Civil Engineering Reference
In-Depth Information
Figure 3.28
Tabulated data for web-encased beams, class R60
Beams rarely have insulation or integrity functions, and have then to be
designed only for the load-bearing function,
R
. The fire resistance class is
normally the same as that of the slab that acts as the top flange of the
beam, so only the structural steel section needs further protection. This
may be provided by full encasement in concrete or a lightweight fire-
resisting material. A more recent method, included in EN 1994, is to
encase only the web in concrete. This can be done before the beam is
erected (except near end connections), and gives a cross-section of the
type shown in Fig. 3.31.
In a fire, the exposed bottom flange loses its strength, but the protected
web and top flange do not. For the higher load levels, given by
η
fi,t
(defined in Section 3.3.7.2), and longer periods of fire resistance, min-
imum areas of longitudinal reinforcement within the encasement,
A
s
, are
specified, in terms of the cross-sectional area,
A
f
, of the steel bottom
flange. The minimum depth,
h
a
, and breadth,
b
f
, of the steel I-section are
also specified, for each standard fire resistance period. The notation for
the steel section is as in Fig. 3.15.
The requirements of EN 1994-1-2 for 60 minutes' fire exposure (class
R60) are shown in Fig. 3.28. The minimum dimensions,
h
a
and
b
f
, in-
crease with
η
fi,t
as shown by the three sets of lines in Fig. 3.28(a). For
other values of
η
fi,t
, interpolation may be used.
The minimum ratios
A
s
/
A
f
are zero for
η
fi,t
=
0.3 (ABC) and
η
fi,t
=
0.5
(ADE). For
0.7, they are indicated within the regions where they
apply. To ensure that the additional reinforcement maintains its strength
for the period of fire exposure, minimum distances
u
1
and
u
2
are specified,
in terms of the specified minimum
b
f
and the fire class. Those for class
R60 are shown in Fig. 3.28(b).
η
fi,t
=
