Civil Engineering Reference
In-Depth Information
The value of
η
fi
falls from 0.74 to 0.55 as the ratio
q
k
/g
k
rises from zero to
2.0. A note in EN 1994-1-2 gives, as a simplification, the recommended
value
0.65.
It often occurs in cold design that the resistance provided,
R
d
, exceeds
the relevant action effect,
E
d
. This is allowed for in fire design as follows.
A resistance ratio
η
fi
=
η
fi,t
is calculated from
η
fi,t
=
η
fi
(
E
d
/
R
d
)
(3.32)
Using Equation 3.30, the verification condition (
R
fi,d
≥
E
fi,d
) then becomes
R
fi,d
≥
η
fi
E
d
=
η
fi,t
R
d
(3.33)
This enables tabulated design data to be presented in terms of
η
fi,t
, which
typically lies between 0.3 and 0.7.
3.3.7.3
Thermal properties of materials
Stress-strain properties for concrete and reinforcement, and for steel are
given as functions of temperature,
, in ENs 1992-1-2 and 1993-1-2,
respectively. These are used as required in the worked example.
θ
3.3.7.4
Design methods for resistance to fire
The three methods given in EN 1994-1-2 are outlined below. The first
(given last in the code) has the widest scope, but is the most complex. It
is primarily a research tool, used to validate simpler methods.
(1) Advanced calculation models
These methods rely mainly on finite-element computations, which are done
in three stages, starting with a given structure, materials and fire exposure.
(a)
The theory of heat transfer is used to obtain distributions of temp-
erature,
, throughout the structure as functions of the time,
t
, since
the start of the fire.
θ
(b)
From the temperatures, distributions of thermal strains and of the
stiffness and strength of the materials throughout the structure are
computed, for various times,
t.
(c)
The design resistances of the structure are computed at various times,
t
, using data from stage (b). These resistances diminish as
t
increases,
and eventually one of them falls below the corresponding design
action effect. The structure satisfies criterion R if the time when this
occurs exceeds the specified failure time.
