fire safety & steel structures - october 2015
TRANSCRIPT
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Structural fire safety & modern buildings
Dr. Danny Hopkin CEng MIFireE
Head of Fire Engineering, Trenton Fire Ltd.
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OverviewFire resistance – A quick history lessonModern buildings – Where are we going?Rising to the challengeDesigning at the interfaceQuestions
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‘Fire resistance’A history lesson
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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
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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
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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”.
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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…
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A divergenceFuture trends
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Where are we going?
Into cities & up
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Timber is on the rise
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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?
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Accidental & variable load-cases
Wind – performance based assessmentSeismic – performance based assessmentFire?............................
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Fire – apathetically….Solution – protect all steel members to a 120 minute standard for a limiting temperature of X°C
Engineering…..Done!
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The apathy part…
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Rising to the challengeModern Buildings
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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
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Fire safety engineering
Failure at x°C
Structural engineering
Time
Tem
pera
ture
Failure at x°C
With thanks to G. Rein (Imperial College)
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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
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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…
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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…
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Design at the interface4 Pancras Square
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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
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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
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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’
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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
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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
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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
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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
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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!!!)