Fire performance of engineered timberDanny HopkinResearch Engineer

Content

• What is engineered timber? Why use it?• Research drivers

– Why are we interested in E.T.?• DCLG- Fire performance of SIPs

– What is a SIP?• Summary• Forward look

What is engineered timber?• Any form of timber element not formed from

a continuous solid piece of wood.– Categories:

• Light timber (UK)• Large section (Scandinavia)

Examples?

• Light timber frame:– Engineered floor joists/studs– Structural Insulated Panels (SIPs)

Examples?

• Large section timber:– CLT – LVL– Glulam

Why use it?

• Structural drivers:– Less defects– Less material for equivalent stiffness

• Construction drivers:– Easier to work with– Less time on site– Prefab = Less skilled labour

• Environmental drivers:– Less waste– Better thermal performance

Why are we interested?• Research drivers:

– Redundancy = Fire Resistance (FR)• Excess material is a good thing• Removing material reduces inherent FR

– Improved thermal performance means:• Introduction of combustible insulation• Different fire dynamics• Mechanisms for fire spread

– Structural integrity• Type of failure (predictable? sudden?)

The outcome

• 2 year CLG funded research project into the fire performance of SIPs and engineered floors.

• 7 full scale natural fire experiments• 20+ laboratory fire tests• Development of numerical models (thermal

& structural)

STRUCTURAL INSULATED PANELS

(DCLG) The performance in fire of

What is a SIP?

• Prefabricated lightweight units• Used as a principal load bearing element

– External & internal walls– Roofs– Floors (rarely)

• Sandwich construction comprising two structural veneers bonded to a light weight insulating core

70 to 250 mmFace layers

Insulated core

What is a SIP? (2)

• SIPs currently approved for use up to 4 storeys in the UK (limited by FR)

• Can be viewed as a single structural element or an entire system

• Typically residential applications • Increased use in commercial applications

i.e. hotels, schools, offices. • Can be used as infill panels on multi-storey

framed structures.

What is a SIP? (3)• SIPs assembled on site to form volumes• In the UK SIPs almost exclusively OSB PUR/PS

composites• Traditionally adopted with an engineered timber floor plate

Project objectives• To determine how SIPs & EFJs fail when

exposed to a ‘realistic’ fire scenario.– 60 mins FR ≠ 60 min survivability in a real

fire• Hypotheses:

– (a) SIPs- combustible veneer & combustible core forming a structural composite……………..Not good!

– (b) EFJs- less redundant material…….sudden brittle collapse

Objectives contd…

• Typical SIP system could not be defined by a single large scale fire test so we did 4!

• Target to represent multi-occupancy vs. single dwellings and EPS vs. PUR insulants

• Overarching objective- establish how the system fails.

What did we do?

• 4 x full scale fire tests on SIP buildings– F1- 60 min EPS building– F2- 30 min EPS building– F3- 60 min PUR building– F4- 30 min PUR building– All with I section joist floors

• FR 30- Designed to fail• FR 60- Designed to survive burnout• How? Fire severity designed as 60 min equiv.

EPS

PUR

What did we do? (2)• Additional 3x full scale natural fire tests on

engineered floors– (J1) Traditional solid floor– (J2) I joist floor– (J3) Truss steel web floor

• Comparative performance

The fires:

• 7 experiments conducted between Jan and Dec 2009

What did we find out? (F1-F3)• Buildings survived burnout* without

collapse.• Passive fire protection stayed in place for

the duration of the experiment• Nominal deflection of the floor plate• But some interesting ‘post experiment

findings’*Fire brigade intervened after 80ish min to

accelerate cooling

F1 and F3• Outcome generally positive but….

What did we find out? (F2-F4)

• Mismatch between PFP spec and fire design….failures.

• Imminent floor collapses in both cases (terminated to save instruments etc)

• Extensive damage to wall panels and involvement of insulation

• Identification of system redundancy which prevented major collapse

F2 & F4

Summary of findingsEngineered floors:

– fail in a more sudden and brittle manner than ‘traditional’ joist floors

– have little inherent fire resistance. What does exist is afforded by the membrane mechanism whereby joists are tied together via MDF sheathing

Summary of findings (SIPs)• Where PFP is appropriately specified and

installed: no failures and compartment is capable of surviving burnout without collapse or significant deformation (subject to exemplary workmanship)

• Where thermo-plastic insulants are used i.e. PS, significant damage/loss of composite action can occur as a result of heat transfer through internal linings without direct exposure

• However collapse didn’t occur due to redundancy i.e. alternative load paths through timber ring beams, corner studs and intermediate timber studs.

SIPs…• Without studs whole building collapse would

have occurred.

Concerns of stakeholders- myths and facts

Engineered floors fail in a more brittle/catastrophic manner than solid timberPotential for unseen fire spread (more pronounced in PUR panels)Potential for re-ignition after FRS intervention.

x SIP structures are not robust (they are if solid timber is present)

x Increased fire spread due to service penetrations (insufficient oxygen present)

x Pool fires as a result of melted EPS (low volume of polymer present)

Forward look• Timber is here to stay• Buildings will get higher• Detailing and workmanship becomes more critical• Potential for site fires to become more common• Solutions?

– Invest in research– Promote performance based design

• The standard fire test is only a yard stick.• What about real fires?• What about system interactions?• Buildings in fire fail at interfaces.

Thanks for your time

• Danny Hopkin MEng AIFireE– HopkinD@bre.co.uk– +441923664662– http://www.lboro.ac.uk/cice/people/engine

ers/90.html