quantitative fire engineering assessment methods … · • the main issue is the timber building...

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QUANTITATIVE FIRE ENGINEERING ASSESSMENT METHODS FOR MASS TIMBER

Kapilan SarmaFire Safety EngineerScientific Fire Services Pty Ltd


ACKNOWLEDGEMENTS

• Co-Authors:• Parkan Behayeddin• Russell Kilmartin


AIMS

• Introduce a framework to appropriately and accurately determine the fire resistance performance of mass timber elements such as Cross Laminated Timber (CLT)

Cross Laminated Timber (Stora Enso, 2013)


AIMS

• Introduce methods and concepts for fire modelling purposes to determine the contribution to the overall fuel load by the CLT:

• Delamination of CLT; and• The potential for self-extinguishment of

CLT; and• Travelling fires concept within an

enclosure • Contribution of CLT to the fuel load.

Cross Laminated Timber (Stora Enso, 2013)


INTRODUCTION

• There are limited methods to determine an accurate period of fire resistance for an exposed combustible element.

• Current methodologies are required to be reviewed to include the interaction between an exposed mass timber element and the fire.

• What will be the extent of CLT contribution to the fire and what is the suitability of the available methods when applied to mass timber building designs? Charring layer of CLT (Woodworks, 2010)


ASSESSMENT FRAMEWORK

• A structural fire performance assessment framework is commonly adopted in structural fire engineering.

• The framework provides a process for calculating the design strength of a structural element exposed within an enclosure fire where complete burnout is achieved.

• The three (3) phases in determining fire resistance are:

• Fire Modelling• Heat Transfer/Thermal Modelling• Structural Modelling

Flow Chart for predicting structural fire performance (Buchanan, 2002)


FIRE MODELLING

• A precise fire model is a vital part of fire-structure modelling.

• The aim is to ensure that the fire resistance of the structure is greater than the severity of the fire to which the structure is exposed.

• The fire severity is a function of the fuel load density and type, ventilation available and thermal properties of the fire cell.

• With respect to prescriptive design, fire severity is the structural element’s ability to withstand the code prescribed standard time-temperature curves for furnace test fires.

Standard Temperature-time curves (University of Edinburgh, 2001)


FIRE MODELLING

• In a performance based design regime, engineers are less constrained and as such have the ability to design to real fires.

• In order to compare compartment fires to standard furnace fire tests, the concept of equivalent fire severity has traditionally been used.

• The concept is to compare the time-temperature profile of a compartment fire with the standard heating regime. In this case, fire severity is usually defined as the time for complete burnout or the equivalent time of complete fire burnout.

Typical time-temperature fire development (Crielaard, 2015)


EQUIVALENT FIRE SEVERITY

• One of the common misunderstandings when adopting the equivalent fire severity methodology is the assumption that all construction materials respond similarly.

• The current time-equivalent methods are deemed not suitable for combustible structural elements such as mass timber (i.e. CLT). Equivalent fire severity on a temperature basis

(University of Edinburgh, 2001)



• The research and understanding in timber structures in fires, particularly on large scale developments, is limited yet developing at considerable rate.

• Further study in compartment fires within large buildings where combustible building elements and contents are present is needed to further strengthen the understanding across the industry. Equivalent fire severity on a temperature basis

(University of Edinburgh, 2001)



• The most commonly used time-equivalence method is the Eurocode formula.

• The Eurocode formula was developed based on comparisons with energy balance and heat transfer calculations for a protected steel member using a particular set of time-temperature profiles for small rooms.

• Its an empirical formula and may not be applicable to other time-temperature curves, to larger rooms or other structural materials.

4 1/2 storey building with total of 700 m2 facade in timber construction (Heitaniemi 2005 and Korhonen 2006)



• The Eurocode formula does not categorically allow for timber building elements which burn or char during the consumption of the fuel load within the compartment.

• The main issue is the timber building element continues to burn along with the fuel load present in a compartment fire and the uncertainty of whether the timber building elements will be totally consumed in the event of a fire or continue to burn after the remaining contents in the compartment suffer burnout.


FIRE MODELLING CONCEPTS AND METHODS

• Continued research is required to be conducted with regards to fire behaviour of CLT in compartment fires.

• In particular to post-flashover fires with combustible structural materials available to fuel the fire post burnout.

• It is essential that further research consider the certain behavioural characteristics of CLT are taken into consideration as part of the quantitative methodology.


FIRE MODELLING CONCEPTS AND METHODS

• The main concepts to be discussed:• Delamination of CLT; and• The potential for self-extinguishment of CLT; and• The concept of travelling fires within an enclosure.

• A combination of the above is considered to make up the CLT contribution to the fire.

• The data output following fire modelling is input into the thermal and structural phase.


DELAMINATION OF CLT

• Delamination is termed as the separation of layers in a laminate due to failure of the adhesive either in the adhesive itself or at the interface between the adhesive and the wood layer.

• Failure modes of CLT have focused on the reduced cross section by charring whereas the failure mode by delamination is often overlooked and understudied.

• The relationship the glue plays with interfacial shear and normal stresses which form the composite action need to be understood.

Adhesive to bond the CLT Panels (Courtesy of Google Images)


DELAMINATION OF CLT

• Its common knowledge that CLT usually chars around 300°C. • The tests demonstrated that complete cohesion failure of the adhesive is

between 70°C and 190°C. • Structurally designed composite action of the mass timber product has been

lost.• Delamination can lead to lamella failure potentially increasing the fuel load in

the fire and exposing a new layer.


DELAMINATION OF CLT

• Currently, the phenomenon of delamination is yet to be adequately studied. • The current codes and standards focus predominantly on charring and

structurally stability.• By undertaking further research, CLT buildings can be designed to withstand

delamination and remain structurally stable after complete fire burnout.• A common technique is to provide an outer lamella to prevent delamination


POTENTIAL FOR SELF-EXTINGUISHMENT

• Unprotected CLT is likely to contribute to the overall fuel load in a compartment fire and may lead to structural instability.

• Research on self-extinguishment of cross-laminated timber has been undertaken to gain a broader understanding of fire behaviour of unprotected CLT

Smouldering zones, heat transfers, and the effect of additional air flow (Crielaard, 2015)



• Determine the conditions required for self-extinguishment of cross-laminated timber to potentially occur.

• It should be noted that the self-extinguishment is a concept and is not currently recognised in any of the standards in structural fire design.

Smouldering zones, heat transfers, and the effect of additional air flow (Crielaard, 2015)



• Crielaard (2015) describes a model that could be formulated to describe the route to self-extinguishment. The model is based on a set of assumptions:

• Firstly, the unprotected CLT is involved via flaming combustion with the building enclosure contents within the compartment.

• After a period of time, the fire would undergo the decay stage whereby the contribution of CLT would naturally decrease and transform from flaming to smouldering.

• If a potential for self-extinguishment is present, it would follow smouldering combustion.

• Smouldering inward in to the CLT must cease for self-extinguishment to occur.

The involvement of the CLT in flaming combustion and transformation from flaming to smouldering (Crielaard, 2015)



• In order for the transition from smouldering to self-extinguishment to occur, two conditions need to be satisfied:

• Firstly, the smouldering combustion is required to cease at a sufficiently low externally applied radiant heat flux on the CLT.

• If this does not occur, the fire will remain to smoulder

Two conditions required to transform from smouldering to self-extinguishment (Crielaard, 2015)



• Delamination can occur and interfere in the process by continuing the flaming combustion or transforming the smouldering combustion back to flaming.

• If flaming combustion is sustained due to delamination or if smouldering is sustained due to a too high radiant heat flux or unfavourable airflow, the CLT will not reach a potential to self-extinguish and the CLT will continue burning.

• In order to prevent delamination, a sufficiently thick outer lamella can be designed to delay delamination and assist in self-extinguishment.

Heat flux too high and unfavourable airflow can lead to delamination (Crielaard, 2015)



• The concept method for self-extinguishment consists of two steps.

• The first step is to ensure the CLT can transform from flaming to smouldering combustion, by preventing delamination by means of sufficiently thick outer lamella.

• The second step is to ensure the smouldering CLT is able to make the transition to self-extinguishment, depending on the conditions of heat-flux and air-flow.

Proposal for assessment method for self-extinguishment of CLT structures (Crielaard, 2015)


TRAVELLING FIRES WITHIN AN ENCLOSURE

• Methods and concepts for fire modelling have predominantly based on small compartment fires.

• Such methods specify thermal inputs but assume uniform burning and similar temperature conditions throughout a compartment, regardless of its size.

• This is in contrast to the observation that fires in large, open-plan compartments tend to travel across floor plates, burning over a limited area at any one time.

Open Plan Office Compartment (Courtesy of Google Images)



• The main reason to investigate the use of travelling fires theory is due to modern open plan building design.

• Modern buildings adopt large open spaces and glazed facades.

• Limitations within the current equivalent fire severity methods such as area, height and thermal properties for large compartments.

• Common features such as high ceilings, large open office spaces, multiple floors connected by voids and glass façades cannot be assessed.

Office Building Glazed Façade (Courtesy of Google Images)



• Stern-Gottfired’s methodology aims to replace the current methods for small compartments with a more realistic fire scenarios in a large open plan compartment.

• The theory is based on a thermal environment being divided into two regions:

• the near field (flames); and • the far field (smoke away from the

flames).

Illustration of a travelling fire (Jamie Stern-Gottfried , 2011)



• Smaller fires travel across a floor plate for long periods of time with relatively cool far field temperatures

• Larger fires have hotter far field temperatures however burn for shorter durations.

Illustration of a travelling fire (Jamie Stern-Gottfried , 2011)


FIRE MODELLING TECHNIQUES

• In order to acquire the relevant data from the fire model as input data into the thermal modelling stage, an appropriate analysis tool is required.

• The input parameters associated with the fire model consist of:

• geometry of the enclosure; and• ventilation conditions; and • fuel load present within the enclosure

(i.e. exposed CLT), material properties and critical fire locations for analysis. Examples of CFD Fire Models



• Due to the complexity of these input parameters Computational Fluid Dynamics (CFD) modelling would be required for the purposes of fire modelling.

Examples of CFD Fire Models



• The main output data to be acquired from the fire model is fire temperature and radiant heat flux readings as well as the location, size and extent of the fire.

• The fire modelling output should be compared to research findings, validated and modified prior to using the data as input into the thermal modelling phase.

• By acquiring accurate and validated output data in relation to fire size, location, extent and temperature profiles from the fire model the thermal modelling phase can commence. Examples of CFD Fire Models Output


CASE BY CASE BASIS

• The concepts and methods presented are required to be adopted on a case-by-case basis.

• Currently, there is yet to be a recognised framework to determine the contribution of mass timber to a fire.

• Industry practitioners are required to apply knowledge such as:• the specific characteristics of the mass timber product (i.e. CLT);

and • the science behind the fire properties/ behaviour of the specific

mass timber product.


CASE BY CASE BASIS

• Project specific application of mass timber is due to the range of buildings classifications, function and use:

• Residential• Commercial Office• Civic (community centre and

library)• Heritage • Hotel/Serviced Apartments• Independent Aged-Care Living


CONCLUSION

• Currently, there are limited validated methods to determine an accurate period of fire resistance for an exposed combustible element within the traditionally known fire resistance framework.

• A conceptual framework has been presented to determine the structurally stability of CLT structures under fire conditions with particular emphasis on quantitative fire modelling such as:

• Delamination of CLT; and• The potential for self-extinguishment of CLT; and• The concept of travelling fires within an enclosure.


CONCLUSION

• The concepts and methods presented are required to be adopted on a case-by-case basis since their currently is no validated and recognised framework to determine the contribution of mass timber to a fire.

• Moving forward, further full-scale tests are required to provide more accurate information on fire severity when mass timber is exposed within a fire.


QUESTION AND ANSWERS

Thank you for time and attendance

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quantitative fire engineering assessment methods … · • the main issue is the timber building...

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