Insights on modelling structures in fire and recent developments in OpenSees
1. OpenSees Days Europe, 19-20 June, 2017
Insights on modelling structures in fire and
recent developments in OpenSees
Asif Usmani
Department of Building Services Engineering
Faculty of Construction and Environment
HONG KONG POLYTECHNIC UNIVERSITY
With acknowledgements to (other previous and current PhD students):
Liming Jiang, Jian Jiang, Jian Zhang, Yaqiang Jiang,
Panagiotis Kotsovinos, Shaun Devaney, Praveen Kamath,
Xu Dai, Jiayu Hu, Ahmad Mejbas Al-Remal, Payam Khazaienejad,
Mian Zhou, Zhujun Zhang and Mustesin Khan
& special acknowledgement to Frank McKenna !!
6. Shanghai 28 storey residential building fire (2010)
"Shanghai jiaozhou road fire" by monkeyking (Peijin Chen).
7.
8. Traditional philosophy on protecting structures from fire
♦ Insulation
♦ Integrity
♦ Stability
Fire safety requirements are usually expressed as
Fire resistance:
length of time a structural component
withstands exposure to a “standard
fire” while retaining adequate
capacity to resist fire limit state load
For structure/structural members
Compartmentation:
maintaining structural and thermal
barriers to prevent spread for a
sufficient length of time to enable safe
exit of all occupants
For general building fire safety
9. The basis for traditional approaches
CONCRETE
STEEL
Observation
Fire heats steel, steel rapidly loses stiffness
& strength at temperatures above 400oC with
only half the strength remaining at 550oC
STEEL
CONCRETE
While concrete also loses strength and stiffness,
its low thermal conductivity means that fire
only affects the surface layers
Observations
Fire heats steel, steel rapidly loses stiffness
& strength at temperatures above 400oC with
only half the strength remaining at 550oC
10. Proposed (traditional) solution
0
250
500
750
1000
1250
0 30 60 90 120 150 180
time [minutes]
Temperature[oC]
Fire (BS-476-Part 8)
?
Protect ALL steel for a long enough period
Standard fires specify a fixed temperature-time curve (originally developed over 100
years ago in USA in a 2.9mx2.9mx4.4m compartment to reach 926 Celcius in 30 mins).
Provide sufficient cover to reinforcing bars based on duration of exposure
11. Consequences
(design philosophy based entirely on material behaviour)
Reinforced concrete structures are considered to
possess inherent resistance to fire
Steel framed structures are considered to require
protection against fire
12. Concrete structures do fail in fire!
Gretzenbach, Switzerland (Nov, 2004)
Delft University Architecture
Faculty Building, May 2008
14. The Fully Developed Compartment Fires
Ventilation controlled fire. Fuel load,
fibre insulation board, 7.5 kg/m2
Fuel controlled fire. Fuel load,
wood cribs, 15 kg/m2
As the “burning” of 1 kg air releases 3 MJ of energy, in the post-flashover
fire, the rate of heat release (RHR) in the compartment is:
MW52.03RHR HAw×≈
15. Experimental data shows that the ventilation controlled fire is the most
“severe” if judged by the maximum temperatures
Limit of ventilation control
“Opening factor” AT/AwH1/2
The Fully Developed Fire
16. Natural fires in a compartment
Time
long-cool (parametric) fire
Temperature
short-hot (parametric) fire
0
250
500
750
1000
1250
0 30 60 90 120 150 180
time [minutes]
Temperature[oC]
Fire (BS-476-Part 8)
?
18. Simplest “realistic” model of composite beam
Temperature, T(z)
Distance,z
Ambient
Temperature, To
CONCRETE
STEEL
Structure subjected
to the illustrated
temperature
distribution
A
A
Section AA
kr1 kr2 kt
19. Decompose into simpler effects
= +
A uniform increase
In Temperature
∆T
A uniform through-
depth thermal gradient
T,z
z
T (z)To
20. Governing parameters
TT ∆=αε
Thermal expansion induced by mean temperature increment ∆T
l
Tεl
2
2sin
1 φ
φ
φε l
l
−=
yT,αφ =
φεl
Thermal curvature φ induced by through depth thermal gradient T,z
z
x
y
x
Combination of the two effects
leads to large deflections and often
very low stresses (internal forces)
φεε +T
21. Effect of end-restraint
TEAF ∆= α
Restraining thermal expansion
Restraining thermal curvature or “bowing”
F F
M M
zTEIM ,α=
T T
T is highly nonlinear
compression
tension
compression
tension
22. CONCRETE
STEEL
Fire 2
Temperature
Fire 1
T(z)
z
higher ∆T
lower T,z
therefore more
compression
Fire 2
z
Fire 1
lower ∆T
higher T,z
therefore more
tension
T(z)
Therefore different mechanical
responses are produced
0
250
500
750
1000
1250
0 30 60 90 120 150 180
Temperature[oC]
Fire (BS-476-Part 8)
Effect of fire history on response
24. Cardington frame
8 Storey steel frame composite structure
2 tests by BRE
4 tests carried out by “British Steel”
(now TataSteel), shown on building plan
below
Restrained
beam test
Corner test
Frametest
Demonstration test
Download report from:
www.mace.manchester.ac.uk/project/research/structures/strucfire/DataBase/References/MultistoreySteelFramedBuildings.pdf
35. Why are tall building fires different?
Taller buildings
More adventurous architecture
Open plans offices
Larger number of occupants
City centre locations
Multiple-floor fires
Complex structural response
Non-uniform “travelling” fires
Extended evacuation times
Delays in emergency response
Significantly increased risk
(probability x consequence)
NO CURRENT REQUIREMENT
FOR TREATING TALL BUILDINGS
DIFFERENTLY
Except that usually higher fire resistance
times are specified
or
the recommendation
to use
PERFORMANCE-BASED DESIGN
(or P-B ENGINEERING)
36. Fires in large compartments
Source:
NIST NCSTAR 1-5
Fire tends to
travel in large
spaces
37. Performance based engineering for structural fire resistance
NIST recommendation in WTC investigation reports
Code recommendations starting from Approved Document B (UK, 1991)
and followed by many other international codes including Eurocodes
ask for Performance-based Design, where building and fire compartments
are outside the limits of prescriptive design
No coherent guidance provided
Engineers left to own devices
47. 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
Restrained
beam test
Corner test
Frametest
Demonstration test
Download report from:
www.mace.manchester.ac.uk/project/research/structures/strucfire/DataBase/References/MultistoreySteelFramedBuildings.pdf
48. 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
Midspandeflectionofjoistatgridline1/2(mm)
Temperature of the joist lower flange at mid span (o
C)
Experiment
OpenSees
0 100 200 300 400 500 600 700 800 900
-250
-200
-150
-100
-50
0
Midspandeflectionofjoist(mm)
Temperature of the joist lower flange at mid span (o
C)
Experiment
OpenSees
Restrained beam test Corner test
49. OpenSees model of WTC collapse and generic mechanisms
2D model of WTC collapse
50. Integrated computational environment for structures in fire
Fire models
Standard
Natural/parametric (short hot/long cool)
Localised
Travelling
CFD (FDS, OpenFOAM, ANSYS-CFX)
fire – heat transfer
middleware
heat transfer - thermomechanics
middleware
Structural response
OpenSees, ABAQUS, ANSYS (general)
SAFIR, VULCAN (special purpose)
Heat transfer
OpenSees, ABAQUS, ANSYS
Integrity
failure
structure –
fire
coupling
OpenSees
OpenSees
Full 3D “structures in fire” modelling code provided to Frank
to upload to next release of OpenSees (integrated version later)
& later FDS (NIST) & OpenFOAM
Eventually all to be packaged
within a “wrapper” software
capable of carrying out PBE
and probabilistic analyses
currently Matlab
later OpenSees
OpenSees
51. 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 do we need an integrated environment ?
Example showing complexities
(currently ignored in practice !)
of modelling even a simple real
structure
52. 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
53. 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
54. ♦ 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 (idealised fires), i.e.,
Standard fire, Parametric fire (uniform)
Localised fire, Travelling fire (nonuniform)
Heat transfer to the structural members
due to fire action
Structural response to the elevated
temperatures
First version of the “integrated enviroment” in OpenSees
Dr Liming Jiang will provide more details of this in his talk
55. Evolution of the design fires
IDEALISED UNIFORM FIRES
Standard fires (based on tests dating back over 100 years), specifies a fixed temperature-time
curve (originally developed in USA a 2.9x2.9x4.4 m3 compartment to reach 926 Celcius in
30 mins).
– still remains widely used as a “PRESCRIPTIVE” approach for ALL types of construction
Parametric fires (initiated in Sweden, now used widely in Eurocode as it brings in to play
parameters such as fuel load, ventilations and compartment dimensions (limited to 500m2
and 4m height by EN1991)
– beginning to be used in “PERFORMANCE-BASED” approach, often disregarding limits
IDEALISED NON-UNIFORM FIRES
Localised fires available in Eurocodes (e.g for car park type structures)
- rarely used for structural fire resistance design
Zone models and CFD based fire scenarios
- rarely used for structural fire resistance design
Newly emerging proposals
- Using multiple parametric fires (short-hot/long-cool fires - Lamont et al)
- Travelling fires in large compartments
56. Where is this leading to ?
“SCENARIO” FIRES (to enable PBE)
In Buildings:
♦ Post-flashover fires in “small” compartments (relatively well established)
♦ Localised fires (such as a burning car in a parking building – also well established)
♦ Travelling fires in “large compartments” (still developing – Xu Dai (UoE) will present this)
♦ Façade fire (models exist for external flaming but no scenarios for whole façade fire)
In Transport infrastructure:
♦ Bridge Fires (none exist, PhD student Jiayu Hu working in Edinburgh to develop suitable
scenarios)
♦ Tunnel Fires (Hydrocarbon and RWS curves normally used – but no scenarios)
♦ Train carriages
♦ Aircraft
59. Multi-scale hybrid analysis (OpenSees/OpenFresco)
• Composite Beam (consisting of a cellular beam and composite concrete slab
is modelled using thermal shell elements and exposed to ISO-834 fire.
• Drucker Prager Thermal and J2 Plasticity Thermal material used for concrete
and steel respectively.
• Rest of the structure is modelled as Displacement beam column element.
• 18 Node Adapter element and 18 Node Super element are defined in Slave
and master assembly respectively.
Mustesin Khan (Brunel University London) working on this
62. Bridges in fire (Jiayu Hu - University of Edinburgh)
20 minutes of Hydrocarbon fire
63. Bridges in fire
Models being reproduced in OpenSeees
Also working of “scenario” fires for bridges
64. LITS (load induced thermal strain) in concrete
LITS is the difference between strains in heated concrete with and
without preload.
Zhujun Zhang at University of Edinburgh
