Fire Safety Needs for Mass Timber Buildings _Arup_UoE

Needs for Total Fire Engineering of Mass Timber Buildings

WCTE 2016 – Prof Luke Bisby & Dr Susan Deeny

Great Fire of London 1666Major conflagrations have shape our fire safety regulations.

These now hinge on the assumption of using non-combustible materials in our building fabric

As the consequence of structural failure is proportional to the building height our smallest buildings require a low level ofperformance and our tallest buildings the highest; this produces a consistent level of risk across buildings.

Globally structural fire performance requirements are defined in terms of fire resistance which is always express in minutes of exposure to the standard fire.

Fire performance requirements for buildings

Garden Festival Metropol Parasol Framework, Portland

Origins of ‘Fire Resistance’ Testing & Design

Stewart & Woolson (1902)

Standard fire tests were originally conceived as comparative tests of alleged ‘fireproof’ building systems in the late 1800s

•Before temperatures in real fires had been properly characterised

•Without the intent to assign fire resistance ratings

ASTM E119 (1918)

FTT (2016)

Ingberg’s 1st Insight: ‘Fire Resistance’ (c. 1922-1928)

Relating real fires to standard fires?•The full history of a compartment is related to the duration of standard fire that gives the same enclosed area

•This area is the ‘equivalent’ fire resistance time

Tem

pera

ture

Time

1000oC

Standard fire

AREA 1

1 hr

Threshold Temperature

Equivalent Standard Fire

Resistance Time

Natural (real) fire

0oC

AREA 2

2 hrs 3 hrs

AREA 1 = AREA 2

Ingberg’s 2nd Insight: Defining the ‘Fire Severity’ (c. 1928)

Ingberg (incorrectly) assumed that equivalent fire severity depends only on fuel load:•i.e. How long can a fire burn?

•The required fire resistance was explicitly linked to fuel loads and ‘fire resistance’ originally implied burnout without intervention

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1/The structure should withstand burnout

Broadgate Phase 8,

Office 1990

Nottingham University

GSK Carbon Neutral

Lab 2014

Taichung Metropolitan Opera House Sky Believe in Better

2/The structure is assumed to not contribute to the fire fuel load…

3/Conventional furnaces were not intended for timber …

Gas Temperature Gas Temperature

The test on concrete will use more fuel than tests on exposed timber to yield the same gas temperatures in a furnace:•Do timber buildings have less fuel in them than concrete buildings?

•Is this a ‘fair’ comparison of candidate structural framing systems?

Concrete Beam Timber Beam

Structural design for fire is different when ‘exposed’ mass timber is used

475 West 18th, New York

(12 Storeys, residential)

Our Aspiration

Where we are

Knowingly

The key to start truly pushing boundaries in tall and mass timber construction is to identify and address them as engineers…

The methodology for doing so is already well established for non-combustible construction

Total Fire Engineering Design:Non-Combustible Construction

Develop the design

fire(s)

Fire duration

Tem

per

atu

re

Established compartment fire models : (Cardington Natural Fire Safety

Concept 3)


Develop the design

fire(s)

Fire Analysis: Thermal

Exposure

Incident Heat Flux / Cardington Natural Fire Safety Concept 3


Develop the design

fire(s)


Exposure

Structural heat transfer

analysis

Steel connection / 3D Heat transfer analysis


Develop the design

fire(s)


Exposure


analysis

Material response at

high temperatures

Concrete Spalling / Channel Tunnel ‘96


Develop the design

fire(s)


Exposure


analysis


high temperatures

Structural analysis;

calculation of responseLong span cellular beams – finite element stress analysis


Develop the design

fire(s)


Exposure


analysis


high temperatures


calculation of response

Design meets

performance objectives

Long span cellular beams – finite element stress analysis


Develop the design

fire(s)


Exposure


analysis


high temperatures


calculation of response

Design meets


Design fails


Redesign

structure &

fire protection

Mitigate the

fire hazard

Fire duration

Tem

per

atu

re

Redesign of structure and/or

its fire protection

Mitigate the fire hazards

Total Fire Engineering Design:Combustible Construction

Develop the design

fire(s)

Arup – University of Edinburgh CLT Compartment fire tests, 2016

New knowledge and tools needed…


Develop the design

fire(s)

Thermal exposure &

heat transfer

Timber heat transfer model


Develop the design

fire(s)

Thermal exposure &

heat transfer


high temperatures

Inter-d

epen

den

ce

Normal Wood

Pyrolysis zone

Char layer

Material tests of engineered timber under radiative heat flux



Develop the design

fire(s)

Thermal exposure &

heat transfer


high temperatures

High temperature

structural analysis

Single element structural tests of engineered timber at high temperature

Inter-d

epen

den

ce



Develop the design

fire(s)

Thermal exposure &

heat transfer


high temperatures

High temperature

structural analysis

Does the structure

survive burnout?

Inter-d

epen

den

ce

Extinction

Design needs to satisfy both stability and burnout performance criteria


Develop the design

fire(s)

Thermal exposure &

heat transfer


high temperatures

High temperature

structural analysis

Does the structure

survive burnout?

Inter-d

epen

den

ceMitigate the

fire hazard

Design fails

performance

objectives

Redesign

structure &

fire protection

Design meets

performance objectivesRedesign the structure and/or its

fire protection

Mitigate the fire hazard

Designing Knowingly:Six Linked Areas

1. Pyrolysis and charring

2. Fire dynamics

3. Delamination

4. Smouldering

5. Thermo-mechanical properties

6. Real (full frame) structural response

C.F. Møller – HSB Stockholm

(34-stories, residential)

Knowledge Gap 2 – Fire Dynamics with Exposed Timber

Coupling between burning of fuel and burning of

the structure:•Faster fire growth & time to flashover

•No increase in gas temperatures (if ventilation controlled)

•Increased production of volatiles and smoke

•Increased severity of external flaming

•Increased total heat release rate

•Longer burning duration

•Potential for secondary flashover

•Does ‘fire resistance’ concept make sense?

Knowledge Gap 1 – Pyrolysis & Charring

vs.

Factors affecting charring rates:•Density

•Moisture content

•Grain direction/angle

•Test method (system properties)

•Fire protection (‘falling off’)

•Severity of heating

•Splitting/cracking/fissures

•Species

•Chemical composition

•Anatomy

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

300 400 500 600 700 800 900 1000

Ch

arr

ing

Rate

(m

m/m

in)

Density (kg/m3)

Yang et al. (2009) White and Nordheim (1992) Frangi and Fontana (2003)

Friquin et al. (2010) Hugi et al. (2007) Njankouo et al. (2004)

Frangi et al. (2008) White (2000) Collier (1992)

Hall (1970) Fragiacomo et al. (2012) Cachim and Franssen (2009)

AS 1720.4 BS:EN 1995-1-2 (softwoods) BS EN 1995-1-2 (hardwoods)

e.g. Variation of Average Charring Rate with only Density(standard fire exposures in furnaces)

Knowledge Gap 3 – Delamination

Falling off or loss-of-stickability of charred

timber lamellae

• Effects on effective charring rates in furnace

tests can be accounted for

• Causes and contributory factors not understood:

• Adhesive type (MUF vs. PUR)?

• Critical temperatures?

• Orientation?

• Loading?

• Grain direction, species, moisture, lamella

thickness,

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

Tem

pe

ratu

re [

°C]

Time [min]

Burnout of contents

Initial cooling

Delamination during cooling

Secondary flashover

e.g. Secondary Flashover due to Delamination

Courtesy Roy Crielaard

Knowledge Gap 4 – Smouldering

The potential for continuing smouldering once the compartment

contents have burned out:•Within the compartment

•Within concealed or encapsulated adjacent spaces

Self sustaining when timber is exposed to heat fluxes of 6-10kW/m2,

and depends on convective currents

Yield of toxic is higher during smouldering as compared with

flaming combustion

Particular care is needed in a ‘fire resistance’ design framework

0

20

40

60

80

100

0 50 100 150 200 250 300

Rel

ati

ve

Yo

un

g's

Mo

du

lus

(%)

Temperature (°C)

Konig and Walleij [55] Ostman [60]

Thomas [57] Sulzberger [64]

Nyman [69] Lie [58]

Kollman [63] Preusser [70]

Young [66] Janssens [68]

James [71] Schaffer [52]

Knowledge Gap 5 – Thermo-mechanical Properties

0

20

40

60

80

100

0 50 100 150 200 250 300

Rel

ati

ve

Ten

sile

Str

eng

th (

%)

Temperature (°C)

Konig and Walleij [55] Knudson & Schneiwind [56]

Thomas [57] Schaffer [52]

Lie [58] Lau & Barrett [59]

Ostman [60] Schaffer [61]

Kollmann [62]

0

20

40

60

80

100

0 50 100 150 200 250 300

Rel

ati

ve

Co

mp

ress

iveS

tren

gth

(%

)

Temperature (°C)

Konig & Walleij [55] Knudson & Schneiwind [56]

Thomas [57] Schaffer [52]

Kollmann [63] Sulzberger [64]

Youngs [65] Young [66]

Ingberg [67]

Tensile Strength Compressive Strength Elastic Modulus

Knowledge Gap 6 – Structural Response(as distinct from single element response)

Mass timber elements are widely considered to perform well in fire but…

There are uncertainties:•Full frame structural response (is a dominant factor for steel buildings but widely ignored for timber)

•Both full frame and element level failure modes (e.g. rolling shear)

•Compression elements and stability failures

•Connection response and failure modes

Pedro Palma, Andrea Frangi, Erich Hugi, Paulo Cachim and Helena Cruz

A.1 (10 mm gap) A.2 (20 mm gap) A.3 (0 mm gap)

Figure 7. Influence of the gap between the beam and the column: connections A.1, A.2, and A.3 after the fire tests.

The common commercially available concealed beam-hanger (A.5) had gap of only 6 mm between

the beam and the column (Figure 2.1 and Table 2), but a much lower estimated load-carrying capacity of

the column-side of the connection (Table 3 and Figure 4a), although it failed on the beam-side at normal

temperature. This commercial connection exhibit a fire resistance 5 minutes lower than the custom A.1

connection and failed in the column-side, with a failure mode similar to that of the connections A.2.

However, the load in connection A.5 during the fire test was about 40% of the load-carrying capacity of

the beam-side at normal temperature, instead of the 30% in connections A.1-3.

A.4 (reinforced) A.5 (commercial beam-hanger)

Figure 8. Connections A.4 and A.5 after the fire tests.

The influence of the failure mode can be analysed in the connections B.1 and B.2. These connections

had smaller sized dowels (diameter of 8 mm) and showed brittle splitting (B.1) and ductile

embedment/dowel failures (B.2) at normal temperature. In the fire tests, however, both typologies

exhibited similar extensive embedment failures, followed by splitting. In fire, the smaller minimum dowel

spacing perpendicular to the grain prescribed by EN 1995-1-1 (only 3·d, compared to 5·d parallel to the

grain) leads to a premature failure when all the wood between the dowels is charred (Figure 9). Also the

smaller minimum unloaded edge distances (connection B.2) perpendicular to the grain (only 3·d, compared to 7·d parallel to the grain) result in the complete charring of the wood surrounding the

outermost dowels. Regardless of the failure mode at normal temperature, both connection typologies

exhibited approximately the same fire resistance.

B.1 B.2

Figure 9. Connections B.1 and B.2 after the fire tests.






















B.1 B.2























B.1 B.2



Connections C.1 exhibited shear/splitting failures at normal temperature and the highest load-carrying

capacities of the R30 connections (Table 3). In the fire tests, they reached approximately the same fire

resistance as connections A.1. In fire, the dowels remained mostly straight and the wood surrounding the

dowels closer to the unloaded edge charred completely. After failure, splitting cracks could be observed

in the beam side.

C.1

Figure 10. Connections C.1 after the fire tests.

Connections A.6 and C.2 reached more than the estimated minimum 60 minutes of fire resistance [8].

Regarding connections A.6 (with 12 mm dowels), the long fire exposure charred the wood below and

above the header steel plate nailed to the column, affecting the tension (nail withdrawal) and compression

zones and allowing the steel plates to rotate (Figure 11, left). The dowels’ end distance in connections C.1

(8 mm dowels) was smaller than in connections A.6 and, consequently, so was the moment in the header

plate. Therefore, the rotation of the header plate was negligible. On the other hand, the charred depth in

the zone between the dowels and the end of the beam was significantly higher than elsewhere in the

beam, due to additional heat coming from the burning column-member and transferred by the dowels into

the cross-section. After the wood between the dowels charred, the load was mostly transferred through the

last dowel and it ultimately bent (Figure 11, right).

A.6 C.2

Figure 11. Connections A.6 and C.2 after the fire tests.

Finally, the commercial aluminium dovetail connection D.1 also reached more than 30 minutes of fire

resistance, failing in the connector itself after 36 minutes. This connection had a gap of 18 mm between

the beam and the column (thickness of the connector), which is larger than the 10 mm of most of the other

connections.

D.1

Figure 12. Connection D.1 after the fire test.

From Palma et al. (2014)

“we cling to the myth that timber

construction presents risks, while concrete

and steel do not. Nonsense. Every material

presents risks, but we manage them in

different ways”

Russell Fortmeyer, Arup, 2011

Requested topics: - Recent developments- Behaviour of structural timber elements

in fire- Fire design of insulating bio based

building products (BBBP)- Modification methods, treatments and

durability aspects of fire resistant BBBP- Consideration of additional fire load due

to BBBP- Interaction among Fire Safety Engineers,

Material Scientists & Structural Engineering

- Performance Based Design with combustible products

- Fire Risk and Safety of building with BBBP

- BBBP and their impact on regulations –background for todays’ rules and possible design procedures

Fire Safety Journal special issue

Deadline for papers: 2017-02-28

Life time: 2014/12 – 2018/12

Networking platform for researchers dealing with combustible building products, performance based design and fire safety engineering.

Further information: www.costfp1404.com

COST – European Cooperation in Science and Technology – is one of the longest-running European

instruments supporting cooperation among scientists and researchers across Europe.

Working Group 1

Contribution of bio-based materials to

the fire development

Working Group 2

Structural Elements made of bio-based

materials and detailing

Working Group 3

Regulations and standards for fire

safety of bio-based building materials

Working Group 4

Dissemination

Fire

Safety

engineers

Material

scientists

Structura

l

engineers

COST Action FP1404 Fire Safe Use of Bio-based building products

http://www.costfp1404.com/

Fire Safety Needs for Mass Timber Buildings _Arup_UoE

Engineering

Transcript of Fire Safety Needs for Mass Timber Buildings _Arup_UoE