Civil Engineering Reference
In-Depth Information
The validity of tabulated data of this type is inevitably limited. The
principal conditions for its use, given in the Eurocode, are as follows.
(a)
The composite beam must be simply-supported, with
t
w
≤
b
f
/15
t
f
≤
2
t
w
f
y
≤
355 N/mm
2
b
eff
≤
5m
(b)
The thickness of the concrete slab must be at least 120 mm, but
'thickness' is not defined. It is not clear if a value exceeding
h
c
in
Fig. 3.15 could be used.
(c)
If the slab is composite, the voids formed above the steel beam by
trapezoidal profiles must be filled with fire-resistant material.
(d)
The web must be encased in concrete, held in place by stirrups, fabric
or stud connectors that pass through or are welded to the steel web.
The data given in Fig. 3.28 are used for the design example in Section
3.11.4. The Eurocode also gives both simple and advanced calculation
models, which are often less conservative than the tabulated data, and
have wider applicability. Those for beams and columns are outside the
scope of this topic. Other guidance is available [2].
3.11
Example: simply-supported composite beam
In this example, a typical composite T-beam is designed for the floor
structure shown in Fig. 3.1, using the materials specified in Section 3.2, and
the floor design given in Section 3.4. Ultimate limit states are considered
first. An appropriate procedure that minimises trial and error is as follows.
(1)
Choose the types and strengths of the materials to be used.
(2)
Ensure that the design brief is complete. For this example it is
assumed that:
•
no special provision of holes for services is required;
•
the main source of vibration is pedestrian traffic on the floor,
and occupants' sensitivity to vibration is typical of that found in
office buildings;
•
the specified fire resistance class is R60.
(3)
Make policy decisions. For this example:
•
the steel member is to be a rolled universal beam (UB) section;
•
propped construction is to be avoided, even if this involves pre-
cambering the steel beam;
•
fire resistance is to be provided by encasing the web, but not the
bottom flange, in concrete;
•
nominally-pinned beam-to-column joints are to be used.
