Civil Engineering Reference
In-Depth Information
flange with fire-resistant material. This satisfies conditions (b) and (c) of
Section 3.10; and all of conditions (a) are satisfied.
For the steel beam,
h
a
=
406 mm and
b
f
=
178 mm. This plots as point
H in Fig. 3.28(a). As
η
fi,t
<
0.5, these dimensions qualify, and no addi-
tional reinforcement
A
′
is required.
Web encasement for fire resistance is specified in EN 1994-1-2. Stir-
rups should be at least 6 mm in diameter, with cover not exceeding 35 mm,
at spacing not exceeding 250 mm, with longitudinal bars at least 8 mm in
diameter at their corners. The concrete should be connected to the steel
web by one or more of:
•
welding of the stirrups,
•
bars passing through holes in the web,
•
stud connectors.
The encasement does not increase the resistance of the composite sec-
tion to sagging bending, because all of the concrete is in longitudinal
tension and the additional reinforcement is neglected. The width of cracks
in the concrete should be controlled, and Eurocode 4 gives rules for this
purpose. Where the design crack width is 0.5 mm, it requires that the
spacing of longitudinal bars should not exceed 250 mm.
A possible design for the web encasement, in accordance with these
rules, is shown in Fig. 3.31. The 6-mm stirrups are either welded to the
web or passed through holes in it.
In a fire, the web encasement preserves the resistance of the beam to
vertical shear, and the slab is thick enough to protect the top transverse
reinforcement and the shear connection.
