interflam 2016 - design at the interface
TRANSCRIPT
Design at the interfaceA structural fire engineering strategy for an expressed Cor-Ten frame
Dr. Danny Hopkin – Associate Director
Scope
• What interfaces? And why?• Our project;• The challenges we faced; • The lessons we learned; &• Convention.
Fire safety engineering
Structural engineering
Structural fire engineering
The interfaces
What?
Who? How?
An interface between disciplines
The interfaces between facets of competence
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
Competence – A prerequisite for success
4 Pancras SquareFor an industrial building
An industrial site
The building• A 10 storey office – 46 m in height;• Predominantly a concrete frame – cast
insitu & PT;• Architectural feature – external Cor-Ten
frame;• A huge Cor-Ten transfer structure;• Tricky interfaces.
A successful solutionA melange of competing goals, obligations & constraints, of varying intelligibility
The life safety goal
• "Stability for a reasonable period";• Consistency of risk – Kirby, et. al;• Overall reliability requirement of 97%;• Active reliability contribution of 93%;• Passive reliability requirement of 49%;• All 50% have the potential to fully
develop.
0 20 40 60 80 100 120 140 160 180 2000
20
40
60
80
100
Height (m)
Frac
tile
(%)
Fire manifestation
0%
20%
40%
60%
80%
100%
Peak steel temperature (°C)
Perc
entil
e (-)
Thermal conditions
• A lack of guidance – Law & O’Brien – SOA;
• Steady state analysis – overly conservative;
• A need to quantify transient behaviour;
• Consider the impact of wind;• Quantifying thermal gradients,
etc., key.
Thermal conditions
Side 1 Side 2 Rear Front0.00
0.20
0.40
0.60
0.80
1.00
Elevation of elementRela
tive
prop
ortio
n of
com
part
men
t te
mpe
ratu
re (-
)
0 30 60 90 120 150 180 2100
200
400
600
800
1000
1200Fire CompartmentSidesRearFront
Time (min)AS
T (°
C)
- BS EN 1991-1-2 Annex B as a ‘scalar’- Benchmarked against CFD models- Adequately conservative.
Element orientation influences exposure:
• Location ‘manages’ exposure;
• Sections still very hot;• Concrete filling, where
practicable;• Shielding, where
permissible; &• Otherwise, plate sizing.
Fire Floor
Floor Above
Managing temperature
Materials – Cor-Ten• Cor-Ten is not a typical material;
• The scale of the section is not typical;
Peak Temperature
(°C)
Yield Strength Reduction
(MPa)
Residual Yield Strength
(MPa)700 30 325800 60 295900 90 265
Structural response
• Two key areas:• Vierendeel transfer; &• Columns
• Other complications:• Connections;• PT;• Bi-metallic corrosion & PFP.
Vierendeel behaviour
0 5000 10000 15000 20000
-4000000
-3000000
-2000000
-1000000
0
1000000
2000000
3000000
Time (s)
Axia
l for
ce (k
N)
• Expansion governed;• Very sensitive to TFs;• Doesn’t deflect excessively;• Plastic strain tension;• A building that needs to ‘breathe’;• Matching ‘actual’ vs. ‘idealised’.
Column behaviour
• Concrete filling;• Explored rebar vs. T;• T more ‘buildable’;• UC 254x127x84 (S355);• Actions influenced by
curvature & slab ‘push-out’;• Sensitivity to vertical fire
spread explored;
0 30 60 90 120 150 180
-150%-100%
-50%0%
50%100%150%
BF WEB_CTF MAXMin
Time (min)Inne
r Tee
Util
isati
on (%
)
Lessons & key points
• Struct. Eng. understood their responsibility & limits;• “Stability for a reasonable period” not FR120 + sprinklers;• They understood the expertise req’d & delegated;• Those responsible for delivery were involved in design.
• Quantification of the goal -> rational basis -> rational process; • Thermal tools are inadequate for external exposure;• Cor-Ten does not behave like regular carbon steel;• Bigger is not always better;• A need to be pragmatic about what you can’t fully quantify.
Design by convention
Skill & care
• Successful fire engineering doesn’t end when a report is issued….
Acknowledgements
• Co authors – D. Illingworth, E. O’Loughlin, B.McColl & S. Anastasov;• Design team – Eric Parry Associates, BAM, Grontmij• Client – Argent; &• AHJ – Camden LABC, London FB and BRE (reviewer).
Thanks for your time
"If you always do what you've always done, you'll always get what you've always got.“ - H. Ford
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