CE RTEC

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016

Performance-based solutions at the WUI microscale

Barcelona, October 2016

Elsa Pastor & Eulàlia Planas

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016

WUI FIRE PROBLEM STATEMENT

Fires at the WUI have large economic, social and environmental impact. Somerecent examples: Victorian Black Friday wildfires (Australia, 2009); Fort McMurrayFire (Canada, 2016); Southern Europe 2016 fire season (Madeira island –Portugal,Comunitat Valenciana – Spain, Marseille – France).

New WUI-fire prone areas are expected to appear in the coming years. Maincauses: human pressure, ignition potential, climate change, etc.

The WUI fire problem is inherently complex, as it is characterized by theinteraction of multiple phenomena of diverse nature occurring at differentobservation scales: macroscale (landscape scale), mesoscale (settlement scale)and microscale (plot scale) [1]

Vallirana, 2013 © Bombers

Microscale

Mesoscale

Macroscale

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016

The WUI microscale is often characterized by the presence of combustibleelements that may jeopardize the main structure: ground fuels, stored material,ornamental vegetation, fences, outbuildings or even adjacent structures, that incase of ignition might be responsible of severe impact.

Houses at the WUI have always weak elements to fire exposure (openings, glazingand flooring systems, decks, verandas, gutters, etc.) responsible of houses’vulnerability, either because they are combustible or made of materials sensitive tofire or either because their geometry enhances heat transfer.

CHARACTERIZING THE MICROSCALE

In addition, fire at the WUImicroscale may also involvepeople’s presence. Actually,residents in some WUIareas have beenencouraged to activelyprepare, to stay and defendor shelter in place in caseof fire.

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTION

Fire agencies of WUI-fire prone countries have issued recommendations to reducerisk in buildings in close proximity to the wilderness:

Good practices to keep house surroundings in good conditions, based on fire-fighters experience, empirical evidences or simplified heat transfercalculations.

From the building perspective, there are just a reduced number of regions that havealready issued regulations to decrease houses vulnerabilities at the WUI (i.e.Building Codes of Australia BCA-2011 and California CBC-2007):

Conservative, based on simple fire models, with room of improvement with abetter scientific analysis. [2,3]

REGULATORY GAPS AND NEEDS

Current recommendations and regulationsprovide only general solutions to very specificproblems. They cannot cover the large degrees offreedom of the WUI microscale scene.

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTION

Performance-based (PB) fire safety design is a common (USA, Canada, Australia) andpowerfully emerging approach (e.g. Mediterranean countries) in urban buildings,which allows assessing singular and highly specific scenarios.

PB methodology is based on three key aspects [4]:

WHAT IS A PERFORMANCE-BASED APPROACH?

Performance level/type

Fire Scenarios

Quantitative assessment

PB is goal-oriented:To protect people to harmless effects of fireTo protect propertiesTo facilitate fire-fighters operations

Fires are prescribed, according to:Types of fuels involvedEnvironmental conditions

Level of performance of the proposeddesign can be quantified by means ofCFD fire modelling

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTION

CFD (Computational Fluid Dynamics) simulation toolssolve numerically the conservation equations of mass,energy and momentum in discretized volumes torepresent fluids flow in certain systems.

CFD fire simulation tools can be used to calculate theeffect of flames impingement and heat exposure toproperties and people. Moreover, CFD has animportant contribution to make in betterunderstanding the fire physics fundamentals.

In most CFD codes fire is usually predetermined (bymeans of the heat content and the mass loss rate of thefuel burning). Dwellings configuration can be easilydefined combining CAD (Computer-Assisted Design)tools.

Provided i) fire is correctly set, ii) dwellingscharacteristics are finely represented and iii) enoughcomputational resources are available, CFD tools offera great potential to improve safety in WUI microscale.

THE ROLE OF CFD FIRE SIMULATION

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTION

Microscale overall designs or particular systems can be evaluated by means ofprocedures “PB-inspired”:

Setting goals and levels of performance

Setting fire scenarios to be assessed

Analyzing fire impact with CFD tools:

Temperatures, velocities, heat fluxes, smoke exposure, etc.

HOW CAN SAFETY BE IMPROVED AT THE MICROSCALE?

Also, CFD modeling can be used to…

Get new insights on fire dynamics, e.g.:

Re-radiation effects in re-entrant corners

Heat transfer enhancement by local turbulences/flamesentrainment in angled/edgy façades

Forensic studies: fire origin, fuel sources, main fire drivers, etc.

Let’s see some examples…

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTIONEXAMPLE 1: Individual System performance

A PB approach is used to study a particular double-glazed window performanceexposed to a flame from ornamental vegetation. Only one fire scenario will beshowed. A complete study will imply the analysis of different fires scenarios(different fuels, environmental conditions, geometric configurations) toobtain/assess the limiting variables to achieve performance (e.g. fuel load, wind,tree-house distance, etc.)

(1)

Glazing system performance exposed to fire at WUI microscale [5]

PB items Description

GoalsGuarantee double-glazed window integrity at all times

Guarantee inhouse tenability conditions

Particularobjectives /

Levels of performance

1. Glass should not reach T<150 ºC (crack point) [6]

2. PVC gutter should not reach T< 140º C (deformation point) [7]

3. Inhouse internal temperatures T< 60º C (maximum survival temperature) [4]

Fire scenario#1

Flame source is 2-m Douglas fir tree [8] at 30 cm from façade

Ambient temperature T= 28ºC

Wind atmospheric profile U = 30 km/h (10 m)

Figures: Up: Pyrosim model; down: Simulated net heat flux footage

(1)

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTIONEXAMPLE 1: Individual System performance

0

20

40

60

80

100

120

140

160

0 4 8 12 16 21 25 29 33 37 41

T (º

C)

Time (s)

External Glass Temperature

Wall Temp Ext 1

Wall Temp Ext 2

Wall Temp Ext 3

0

50

100

150

200

250

300

0 4 8 12 16 21 25 29 33 37 41

T (º

C)

Temps (s)

External Shutter Box Temperature

Temp SB 1

Temp SB 2

Temp SB 3

CFD analysis of fire scenario #1External glass reaches crackingtemperatures. PVC gutter reachesdeformation temperatures. Objective 1and 2 not fulfilled.

In house temperature does not varysignificantly from initial temperature.Objective 3 fulfilled.

27.96

27.98

28.00

28.02

28.04

28.06

28.08

0 4 8 12 16 21 25 29 33 37 41

T (º

C)

Time (s)

Air Temperature In House

THCP Int 1

THCP Int 2

THCP Int 3

THCP Int 4

THCP Int 5

The system does not fulfill the performance requirements at the desired levels

Glazing system performance exposed to fire at WUI microscale [5]

Temperatures at the top (3), center (1) and bottom of the glass (2) Temperatures at the top (3), center (1) and bottom of the glass (2)

Temperatures different points of the internal glass (internal face)

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTION

A PB approach is used to assess an overalldesign of a house exposed to flames froma hedgerow. An specific baseline scenariocombining different closing systems statesis used to show different PB capabilities.Two scenarios are computed byconsidering the distance of the hedgerowto the house as limiting variable. Acomplete study will imply the analysis ofdifferent fires scenarios (different fuels,environmental conditions, geometricconfigurations) to obtain/assess otherlimiting variables to achieve performance(e.g. fuel load, wind, etc.)

(2)

House vulnerability assessment to a flaming hedgerow

PB items Description

GoalsGuarantee window 1 and 3 integrity at all times

Guarantee inhouse tenability conditions

Particularobjectives /

Levels of performance

1. Glass should not reach T<150 ºC (crack point) [6]

2. PVC gutter should not reach T< 140º C (deformation point) [7]

3. Inhouse internal temperatures T< 60º C (maximum survival temperature) [8]

Fire scenarios

Flame source is hedgerow of 2-m Douglas fir tree[9] at d= 2 m and d=3 m

Ambient temperature T= 28ºC

Wind atmospheric profile U = 30 km/h (10 m)

(2)

(2) Open window

(3) PVC drawn shutter

(1) Closed window (glass)

d

EXAMPLE 2: Overall design performance

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTION

House vulnerability assessment to a flaming hedgerow

EXAMPLE 2: Overall design performance

Simulated net heat flux footage(kW/m2)

Simulated Smoke evolution footage(saturated pattern)

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTION

House vulnerability assessment to a flaming hedgerow

EXAMPLE 2: Overall design performance

d= 3 m; PVC shutter external temperaturesd= 2 m; PVC shutter external temperatures

d= 2 m; Glass temperatures d= 3 m; Glass temperatures

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTION

House vulnerability assessment to a flaming hedgerow

EXAMPLE 2: Overall design performance

d= 2 m; In-house 1.5-temperatures at different rooms

d= 3 m; In-house 1.5-temperatures at different rooms

Thermocouples position

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTION

House vulnerability assessment to a flaming hedgerow

EXAMPLE 2: Overall design performance

CFD analysis of fire scenariosHedgerow distance acts as limiting variable to fulfill performance requirements

External glass reaches cracking temperatures, when hedgerow is located at 2 m from the façade.Glass is undamaged when hedgerow is at 3 m.

PVC shutter deforms when hedgerow is located at 2 m from the façades. Shutter is undamagedwhen hedgerow is at 3.

Tenability (i.e. temperature) is assured in rooms 4.5 and 6.5 in all cases. When the hedgerow is at3 m, tenability is also assured in room 5.5.

The system fulfills performance requirements at the desired levels with hedgerow distances above 3 m.

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTION...AND MORE...

Other common WUI-microscale fire-related casuistries/problems can be studiedwith this method:

Radiant heat barriers/enhancers performance

The role of elements acting as barriers (backyard fencing, isolated trees or hedgerows) for heattransfer can be assessed. Also, radiation enhancement of roofs or cover boards can be studied.

New/alternative materials performance

Comparative analysis of materials used in closing systems can be assessed.

Natech (natural-technological) risk interaction

Heat exposure to flames coming from certain devices, appliances or equipment can be assessedprovided mass loss rate is known.

Fire-fighters exposure

Heat flux fields can be obtained in WUI microscale scenarios to assess fire-fighters/residentsexposure and safety distances.

Overall sheltering conditions

Sheltering conditions of ad-hoc designs can be assessed by means of performance-basedsimulations.

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTIONCONCLUDING REMARKS

The complexity of the WUI-fire problem forces a trans-scalar approach to rationalize thedifferent threats and opportunities in terms of fire management.

At the microscale, the interaction between the building, the surrounding elements, theresidents and the fire makes vulnerability assessment a very complex task. Moreover, theinherent specificity of the microscale makes this job even more challenging.

Current recommendations and regulations, based on simple fire models and limitedexperiments, provide general solutions to very particular problems. Although there are alreadymore detailed and sophisticated guidelines they are still limited to cover the large degrees offreedom of the problem at hand.

A novel performance-based approach can be applied to assist fire safety design at themicroscale, grounded on physically-based CFD modelling linked to fire characteristics andbuilding specification.

This method will allow covering diverse fire safety needs at the WUI microscale, like analysingunique designs, testing current regulations validity, inferring new recommendations and eveneducating residence.

CERT

EC W

orki

ngDo

cum

ent_

DV1

PBD

at W

UI m

icro

scal

e10

/20t

h -2

016 INTRODUCTIONREFERENCES

[1] Caballero, D. 2016. Webinar: prevention in the WUI, a trans-scalar approach. Wuiwatch EC ECHO.https://www.youtube.com/watch?v=9ynJmccXYDI

[2] Stephens, S., Adams, M.A., Handmer, J., Kearns, F.R., Leicester, B., Leonard, J., Moritz., M. 2009. Urban-wildland fires: how California and other regions of the US can learn from Australia. Environmental ResearchLetters, 4, 1-5-

[3] Siggins, A., Newnham, G., Blanchi, R., Leonards, J. 2013. A 3 dimensional ray tracing approach tomodelling bushfire radiant heat flux for houses using lidar derived vegetation voxel data and quadraticpolygonal fire fronts. Geoscience and Remote Sensing Symposium (IGARSS), 2013 IEEE International.

[4] Hurley, M.J. 2016. SFPE handbook of fire protection engineering. Society of fire protection engineers,Springer. 3512 pp.

[5] Fanlo, J., Rios, O., Valero, MM, Planas, E., Navalón, X., Escolano, S., Pastor, E. 2016. DesigningComputational Fluid Dynamics simulation scenarios for the assessment of dwellings vulnerability in thewildland urban interface. Int. Conf Forest and WUI Fires. 25-27/05/ 2016, Aix en Provence, France.

[6] Mowrer, F.W. 1998. Window breakage induced by exterior fires. Society of fire protection engineers,Boston, MA:

[7] Ribeiro, L.M., Oliveira, R., Raposo, J., Caballero, D., Viegas, D.X., Exploratory tests on structures’ resistanceduring forest fires. . Int. Conf Forest and WUI Fires. 25-27/05/ 2016, Aix en Provence, France.

[8] Mell, W., Maranghides, A., McDermott, R., Manzello, S.L. 2009. Numerical simulation and experiments ofburning Douglas fire trees. Combustion and Flames, 156, 2023-2041.