Structural design for fire safetyWhere have we come from? & where are we going?

Dr. Danny Hopkin – Associate Director

London Fire Brigade – July 2016

Scope

• Fire resistance – why?• Future trends – where next?• Design at the interface – an

alternative• Summary• Questions

Fire resistance - HistoryModern application, dated origins

Why do we have FR?

• Great conflagrations in the late 1800s and early 1900s;

• Fire resilient buildings became a social expectation;

• ‘Fire proof’ materials emerged;• A lack of trust in private testing;• A need to independently verify;• Emergence of federal &

municipal fire test laboratories

The start of ‘standard’

• Earliest references of a ‘standard fire test’ – New York – late 1800s;

• Five hours at 2000°F;• Post Baltimore - “no ordinary room

would have enough inflammable material in it to maintain a 1700°F fire for more than 30 minutes”;

• Anecdotal FF experience;• Still ‘non-standard’.

A benchmark test• Ira Woolson - NFPA -

“unify all fire tests under one single standard and remove an immense amount of confusion within the fire testing community” (1917);

• By the 1920s the time-temperature curve is standardised and fire resistance is born;

A concept discredited

Ingberg 1928

Relates real fire severity to equivalent ‘standard’ durations

Woolson 1917 NFPA meeting –

“We want to get it as nearly right as possible before it is finally adopted, because, after it is adopted by these various associations, it will be pretty hard to change it”

Woolson’s premonition

Fast forward a century

Structural FR - Defined

• Tests whether an isolated structural element does not violate particular performance criteria after a set period of time in a furnace, when subject to the standard time-temp curve;

• Deflection limit span/20;• It cannot ever be a measure of

survivability in a real fire; • The standard fire is not a

standard fire, it’s not even a fire!

• Energy flow in is balanced against the losses to achieve ‘the standard fire curve’

Is it a ‘fair’ test?

Pump some energy in

Lose some energy to the furnace walls

Energy absorbed into the specimen

A concrete slab

A CLT slab

• Less energy is required to balance the losses because the specimen is contributing

Is it a ‘fair’ test?

Pump some energy in

Lose some energy to the furnace walls

Energy absorbed into the specimen

Specimen produces energy as it burns

Future trends, divergenceand the renaissance of a familiar foe

Going up & urban

Timber renaissance

Sustainability

• 400+ towers (>20 storeys) proposed in London…

• There will be features that are ‘unusual’ or sensitive to fire…

• How will we approach their design?

• Wind – performance based assessment

• Seismic – performance based assessment

• Fire?............................

Lame substitutions*

Fire safety engineering

Structural engineering

Structural design for fire safety

*Credit – Guillermo Rein

The 1st kind

Struct. engineer is replaced by pseudo-science

Fire safety engineering

Failure at x°C

Fire eng. replaced by pseudo-science

Structural engineeringTime

Tem

pera

ture

Failure at x mins

The 2nd kind

Both eng. replaced by pseudo-science

Time

Tem

pera

ture

Failure at x°C

Sound familiar?

The 3rd kind

Solution – protect all steel members to a 120 minute standard for a limiting temperature of X°C

Engineering…..done

• “intended to provide guidance for the more common building situations…”

• “need to take into account the particular circumstances of the individual building…”

A health warning

Apathy?

1940s

1990s

2020s

Familiar magic numbers

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

Fire safety engineering

Structural engineering

Structural fire engineering

The interfaces

What?

Who? How?

An interface between disciplines

The interfaces between facets of a successful

delivery

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;

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.

Design by convention

Skill & care

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

SummaryConsidered structural design for fire safety

Final remarks• Fire resistance is ‘old hat’;• Prescriptive guidance caters for

the simple;• The legal requirement is stability

for a reasonable period not a predefined level of FR;

• An approach commensurate with complexity;

• Competence is a prerequisite for successful design;

• A great report ≠ a great solution.

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

H. Ford

Thanks for your time• Danny.Hopkin@trentonfire.co.uk

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

• https://twitter.com/DannyHopkin

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