Civil Engineering Reference
In-Depth Information
Until recent times it has been the practice to allocate fire
resistance ratings to individual elements of construction, these
ratings being relevant to the building or structure. Thus for a
typical office block a rating of two hours' fire resistance might
be deemed suitable. All structural elements would then be
treated to meet that criterion. Steel members would be cased
in concrete or other materials such as asbestos (now banned) or
vermiculite boarding. As an alternative, a coating of intumes-
cent paint of the appropriate thickness might be applied to steel
members. However, a more modern approach termed
fire engin-
eering
takes a more holistic view of a structure taking account,
for example, of large-scale fire tests carried out on a 12-storey
steel-framed building at the BRE testing facilities at Cardington.
These tests proved that a steel-framed building designed for a
specific resistance did not necessarily immediately collapse
after the expiry of that time. This is due to the structural con-
tinuity and inherent robustness of the frame. It is also apparent
that a heavy, massive steel section will heat up more slowly than
a light slender section. Modern fire engineering methods permit
the calculation of the fire resistance of uncased steel.
Under the leadership of Professor Colin Bailey of Manchester
University a one-stop facility for dealing with Fire in Structures
is available at www.mace.manchester.ac.uk.
Magnesium
Aluminium
Zinc
Chromium
Iron (Fe
2+
)
Nickel
Tin
Lead
Iron (Fe
3+
)
Hydrogen reference
Copper
Silver
Gold
Anodic
Cathodic
Figure 14.4
Electrochemical series for pure metals
them and the metal with the lower potential value (the anode)
corrodes. Some metals, for example, copper and nickel, therefore
accelerate the corrosion of steel; others such as zinc corrode pref-
erentially and actually protect the steel. The rate of bi-metallic
corrosion increases with the relative separation of the two metals
in the electrochemical series (see
Figure 14.4
). It also depends
on the nature of the electrolyte and the contact area.
Bi-metallic corrosion can be most severe in immersed or
buried structures. In less aggressive environments where,
for example, stainless steel brick support angles are attached
to mild steel structural sections no special precautions are
required. The problem can be avoided altogether by isolating
the two adjacent metals with gaskets of neoprene or similar
material.
14.4.5 Design
Structural design techniques are dealt with elsewhere in this
topic. However, it is relevant to point out that two develop-
ments have radically changed the way the material is designed.
These are:
The change from permissible stress to limit state methods.
■
The advent of Eurocodes; in particular the issue of BS EN1993
■
14.4.3.3 Liquid metal assisted cracking (LMAC)
LMAC is a rare phenomenon that can take place when steel-
work is galvanised to provide protection from corrosion.
Certain solid metals with other liquid metals can give rise to a
reaction which may affect the parent solid material. This reac-
tion is termed liquid metal embitterment (LME) and may lead
to cracking of the steel. For example, when structural steel is
stressed and temporarily in contact with liquid zinc in the gal-
vanising process then LME/LMAC may occur. More research
will identify more clearly the critical factors but stress level,
material susceptibility and the presence of a liquid metal are
thought to be the main elements of the problem.
Eurocode 3: Steel.
14.4.6 Structural sections available to designers
The range of sections available to design engineers is consid-
erable and includes:
Universal beams and columns
■
Universal bearing piles
■
Sheet piles
■
Joists
■
Parallel flange channels
■
Angles (equal and unequal)
■
14.4.4 Performance in fire
Hot finished carbon steel begins to lose strength at tempera-
tures above 300°C and then reduces in strength at a steady rate
up to 800°C. The small residual strength then reduces grad-
ually until meltdown at around 1500°C. For cold worked steels
there is a more rapid decrease in strength after 300°C. The
thermal properties of steel at elevated temperatures are found
to be dependent on temperature rather than stress level and rate
of heating.
Z sections
■
Slimdeck beams
■
Tees
■
Structural hollow sections (square, rectangular and circular)
■
Pressed metal sections (available from specialist suppliers)
■
Structural section in accordance with European, Japanese wide
■
flange and ASTM specifications
