an integrated computational environment for simulating structures in real fires
DESCRIPTION
Presentation made by Prof. Asif Usmani @ University of Porto during the OpenSees Days Portugal 2014 workshopTRANSCRIPT
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OpenSees Days, Porto, 3-4 July, 2014
An integrated computational
environment for simulating
structures in real fires
Asif Usmani and Liming JiangSchool of Engineering, The University of Edinburgh, UK
&
Jian Jiang, Guo-Qiang Li and Suwen ChenCollege of Civil Engineering, Tongji University, Chi na
With acknowledgements to (other previous and current PhD students):David Lange, Jian Zhang, Yaqiang Jiang, Panagiotis Kot sovinos, Ahmad Mejbas Al-Remal, Shaun Devaney and Payam Khazaie nejad+ the IIT Roorkee and Indian Institute of Science te ams!
& special acknowledgement to Frank McKenna for
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Key components of simulating structures in fire
modelling fire and smoke
1.
750,0°C
932,2°C
750
800
850
900
400°C
980°C
600
800
modelling structural membertemperature evolution
2.
modelling structural responseto the point of collapse
3.
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Integrated computational environment for structures in fire
Fire modelsStandardNatural/parametric (short hot/long cool)LocalisedTravellingCFD (FDS, OpenFOAM, ANSYS CFX)
fire – heat transfermiddleware
heat transfer - thermomechanicsmiddleware
Structural responseOpenSees, ABAQUS, ANSYS (general)SAFIR, VULCAN (special purpose)
Heat transferOpenSees, ABAQUS, ANSYS
Integrityfailure
structure –firecoupling
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Why do we need an “integrated computational enviroment” ?
Current widespread practice is “prescriptive ” (standard fire + isolated member)
Architects are pushing boundaries with larger and taller buildings, internal partitions aredisappearing, more unusual architectural shapes are being used, demand forsustainability is bringing in new materials (and hazards), built-environments are getting more complex and dense creating higher risk (consequences of disaster are increasing) => “alternative” or performance based engineering (PBE) approaches
Even when PBE approaches are used in general uniform compartment fires are assumed (a single compartment temperature at a given instant in time – no spatial variation)
But even if one wanted to make a realistic estimate of the fire, there are no tools tosimulate the whole process , (if commercial vendors make them they would betoo expensive – furthermore researchers will have no control over the tools)
Yes it is very unlikely that such an environment will be used in routine engineering – but routine engineering can benefit from research to create a better understanding of structural response in real fires – IF ONLY we had such a tool! Currently the only way to do a fully coupled simulation is to “conduct an experiment”
How do we learn from our failures (aviation industry is a great example!)
Just like the seismic hazard and the
structure interact in earthquake engineering
(necessitating e.g. soil-structure interaction
for improved fidelity of modelling with
reality) – fire and the structure also interact!
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“Performance” in a general structural engineering context
Modern structural engineering defines “limit states”
δExpressed quantitatively in termsof the fundamental parameters
Load : “maximum” load sustainable (ultimate limit state)
Displacement : “maximum” allowable displacement (serviceability limit state)
Clear and quantifiable link between cause (load) and effect (displacement)
Concept easily extended to general structural frames and extreme loadings,
inter-storey driftin an earthquake
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Performance in the context of struct. fire resistance
CONCRETE
STEEL
ObservationFire heats steel, steel rapidly loses stiffness& strength at temperatures above 400oC withonly half the strength remaining at 550oC
SolutionProtect all steel for a long enough period
Issues with this approach1. How long should a structural member be protected for?
but not for large redundant structures
2. Cause (heating) and effect (reduced load capacity and displacements) works reasonably well for simple determinate structures such as
0
250
500
750
1000
1250
0 30 60 90 120 150 180time [minutes]
Tem
pera
ture
[oC
]
Fire (BS-476-Part 8)? But time on a standard fire curvehas no physical meaning!
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Responses of real structures depend upon the fire
0
200
400
600
800
1000
1200
0 2000 4000 6000 8000 10000 12000 14000
Time (sec)
Atm
osph
ere
Tem
pera
ture
(°C
)
Well-ventilatedOF=0.08
Under-ventilatedOF=0.02
Short hot fire
Long cool fire
Behaviour of a small composite steel frame structur e in ‘long-cool’ and ‘short-hot’ fires,Fire Safety Journal, 39:327–357, 2004
Eurocode model of “natural fires” (valid for < 500 m 2 & < 4m ceiling height)
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e.g. 2x2 generic frame (columns & edge beams protected )
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Deflections - 2x2 generic frame
OF=0.02
max. defl.=310 mm @ 78 minsmax. steel tempr.=750 O C
OF=0.08
max. defl. = 365 mm @ 23 minsmax. steel tempr.=950 O C
Short hot fire Long cool fire
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OF=0.02OF=0.08
TENSILE
Strains (in the horizontal direction) - 2x2 generic frame
COMPRESSIVE COMPRESSIVETENSILE
Different fires produce different structural responsesShort hot fire Long cool fire
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Fires in large compartments
Source:NIST NCSTAR 1-5
Fire tends to travel in largespaces
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Therefore …
Also, we cannot properly address this issue
in a deterministic framework
Permutations are potentially endless!
an “integrated computational environment”
is required to deal with them
(earthquake engineers realised this long ago!)
However,
there are no tools available to easily implement
“proper” Performance Based Structural Engineeringfor Fire Resistance
Our aim is to develop a prototype of such a tool
based on the OpenSees framework
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Plantation Place (Arup Fire)
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Two sub-structure models
10m
9m
9m
Model 1
Model 2
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Fire scenarios
Time
long-cool (parametric) fire
Temperature
short-hot (parametric) fire
0
250
500
750
1000
1250
0 30 60 90 120 150 180time [minutes]
Tem
pera
ture
[oC
]
Fire (BS-476-Part 8)
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Results from Model 1
470mm max deflection380mm max deflection
All beams protected Only secondary beamsunprotected
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Unprotected 10m panel (Model 2)
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Protected 10m panel (Model 2)
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Final Proposal (accepted)
Saving of £250K on Plantation Place
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Engineers learn from failures
Ronan Point,UK (May, 1968) I35W Bridge, Minneapolis, USA (Aug, 2007)
“disproportionate” and/or “progressive collapse” = > “robustness requirements”
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Why some structures collapse
and others don’t in large fires ?
Have we learnt anything?
If so, what has changed?
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Concrete structures do fail in fire!
Gretzenbach, Switzerland (Nov, 2004)
Delft University Architecture
Faculty Building, May 2008
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How can we learn from failures?
Modelling and Simulation
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The “spectrum” of modelling (analytical solutions)
P P
M M
L, E, A, I ∆T, T,zP = EA(ǫT − ǫφ)M = EIφ
L, E, A, I ∆T, T,z
z
x
δz
δx
δx = (ǫT − ǫφ)l
P P
δz
L, E, A, I ∆T, T,zP = EA(ǫT − ǫφ)
Deflections assuming ǫT > ǫφ
δz = δcirc(ǫφ) ≈ δsin(ǫφ)
Post-buckling deflections
where, ǫp = ǫT − ǫφ −π2
λ2add to δz above a δsin(ǫp) calculation
Pre-buckling deflections
where, C =0.3183l2A
πI(ǫT − ǫφ)δz = δcirc(ǫφ) +
Cδcirc(ǫφ)
1−Cfrom thermal bowing from P-δ moments
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The “spectrum” of modelling (computational)
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Is deterministic analysis satisfactory in this context?
The “spectrum” of modelling (multi-hazard)
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Set pass/fail criteria
Monte Carlo Method
Run “n” number of deterministic analyses by randomly selecting values of random parameters
(reduce “n” using a variance reduction technique)
Determine the probability of failure
Identity random parameters and their pdfs
(sensitivity analysis may be necessary)
Determine the acceptable level of “confidence”
How to deal with uncertainty ?
The “spectrum” of modelling (multi-hazard probabilistic)
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The “spectrum” of modelling (probabilistic - whole life)
Pe
rfo
rma
nce
Time
100%
Initial
design life
Normal deterioration
(no repair or maintenance)
material durability and
cyclic load/deformation
induced cumulative damage
Accelerated deterioration
(no repair or maintenance)
following a short duration extreme
event, e.g. blast, fire, windstorm,
earthquake)
Extended life & near 100%
performance with regular
repair/maintenance
Likely scenarios in current
practice
Extreme
eventRepair
Maintenance
Tolerance
threshold
for deterioration
in performance
Life-cycle analysis
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Integrated computational environment for structures in fire
Fire modelsStandardNatural/parametric (short hot/long cool)LocalisedTravellingCFD (FDS, OpenFOAM, ANSYS-CFX)
fire – heat transfermiddleware
heat transfer - thermomechanicsmiddleware
Structural responseOpenSees, ABAQUS, ANSYS (general)SAFIR, VULCAN (special purpose)
Heat transferOpenSees, ABAQUS, ANSYS
This is only part of the big picture
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The “spectrum” of modelling
Model complexity
Com
puta
tiona
l cos
t(lo
g sc
ale)
Analytical
2D sub-framesIdeal fires (unif.)
3D sub-framesNon-uniform fires
3D sub-framesReal fires (CFD)
3D sub-framesReal fires (CFD)
Uncertainty
Whole building
Multi-hazard/multi-scale
UncertaintyLife-cycle analysis
3D sub-framesUncertainty
Non-uniform fires
CURRENT FOCUS
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Previous modelling work
reproduced using OpenSees
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Cardington frame
8 Storey steel frame composite structure
2 tests by BRE
4 tests carried out by “British Steel” (Corus),shown on building plan below
Restrainedbeam test
Corner test
Fra
me
test
Demonstration test
Download report from:www.mace.manchester.ac.uk/project/research/structures/strucfire/DataBase/References/MultistoreySteelFramedBuildings.pdf
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Modelling of Cardington tests using OpenSees
0 100 200 300 400 500 600 700 800 900 1000-450
-400
-350
-300
-250
-200
-150
-100
-50
0
Mid
span
def
lect
ion
of jo
ist a
t grid
line
1/2
(mm
)
Temperature of the joist lower flange at mid span (oC)
Experiment OpenSees
0 100 200 300 400 500 600 700 800 900-250
-200
-150
-100
-50
0
Mid
span
def
lect
ion
of jo
ist (
mm
)
Temperature of the joist lower flange at mid span (oC)
Experiment OpenSees
Restrained beam test Corner test
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WTC Towers Collapse models (3 Floor Fire – no damage)
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WTC Towers Collapse models (3 Floor Fire – no damage)
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3D model: Truss deformations
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Collapse mechanism from 3D model
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Photograph from NIST report
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Photograph from NIST report
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Strong floorWeak floor
Are there generic collapse mechanisms?
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Model to test generic mechanisms
10m
6m
UniversalBeam
UniversalColumn
Beam udl(N/mm)
Columnload (N)
Floorspan
Strongbeam 533x210x92 305x305x198 45 6000 10
Weakbeam 305x102x28 305x305x198 45 6000 10
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Weak floor mechanism Strong floor mechanism
Model results
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OpenSees models
2D model of WTC collapse
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Current status of the work on the
“Integrated Computational
Environment for Structures in Fire”
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Integrated computational environment for structures in fire
Fire modelsStandardNatural/parametric (short hot/long cool)LocalisedTravellingCFD (FDS, OpenFOAM, ANSYS-CFX)
fire – heat transfermiddleware
heat transfer - thermomechanicsmiddleware
Structural responseOpenSees, ABAQUS, ANSYS (general)SAFIR, VULCAN (special purpose)
Heat transferOpenSees, ABAQUS, ANSYS
Integrityfailure
structure –firecoupling
OpenSees
OpenSees
Open source and freely downloadable from UoE andlater main OpenSees site at PEER/UC Berkeley
& later FDS (NIST) & OpenFOAM Eventually all to be packagedwithin a “wrapper” softwarecapable of carrying out PBEand probabilistic analyses
currently Matlab
later OpenSees
OpenSees
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https://www.wiki.ed.ac.uk/display/opensees
o Command manualo Demonstration exampleso Downloading executable applicationo Browsing source code
UoE OpenSees Wiki site
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♦ Scheme for Modelling Structure in fire
SIFBuilder
Fire
Heat Transfer
Thermo-mechanical
User-friendly interface for creating (regular) structural models and enable consideration of realistic fire action
Models of fire action (only idealised fires), i.e., Standard fire, Parametric fire, EC1 Localised fire, Travelling fire
Heat transfer to the structural members due to fire action
Structural response to the elevated temperatures
First version of the “integrated enviroment” in OpenSees
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� Developed for creating large models� Driven by Tcl� Minimum input required
Geometry information-XBays,Ybays,Storeys
Structural information-Material, Section
Loading information-Selfweight, Horizontal loading-Fire action
SiF Builder
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Floor
Building Sub-frame
Partitions
Heat flux
distribution
from a localised
Fire (at a single
instant)
Typical beam X-sections
Typical column X-sections
Fire exposed surface
Fire exposed surface
Why is this needed?
Example showing complexities(currently ignored in practice !) of modelling even a simple realstructure
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Temperature distribution in
the RC column after 1 hour
of EC1 localised fire
Temperature distribution in
the RC column after 2 hours
of EC1 localised fire
OpenSees heat transfer analysis of RC column X-section
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Temperature distribution in
the RC beam after 1 hour
of EC1 localised fire
Temperature distribution in
the RC beam after 2 hours
of EC1 localised fire
OpenSees heat transfer analysis of RC beam-slab X-section
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� Uniform (no spatial variation) fires � Standard fire: ISO- 834 fire curve� Hydro- carbon fire: EC1 � Empirical Parametric fire: EC1 Parametric fire mode l
� non- uniform (spatial and temporal variation) fires� Localised fire� Alpert ceiling jet model� Travelling fire
Idealised Fires
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� HT materials - CarbonSteelEC3, ConcreteEC2- Steel ASCE- easy to extend the library,- Entries for conductivity, specific heat
� HT elements - 1D, 2D, 3D heat transfer elements
� HT recorders (for structural analyses)
� Simple Mesh - I Beam, Concrete slab, Composite beam
Fire
Heat Transfer
� Heat flux BCs- Convection, radiation, prescribed heat fluxes
Tcl interpreter
� Still under development
� Tcl commands available
� Easy to extend
Heat Transfer modelling
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Tcl commands for Heat transfer analysis
Fire
� Initialization of heat transfer moduleHeatTransfer 2D<3D>; --To activate Heat Transfer module
� Definition of Heat Transfer MaterialsHTMaterial CarbonSteelEC3 1;.HTMaterial ConcreteEC2 2 0.5;
� Definition of Section or EntityHTEntity Block2D 1 0.25 0.05 $sb 0.10;
� Meshing the entity#SimpleMesh $MeshTag $HTEntityTag $HTMaterialTag $eleCtrX $eleCtrY;
SimpleMesh 1 1 1 10 10;
� Definition of fire modelFireModel Standard 1;……..
Heat Transfer
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Strategy for efficient heat transfer modelling
Fire
Heat Transfer
3D 2D 1D
q heat flux input
q convection
+radiation
Idealised uniform fires, T(t):
Heat flux input is spatially invariant over structural member surfaces;
2D heat transfer analysis for beam section, 1D for concrete slab
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2D section with localised BC
q heat flux input
q convection + radiation
Idealised non-uniform fires, T(x,y,z,t):
oHeat flux input varies with the location ;
oComposite beam: a series of 2D sectional analyses
oConcrete slab : using localised1D Heat Transfer analyses
Section 1 Section 2 Section 3
1D section with localised BC
Localised heat flux
Strategy for efficient heat transfer modelling
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Composite beam - 2D X-sectional versus full 3D model
0.0 0.5 1.0 1.5 2.0 2.5 3.00
100
200
300
400
500
600
700
800
Bottom flange of steel I-beam
Bot
tom
flan
ge T
empe
ratu
re (
o C)
Section Location (mm)
3D-300s 2D-300s 3D-600s 2D-600s 3D-900s 2D-900s 3D-1200s 2D-1200s 3D-1500s 2D-1500s 3D-1800s 2D-1800s
0.0 0.5 1.0 1.5 2.0 2.5 3.00
20
40
60
80
100
120
140
Mid-depth of Concrete Slab
Tem
pera
ture
in c
oncr
ete
slab
(o C
)
Section location (m)
3D-300s 2D-300s 3D-600s 2D-600s 3D-900s 2D-900s 3D-1200s 2D-1200s 3D-1500s 2D-1500s 3D-1800s 2D-1800s
Composite beam
Length: 3m
Steel beam: UB 356 × 171 × 51
Concrete slab: 1.771× 0.1m
Material with Thermal properties according to EC2 and EC3
EC localised fire
Heat release rate: 3MW
Diameter: 1m, Ceiling height:3m
Fire origin: under the beam end
What we found
Exactly the same temperature profile!
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Concrete slab:
Dimension: 5m×5m× 0.1m
Material with Thermal properties according to EC2
EC localised fire
Heat release rate: 5MW
Diameter: 1m
Ceiling height:3m
Fire origin: under the slab corner
What we found:
Localised 1D analysis produces identical temperature profile as 3D analysis
0 1 2 3 4 5
0
200
400
600
800 Concrete slab exposed to localised fireDiameter:1m
Storey height:3mHRR=5MW
Con
cret
e sl
ab te
mpe
ratu
re (
o C)
Section Location (mm)
Bottom_3D Bottom_1D Mid depth_3D Mid depth _1D Top_3D Top _1D
Temperature plot for slab bottom
Concrete slab – 1D (through-depth only) versus full 3D model
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♦ Thermo-mechanical classees
Heat Transfer
Thermo-mechanical
analysis
� HT recorders (for structural analyses)
� Thermomechanical materials- With temperature dependent properties � Thermomechanical sections-Beam sections & membrane plate section� Thermomechanical elements-Disp based beam elements, MITC4 shell elements� Loading:Thermal action-2D&3D BeamThermalAction, ShellThermalAction- NodalThermalAction
Thermo-mechanical modelling
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uniaxialMaterial SteelECThermal $matTag <EC3> $fy $E0;…
section FiberThermal $secTag {Fibre..Patch..Layer..}…
element dispBeamColumnThermal $eleID $node1 $node2 $NumIntgers $secTag $GeomTransTag;…
block2D $nx $ny $NodeID0 $EleID0 ShellMITC4Thermal $SecTag {….}
♦ Tcl commands for material, section, and elements
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pattern Plain $PatternTag Linear {…eleLoad –ele $eleID -type - beamTherma l $T1 $y1 $T2 $y2 <$T3 $Y3 … $T9 $Y9>…}
Thermo-mechanical analysisThermo-mechanical analysis
♦ Tcl commands for defining beam thermal actions
� Uniform along beam length, non-uniform through depth
pattern Fire $PatternTag $Path $Path $Path $Path $Path $Path $Pa th $Path $Path {…eleLoad –ele $eleID -type -beamThermal $T1 $y1 $T2 $y2 <$T3 $Y3 … $T9 $Y9>…}
Using Linear Load pattern
Using Fire Load pattern for further non-uniform profile
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pattern Plain $PatternTag Linear {…eleLoad -ele $eleID -type -beamThermal -source $filePath $y1 $y2 <$y3…$y9>;…}
Thermo-mechanical analysisThermo-mechanical analysis
♦ Tcl commands for defining beam thermal actions
� Importing external temperature history file
“BeamTA/element1.dat”
60 47.2327 46.4181 47.3834 47.5978 47.6355 47.5978 47.3834 46.4181 47.2327120 75.5848 74.8791 76.5169 77.0164 77.1223 77.0164 76.5169 74.8791 75.5848180 104.494 103.735 105.762 106.516 106.698 106.516 105.762 103.735 104.494….….
Time T1 (corresponding to y1)……………………………T9(corresponding to y9)
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pattern Plain $PatternTag Linear {…eleLoad -range $eleTag0 $eleTag1 -type -beamThermal -source -node ;load $nodeTag - nodalThermal $T1 $Y1 $T2 $Y2;…}
Thermo-mechanical analysisThermo-mechanical analysis
♦ Tcl commands for defining beam thermal actions
� Non-uniform along beam length, either through depth
Apply Nodal thermal action
…load $nodeTag - nodalThermal –source $filePath $y1 $y2;…
Source external temperature file
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…eleLoad –ele $eleID -type - beamThermal $T1 $y1 …T5 $Y5< $T6 $T7 $Z1 $T8 $T9 $Z2 … $T14 $T15 $Z5>…
Thermo-mechanical analysisThermo-mechanical analysis
♦ Tcl commands for defining beam thermal actions
� ThermalAction for 3D I section beams
Web TemperatureT1,2,3,4,5
Lower Flange TemperatureT6,8,10,12,14
Upper Flange TemperatureT7,9,11,12,15
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Examples [Available@UoE Wiki]
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♦ A simply supported steel beam;
♦ Uniform distribution load q= 8N/mm
♦ Uniform temperature rise ∆T;
♦ Using FireLoadPattern
� Temperature-time curve defined by FireLoadPattern:
element dispBeamColumnThermal1 1 2 5 $section 1;
uniaxialMaterial Steel01Thermal 1 308 2.1e5 0.01;
Tcl script
Examples-Simply supported beamExamples-Simply supported beamSimply supported beam
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1) without thermal elongation?
2) UDL removed?
� Deformation shape (without UDL)
� Deformation shape (with UDL)
Examples-Simply supported beamExamples-Simply supported beamSimply supported beam
Material degradation
Beam end displacement
UDL applied
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♦ 2D elements, Fixed ends;♦ Element 1 with ∆T ≠0 , only one free DOF at Node 3
- The effects of Thermal expansion;-stiffness degradation, strength loss;-and restraint effects;
Examples-Restrained Beam under thermal expansionExamples-Restrained Beam under thermal expansionRestrained Beam under thermal expansion
set secTag 1;section FiberThermal $secTag {
fiber -25 -25 2500 1;fiber -25 25 2500 1;fiber 25 -25 2500 1;fiber 25 25 2500 1;
};…pattern Plain 1 Linear {eleLoad -ele 1 -type -beamThermal 1000 -50 1000 50};
set secTag 1;section FiberThermal $secTag {
fiber -25 0 5000 1;fiber 25 0 5000 1;
};…
pattern Plain 1 Linear {eleLoad -ele 1 -type -beamThermal1000 -50 1000 50};
2D beam element 3D beam element
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♦ 2D elements, Fixed ends;♦ Element 1 with ∆T ≠0 , only one free DOF at Node 3
� No strength loss in heated part (stiffness loss considered)
� Considering strength loss
- The effects of Thermal expansion;-stiffness degradation, strength loss;-and restraint effects;
Examples-Restrained Beam under thermal expansionExamples-Restrained Beam under thermal expansionRestrained Beam under thermal expansion
Thermal expansionIn heated element
Material softening
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� Composite beams with column connected
� Deformation shape
1) Column was pushed out by thermal expansion;
2) Being pulled back by Catenary action
Examples-Composite BeamExamples-Composite BeamComposite Beam
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Thank you