk – Kirby, et. al;• Overall reliability requirement of 97%;• Active reliability contribution of 93%;• Passive reliability requirement of 49%;• All 50% have the potential to fully

develop.

0 20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

Height (m)

Frac

tile

(%)

Fire manifestation

0%

20%

40%

60%

80%

100%

Peak steel temperature (°C)

Perc

entil

e (-)

Thermal conditions

• A lack of guidance – Law & O’Brien – SOA;

• Steady state analysis – overly conservative;

• A need to quantify transient behaviour;

• Consider the impact of wind;• Quantifying thermal gradients,

etc., key.

Thermal conditions

Side 1 Side 2 Rear Front0.00

0.20

0.40

0.60

0.80

1.00

Elevation of elementRela

tive

prop

ortio

n of

com

part

men

t te

mpe

ratu

re (-

)

0 30 60 90 120 150 180 2100

200

400

600

800

1000

1200Fire CompartmentSidesRearFront

Time (min)AS

T (°

C)

- BS EN 1991-1-2 Annex B as a ‘scalar’- Benchmarked against CFD models- Adequately conservative.

Element orientation influences exposure:

• Location ‘manages’ exposure;

• Sections still very hot;• Concrete filling, where

practicable;• Shielding, where

permissible; &• Otherwise, plate sizing.

Fire Floor

Floor Above

Managing temperature

Materials – Cor-Ten• Cor-Ten is not a typical material;

• The scale of the section is not typical;

Peak Temperature

(°C)

Yield Strength Reduction

(MPa)

Residual Yield Strength

(MPa)700 30 325800 60 295900 90 265

Structural response

• Two key areas:• Vierendeel transfer; &• Columns

• Other complications:• Connections;• PT;• Bi-metallic corrosion & PFP.

Vierendeel behaviour

0 5000 10000 15000 20000

-4000000

-3000000

-2000000

-1000000

0

1000000

2000000

3000000

Time (s)

Axia

l for

ce (k

N)

• Expansion governed;• Very sensitive to TFs;• Doesn’t deflect excessively;• Plastic strain tension;• A building that needs to ‘breathe’;• Matching ‘actual’ vs. ‘idealised’.

Column behaviour

• Concrete filling;• Explored rebar vs. T;• T more ‘buildable’;• UC 254x127x84 (S355);• Actions influenced by

curvature & slab ‘push-out’;• Sensitivity to vertical fire

spread explored;

0 30 60 90 120 150 180

-150%-100%

-50%0%

50%100%150%

BF WEB_CTF MAXMin

Time (min)Inne

r Tee

Util

isati

on (%

)

Lessons & key points

• Struct. Eng. understood their responsibility & limits;• “Stability for a reasonable period” not FR120 + sprinklers;• They understood the expertise req’d & delegated;• Those responsible for delivery were involved in design.

• Quantification of the goal -> rational basis -> rational process; • Thermal tools are inadequate for external exposure;• Cor-Ten does not behave like regular carbon steel;• Bigger is not always better;• A need to be pragmatic about what you can’t fully quantify.

Design by convention

Skill & care

• Successful fire engineering doesn’t end when a report is issued….

Acknowledgements

• Co authors – D. Illingworth, E. O’Loughlin, B.McColl & S. Anastasov;• Design team – Eric Parry Associates, BAM, Grontmij• Client – Argent; &• AHJ – Camden LABC, London FB and BRE (reviewer).

Thanks for your time

"If you always do what you've always done, you'll always get what you've always got.“ - H. Ford

• Danny.Hopkin@trentonfire.co.uk

• http://uk.linkedin.com/in/dannyjhopkin

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