Technical
Temperature profiles through
composite slabs at elevated
temperatures
Callum Heavens of the SCI discusses different methods for determining temperature profiles
through composite slabs at elevated temperatures.
38 NSC
October 18
Introduction
The resistance of composite slabs at elevated temperatures is
dependent on the temperatures that the critical regions of the
slab reach within a given period of time. The strength of the steel
in the deck, and in any bar or mesh reinforcement, reduces with
temperature, as does the strength of the concrete within the slab.
It is imperative that the temperatures of these components be
determined accurately when designing the slab in order to satisfy
the requirements of a given fire resistance period.
The temperature profile through the slab is dependent on the
depth into the slab from an exposed surface but determining
exact temperatures at a given depth is complicated by the shape
(re-entrant or trapezoidal) and dimensions of the deck profile, as
well as the moisture content of the concrete.
The components contributing to the sagging and hogging
resistance of the slabs will depend on the particular design
method being employed (such as the “Mesh & Deck” method or
the “Bar” method). However, regardless of the chosen method, the
temperature is obtained by a temperature profile which may be
tabulated or given by equation.
BS 5950-8 and BS EN 1994-1-2 provide tabulated temperature
data for light and normal weight concretes for different fire
resistance periods, whilst NCCI document PN005c provides a
set of calibrated equations which give temperature as a function
of depth, considering the fire period and concrete type. By
determining the temperature at the location of interest, the
reduction in strength of the material can then be evaluated for
British Standard design using BS 5950-8 Table 1 to Table 3 or for
Eurocode design using BS EN 1994-1-2 Table 3.2 to Table 3.4.
BS 5950-8
Fire resistant design to British Standards is given in BS 5950-
8:2003, with composite slabs being discussed in Section 8.9.
Here, the temperature profile through a 100mm composite
floor slab with a profiled steel deck is given in Table 12. This table
provides temperatures at 10mm intervals through the slab for
both normal and lightweight concretes and for fire resistance
periods in steps of 30 minutes from a minimum duration of 30
minutes up to a maximum duration of 240 minutes.
This data is plotted in Figure 1 for normal weight concrete.
BS 5950-8 only provides temperature data up to a maximum
temperature of 800°C. No distinction is made between the
temperature profiles of re-entrant or trapezoidal slab decks.
Instead, it is stated that the temperature relates to a location
at the prescribed distance from the nearest exposed surface,
whether that be the deck soffit, rib wall or top of a re-entrant
dovetail.
No limits are placed on the scope of this tabulated data,
however, the limits on the applicability of the insulation criteria
should be noted.
Figure 1: Temperature profiles through a 100mm composite slab according
to BS 5950-8 for normal weight concrete.
BS EN 1994-1-2
The temperature model for composite slabs in the Eurocodes
is given in BS EN 1994-2:2005 Annex D. This annex provides
methods for separately considering the thermal insulation
requirements, the sagging resistance and the hogging resistance.
As part of determining the hogging resistance, Table D.5
provides tabulated temperature data throughout a 100mm slab
for fire durations of 30 to 240 minutes for normal weight concrete
(it is stated that for lightweight concrete, the temperatures should
be reduced to 90% of the normal weight values). As with BS 5950-
8, only temperatures up to 800°C are presented in the table.
The use of this table differs slightly from BS 5950-8 in that the
depth is measured from the base of an “effective slab height”
which is equal to the cover depth of concrete above the deck in
addition to some portion of the rib height (determined by the rib
dimensions).
The scope of this data is limited by a set of dimensional
requirements which are given in Table D.7 of Annex D.
Annex D is an informative Annex. Its use is determined by the
National Annex which states that it “should not be used” with
“alternative guidance given” by “reference to non-contradictory
complementary information”.
NCCI PN005c
PN005c is non-contradictory complementary information that
provides an alternative to the guidance presented in Annex D
of BS EN 1994-1-2. Rather than provide tabulated data as in the
case of BS 5950-8 and BS EN 1994-1-2, the NCCI provides three
calibrated equations for determining the temperature profile
through a composite slab for each of normal and lightweight
concrete.
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/Design_of_composite_steel_deck_floors_for_fire
/Design_codes_and_standards#NCCI
/Floor_systems#Composite_slabs
/Design_codes_and_standards#Introduction_to_Eurocodes