byl rainham cpd - structural design for fire safety - nov 15
TRANSCRIPT
Structural fire safety & modern buildings
Dr. Danny Hopkin CEng MIFireE MIMechE PMSFPE
Head of Fire EngineeringTrenton Fire Ltd.
Who am I?Struct. Eng. graduate,Joined the BRE (2007-2011),Doctorate “Fire resistance of engineered timber”,Chartered Fire/Mech Engineer,Fire engineering nerd:– Chair IFE FR SIG, IStructE FESG– Member BSI B/525/5 & FSH/24
Lead a team of 12 FEs across 2 offices
Trenton Fire LtdFire & risk consultancy,Specialist & independent,Facilitators and not barriers to successful design;– Code consulting,– Advanced performance based design.
Committed to making sure our design’s are implemented as intended;– Engagement with main contractors– Site visits, etc.
Award winners
OverviewFire resistance – A quick history lessonModern buildings – Where are we going?Rising to the challenge– Competency– Success– Engineering
Designing 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…Key question – will this continue?
A divergenceFuture trends
Where are we going?
Into cities & up
Timber is on the rise
Sustainability
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?............................
Lame substitutions*
Fire safety engineering
Structural engineering
Structural design for fire safety
* Credit Dr. Guillermo Rein (Imperial College)
Lame substitution of the 1st kindStructural engineer is replaced by pseudo-science
Fire safety engineering
Failure at x°C
Lame substitution of the 2nd kind Fire engineer is replaced by pseudo-science
Structural engineeringTime
Tem
pera
ture
Failure at x mins
Lame substitution of the 3rd kind Both engineers are replaced by pseudo-science
Time
Tem
pera
ture
Failure at x°C
Sound familiar?
Fire – apathetically….Solution – protect all steel members to a 120 minute standard for a limiting temperature of X°C
Engineering…..Done!
“intended to provide guidance for the more common building situations…”
Prescriptive FR – a health warning
“need to take into account the particular circumstances of the individual building…”
Progress - reviewing what is built vs. tolerability of performance achieved
The path to contemporary guidance
The apathy part…
Rising to the challengeModern Buildings
CompetenceLow-rise buildings
Medium-rise
High-rise or complex structures
A competent builder?
A structural engineer?
Specialist structural engineering input?
Structural fire safety?
Requirements for success*
What?
Who? How?
Regulations
Responsibility Skill & Care
*Credit: Neal Butterworth
Challenges
Challenges
Successful FR designDefining goals (Regulation & aspirations)Assessing the appropriateness of a prescriptive solution & delegation of responsibilitySkill to deliver performance in tangible terms:– Quantifying the design goals,– Defining what the fires might look like,– Computing how hot the structure might get,– Ensuring adequate structural performance in fire
Care to ensure that the designer’s intentions are achieved in practice
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…
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
Competency revisited…
Regulations
Responsibility Skill & Care
Structural engineers understood they were responsible for ensuring “stability for a reasonable period” in fire
Those responsible for construction were engaged at an early stage and became familiar with the requirements
Design team understood that the fire performance demands were beyond their competency & delegated
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 passive
(structural system) contributions
‘Scale’
Frequency Consequence
‘Risk’
Outcome – building designed to resist fires equivalent to 60 minutes of furnace exposure (or 60 minutes of fire resistance)…
Defining the fires
6 fires selected as a design basis that were at least representative of the 97th percentile confidence limitFires were ‘realistic’ not pseudo representations
Fire safety engineering
Thermal exposure to Cor-Ten
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
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
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!!!)
Care…
Thanks – Questions?
Danny Hopkin
– 07894483449
– http://uk.linkedin.com/in/dannyjhopkin
– https://twitter.com/DannyHopkin
– http://www.slideshare.net/DannyHopkin