Civil Engineering Reference
In-Depth Information
The provision of anti-crack reinforcement, as specified above, should
reduce deflection by a useful amount. For internal spans, the Eurocode
recommends that the second moment of area of the slab should be taken
as the mean of values calculated for the cracked and uncracked sections.
Some of these points are illustrated in the worked example in Section 3.4.
3.3.7
Fire resistance
All buildings are vulnerable to damage from fire, which is usually the first
accidental design situation to be considered in design, and is the only one
treated in this topic.
In the worked example, design is in accordance with the Eurocodes.
The concepts and methods used are now introduced. Italic print is used
for terms that are defined in EN 1991-1-2,
Actions on structures exposed
to fire
[13] or in EN 1994-1-2,
Structural fire design
[16].
Buildings are divided by
separating members
into
fire compartments
.
A fire is assumed to be confined to one compartment only, which must be
designed to contain it for a specified
failure time
(or
fire resistance
time
)
when subjected to a given temperature-time environment or fire exposure.
A
standard fire exposure
is given in EN 1991-1-2, and other curves are
available that depend on the
fire load density
(calorific energy per unit area,
for complete combustion of all combustible materials) within the compart-
ment considered. These temperature-time curves are reproduced in furnaces
for fire testing, and are simplified models of the effects of real fires.
The walls, floor and ceiling that enclose a compartment must have a
separating function.
This is defined using two criteria:
•
thermal insulation criterion
, denoted I, concerned with limiting the
transmission of heat by conduction, and
•
integrity criterion
, denoted E, concerned with preventing the passage
of flames and hot gases into an adjacent compartment.
The structure of a compartment must have a
load bearing function
,
denoted R (resistance), to ensure that it can resist the design effects of
actions specified for the fire situation, including the effects of thermal
expansion, for a period not less than the specified failure time. The
fire
resistance class
of a member or compartment is denoted (for example)
R60, which means that its failure time is not less than 60 minutes.
Criterion I is met mainly by specifying minimum thicknesses of incom-
bustible insulating materials. These also contribute to meeting criterion E,
which has structural implications as well. For example, excessive thermal
hogging of a beam heated from below can create a gap between it and a
separating wall below.
