Civil Engineering Reference
In-Depth Information
The construction time is strongly influenced by the time taken to construct
a typical floor of the building, and here structural steel has an advantage
over
in situ
concrete.
Even more time can be saved if the floor slabs are cast on permanent
steel formwork, which acts first as a working platform and then as bottom
reinforcement for the slab. The use of this formwork, known as
profiled
steel sheeting
, commenced in North America [1], and is now standard
practice in Europe and elsewhere. These floors span in one direction only,
and are known as
composite slabs
. Where the steel sheet is flat, so that
two-way spanning occurs, the structure is known as a
composite plate
.
These occur in box-girder bridges.
Steel profiled sheeting and partial-thickness precast concrete slabs are
known as
structurally participating
formwork. Cement or plastic profiled
sheeting reinforced by fibres is sometimes used. Its contribution to the
strength of the finished slab is normally ignored in design.
The degree of fire protection that must be provided is another factor
that influences the choice between concrete, composite and steel structures,
and here concrete has an advantage. Little or no fire protection is required
for open multi-storey car parks, a moderate amount for office blocks, and
most of all for public buildings and warehouses. Many methods have
been developed for providing steelwork with fire protection.
Design against fire and the prediction of fire resistance is known as fire
engineering [2]. Several of the Eurocodes have a Part 1.2 devoted to it.
Full or partial encasement in concrete is an economical method for steel
columns, since the casing makes the columns much stronger. Full encase-
ment of steel beams, once common, is now more expensive than the use
of lightweight non-structural materials. Concrete encasement of the web
only, done before the beam is erected, is more common in continental
Europe than in the UK, and is covered in EN 1994-1-1 [3]. It enhances
the buckling resistance of the member (Section 4.2.4) as well as providing
fire protection.
The choice between steel, concrete and composite construction for a
particular structure thus depends on many factors that are outside the
scope of this topic. Composite construction is particularly competitive for
medium- or long-span structures where a concrete slab or deck is needed
for other reasons, where there is a premium for rapid construction, and
where a low or medium level of fire protection to steelwork is sufficient.
1.2
Composite columns and frames
When the columns in steel frames were first encased in concrete to protect
them from fire, they were designed for the applied load as if uncased. It
