Steel structuresof the dimensions of elements on site is diffi-
cult, if not impossible, so that a much higher
standard of quality control is required in such
processes as the initial shaping and cutting to
length, than is necessary with, for example,
timber.
Weight
The density of steel is high and this makes
individual components fairly heavy. Elements
such as beams and columns are difficult to
move around on site and cranes are normally
required for the assembly of steel structures.
Cost
The basic cost of a steel structure is normally
greater than that of its timber or reinforced
concrete equivalent. The shorter on-site
construction time can be a compensatory
factor, however.
Durability
Most steels are relatively unstable chemically
and a corrosion-protection scheme is normally
required for a steel structure.
Performance in fire
Steel loses its ability to carry load at a
relatively low temperature (around 500 °C) and
this means that, while a steel structure does
not actually burn, it will collapse in fire unless
it is kept cool. This is normally achieved by
protecting the steelwork with a suitably thick
layer of fire resistant insulating material but
sometimes more sophisticated methods, such
as water cooling systems, are used. The trad-
itional fireproofing material was concrete - the
elements of a steel frame were simply encased
in concrete - but much lighter materials which
are easier to apply have been developed. The
need to provide fireproofing for steelwork
nevertheless increases the complexity of a
steel-frame building and adds to the cost of
the structure.
The need to provide fire protection is
obviously particularly problematic if there is an
intention to expose the structure as part of its
architectural expression. A prominent example
already mentioned is the Centre Pompidou in
Paris where an elaborate system of shutters,
which would operate automatically in the eventof a fire, had to be installed adjacent to the
glass external wall in order to protect the steel-
work on the exterior of the building should a
fire occur. The steel trusses in the interior were
encased in fireproofing material which was
wrapped in a thin skin of sheet metal to
preserve the appearance of a steel structure. A
similar architectural language was proposed for
the Lloyd's Headquarters Building in London.
In this case, however, no satisfactory way of
meeting the fire-resistance requirements could
be found and reinforced concrete was finally
adopted as the structural material.3.2.2.4 Conclusion
Steel is a material whose properties make it
particularly suitable for skeleton-frame struc-
tures. The principal advantage which results
from its adoption is that it releases the
designer from the restrictions on internal
planning and the aesthetic treatment of the
exterior which are imposed by the use of
loadbearing walls. In addition, the high
strength of steel makes possible a very wide
range of frame types; both single-storey and
multi-storey frames can be constructed over a
very wide range of spans and very tall multi-
storey structures are also possible. In all cases
the structures will be lighter than equivalent
reinforced concrete frames. Other advantages
stem from the fact that steel structures are
prefabricated: these include speed of erection,
ease of assembly on difficult sites and, if
required, statical determinacy. Restrictions on
the overall form of a building must normally be
accepted, however, due to the limited range of
components which are readily available, and
the detailing of the structure is likely to be
complicated by the need to provide fire protec-
tion. In addition, the cost of the steel-frame
structure is likely to be higher than that of an
equivalent reinforced concrete frame.3.3 The properties and composition
of steel
Steel is a ferrous metal (its principal
constituent being iron), but it contains a 61