Steel structuresform an insulating layer. Intumescent coatings
are particularly effective in situations in which
the steelwork forms part of the architectural
language of the building.
A further fire protection strategy is the use of
water cooling as a means of maintaining the
temperature of a steel structure at an accept-
able level. This strategy normally requires that
hollow-section elements be adopted and in
practice has only very rarely been utilised. A
prominent example is the Centre Pompidou in
Paris in which the circular cross-section
columns are protected in this way. Even in this
building, however, conventional fire protection
systems were used for all other structural
elements.3.6 Structural forms
3.6.1 Introduction
Steel frames are assemblies of structural
elements which are selected or built up from
the standard range of components. They are
prefabricated structures whose constituents
are prepared to a semi-finished state in the
factory or workshop before being transported
to the site for erection.
The design of the connections between the
elements is an important aspect of the
planning of steel frames. Two types of fasten-
ing element are used in steelwork: bolts and
welding. In most cases, welding provides a
better structural junction than bolts, but bolts
are easier to connect on site. Most steel
frames are therefore designed so that they can
be prefabricated by welding into parts which
are small enough to be transported easily (not
more than 10 m in length) and these are then
connected together on site by bolting; most
steel frames therefore have joints in which a
combination of welding and bolting is used.
So far as the structural performance is
concerned there are two basic types of joint,
hinge-type joints and rigid joints. Hinge-type
joints transmit shear and axial force between
elements but not bending moment. They are,
in other words, incapable of preventing
elements from rotating relative to one another 73fire-rating time has elapsed. Various strategies
can be adopted to achieve this. For example,
the structure can be designed such that the
actual stresses under service loads are
maintained at a low level in parts of the build-
ing which are vulnerable to fire (e.g. columns).
Bearing in mind that the problem with fire is
the reduction in yield strength due to tempera-
ture increase, and that instability will not occur
until the yield strength becomes reduced to
the level of the actual stress in an element, the
lower the stress in the material the longer will
it take for that point to be reached following
exposure to fire. If this method is applied,
therefore, the need for protection of steelwork
by insulating material can theoretically be
eliminated or the amount of the required
protection reduced. The problem with the fire
resistance strategy is the lack of data on which
to base the calculations required to prove that
the necessary resistance time will be achieved.
This is currently an active field of research,
however, and the situation may be expected to
improve. The protection of steelwork ('fire
protection') is, however, the strategy which is
normally adopted at present.
Three types of insulating material are used
to protect steel structures. The first group
comprises the traditional materials of
concrete, masonry and insulating board, such
as plasterboard. Concrete is perhaps the most
effective but is expensive and can add signifi-
cant dead load to the structure. Insulating
boards are also expensive because the fixing of
them is labour intensive. The second type of
system consists of spray-on material based on
rock-fibre or vermiculite. This is currently the
cheapest method and is particularly conven-
ient for complex shapes or connections but it
is generally regarded as unsightly and only
suitable in situations in which the structure
will not be exposed to view, such as above
false ceilings. A third type of product is the
intumescent coating. With this system the
protecting material is very thin (treated steel-
work has the appearance of having been
painted) but on exposure to fire, a chemical
reaction occurs which generates gas and
causes the material to become a foam and