Structural fire safety & modern buildings

Dr. Danny Hopkin CEng MIFireE

Head of Fire Engineering, Trenton Fire Ltd.

OverviewFire resistance – A quick history lessonModern buildings – Where are we going?Rising to the challengeDesigning at the interfaceQuestions

‘Fire resistance’A history lesson

FR – A need identifiedOrigins – 1900s (Gales, et al., Bisby & Maluk)– Intended as a temporary practice correction after the

Baltimore and San Francisco conflagrations– Flooding of market place with proclaimed ‘fire proof

materials’– A lack of trust in ‘private testing’– A need to independently benchmark performance

FR – A level playing fieldEmergence of federal and municipal testing laboratoriesNo ‘standardised’ test method/criteriaIra Woolson – NFPA (1903) – A need to:– “unify all fire tests under one single

standard and remove an immense amount of confusion within the fire testing community”

The concept of fire resistance is bornThe ‘test fire’ defined by anecdotal evidence of NY FF

FR – 112 years on….

At the 1917 NFPA annual meeting, Woolson stated that; “we want to get it as nearly right as possible before it is finally adopted, because, after it is adopted by these various associations, it will be pretty hard to change it”.

Structural fire resistanceTests whether an isolated structural element does not violate particular performance criteria after a set period of time in a furnace.Deflection limit span/20It cannot ever be a measure of survivability in a real fire.However, it hasn’t served us too badly…

A divergenceFuture trends

Where are we going?

Into cities & up

Timber is on the rise

Emerging trends - UK

263 towers (>20 storeys) proposed in London…There will be features that are ‘unusual’ or sensitive to fire…How will we approach their design?

Accidental & variable load-cases

Wind – performance based assessmentSeismic – performance based assessmentFire?............................

Fire – apathetically….Solution – protect all steel members to a 120 minute standard for a limiting temperature of X°C

Engineering…..Done!

The apathy part…

Rising to the challengeModern Buildings

What are we trying to achieve?Legally – B3 – “stability for a reasonable period”Holistically –– Business continuity?– Resilience?– Insurability?– Aesthetics?

Delivering a solution that meets aspirations, fulfils obligations, in cognisance of the constraints

Fire safety engineering

Failure at x°C

Structural engineering

Time

Tem

pera

ture

Failure at x°C

With thanks to G. Rein (Imperial College)

Challenging the adequacy of the ‘magic numbers & golden rules’

Assessing the appropriateness of a prescriptive solution Where necessary delivering performance in tangible terms:– Explicit performance goals– Defining what the fires might look like,– Computing how hot the structure might get,– Ensuring adequate structural performance

considering fire as a load caseStructural fire engineering

Something in common?All considered unusual (un-common)SFE integralMore resilientAll have features sensitive to fire that prescriptive design wouldn’t captureSome more cost effective than…

The right process, the right solutionTabulated/prescriptive fire solutions are not invalid, they’re just not a panacea,The key questions:– Do we only care about life safety?– Can the fire be appropriately represented by a

furnace exposure?– Can the structural response be adequately

represented by isolated element behaviour?Answers direct the path to a solution…

Design at the interface4 Pancras Square

The buildingNot an especially tall building, but unusual10 storeys + roof garden46m in heightRetail use at GF, office elsewhereStructural Cor-Ten framePT concrete floor slabs Internal steel composite columns

Key design challenges

An ‘architectural structural frame’,Inability to protect Cor-Ten,Key structural elements were located outside the fire compartment,Limited international experience – Cor-TenDiscipline integration

Explicit definition of the goalWhat is ‘acceptable’ performance?– Building designed to withstand 97% of ‘real’ fires,– A large proportion addressed by virtue of sprinkler

protection – The remainder must be resisted by the passive

(structural system)

‘Scale’

Frequency Consequence

‘Risk’

Defining the firesMonte Carlo simulation (10,000 fires sampled)Large compartments – a need to consider both travelling and post-flashover fires6 fires selected as a design basis that were at least representative of the 97th percentile confidence limit

Fire safety engineering

Thermal exposure to Cor-Ten

Hand calculations informed by EC1-1-2CFD modelling (FDS)Aim – defining temperatures and thermal exposure for ‘external’ elements

0 30 60 90 120 150 1800

200

400

600

800

1000

1200

Time (min)

AST

(°C)

CFD results (dashed)

Design methodology (solid)

Fire safety engineering

Managing external member temperatures

Finite element analysis of temperature developmentThermal ‘load-case’ for structural analysisMitigation measures

0 60 120 180 240 3000

100

200

300

400

500

600

700

Top flange

Web

Bottom Flange

Shielding Plate

Time (min)

Tem

pera

ture

(°C)

Fire safety engineering

Structural response – performance limits & lessons

Aims– Stability!– Prevention of excessive deformation– Materials stay within ‘ductile’ strain limits

Lessons– Expansion governed– Cooling phase critical– Bigger is not always better

Displacement (m)

Structural engineering

Fire safety engineering

• Successfully define the fire fully• Quantify exposure at the building perimeter• Properly quantify structure temperatures• Complete disregard for thermally induced stresses• Interactions not captured

Structural engineering

• Failure temperature of the structure can be defined….• Some ‘system’ interaction, i.e. thermal expansion,

redistribution, etc.• The fire is ill-defined, heat transfer poorly captured• Sensitivity to cooling doesn’t manifest (critical!!!)

Thanks

Danny Hopkin– 07894483449– Danny.Hopkin@trentonfire.co.uk