BOFIRE – A FEM-programm for non-linear analysis of structural
members under fire conditions
Prof. P. Schaumann, Dipl.-Ing. F. Kettner
DESCRIPTION
BOFIRE is a transient, non-linear, incremental computer code based on the finite element
method. The program includes two main calculation modules: one for calculating the
development and the distribution of the temperature in the structural member (thermal
response model) and another for considering the mechanical behaviour of the structure
taking into account the change of material properties at elevated temperatures (mechanical response model). The mechanical actions in fire situations are taken into account
in the first step, the thermal actions are applied by gas temperature-time curves e.g.
ISO-fire curve. Time steps typically range between 5 and 15 seconds. In each time step
the temperature distribution and the resulting stress-strain distribution in the structure
are calculated. For the material properties the thermal and mechanical definitions of
ENV 1994-1-2 [1] are implemented, thus concrete-, steel- and composite steel and concrete structures can be analysed.
Failure of the structure may occur due to loss of equilibrium (instability, kinematics) or
exceeding the material strengths. The model was developed by Schaumann (1984) [2]
and updated by Upmeyer (2001) [4] and Kettner (2003). Verification of BOFIRE has
been carried out by comparison to numerous fire tests of steel, concrete and composite
structures. An overview of the operation of BOFIRE is illustrated in figure 1.
DATA INPUT
Interactive windows-surfaces can be used to define the cross section, the static system
and the boundary conditions for the numerical calculation. Figure 2 shows the main input window of the user interface HAFRONT.
The following input can be done:
• Geometry:
o System: beam, column, plane frame
o Cross section: Any
•
•
•
Material: Concrete, steel, composite
mechanical loads: Single and distributed loads in/ out of plane
thermal loads: any (prefix: ISO 884)
THERMAL ANALYSIS
For the thermal analysis the cross section is divided in finite elements as shown in figure 3. With the latest versions of BOFIRE all kind of cross sections can be modelled.
With this finite element net a plane transient temperature calculation is done. Therefore
different kind of gas temperatures can be imported.
MECHANICAL ANALYSIS
BOFIRE is able to calculate linear structures like beams, columns or plane frames. Hence
beam elements with up to seven degrees of freedom are used (figure 5).
The calculation of the equilibrium of the internal and external forces is a non-linear and
incremental calculation process considering second order theory and the non-linear material behaviour.
DATA OUTPUT
The spreadsheet program EXCEL is the most popular way to interpret the thermal and
mechanical calculation results of BOFIRE. But the code included data plotting library
DISLIN provides the opportunity to produce colored countourplots of the temperature
distribution or 3-dimensional graphics of stresses or strains as shown in figure 6.
REFERENCES
[1] CEN (European Committee for Standardization) (1994), ENV 1994-1-2 Eurocode 4
– Design of composite steel and concrete structures, Part 1-2: General rules – Structural fire design, CEN, Belgium
[2] Schaumann P. (1984). Zur Berechnung stählerner Bauteile und Rahmentragwerke
unter Brandbeanspruchung (To the Calculation of Steel Members and Frames exposed
to Fire), Technisch-wissenschaftliche Mitteilungen Nr. 84-4, Institut für konstruktiven
Ingenieurbau, Ruhr-Universität Bochum, Germany (in German)
[3] Schaumann, P. and Upmeyer, J. (2000). Behavior of composite structures exposed to
natural fires, 6th. ASCCS International Conference – Steel-Concrete Structures, Los
Angeles, USA
[4] Upmeyer J. (2001). Nachweis der Brandsicherheit von kammerbetonierten Verbundbauteilen über Grenzbrandlasten (Fire Design of Partially Encased Composite
Columns by Limit Fire Loads), Schriftenreihe des Instituts für Stahlbau der Universität
Hannover, Heft 19, Shaker Verlag, Germany (in German)
Ffi,d
Static system and
mechanical action
Discretization of the
cross section
Thermal action
ISO
Thermal Problem
tu
Temperature
distribution in the
structural member at
time t
t [min]
Ffi,d
Mechanical Problem
Failure Time tu
Figure 1: Numerical analysis with BOFIRE
Ultimate loadbearing
capacity at time tu
including the effects
of temperature
Figure 2: Main window of HAFRONT
Figure 4: Finite element net of the cross section
y
w’, My
x
v, Qy
ϑ, MT
z
u, N
w, Qz
v’, Mz
additionally for the warping
of the crossection: ϑ’, Mw
Figure 5: Degrees of freedom for the beam element
Stresses
Temperature
Time:
Figure 6: Stress and temperature distribution over the cross section of a concrete filled tube with an embedded I-section