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8/21/2019 10-1_Gales
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Fire
Engineering
Research:
Key
Issues
for
the
Future
Post ‐tensioned Concrete Structures in Fire
John Gales
Supervision: Luke Bisby,
Co supervision: Martin Gillie‐ modelling, Phase 1 and 3
Tim Stratford ‐ experimentation, Phase 2 and 3
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What are post‐tensioned buildings?
Conventional steel rebar Prestressing (PS) steel
• Advantages of post‐tensioning concrete with PS steel for load balancing
‐ Thin floors (high ceilings)
‐ Increased span lengths
‐ Reduces building materials
‐ Rapid construction
Highly
optimized
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Typical post‐tensioned buildings
Modern BPT
building, UK
Modern UPT
building, USA
Antiquated (1960s)
UPT building, USA
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Novel building optimization
“ Today’s flat ‐slab post ‐tensioned buildings , for example, with columns spaced (12 m) on center
and span
‐depth
ratios
of
40
are
more
complex
and require more engineering attention thantypical flat ‐slab buildings of 40 years ago , with columns spaced at (6 m) on center and span‐
depth ratios
of
20.
” ‐Randall Poston (chair ACI
318)
• Current guidance is dated and has not
kept up with modern optimization trends
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Real PT slab behaviour in fire is debatable
• PT optimization increases
susceptibility to fire:
‐ PS steel more
sensitive
to
strength
loss
in
high
temperature
‐ Spalling of concrete cover (HS concrete,
precompression of slab)
‐ Unbonded tendons run
continuous , local damage
WILL
effect the entire floor (Key Biscayne demolition)
• Code guidance is based on (often
dated)
standard
furnace
tests
of
simple
span slabs:
‐ modern construction?, building materials?, real fires?
PT Standard fire test (Kelly and
Purkiss, 2008)
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The PhD
• Phase 1 Fire code assessment for unbonded PS steel rupture (spalling, and variable heating length)
• Phase 2 High
temperature
mechanical behaviour of modern PS steel (softening, strength and creep)
• Phase 3 three large‐scale continuous PT slab tests under localised heating
• Side projects while I wait for Phase 3 to begin (curing time delayed)
Temperature compensated time (θ)
C r e e p s t r a i n ( e c r )
∆θ
∆ecr Secondary Creep rate=Z= ∆ecr
∆θ
ecr,0
P r i m a r
C r e e p
S e c o n d a r
C r e e p
T e r t i a r
C r e e p
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Phase 1: Localized fire damage to unbonded
PS
steel• 2009 Tests demonstrated unbonded PS steel rupture is more probable under localized heating ‐ influenced by creep
• Localized fires may be due to spalling, travelling, ceiling jets…
Localized heated UPT tendon tests
( strong back tests ) conducted in my
masters
Lower ratio of heating, failed tendons
at equivalent temperatures
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Phase 1: Localized fire damage to unbonded
PS
steel• IBC, and EC2 analyzed with simple tendon rupture
modelling with creep (time, temp, load dependent)
relation and
heat
transfer
( ASTM
E119
curve)
Parametric analysis: Heated length ratio, spalling, specified concrete cover
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Phase 1 results
Performance based
guidance
not
clearly specified in codes with respect to losing unbonded PS steel in a fire
Considerations
to
made;
restraint,
bonded reinforcing, spalling mitigation
American IBC code was unconservative
Real unbonded PS steel behaviour more
severe than
Phase
1 modelling,
new
modelling parameters needed (Phase 2)
Results have tied in directly or inspired
related PhD
projects
at
Edinburgh
(spalling, concrete cover influence using FEM)
0.00
100.00
200.00
300.00
400.00
500.00
600.00
700.00
800.00
900.00
1000.00
0 0.2 0.4 0.6 0.8 1
Time (hr)
T e m
p e r a t u r e ( ° C )
Cover (0mm)
Cover (1mm)
Cover (2mm)
Cover (3mm)
Cover (4mm)
Cover (5mm)
Cover (6mm)
Cover (7mm)
Cover (8mm)
Cover (9mm)Cover (10mm)
Cover (11mm)
Cover (12mm)
Cover (13mm)
Cover (14mm)
F|IR|E
0
100
200
300
400
500
600
700
800
900
1000
0 0.2 0.4 0.6 0.8 1
Time (hr)
T e m p e r a t u r e ( ° C )
Cover (0mm)
Cover (1mm)
Cover (2mm)
Cover (3mm)
Cover (4mm)
Cover (5mm)
Cover (6mm)
Cover (7mm)
Cover (8mm)
Cover (9mm)
Cover (10mm)
Cover (11mm)
Cover (12mm)
Cover (13mm)
Cover (14mm)
F|IR|E
Example heat transfer compensated for spalling input (200mm slab)
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Phase 2: Modern PS steel behaviour in high
temperature
Used Digital Image Correlation (DIC) in uniaxial tensile tests to
measure deformation and cross section reduction
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Phase 2: Modern PS steel behaviour in high
temperature• DIC patch correlations based on HT paint speckle pattern atch A 750Px gauge length = 37.5mm Patch B
Pixel (y)
Pixel (x)
Stress = 0 MPa
Stress = 1950 MPa
Patch A Patch B
Method needed validation for current use……………….
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Phase 2: Modern PS steel behaviour in high
temperature• DIC to bonded foil strain gauges and extensometer
• DIC cross section to Poisson constant volume theory
• DIC to theoretical
thermal
expansion
calculation
(EC2)
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Phase 2: Modern PS steel behaviour in high
temperature• Creep behaviour using temperature compensated time.
• PS steel types considered; ASTM 421‐1970, ASTM 416‐
2008, and
BS
5896
‐2011 (all
of
different
composition,
but
considered structurally equivalent)
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Phase 2 results
• Uniaxial creep tests at Steady state and Transient
investigating equivalency
Results appeared similar (creep parameters were
identical magnitudes; at
690MPa and
1000MPa
stress levels)
Change in transient test
heating rate had same magnitudes
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Phase 2 results
• Tertiary creep as manifestation of localized yielding
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0 1E-19 2E-19 3E-19 4E-19 5E-19 6E-19
ε c r
Virtual creepstrain (steady
state)
Harmathy
equation
w/ASTM 416
params and area
reduction
690 MPa
12
13
14
15
16
0 1E-19 2E-19 3E-19 4E-19 5E-19 6E-19
Temperature compensated time, θ (x 10-19
hrs)
A r e a ( m m
2 )
Necking region
3300Px distance
1 2 3 4 5 6
Creep curve initiates runaway (tertiary) failure
when a local necking
region develops
Result appears in
transient test
Possible to model, but relations produce error
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Phase 2 results
• Strength tests with true stress in steady state; Implicit
creep strength tests comparison underway (post peak
softening).
Reduction ratios matched well
to Eurocode
Loading
rate
decrease,
decreased yield point
True strength retention at
elevated temperature
better
than EC2 until post peak
softening occurs
0
500
1000
1500
2000
2500
0 0.05 0.1 0.15 0.2 0.25 0.3
S t r e s s ( M P a
)
Strain
100ºC (true)
300ºC (true)
400ºC (true)
500ºC
(true)
100
500ºC
400ºC
300ºC
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Phase 2 results
• Creep models were compared with the results of the
locally heated strong back tests (varied transient and
steady state heating with cooling)
60 0
65 0
70 0
75 0
80 0
85 0
90 0
95 0
1000
1050
0 1 2 3 4 5 6
Test duration (hrs)
U n b o n d e d P S s t r e s s l e v e l ( M P a
ASTM 416 (2008)
actual stress
ASTM 416 (2008)
model
ASTM 421 (1970)
modelBS 5896 (2011)
Transient heating (2°C/min) Steady
heating
(400°C)
Cooling (natural)
Creep model accuracy
function of heating rate
and metallurgy
Error at 2% for2⁰ C/min
growing to 7% error at
30⁰ C/min
Third creep phase not
considered yet
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Phase 3: Continuous post‐tensioned
concrete slabs under localized fire• Two UPT and One BPT , 1‐hour rated EC2 slabs
Tests planned for this summer (6+ months, low MC%)
Restraining forces measured from steel columns (stiffness based on
representative concrete columns
Applied loading
Realistic
span
to
depth
ratio
(>40)
Bonded steel provided
Thermocouples (x24), Linear Potentiometers (x8), Load cells (x2)
Radiant panel heating (locally heated)
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Phase 3: Continuous post‐tensioned
concrete slabs under localized fire
Issues and problems with Phase 3:
• What do we want to do with the results…..
‐ Apriori and Aposteriori round robin modelling?
‐ In house modelling (FEM packages)?
• Instrumentation
‐ What should we be measuring and what does it mean?
‐ Motion imaging? (2D DIC, 3D tracking?)
• Pretesting‐ Ambient tests before heating?
• Intangibles; prestressing the slabs?
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Current collaborative side projects
• Project 1: The History of Fire Safety Engineering (The full story is not recorded)
Traditional and non traditional construction
Large scale testing (Modern and antiquated)ICEM15 conference this July in Porto
Fire behaviour, dynamics and design philosophy
• Project 2: Axis
distance
vs.
clear
cover of
miniature PS slabs exposed to ISO 834. Should this design rule change?
• Project 3: Open
access
repositories for
historical fire engineering photographs and articles
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Thank you
For additional information
Email: [email protected]
Further reading:
•http://www.eng.ed.ac.uk/fire/2009‐ phd ‐ john.html
•Results of Phase 1 can be consulted in the Journal of Structural Fire Engineering and
Fire Safety Journal (see web link for references)
•Some preliminary results of Phase 2 will be presented at SIF 2012 conference in Zurich
•Phase 3 is currently in progress targeting 2013 for completion.