Civil Engineering Reference
In-Depth Information
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Figure 11.7
0.2% strength ratios (lower limits) for aluminium alloys
11.4 Mechanical (structural) response
Once the thermal analysis has been carried out to ascertain the
compartment atmosphere temperatures and the heat transfer to
the structure has been completed it is then necessary to assess
the effect of the increased temperatures on the resistance of the
structural members. In reality, steps 2 and 3 of the fire engin-
eering design (heat transfer and structural response) will gen-
erally be undertaken in tandem, with the rules for calculating
or looking up member temperatures within the same standards
as the rules for evaluating member capacities.
The most comprehensive suite of design standards for
undertaking structural fire engineering design are the struc-
tural Eurocodes. The fire codes cover actions on structures
exposed to fire as well as design procedures for concrete,
steel, composite steel and concrete, timber, masonry and alu-
minium. All these codes have now been published by BSI
for use in the UK along with a National Annex setting out
Nationally Determined Parameters for those areas where
National choice is allowed. Before looking at the methods
for determining structural response it is necessary to look at
the relationship between design loading at ambient tempera-
ture and the design load case for the ultimate limit state for
the accidental design situation of a fire. This is the subject of
the next section.
11.4.1 Load effects at the fire limit state
Traditional design procedures for steel structures are based on
limiting the temperature rise of the steel section to a set value
generally termed the 'critical' temperature for steel. Similarly
tabulated values in the National code for the fire design of con-
crete structures specify minimum cover distances to ensure
that the temperature of the reinforcement does not exceed a
specified limiting value. Such methods are independent of the
load applied under fire conditions and offer simplified often
conservative solutions to the majority of fire design scenarios.
The development of structural fire engineering has high-
lighted the importance of load in determining the fire resist-
ance of structural elements. A major change in the design
methodology for steel structures in fire came about with the
publication in 1990 of BS 5950 Part 8. Although this code is
still based on an evaluation of the performance of structural
steel members in the standard fire test it allows architects and
engineers an alternative approach of designing for fire resist-
ance through calculation procedures. It recognises that there
is no single 'failure temperature' for steel members and that
structural failure is influenced not only by temperature but also
by load level, support conditions and the presence or other-
wise of a thermal gradient through and/or along the member.
The code allows for the consideration of natural fires but does
