Fire Severity for Structural Design A UK Perspective · 2016. 4. 8. · 52 Fires for Structural Analysis •Range of fires: parametric curves, travelling fires and standard •Engineering

Fire Severity for Structural DesignA UK Perspective

Susan Deeny, PhD

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Broadgate Phase 8

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Shaping a better world

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Experience of working in

Abu Dhabi

UAE

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Fire Severity for Structural Analysis

• Building Fire Behaviour

• Fire Severity

• Structural Response

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Building Fire Behaviour

Fuel Ventilation Geometry Boundaries

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0

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Small-medium compartments

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Post flashover fires

L

• All fuel within enclosure is burning;

• Gas temperatures are ‘generally’ uniform

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Spatial variation compartment temperatures

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• Considering the impacting factors again…

- Fuel: Well distributed – affects duration.

- Ventilation: affects duration and peak temperatures

- Geometry – affects growth rates and peak temperatures


Fire duration

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Fire durationAv

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Fire duration

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Post-flashover fires – Tools

Parametric design fires

Single gas temperature – time relationship

Heat and cooling phase

Available in national design documents

Considers

Ventilation

Fuel load

Thermal boundaries

Compartment size

Validated up in tests up to 100~144m2 compartments.

Likely to be unphysical for large compartments (1000m2)

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Post Flashover – Flame Projection

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6

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Open plan compartments

Vertical Villages

Typical Village

Compartment floor

Atrium

floors

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Travelling fires

• Witnessed in real building fires- World Trade Centres, Torres Windsor, Delft Faculty of Architecture

• Little to no experimental data (Current programmes in Europe)

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Travelling fires

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• Fuel: Distributed

• Ventilation: Large, fuel controlled fires

• Geometry: Large (100m2 +)

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Travelling fires - Tools

• Arup – UoE Methodology (Stern Gottfried & Rein)

• Near and Far field temperatures

• Fuel load density and burning area determine ‘travel speed’

• Family of fire curves required

Near Field

1200°C

Far Field

Alpert

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5

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Very Large

Compartments

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Localised fires

• Fuel: Low/localised fuel source

• Ventilation: Large – fuel controlled burning

• Geometry: Large volume – low feedback

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Localised fires – Structural Effects

• Exposure:

- High temperatures/heat flux local to

flame

- Limited duration due to restricted fuel

source

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Localised fires – Tools

• Plume Temperatures:- Heskestad (SFPE Handbook/EC1)

- Hasemi (SFEP Handbook/EC1)

- Alpert Ceiling Jet Correlations

- TM19 Plume Correlations

Heskestad method (left) and Hasemi method (right)

• Inputs:- Heat Release Rate (kW)

- Fire Area (m2)

- Fuel Load (MJ)

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Combustible Construction

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0 50 100 150 200 250 300 350 400 450

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Long Duration Travelling Fire

Medium Duration Travelling Fire

Short Duration Travelling Fire

Parametric Fire

Standard Fire

Design Fire Severity

ASTM E-119

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Time (minutes)

Goals Solution

Time

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Ingberg’s Goal…T

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ature

Time

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TimeEqual

areas

Goals Constraints

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Time

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Time

Solution

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Design Fire Selection - Current method…

Design Fire

Heat transfer analysis

Structural model

How do we pick a design fire?

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Fire duration

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Fire durationAv

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Fire duration

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Design Fire


Structural model


Its not practical to design our building

to resist every possible fire scenario

BS 9999 acknowledges this: an acceptance

criteria for design is defined (based on

consequence of failure)

Con

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ilure

Building type

All

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Building type

Building type

Likelihood

Consequence

likelihood consequence Risk× =

Risk

Low

Hig

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Design Fire


Structural model


Its not practical to design our building

to resist every possible fire scenario

BS 9999 acknowledges this: an acceptance

criteria for design is defined (based on

consequence of failure)

BS 9999 recognises that the standard fire is

inadequate, and adopts parametric curves

We now recognise that parametric fires are not

always appropriate

This approach will allow us to discard the most

onerous fires, and select specific fires for

structural design

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Risk Based Approach to Design Fire Selection

1) Physical Inputs based on

probabilistic distributions

2) Maximum protected steel

temperature used to characterise

fire severity

3) Acceptance criteria (allowable

failure rate) and selection of key

design fires

Hp/A analysis

Design Fire


Structural model

Compartment geometry

Fuel Load

Fire size

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fire severity



design fires

Design Fire


Structural model

Compartment geometry

Fuel Load

Fire size

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(1) Physical Inputs – Key Variables

Key Variables

• Fuel Load• Compartment Area• Fire Burn Area• Heat Release Rate• Flame Temperature• Ventilation

Monte Carlo Analysis to consider potential variation

Controlled for

probabilistic

distribution and

confidence

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(1) Physical Inputs - Possible Distributions

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HRR/UA HRR/UA HRR/UA

More low HRR/UA: More medium HRR/UA: More high HRR/UA:

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Max HRR/UA Min HRR/UA

250kW/m² 550kW/m² 250kW/m² 550kW/m² 250kW/m² 550kW/m²

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fire severity



design fires

Hp/A analysis

Design Fire


Structural model

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(2) Fire Severity – Maximum Steel Temperature

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fire severity



design fires

Design Fire


Structural model

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(3) Acceptance Criteria & design fire selection

At 18m design fractal is 80% giving 60 minutes FR

At 40m design fractal is 96%

With sprinklers, this is reduced to ~80%

Fires that the structure

must be able to resist

Fires that the structure will

not be designed to resist

Most onerous design fires

selected for input to FE model

90 minutes of fire

protection

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100%

0%

Limiting temp

Target

reliability

Cum

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tive F

requency

Limiting temp

Target

reliability

Range of worst case

design fires

(3) Acceptance Criteria & design fire selection

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Fires for Structural Analysis

• Range of fires: parametric curves, travelling fires and standard

• Engineering judgement required to determine appropriate range

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Parametric Fire

Standard Fire

Structural Response

Structural Response

Office Tower

Typical Village

Compartment floor

Atrium

floors

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Multi-storey fire

file://Global.arup.com/london/FIR/FIR-Jobs/new_sys/structures in fire/Conference_seminar papers and presentations/Presentations_Architects_Structural_Enginners/101117 BEL2 BD Pres/VilMk10U3Cont.avifile://Global.arup.com/london/FIR/FIR-Jobs/new_sys/structures in fire/Conference_seminar papers and presentations/Presentations_Architects_Structural_Enginners/101117 BEL2 BD Pres/VilMk10U3Cont.avi

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Fire spread to multiple floors - Columns

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Long-cool & Short-hot

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Travelling Fires

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An envelope of fire behaviour

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Parametric Fire

Standard Fire

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An Envelope of fire behaviour• Buildings structures and fire

hazard are increasingly complex

will require

• Risk based methods can remove

subjectivity in selecting design

fires

• Structural fire behaviour should

be tested under an envelope of

design fires

• Combustible construction

presents a new challenge to fire

severity

Documents

Fire Severity for Structural Design A UK Perspective · 2016. 4. 8. · 52 Fires for Structural Analysis •Range of fires: parametric curves, travelling fires and standard •Engineering