Temperature profiles through
composite slabs at elevated
Callum Heavens of the SCI discusses different methods for determining temperature profiles
through composite slabs at elevated temperatures.
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
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 5950-81 and BS EN 1994-1-22 provide tabulated temperature data
for light and normal weight concretes for different fire resistance periods,
whilst NCCI document PN005c3 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.
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
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.
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.
Technical Digest 2018
Figure 1: Temperature profiles through a 100mm composite slab according to BS 5950-8
for normal weight concrete.
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
Annex4 which states that it “should not be used” with “alternative guidance
given” by “reference to non-contradictory complementary information”.
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
The equations are quadratic curves fitted to data obtained from
a combination of physical test results and finite element (FE) results
calibrated against these tests. The temperature profile within the width
of the rib (i.e. over the full slab depth) is described by two curves. The first
curve applies over the first 80mm above the deck soffit and the second
applies to the remainder of the slab depth. Over the reduced depth of the
slab, the profile is described by a third curve.
The coefficients of each of these quadratics are tabulated for fire
resistance periods of 30, 60, 90 and 120 minutes, with some of these values
dependent on parameters determined by the profile height, slab depth or
trough width. These equations lead to temperature distributions through